Part 1: Overview

1. Background

This report was prepared by the Ozone Transport Assessment Group (OTAG) Air Quality Analysis (AQA) Workgroup to aid the deliberations of the OTAG Policy Group with policy-relevant information. The ozone problem addressed by the workgroup stems from the existence of nonattainment areas in the OTAG domain and the fact that some nonattainment areas experience considerable influx of ozone (O₃) across their boundaries.

The analysis of data from more than 600 monitoring stations shows that the highest average concentrations of ozone (60–80 ppb) within the domain roughly coincide with the highest emissions densities of anthropogenic nitrogen oxides (NOx) and volatile organic compounds (VOCs) near major metropolitan areas and along the industrial Ohio River valley. The ozone levels at the edges of the OTAG domain correspond to the tropospheric background range of 30–50 ppb. A 10-year trends analysis shows a decline of the number of 120-ppb and 80-ppb exceedances in the northeast. However, the reductions of these exceedances over the entire domain is less pronounced, particularly if one disregards the anomalous high ozone year of 1988.

Low wind speeds (< 3 m/sec) enable the accumulation of ozone near local source areas. High winds (> 6 m/sec) reduce the concentrations but contribute to the long-range transport of ozone. The average range of ozone transport implied from an array of diverse methods is between 150 miles and 500 miles. However, the perceived range depends on whether one considers the average concentrations (300–500 miles) or peak concentrations (tens of miles at 120 ppb). The relative importance of ozone transport for the attainment of the new 80 ppb 8-hour standard is likely to be higher due to the closer proximity of nonattainment areas.

The modeling results have been evaluated based on model performance and the representativeness of the selected modeling periods compared to climatological conditions. The model simulations have captured the large-scale features of each episode. The model, however, had a tendency to underpredict (10–20 ppb) the daily maximum regional ozone concentrations in the north and overpredict in the south. The transport wind fields and the average ozone concentration pattern during the four episodes (36 days) are representative of OTAG domainwide episodes that occur three to eight times a year. Stagnation over multistate areas, followed by swift transport, is an important characteristic of O₃ episodes that are prevalent over the central portion of the OTAG domain, where the NOx emissions are also high. The modeling periods are not particularly representative of the highest ozone concentrations occurring locally. In particular, high O₃ events over the Gulf states are underrepresented in the modeling periods.

The anthropogenic ozone in the OTAG domain originates from within the domain and, therefore, it is controllable with measures inside the domain. The strong weekly cycle of peak ozone concentrations together with the observed parallel 10-year trends of ambient ozone and precursor emissions in some subregions suggest that ozone reductions are feasible. Urban areas contribute to their own ozone problems, and VOC controls appear to be effective for "peak shaving" of the 120-ppb standard. NOx emissions reductions are likely to be effective for regional ozone reductions and for attaining the 80-ppb standard. Emissions in the central part of the OTAG domain along the industrial-urban Ohio River valley appear to be associated with many regional-scale ozone episodes. Reductions in that area could benefit many receptor areas downwind.

The AQA Workgroup recommends preserving the air quality analysis infrastructure and the stakeholder-based approach. Future monitoring and assessment programs should set as further goals the quantification of ozone source attribution, flow across political boundaries, and routine evaluation of photochemical models.

2. Introduction

The AQA Workgroup was formed to provide assessments of air quality and meteorological data relevant to OTAG’s mission. The workgroup placed special emphasis on ozone transport because some nonattainment areas experience considerable influx of ozone across their boundaries, and they cannot demonstrate attainment by local measures alone. Specifically, the workgroup’s purpose statement read as follows:

The Air Quality Analysis Workgroup shall identify, characterize, compare, and assess observational data and studies, including but not limited to air quality trends analysis, overflight data, and meteorological studies for the purpose of evaluating the effects of the transport of ozone and its precursors on ozone nonattainment in the eastern United States.

The workgroup process entailed the development of individual work products and the presentation of results at workgroup meetings, which was followed by open review by the group. The workgroup collectively developed policy-relevant summaries and subjected these summaries to repeated group reviews. This process was greatly facilitated by the early development of an interactive Internet Web site, which has been used to communicate data sets, analytical tools, results, interpretations, and critical feedback.

The workgroup members were affiliated with the U.S. Environmental Protection Agency (EPA), state agencies, industry (power and transportation), consultant firms, and academia. The specific activities undertaken by this group included reviewing existing air quality studies and analyses; developing analyses and visualizations of air quality and meteorological data to help in the understanding of ozone formation and transport; comparing modeling results with available air quality data; and integrating air quality analyses and modeled results into conceptual interpretations of ozone transport for use in policy development. The multiple types of analyses included spatial and temporal pattern and trends analyses, trajectory and residence time analyses, correlation and cluster analyses, detailed evaluations of intensive field studies (Southern Oxidants Study (SOS) and North American Regional Strategy for Tropospheric Ozone-Northeast (NARSTO-NE)), and comparison of model results with the data.

The efforts of the Regional and Urban Scale Modeling (RUSM) and AQA Workgroups were intended to provide complementary input to support OTAG policy development. The modeling effort evaluated the effects of future control strategies using photochemical grid modeling for specified episode periods. The air quality analysis results assessed the ozone problem using long-term measurements from monitoring networks. A comparison of the modeling results for the four modeling episodes with corresponding observations and with climatological values provided a good indication of the strengths and weaknesses of photochemical grid modeling, including simulation performance and representativeness.

The air quality assessments helped "set the stage" for the Policy Group by providing broader perspectives on the current ozone problem and its characteristics. While modeling is uniquely suited for evaluating the consequences of future emissions scenarios, the complexities of the ozone problem are such that comparisons with data are a necessity. The combination of modeling and air quality analysis results can help to improve confidence, and identify uncertainties, in the outcome of future control scenarios.

The paragraphs that follow present the major policy-relevant results, conclusions, and recommendations of the AQA Workgroup. Part I of this chapter is divided into the following major categories:

• Origins and Patterns of the Ozone Problem in the OTAG Domain
• Ozone Transport in the OTAG Domain
• Air Quality Comparisons to UAM-V Episodic Model Results
• Air Quality Management Implications of the Data Analysis Results
• Recommendations To Foster Future Air Quality Analyses

In each major category, a set of policy-relevant technical questions are posed, followed by brief statements of pertinent results. The supporting materials for this part of the report can be found in Part II: Summary and Integration of Results and in Part III: Summaries of Individual AQA Workgroup Analyses, all accessible through the AQA Workgroup Web site at: http://capita.wustl.edu/OTAG/.

3. Origins and Patterns of the Ozone Problem in the OTAG Domain

What Is the OTAG Ozone Problem?

Recent health and ecological studies suggest that adverse biological effects can result from ozone exposures at any level above natural background. All sections of the OTAG region periodically experience ozone levels higher than the natural background, and they share a common interest in addressing the ozone problem. However, the severity, frequency, and duration of high ozone events exhibit complex patterns and source receptor relationships, such that the most efficient strategies to reduce local or regional ozone exposures are not obvious.

Control strategies depend directly on how the OTAG ozone problem is defined. Current federal health standards focus on peak 120-ppb, 1-hour concentrations; proposed health standards focus on 80-ppb, 8-hour averages. Ecological effects result from chronic exposures accumulated over the entire growing season.

The spatial pattern of the current (1993–1995) exceedances (120-ppb, 1-hour) shows that the following major metropolitan areas exceed the current standard: the Washington, D.C.-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, and St. Louis (Figure 4-1). Other metropolitan areas throughout the OTAG region also remain in nonattainment with the current standard.

Virtually all areas where exceedances of the current standard occur are confined to the near vicinity (less than 150 miles) of metropolitan areas. In contrast, areas exceeding the proposed 8-hour, 80-ppb ozone standard are more numerous, extend further from metropolitan areas, and include a large portion of the central OTAG domain. The distances between nonattainment areas projected under this proposed standard are significantly shorter than those under the current standard and, therefore, the likelihood is greater that one area influences the exceedances in its neighboring nonattainment areas.

What Is the Pattern of Ozone Precursor Emissions?

Ozone precursors are VOCs and NOx from area and point sources. Anthropogenic area sources of both VOCs and NOx are most dense in large urban metropolitan areas (e.g., the Washington, D.C.-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, and St. Louis). The largest, elevated point sources of (predominantly) NOx emissions are prevalent in industrial regions including the Ohio River valley (Figures 4-2(a) and (b)). Anthropogenic NOx area sources tend to have strong diurnal and weekly cycles, while point sources typically vary less in time. VOC emissions from both anthropogenic and biogenic sources are heavily influenced by sunlight and temperature; therefore, they tend to exhibit stronger diurnal and seasonal variations than NOx emissions.

What Are the Spatial Patterns of Ozone?

The AQA workgroup evaluated the regional ozone concentration patterns using a comprehensive data set of more than 600 monitoring stations from routine EPA and research networks. The entire data set includes 102 urban sites, 259 suburban sites, and 238 rural or remote sites (13 sites in the network are not classified).

During the summer ozone season, the OTAG domain is periodically ventilated by air coming from outside the domain where ozone concentration averages range between 20 ppb and 50 ppb, corresponding to typical tropospheric background levels measured in the northern hemisphere (Figures 4-3(a) and (b)). It is reasonable to assume that in the absence of anthropogenic emissions, the average summertime ozone concentration would be approximately 20–50 ppb throughout the OTAG domain. Thus, with the notable exception of the Canadian border along the Windsor-Quebec corridor, there are no significant external source impacts on the domain, at least on a regional scale. Large sections of the domain experience average daily maximum ozone levels of 60–80 ppb, which is double the tropospheric background. The highest average concentrations are observed near major metropolitan areas and in a large central subregion of the of OTAG domain along the Ohio River (Figures 4-3(a) and (b)).

What Is the Range of Ozone Concentrations?

Ozone exhibits strong day-to-day variation that can be quantified by examining "cleaner" and "dirtier" days across the domain (Figures 4-4(a) and (b)). The urban impact is virtually undetectable during cleaner, low-ozone days (i.e., the lowest 10th percentile of all measured ozone concentrations). However, even these cleanest days exhibit a broad area of higher average ozone (> 40 ppb) from Illinois to Pennsylvania relative to the remainder of the OTAG domain. During the dirtier, high-ozone days (the 90th percentile) the urban influence is very pronounced but confined to about 100–200 miles from major metropolitan areas. It is therefore observed that urban areas make substantial local contributions to the highest 1-hour daily maximum ozone concentrations as well as exhibit the highest range in ozone concentration as measured by difference between 90th and 10th percentiles.

How Does Ozone Vary in Time?

Ozone exhibits temporal variability over hourly, diurnal, synoptic (3–5 days), weekly, seasonal, and long-term (5–20 years) time scales. The ozone changes on weekly and long-term scales are caused primarily by anthropogenic emissions changes; changes at the hourly, diurnal, synoptic, and seasonal scales are also influenced by meteorology.

Ten-year trends show that the number of 120-ppb, 1-hour exceedances has declined markedly over the past decade in areas like the northeastern Ozone Transport Region (OTR) (Figure 4-5(a)) and southern California. The percentage reductions of the 80-ppb, 8-hour exceedances were less.

These areas have also experienced substantial emissions reductions of both VOC and NOx. For the OTAG region as a whole, improvements have been much less dramatic (Figure 4-5b). If 1988 is considered an anomalous ozone year, the OTAG region has experie nced relatively small changes in the number of 120-ppb and 80-ppb exceedances and OTAG-wide average ozone concentration over the past 10 summers (Figure 4-5(b)). Evidently, historical reductions of VOC and NOx emissions have been successful in reducing pe ak ozone levels on a local or subregional scale. However, broader control approaches may be necessary to reduce the more regionally distributed 80-ppb, 8-hour average ozone levels.

4. Ozone Transport in the OTAG Domain

Ozone Transport: Beneficial or Harmful?

Atmospheric conditions can exert a powerful influence on the distribution of pollutant concentrations in space and time. Low wind speeds lead to the buildup of high local pollutant concentrations (Figure 4-6(a)). Strong ventilation with high wind speed s prevents the local buildup near the sources (Figure 4-6(b)), but contributes to long-range transport and regional ozone, particularly during directionally persistent wind conditions. For southern urban areas where episodes are caused by local stagnation , ozone levels decline rapidly with increasing wind speed (Figure 4-7(a)). In northern cities, which are more heavily influenced by transport, ozone levels decrease much less rapidly with increasing wind speeds.

Vertically, ozone transport takes place in synoptic (> 800 m), channeled (200–800 m), and near surface (< 200 m) flow regimes (Figure 4-7(b)). The potential for transport from elevated (> 100m) emissions sources is substantially higher th an for low-level sources, due to higher wind speeds aloft, particularly at night during channeled, "nocturnal jet" conditions.

Examination of dispersion conditions during locally high-ozone days (90th percentile) shows that dispersion in the southeast is typically poor due to stagnating air masses (Figure 4-8(a)). However, the western and northern sections of the domain exper ience stronger and more persistent southerly observation and westerly winds, respectively. This observation supports the notion that ozone exceedances in the central and southeastern areas are predominantly "homegrown," while exceedances in oth er sections of the OTAG domain are also influenced by regional transport. In contrast, on low-ozone days, transport is predominantly from the outside (e.g., Canada and the Gulf of Mexico) into the OTAG domain (Figure 4-8(b)).

The widespread, regional-scale ozone transport episodes result from several days of stagnation over the central portion of the OTAG domain, followed by strong unidirectional flow, generally to the northeast. Three (1988, 1991, 1995) out of four episodes chosen for OTAG modeling were such stagnation-followed-by-transport regional episodes.

In summary, the "good news" about transport is that it can disperse, or clean up, the ozone formed in an area during a stagnation event. The central and southeastern portions of the OTAG domain, which experience relatively more stagnation, ca n benefit from this aspect of transport. The "bad news" about transport is that it can carry significant ozone concentrations from one portion of the domain to another, causing influx of regional ozone across the boundaries of the nonattainment areas, particularly over the midwestern and northeastern portions of the domain.

AQA Workgroup researchers have assessed the transport issue based on examination of meteorological data, trajectory analyses, and field observations of ozone and ozone precursors. These analyses have not estimated ozone transport at future times.

What Are the Implied Ranges of Ozone Transport?

The distances of ozone impact deduced from multiple types of analysis range from 150 miles to 500 miles (Figure 4-9(a)). The directly attributable influence of specific urban areas can be traced 150–200 miles before the urban-industrial plumes mer ge indistinguishably into the regional ozone pool. Ozone and precursors transported at night can have a significant impact hundreds of miles downwind the next day.

Statistical correlation analyses of the regional ozone pattern suggest ozone transport distances of up to 300–500 miles, but it is not clear to what extent this actually represents transport of ozone and/or precursors, or is a meteorological corre lation. Time-lagged correlation analyses also suggest linkages between ozone concentrations separated in time by 1–2 days. Interpretation of such a time-lagged correlation as an ozone lifetime of 1–2 days corresponds to a transport distance of a bout 400 miles.

While an estimate of ozone impact distance based on any single method is rather uncertain, the coinciding range of the various methods increases confidence in the accuracy of these estimated ozone transport distances.

The perceived distance of ozone impact can vary considerably depending on the measures used to describe the ozone impact. In general, the spatial scale of perceived ozone transport decreases rapidly with an increasing concentration threshold (or incre asing percentiles of the ozone distribution). For example, the average daily maximum ozone has a scale of impact of hundreds of miles, while ozone exceedances of 1-hour, 120-ppb thresholds may extend only to tens of miles. This finding underscores the not ion that an 8-hour, 80-ppb ozone standard will implicate larger areas and longer transport scales than the 120-ppb, 1-hour standard.

5. Air Quality Comparisons to UAM-V Episodic Model Results

How Do Model Results Compare to Measured Concentrations?

From the point of view of AQA Workgroup, the utility of modeling results as a foundation for policy making can be evaluated using two criteria: (1) model performance during the selected (36-day) episodes and (2) the representatives of these episode con ditions for "typical" or climatological conditions.

Visual comparison of maps of measured and modeled ozone values shows that the simulations capture the large-scale dynamic and spatial features of each episode. Although not seen in every episode, the model appears to have tendencies to underpredict da ily maximum ozone levels in the northern portion of the domain by 10–20 ppb (Figures 4-10(a) and (b) and 4-11(a)), where high ozone is frequently associated with strong, synoptic-scale flows. The model tends to overpredict the ozone for the southern portion of the domain where high ozone is typically associated with local stagnation. Further, aircraft measurements in the northeast for a few simulation days indicate that ozone levels above the surface layer (> 200 m) may be underpredicted by the m odel. One possible interpretation of these observations (although not the only one) is that transport impacts may be understated by the model. This interpretation, and the generally longer scales of transport indicated by the climatological air quality an alyses, should be kept in mind when analyzing model results regarding transport distances.

Comparison of model predictions to ozone precursor data, while limited by the availability of measurements, shows marginal to poor agreement, especially for isoprene from biogenic VOC emissions. In addition, comparison of modeled and observed carbon mo noxide (CO) (a tracer of automotive emissions) shows poor agreement for the 1995 episode. Comparison of ozone to total reactive nitrogen ratios in non-urban locations tends to agree with model predictions.

The ability of the model to capture the large-scale features of the ozone concentration patterns and non-urban ozone to total reactive nitrogen ratios suggests that the Urban Airshed Model, Version V (UAM-V) is a useful tool for evaluating the gener al features of future large-scale control options. However, the limitations of model applications, such as those suggested by these analyses, reinforce the notion that modeling results should be interpreted not as definitive or absolute, but rather as on e part of a full assessment. The full assessment, relative to the mission of OTAG, would thus include modeling, air quality analysis, and information relating to control strategy feasibility and cost.

How Does Transport During Modeling Episodes Compare to Episodes in General?

The transport conditions during the selected OTAG modeling episodes are similar to transport during OTAG domainwide regional ozone episodes in general (Figure 4-12(a)). Such episodes are characterized by slow meandering transport over Kentucky, Tenness ee, and West Virginia, with a strong clockwise transport around this region of stagnation.

However, the transport during OTAG modeling periods differs significantly from the conditions of highest local ozone concentrations (Figure 4-12(b)). The main difference is that the net transport speeds are predominately higher during the OTAG modeling episodes, and the direction of net transport over the Illinois-Pennsylvania corridor is from the west, rather than the typical southwesterly flow.

What Kind of Episodes Do Model Results Represent?

Within the OTAG domain, high-ozone concentration (above 80 ppb or 120 ppb) can occur during regional, subregional, or local episodes. The OTAG domainwide regional episodes are caused by slow-moving or recirculating airmasses over the center of the OTAG domain (Figure 4-9(a)). Coincidentally, the central Ohio River region is a high NOx emissions area (Figure 4-2(b)). A regional episode is defined here as the condition when the daily maximum ozone concentration averaged over all the OTAG domain monitorin g sites exceeds 70 ppb (Figure 4-9(b)). All four periods selected for OTAG modeling are regional. During the 1988 episode, the OTAG domain average ozone exceeded 70 ppb for 9 consecutive days with an OTAG domain average peak of 103 ppb. The 1991 and 1995 episodes lasted for approximately 6 days with peaks between 80 ppb and 90 ppb. The 1993 episode is best characterized as a subregional episode over the southeast because the OTAG domain-average ozone barely exceeded 70 ppb on 1 day.

The pattern of average measured ozone concentration (Figure 4-10(a)) shows that during the four modeling episodes ozone is high over the industrial midwest, Pennsylvania, and New Jersey, and low along the Gulf coast.

The OTAG domain-scale regional episodes occur about three (1993) to eight (1988) times, covering approximately 10 percent of the April-to-September ozone season. Multiday ozone accumulation causes about one-half of the OTAG-wide recorded 120-ppb excee dances to occur during these regional episodes. However, these episodes account for only 30 percent of the 80-ppb exceedances. Hence, their role for the proposed new standard is diminished compared to the old standard.

How Do Model Episode Concentrations Compare to Episodes in General?

The modeled impacts derived from any specific historical episodic periods cannot be taken as representative of the full range of meteorological flow conditions that may be anticipated in the future. However, the average ozone during the four modeling p eriods (Figure 4-10(a)) appear to be typical for regional ozone episodes that occur three to eight times during every ozone season. The average modeled episode concentrations are within about 5 ppb of the averages for climatological (1991–1995) re gional episode conditions.

The concentrations during the OTAG modeling episodes do not appear to be representative of high concentrations that occur locally. A measure of modeling period overall representativeness is a comparison with the highest ozone (90th percentile) concentr ations (Figure 4-4(b)). In the Pennsylvania-New Jersey region, the modeling periods roughly correspond to the 90th percentile values. However, near the Gulf coast (including Texas) the modeling episodes represent only approximately the 60th percentile o f ozone. Thus, the high ozone concentrations in the Gulf coast belt and Texas are underrepresented in the OTAG domain modeling selection days (Figure 4-11(b)). This should be kept in mind when using the episode simulations for policy development.

6. Air Quality Management Implications of the Data Analysis Results

Is Ozone Controllable by Measures Within the OTAG Domain?

Ozone is indeed controllable by measures within the OTAG domain. The high ozone concentrations in excess of the tropospheric background originate from within the OTAG region (Figures 4-13 (a) and (b) and 4-14); tropospheric background levels of ozone a re found at nearly all of the borders of the OTAG domain (Figure 4-3(a)), with the exception of the Windsor-Quebec corridor. This finding means that the high ozone concentrations are not due to external influences but contributed by sources internal to th e OTAG domain. Furthermore, the high ozone concentration regions roughly coincide with the pattern of anthropogenic precursor emissions as modified by atmospheric dispersion. Consequently, it can be inferred that most of the excess ozone concentrations o bserved within the domain result from anthropogenic emissions within the domain.

Is There Evidence That Precursor Emissions Changes Cause Ozone Concentration Changes?

Empirical evidence suggests that anthropogenic emissions changes do cause changes in ambient ozone concentrations. The data show a pronounced weekly cycle of ozone exceedances, with the 1-hour, 120-ppb exceedances on Sundays reduced by a factor of 3 compared to Friday exceedances. This reduction is most pronounced in urban areas, while in the central portion of the OTAG domain, the weekly ozone cycle is virtually nonexistent (Figures 4-15(a) and (b)). Hence, any control scenario that simulates the w eekday-weekend emissions changes would be effective in reducing the 1-hour, 120-ppb exceedances. It should be noted that the 8-hour, 80-ppb exceedances show less weekly fluctuation, indicating that such a control scenario would be less effective in reduci ng exceedances of the new standard. While this natural "emissions control" experiment provides interesting results, the workgroup does not have good information on the actual emissions changes that occur between weekdays and weekends.

Examination of long-term trends of ozone as well as NOx and VOC emissions show that the largest ozone declines occur over approximately the same geographic area where simultaneous VOC and NOx emissions reduction occur. This observation provides furthe r evidence, but not proof, that emissions reductions cause declines of ozone concentrations.

Do Air Quality Data Suggest High-Leverage Means for Controlling Ozone?

Looking at spatial pattern, temporal pattern, and transport considerations, some general control approaches appear to have higher leverage. Clearly, urban nonattainment areas contribute significantly to their own ozone problems as well as to downwind a reas within their area of influence, 150–500 miles away. Evidently, urban VOC controls are effective for peak shaving near metropolitan areas.

Spatial pattern analysis indicates that the central area of the OTAG domain along the Ohio River is chronically exposed to moderately high ozone levels (Figures 4-4(a) and (b)). Forward and backward trajectory analyses (Figures 4-13(a) and (b)) and sur face wind-ozone rose analyses (Figure 4-14) implicate the central portion of the OTAG domain for being involved in transport-related ozone events more than any other portion of the domain, although the percentage of impact cannot be quantified from this t ype of analysis. Nonetheless, controls implemented in this area may be effective in reducing ozone transport more often than controls elsewhere. Furthermore, given the density of NOx-rich point sources in this portion of the domain and the observation tha t regional ozone formation appears to be determined by the magnitude of NOx emissions, it follows that NOx controls may be more effective for regional ozone reductions. This suggestion is consistent with regional modeling results to date.

7. Recommendations To Foster Future Air Quality Analyses

A sizable data analysis infrastructure has been created by the OTAG process, which consisted of a community of analysts, data resources, and a set of new analysis methods and tools. The associated communications capabilities through regular meetings, c onference calls, mailing lists, and Web sites have proven to be an effective way of integrating scientific research and policy-making. Efforts should be made to maintain this data analysis and communications infrastructure in the future.

EPA is in the process of reviewing its current attainment status and trend networks, and it is involved in the planing of future monitoring programs. Now that the ozone transport problem has been recognized, the workgroup urges the Agency to consider e nhancing the existing monitoring and assessment programs to allow (1) quantification of ozone formation and source attribution, (2) quantification of transport across political boundaries, and (3) routine evaluation of photochemical model performance.

Additional data are needed over less-populated areas, along key political boundaries, as well as for ozone aloft (including ridgetops and tall buildings). Spatially representative sampling and more accurate measurement methods (notably for totally reac tive nitrogen (NOy)) should be pursued. Co-located measurements of ozone, precursors, and fine particle composition are needed to better understand pollutant interactions and source contributions.

The workgroup recommends further development of data analysis methods to quantify ozone transport and to better evaluate the performance of photochemical models. In particular, the analysis of the Photochemical Analysis Monitoring System (PAMS) network data should be enhanced and used in the evaluation of control strategy effectiveness.

Finally, because of the inherent scientific uncertainties in air quality analysis and modeling and the multiple legitimate interests of the participants, the workgroup recommends the continued development of and a commitment to the stakeholder-based ai r quality management process undertaken through OTAG.

1. Introduction

Background

During the summer months, high ground-level ozone concentrations are observed within and downwind of many of the large urban areas in the eastern United States. Peak hourly average concentrations exceed the current National Ambient Air Quality Standar ds (NAAQS) for ozone, which is 0.12 ppm, and peak 8-hour average concentrations rise above the recently proposed revised ozone NAAQS level of 0.08 ppm. Figure 4-16 shows counties containing monitors with design values in excess of the current 1-hour NAAQ S; Figure 4-17 shows those areas with design values in excess of the proposed 8-hour standard. In the east, violations of the 1-hour standard are restricted to the northeast urban corridor, the Lake Michigan area, and the immediate vicinity of other majo r urban areas. The number and geographic extent of counties violating the 8-hour standard are significantly greater. A significant feature of ozone concentrations in the east is that, in contrast to other parts of the country, ozone concentrations well in excess of the 30–40 ppb tropospheric background level are observed throughout most of the eastern United States, including locations outside of the major urban nonattainment areas. As a result of these large-scale elevated ozone events and the pr oximity of nonattainment areas to one another, many states have found that NAAQS attainment cannot be achieved with any reasonable level of local emissions control measures and, therefore, that a regional, multistate emissions control strategy is necessar y. The Ozone Transport Assessment Group was formed to deal with this issue.

Air Quality Analysis Workgroup Activities

Within OTAG’s Modeling and Assessment Subgroup, the Air Quality Analysis Workgroup was formed to evaluate and perform analyses of air quality and meteorological data relevant to OTAG’s mission. Data analyses conducted by AQA Workgroup member s complement work performed under the auspices of the RUSM Workgroup. AQA Workgroup members contributed numerous analyses, as listed in Table 4-1. Some of the analyses were based on ozone and precursor data from intensive field measurement programs such as the Nashville/Middle Tennessee Ozone Study and the NARSTO-NE Study that provide detailed spatial and temporal coverage of ozone, ozone precursors, and meteorological conditions both at the surface and aloft during a limited number of individual ozone episode events. Other analyses were based on research-grade ozone and precursor monitoring programs conducted over multiyear periods at a limited number of locations, such as the Southern Oxidants Study/Southeastern Consortium Intermediate Oxidant Networ k (SOS/SCION) network. Still other analyses were based on the routine ozone and meteorological data collected at regular time intervals at a large number of sampling sites located throughout the OTAG domain. The latter two types of analyses were designe d to identify major features of the ozone climatology within the OTAG domain that provide insights into ozone/precursor relationships and transport issues. A major advantage of these results is that, in contrast to modeling studies and analyses based on intensive measurements of individual ozone episodes, they provide information on ozone, ozone/precursor relationships, and ozone and precursor transport over the full range of meteorological conditions in the OTAG domain.

Analyses contributed by AQA Workgroup members were presented and discussed during monthly workgroup meetings. Comments made during these meetings frequently resulted in significant revisions and additions. Formal written reports are currently availab le for many of the analyses. Most of these reports have been posted on the AQA Workgroup Web site (http://capita.wustl.edu/OTAG) for widespread distribution and additional discussion and review. Summaries of many of the analyses were prepared based on these reviews. These summaries, which appear in the final section of this report, include brief discussions of study purpose, methodology, findings, scientific limitations, and policy implications. Draft summaries were provided to the study authors for review and comment before being finalized. Information contained in the final reviews was then used as the basis for an integrated assessment of analysis results as presented in this part of Chapter 4.

Unfortunately, because of resource constraints, it was not possible to prepare summaries for all of the analyses presented to or discussed by the workgroup. Table 4-1 indicates which analyses were summarized and which were not. Although the following discussion is based primarily on the analyses for which summaries were prepared, references are made in some cases to major results from some of the other studies. It should be recognized, however, that these results were not subjected to the same AQA W orkgroup review process as those from studies for which summaries are provided here, although in some cases these studies may have been reviewed by other groups. For clarity, references shown in bold in the following sections are among those for wh ich summaries were prepared; these summaries can be found in Part III: Summaries of Individual AQA Workgroup Analyses.

Overview of Regional Ozone and Transport in the Eastern United States

The regional nature of elevated ozone episodes in the eastern United States has long been recognized (see, e.g., NRC, 1991; OTC, 1994; SOS, 1995; Husar, 1996a). A review of measurement studies and statistical data analyses conducted by the Natio nal Research Council (NRC, 1991) identified elevated ozone events on spatial scales exceeding 600,000 km² lasting from a few days to as long as a week, with concentrations exceeding at least 80 ppb for a major portion of each day. The most wide spread regional episodes were found to be associated with slow moving, high pressure weather systems that produced high temperatures, light winds, limited vertical mixing, and mostly clear skies. Air movement around the high, the slow (typically eastward or northeastward) migration of the high, and the influence of transient weather features on the fringes of the high can result in transport of elevated ozone and precursor concentrations over significant distances. Within this regional "sea" of elevated ozone, imbedded urban ozone plumes consisting of significantly higher concentrations can be found. Surface and aloft observations have shown that these urban plumes can be distinguished over a period of 12 hours covering an area of up to 8,000 km ² (NRC, 1991).

Given the observational evidence cited above, it is clear that transport mechanisms acting to produce the large-scale regional ozone excess above tropospheric background in the eastern United States have the potential to contribute significantly to exc eedances of the current 1-hour or proposed 8-hour ozone standards. The direction and spatial extent of transport and the relative contribution of transported ozone and precursors to individual ozone exceedances is highly variable. On one hand, some episo des, such as the July 11–15, 1995, Nashville episode analyzed by Meagher (1996) may be entirely local and not involve any transport. During this episode, ozone concentrations in downtown Nashville reached 138 ppb, more than 55 ppb above the d aily maximum value at any surrounding site and approximately 80 ppb above values near the boundary of the metropolitan area. On the other hand, as indicated by Blumenthal et al. (1997), a number of 1-day and multiday transport mechanisms have been observ ed along the coastal plain in the northeast, some or all of which may be involved in any given ozone exceedance episode. These mechanisms include—

1. Near surface flows acting within the first few hundred meters above ground level where surface frictional, radiative, and sensible heating processes are important. In coastal areas, this includes offshore, over water, and onshore flows; orograp hic effects (drainage flows, blocking, channeling) are also important in areas with significant terrain.

2. Boundary layer synoptic flows occurring within the daytime planetary boundary layer but above the surface layer at heights (typically 800–1500 meters(m) above the Earth’s surface, where surface influences are less important and syno ptic scale pressure gradient forces are relatively more important.

3. Channeled nighttime flows in low-level jets that form under stable conditions just above the nocturnal boundary layer (typically 200–800 m above sea level under episode conditions in the northeast) and are enhanced by the channeling effe cts of major terrain features (such as the Appalachian Mountains in the northeast).

Within any given episode, each of the above flow features may be characterized by different directions, average speeds, and temporal and spatial variations. Surface flows are typically more sluggish and more localized than the synoptic flows, and they exhibit a strong diurnal pattern. In most cases, winds are calm or near calm during the nighttime and early morning hours under episode conditions. Surface flows are thus primarily involved in near-field transport whereas the synoptic flows are capable of transporting material for 2 or more days over considerably longer distances. The relatively high-speed channeled nighttime flows can move material long distances overnight close to the surface where it is available to be mixed down to the surface as t he mixing depth increases the next morning. From surface and upper air observations of winds and back trajectory calculations during northeastern ozone episodes, Blumenthal and co-workers concluded that near-surface transport along the urban corridor can cover distances of 50–250 km from morning until evening, boundary layer synoptic flows can transport material 200–600 km across the Appalachian Mountains in a 24-hour period, and channeled nighttime flows such as those observed by Ray et al. (19 97) can transport material 200–400 km overnight. While not all of the three basic flow regimes described above will be important at all locations, some or all may play important roles in ozone and precursor transport in various nonattainment regions throughout the OTAG domain. Thus, the factors contributing to transport are quite complex. The AQA Workgroup conducted analyses designed to identify the relative contributions of these transport mechanisms to ozone exceedances at different locations in the OTAG domain.

2. Integrated Summary of Workgroup Analyses

Analyses conducted by the AQA Workgroup provide information in six key areas:

• The spatial and temporal pattern of ozone in the OTAG domain.

• The transport of ozone and ozone precursors within the eastern United States.

• The effectiveness of VOC and NOx emissions control strategies in reducing ozone concentrations.

• The potential impact of an 8-hour average ozone standard on transport and attainment issues.

• The degree to which ozone episodes selected for photochemical modeling of regional control strategies are representative of the range of conditions associated with high ozone events.

• Evaluation of the accuracy of UAM-V photochemical model predictions of basecase ozone and precursor concentrations.

An integrated summary of AQA Workgroup analysis results as they pertain to these six key areas is provided in the following sections. This summary is followed by a discussion of recommendations for future analyses. Summaries of individual analyses are provided in the final section of this report.

Spatial and Temporal Pattern of Ozone in the OTAG Domain

To better understand the temporal and spatial patterns of regional ozone levels associated with transport impacts in the eastern United States, an integrated set of ozone monitoring data from both the primarily population-oriented Aerometric Informatio n Retrieval System (AIRS) network (consisting of the National Ambient Monitoring Station and the State and Local Ambient Monitoring Station networks) and several rural networks Clean Air Status and Trends Network (CASTNet), Interagency Monitoring of PROte cted Visual Environments (IMPROVE), SCION, Eulerian Model Evaluation and Field Study (EMEFS) was constructed (Husar and Husar, 1996). Analyses of this database (Husar, 1996a, b, c) revealed the following key features:

• Exceedances of the current 1-hour, 120-ppb ozone standard are primarily restricted to the northeast urban corridor and the general vicinity (within about 150 miles) of other major metropolitan areas. Exceedances of the proposed 8-hour, 80-ppb standar d extend further from the metropolitan areas and include a band of multiple exceedance days per year just north of the Ohio River stretching from Illinois to Ohio and into western Pennsylvania.

• Average 1-hour daily maximum ozone concentrations in the "corners" of the OTAG domain (south Florida, south Texas Gulf coast, upper midwest/northern plains, and northern Maine) are 30–40 ppb, which are generally consistent with northern hemisphere background levels. Concentrations along much of the southern portion of the western OTAG boundary average slightly higher (55–65 ppb). It is unclear why values are higher in this location, but these results could reflect a relatively lar ger influence of anthropogenic emissions on the sparse monitoring network in this region. Concentrations along the eastern seaboard from Maryland to New York (which is primarily an outflow boundary) are higher, reaching above 70 ppb in the New York/New J ersey area.

• Average concentrations in the "corners" of the OTAG domain boundaries tend to be spatially uniform and exhibit relatively little day-to-day variability, which is consistent with a tropospheric background source.
• Seasonal mean daily maximum 1-hour concentrations within the OTAG domain range from 80 ppb near urban areas to 40 ppb near the domain boundaries. The domain average seasonal mean daily maximum 1-hour concentration is approximately 60 ppb. Seasonal m ean daily maximum 1-hour concentrations are highest in the Washington, D.C. - New York corridor with "hot spots" near several other major metropolitan areas. In addition, a broad region of 65–70 ppb averages covers the area just north of t he Ohio River.

Although no effort was made to estimate the flux of ozone and precursors across the OTAG domain boundary, these observations suggest that, with the possible exception of the international border along the Quebec-Windsor corridor, air masses entering th e OTAG domain contain little or no ozone burden above tropospheric background levels (and precursor fluxes into the domain are likely to be equally negligible). Thus, ozone concentrations above approximately 35 ppb observed within the OTAG domain must be generated by sources within the domain. Because of the pervasive influence of anthropogenic activities, present-day monitoring data cannot provide any information on the likely range of concentrations that would be observed in the OTAG domain in the abs ence of any anthropogenic influences. While seasonal mean daily maximum concentrations along some parts of the OTAG boundary where anthropogenic influences are minimized are in the 30–40 ppb range as noted above, climatic conditions and biogenic inv entories within the central portion of the domain differ markedly from those in these boundary regions and may conspire to produce somewhat higher "natural" background ozone concentrations. On the other hand, some modeling estimates of pre-indu strial ozone levels suggest that lower values might be expected in the absence of any anthropogenic influence (e.g., Levy et al., 1997), but these estimates have not been reviewed for this report.

Additional findings from the spatial and temporal pattern analyses include the following:

• On low-ozone days (10th percentile of the concentration distribution), concentrations are similar in urban and rural areas, are below approximately 30 ppb in the "corners" of the domain, and remain slightly higher (42–47 ppb) over a bro ad area in the central portion of the domain.

• On high-ozone days (90th percentile of the concentration distribution), concentrations exceed 100 ppb along the northeast corridor and near other major metropolitan areas. Concentrations are also above approximately 90 ppb from the Ohio River northwar d through central Indiana, much of Ohio, and western Pennsylvania, in contrast to the lower values seen in most of the surrounding regions.

• The total number of exceedances of a 1-hour average 120-ppb concentration level across the entire OTAG domain increases during the course of the week from Monday through Friday and then declines sharply on the weekends. On Sundays, the number of exce edances are one-third of the number on Fridays. Large decreases are also observed in the 90th percentiles of the daily maximum 1-hour concentrations. The largest Sunday decreases in the 90th percentiles (more than 15 ppb lower than on Fridays) are ob served near the major metropolitan areas, whereas 90th percentiles on Sundays are slightly higher (5 ppb) in a band stretching along and somewhat to the south of the Ohio River valley. This feature may be the result of differences in the weekly pattern o f emissions in this region as compared to those in major urban areas located outside of this region.

Transport Analyses

Several analyses conducted by the AQA Workgroup focused on identifying the spatial and temporal extent and magnitude of the transport of ozone and precursors. Measurements of ozone, precursors, and meteorological conditions conducted as part of both t he 1995 SOS Nashville/Middle Tennessee Ozone Study, and the 1995 NARSTO-NE Field Study were analyzed for this purpose (Edgerton and Hartsell, 1996; Edgerton, 1997a; Ray et al., 1997; Vukovich, 1996; Blumenthal et al., 1997; Hudischewskyj and Dougla s, 1997). In addition, long-term data sets of routine air quality and meteorological measurements were used to examine spatial ozone and temperature correlation patterns and to compute air parcel trajectories (Poirot and Wishinski, 1996a, b, c; Wishinski and Poirot, 1996; Schichtel and Husar, 1996, 1997). Daily back trajectories ending at 23 monitoring sites located throughout the OTAG domain were computed using seven summers (1989–1995) of three-dimensional gridded wind fields and va riations in the trajectory patterns with ozone concentrations at the receptor sites were analyzed. Principal findings of these analyses are summarized below.

One-day transport of ozone within plumes from urban areas and major power plants has been demonstrated using aircraft data (NRC, 1991 and references therein). In addition, impacts of distinct power plant plumes and urban plumes have been noted at seve ral rural sites in the southeast (Edgerton and Hartsell, 1996). Aircraft measurements of NOx, O₃, and the products of NOx oxidation (NOz) made within the Cumberland and Johnsonville power plant plumes located near Nashville (Meagher, 1996) indicated that, during these summer daytime measurement periods, essentially all of the NOx is converted to NOz within 30–100 km of the source. Thus, ozone production due to NOx in the plume ceases within 100 km of the power plant. These results differ somewhat from those in the earlier Tennessee Plume Study in which aircraft measurements of the Cumberland plume conducted in August 1978 indicated ozone production continuing beyond 110 km to at least 160 km (Gillani and Pleim, 1996). Dif ferences in the distance estimates between these two studies may be related to differences in mean plume transport wind speeds, as well as to other meteorological and air quality factors. It is important to note that, under the right conditions, power pl ant plumes may travel relatively long distances overnight with little loss of NOx and thus be available to participate in photochemical reactions at distant locations on the following day. Trainer et al. (1993) shows that ambient O₃/NOz concent ration ratios provide a measure of the efficiency of ozone production, i.e., essentially the amount of ozone produced during the lifetime of each nitrogen dioxide (NO₂) molecule. Planetary boundary layer measurements of O₃/NOz ratio s from aircraft and surface sites made within the power plant plumes and the Nashville urban emissions plume indicate that ozone formation efficiency as measured by this ratio is 65–70 percent greater in the urban plume as compared to the power plant plumes (Meagher, 1996). Thus, on a per-unit emissions basis, urban area source NOx emissions may be more important to same-day ozone formation than elevated point source NOx emissions.

Conditions during ozone episodes are conducive to longer transport distances over 1–2 day periods in the northeastern portion of the OTAG domain (coinciding with higher average wind speeds and a more organized flow pattern) than in the southeaster n portion of the domain, where average wind speeds are lower and the flow pattern is less well organized. Evidence of this fact can be found in the "source regions of influence" and residence time analyses conducted by Schichtel and Husar(19 97, 1996), and in the residence time analysis, autocorrelations, and lagged inter-regional correlations computed by Poirot and Wishinski (1996a, b, c).

Analyses of the July 11–15 ozone episode in Nashville (Meagher, 1996) indicate that this episode was primarily a result of local emissions; the area of high ozone (with concentrations up to 80 ppb above a 60- to 70-ppb regional background) was restricted to the immediate metropolitan area and did not extend away from it in any direction. However, the frequency of such "homegrown" episodes versus regional episodes was not examined.

Porter et al. (1996) present spatial correlation results for the short-term (deseasonalized) ozone component that suggest a spatial scale of 560–640 km (350–400 miles) for same-day formation and transport. Schichtel and Husar’s s ource regions of influence analysis indicate a scale of 300–600 km (for 1-day transport times) and 450–1000 km (for 2-day transport times) in the northern half of the OTAG domain. In the southern half of the domain, 1-day scales were found to b e somewhat less: 200–400 km (for 1-day transport times) and 300–800 km (for 2-day transport times). Similarly, analysis of typical transport distances and trajectory residence times for days in the upper 20 percent of daily maximum ozone in eac h portion of the OTAG domain show relatively large transport distances and short residence times in the northeast and short transport distances and long residence times in the south. Interpretation of these results is subject to two important caveats:

1. Given the somewhat weak but nevertheless significant correlations of Porter’s short-term ozone component with temperature and the likely influence of other meteorological factors on day-to-day variations in the short-term ozone component, it i s not possible to distinguish the degree to which the spatial scales implied by this analysis are representative of actual ozone and precursor transport or merely representative of correlations in meteorological conditions between locations.

2. The spatial scales obtained from Schichtel and Husar’s source regions of influence analysis represent the typical distances air parcels are estimated to travel over 1- and 2-day intervals and provide no information on the relative contribution s of precursor sources to ozone concentrations at the given downwind distances.

Trajectory model results do not show actual spatial transport scales but do indicate potential source regions along the pathways of air parcels associated with above-average ozone at receptor sites. Back trajectory analyses conducted by Poiro t and Wishinski (1996a, b, c) indicate that air parcels originating outside of the OTAG domain are associated with below-average ozone at various locations throughout the domain. In contrast, air parcels associated with above-average ozone concentrat ions at these receptor sites predominantly originate from the heart of the OTAG domain. Remarkably, this was found to be the case no matter where in the domain the receptor site waslocated. Of course, it should be noted that the trajectory analysis does not take into consideration atmospheric chemical processes, the injection of precursor emissions, or the deposition of material along the trajectory path. Therefore, although these results provide consistent circumstantial evidence that emissions from th e central portion of the domain contribute in some way to above-average ozone events throughout the domain, the relative contribution of precursor emissions along various portions of the trajectory paths leading to the receptor sites (i.e., emissions ori ginating near the upwind end of the trajectory versus emissions originating near the receptor site) are not known.

Development of Control Strategies

Comparisons of historical trends in ozone and precursor concentrations with trends in precursor emissions levels can provide insights into the relative effectiveness of control strategies. Unfortunately, trends in the annual extreme ozone concentration s of interest are sensitive to the influences of interannual variations in meteorological conditions, movement of monitoring sites, missing data, and random fluctuations in concentrations that can make identification of consistent trends difficult. Furth ermore, high-quality data on ambient precursor trends is extremely limited and estimates of precursor emissions are subject to potentially significant biases. Despite these difficulties, several important features can be noted in the trends of ozone and precursor emissions between the early to mid-1980s and the mid-1990s as described below. Many of the analyses presented here were summarized by Morris (1996).

After adjusting for meteorological variations, ozone concentrations decreased significantly in all of the major metropolitan areas in the northeast urban corridor from Washington, D.C., to Boston between 1984 and 1995. Total declines were less in Wash ington and Baltimore than in the other cities (Chinkin et al., 1996; Rao et al., 1995).

Porter et al. (1996) showed declines in deseasonalized, temperature-adjusted daily maximum 1-hour ozone concentrations between 1985 and 1995 at a majority of monitoring sites throughout the OTAG domain, although increases were noted for a signif icant minority of sites. No inter-regional differences in trends can be discerned from the Porter et al. report. However, a similar analysis of 1983–1992 trends for the northern half of the OTAG domain (Rao et al., 1995) showed statistically signif icant downward trends at most northeastern sites. Trends were also exclusively or predominantly negative in all parts of Pennsylvania, Ohio, West Virginia, Kentucky, North Carolina, Wisconsin, Iowa, Illinois, and Indiana. Mixed or positive trends were n oted in Michigan, Maryland, Tennessee, and Missouri.

Wolff and Korsog (1994) showed that ozone trends based on various peak hourly statistics (such as the annual maximum 1-hour value and the 95th percentile of 1-hour values) between 1980/1981 and 1993 showed declines in most northeastern cities (one moni toring site was selected to represent each city). Negative trends were also found in Detroit, Chicago, and Atlanta, but a positive trend was noted in Grand Rapids. Trends computed for Boston were somewhat unreliable due to missing data from 1980 to 1983 . These trends were not adjusted for variations in meteorological conditions.

In contrast to trends based on peak 1-hour statistics, Wolff and Korsog (1994) found trends based on running 8-hour averages to be zero or positive in Detroit, Grand Rapids, Atlanta, Philadelphia, Baltimore, and Washington, D.C. Chinkin et al. (199 6) reported lower percent reductions in number of exceedances of a 100-ppb threshold than for higher thresholds. Thus, trends in exceedances of an 8-hour, 80-ppb threshold are generally less negative than trends in exceedances of a 1-hour, 120-ppb th reshold. In some cases the 8-hour trends are positive as compared to negative 1-hour trends.

EPA’s annual trends report (EPA, 1994) includes total NOx, VOC, and CO emissions by EPA region for 1985–1994. VOC emissions declined sharply in all regions after 1988, reaching a total reduction of 10–15 percent below 1985 levels by 199 1. In contrast, NOx emissions, while decreasing by 5 percent between 1989 and 1991 in Region I, increased by more than 5 percent in Regions IV and V. Thus, the largest ozone declines occurred over approximately the same geographic area where simultaneou s VOC and NOx reductions occurred. In general, ozone trends were less negative or of mixed signs in the south and midwest, areas that experienced increases in NOx emissions. Chinkin et al. (1996) compared correlation coefficients for NOx and VOC emissions versus ambient ozone for metropolitan areas in the northeast and found somewhat better correlations between VOC and ozone as compared to NOx, although the statistical significance of these comparisons was not explored. The correlation coefficie nts were low overall, indicating that annual emissions changes fail to explain a significant percentage of the variability in meteorologically adjusted ozone trends.

The above results do not lend themselves to formulation of a definitive conclusion regarding causal relationships between changes in VOC and NOx emissions and ambient ozone concentrations. When looking at emissions and ozone trends from the mid-1980s t o the mid-1990s for various ozone trend measures at various locations, the most consistently (although by no means universally) identifiable feature is the simultaneous decline in peak ozone concentrations and VOC emissions between 1988 and 1991. This re sult by itself, however, does not indicate that the observed decrease in ozone is attributable in whole or even in part to the drop in VOC, although it is reasonable to conclude that VOC decreases contributed at least partially to the ozone decline outsid e of biogenic-dominated southern locations. Comprehensive analyses of ambient NOx or VOC concentration trends, which might validate the emissions trends, have not been conducted. Rich Poirot of the Vermont Department of Environmental Conservation has not ed examples of apparent discrepancies between ambient nitric oxide (NO) and NO₂ concentration trends, nitrate deposition trends, and NOx emissions trends, which warrant further investigation. In particular, EPA (1995) reported a 3-percent incr ease in national total NOx emissions between 1985 and 1994; over the same period, the national composite NO₂ concentrations decreased by 9 percent.

In addition, future ozone/precursor trend relationships may differ from those of the past. For example, if the ozone peak downwind of an urban area results from the superposition of an ozone plume generated under VOC-limited conditions by the urban ar ea with a regional background "cloud" of elevated ozone formed largely under NOx-limited conditions, then urban VOC reductions, such as those that occurred between the mid-1980s and mid-1990s, may reduce the total ozone peak by reducing the urba n area’s contribution. However, as the peak is lowered, the regional, NOx-limited contribution to the total peak becomes relatively more important. Thus, future VOC reductions may be less effective at reducing ozone concentrations than in the past. This effect would be magnified for exceedances of an 8-hour average ozone standard as compared to a 1-hour standard because the 8-hour averages are less sensitive to sharp 1-hour concentration peaks downwind of urban areas.

Impact of an 8-Hour Ozone Standard

EPA’s proposed revisions to the ozone NAAQS is based on an 8-hour average daily maximum concentration instead of the 1-hour daily maximum measure used in the current standard. Although closely related, spatial and temporal patterns of peak 8-hour averages, the role of transport, and the response of 8-hour values to control strategies are somewhat different than for 1-hour averages. AQA Workgroup analyses of these issues produced the following principal findings:

• Over the OTAG domain as a whole, spatial ozone patterns for 1-hour and 8-hour daily maximum concentrations are similar in that areas of high and low 1-hour concentrations are also regions of high and low 8-hour concentrations.

• Eight-hour average daily maximum concentrations range from 75 percent to 95 percent of the corresponding 1-hour daily maximums. The biggest differences are near major metropolitan areas with pronounced diurnal ozone cycles, while the smallest differe nces are in remote rural locations that exhibit relatively weak diurnal ozone cycles. Thus, exceedances of a given daily maximum 8-hour average threshold will be relatively more frequent further away from the urban areas as the diurnal cycle flatt ens out.

No analyses of the relative effectiveness of various control strategies for 1-hour versus 8-hour ozone have been conducted by the AQA Workgroup. Relative effectiveness is dependent on the complex interaction of many factors and can be expected to vary significantly from one situation to the next.

Representativeness of OTAG Episodes

An issue of concern in the interpretation of control strategy modeling results is the degree to which the set of episodes selected for modeling represent the full range of conditions under which ozone episodes occur. If certain types of episodes are n ot well represented by the selected episodes, model results may give a misleading picture of control strategy impacts. The AQA Workgroup examined the representativeness of episodes modeled by the RUSM Workgroup ("OTAG episodes") from two perspe ctives:

1. How does the spatial distribution of ozone on OTAG episode days compare with that observed on high-ozone days in general?

2. How do forward and backward trajectories computed as part of the "source regions of influence" and "trajectory residence time" analyses discussed above compare between OTAG episode days and high-ozone days in general?

While the general spatial patterns of daily maximum ozone concentrations during the modeled episodes in 1988, 1991, 1993, and 1995 are similar to the pattern of 90th percentile daily maximum concentrations for all days during the 1991–1995 ozone s easons, there are clear differences from episode to episode (Figure 4-18). Generally speaking, the 1991 and 1995 episodes are more similar to each other than the 1993 episode, which is characterized primarily by high ozone levels in the south with lower values elsewhere. Similar results are seen in the comparisons of transport vectors and normalized residence times between the modeled episodes in each year (Figures 4-19(a)–(d)). Each episode is characterized by different regions of high residence times; the 1993 episode differed markedly from the other two, with high residence times in the south and low residence times in the north.

OTAG Model Evaluation

Although the primary model evaluation studies for OTAG were conducted by the Regional and Urban Scale Modeling Workgroup, the AQA Workgroup also conducted several diagnostic comparisons of observed and predicted ozone and ozone precursor concentrations that shed light on the suitability of the OTAG photochemical model results for evaluation of regional control strategy impacts (Hartsell and Edgerton, 1996; Edgerton, 1997b; Morris et al., 1997; Imhoff, 1996; Lurmann et al., 1997). The principal findings of the RUSM Workgroup and AQA Workgroup analyses are discussed below.

Ozone

Comparisons of observed surface ozone concentrations with model predictions for all four modeled episodes (1988, 1991, 1993, and 1995) were conducted under EPA sponsorship (OTAG, 1996). Results of this analysis indicated "generally good agreement between simulated and observed values; [with] no large positive or negative biases." (ibid., p. 6–1). Comparisons of observed and predicted concentration fields indicate that the overall spatial and temporal ozone patterns appear to have been p roperly simulated (OTAG, 1996; Husar and Schichtel, 1996). However, the model results show a regional bias underprediction of concentrations on average in the northern half of the domain and an overprediction in the southern half of the domain. Furtherm ore, for some episodes and in some regions, a "bias creep" was observed in which concentrations were underpredicted on average at the beginning of the episode and overpredicted at the end of the episode.

Comparisons of observed surface ozone concentrations during the July 1995 episode with concentrations predicted by the OTAG UAM-V runs for a suburban and a rural site near Nashville reveal a fair amount of scatter but no bias. However, comparisons alo ft indicates that the model consistently underpredicts ozone concentrations above the mixed layer. This result is consistent with underpredictions aloft during the morning hours along the western boundary of the northeast urban corridor noted by Lurmann, 1997. There are several possible reasons for this outcome; current analyses do not allow the workgroup to distinguish among them.

Comparisons of observed and modeled ozone for nine rural sites in the south and northeast for the July 1995 episode reveal a tendency for the model to overestimate the frequency and duration of ozone values in the 80–100 ppb range for these rural sites. If confirmed by analyses for a larger group of sites, this finding would indicate a potential problem with using OTAG results to provide boundary conditions for urban scale modeling.

Isoprene

Comparisons of observed mid-afternoon isoprene concentrations at monitoring sites located in various parts of the OTAG domain with predicted surface layer isoprene concentrations for corresponding model grid cells reveal a tendency for the model to ove rpredict isoprene (Morris et al., 1997; Edgerton, 1997b). More than three-fourths of the sites examined in these studies exhibit biases greater than ±25 percent and nearly half the sites exhibit mean biases of greater than ±50 per cent of mean observed values. Nearly all the biases over ±50 percent are overpredictions. No geographic pattern in the magnitude or sign of the bias is evident in the results, and the magnitude of the overpredictions greatly exceed the level of bias one might expect from comparing a near surface concentration with a (50 m) vertically averaged value for the model grid cell. These results point to a potentially significant problem with the UAM-V/Biogenic Emissions Inventory System (1995) (BEIS2) mode ling system: If isoprene emissions in the OTAG inventory are significantly overestimated (for whatever reason), the overrepresentation of biogenic VOCs would cause the model to underestimate the expected response to anthropogenic VOC controls. In additi on, the model would tend to be overly sensitive to NOx controls in all but the largest urban areas.

Other Precursors

An analysis of CO/NOy and VOC/NOy ratios at a Nashville urban site during 1995 found good agreement between measured and modeled ratios, indicating consistency between the modeled and ambient CO/NOx and VOC/NOx emissions ratios around this site (Imhoff, 1996). The intercepts at zero NOy from these analyses are indicative of the background concentrations of CO and VOC; the analyses indicated that the model tends to underestimate the CO background and overestimate the VOC background at thi s site. The discrepancy between the apparent background for VOC of 90 ppb in the model compared to near zero in the ambient data possibly indicates that a source of VOC (but not CO and NOy) is overestimated in the model. The tendency for the model to un derestimate the CO background at this urban site is surprising given that the model had a tendency to overpredict CO at two rural sites in Georgia and Tennessee; however, these findings are not necessarily inconsistent. The tendency to over predict CO at the rural sites may be indicative of emissions or atmospheric mixing problems in this region.

Chemistry

Comparisons of observed and predicted ozone production efficiencies based on O₃/NOz ratios show good agreement at rural site near Nashville in 1995; both observed and predicted O₃/NOz ratios are about 6, suggesting ozone formation is NOx limited (Imhoff, 1996). At a Nashville suburban site, both the observed and (particularly) modeled O₃/NOz ratios show more variability, making it difficult to draw conclusions about model performance for this location. The aver age observed O₃/NOz ratio (approximately 3) is lower for the suburban site than the rural site, suggesting ozone formation is relatively less NOx limited and more VOC limited at the suburban site. Similarly, a comparison of modeled and observe d O₃/NOy relationships reveal good agreement across nine rural sites in the south and northeast in 1995; both observed and predicted O₃/NOy ratios are about 9, suggesting ozone formation is NOx limited (Hartsell and Edgerton, 1996). The good performance of the model chemistry for O₃/NOy (and O₃/NOz) relationships under NOx-limited conditions supports the use of the model for evaluating regional control strategies. This finding is particularly rel evant to OTAG because ozone formation is NOx limited over much of the OTAG domain.

3. Recommendations for Additional Analyses

Further work is recommended in a number of different areas as outlined below. The OTAG infrastructure—the community of analysts brought together by the OTAG process and associated communications capabilities (regular meetings, conference calls, W eb sites, and mailing lists)—has proven to be a valuable resource for conducting policy-relevant research, and efforts should be made to maintain this infrastructure in the future. The following specific actions are recommended for future inquiry:

• Prepare summaries of meteorological conditions associated with the buildup and dissipation of ozone episodes in different portions of the OTAG domain and contrast with conditions during non-episode days. Of particular interest are differences in surf ace and upper-air pressure fields, which could be used to help interpret the trajectory analysis results.

• Perform a comparison of weekday versus weekend concentration differences for 1-hour and 8-hour daily maximums. This comparison would shed light on any differences in response to a control strategy that mimics the weekday/weekend emissions cycle.

• Examine weekday/weekend differences in diurnal ozone and precursor patterns.

• Evaluate the ability of the photochemical model to reproduce recent observed trends in ozone concentrations.

• Expand the trajectory analyses to include simulation of atmospheric chemical processes and deposition.

Several AQA Workgroup members also suggest that the current ozone, ozone precursor, and meteorological monitoring network be reconfigured and enhanced to better address the issue of regional elevated ozone events, ozone and precursor transport, and pho tochemical model evaluation. Currently, there is a lack of routine air quality and meteorological data suitable for analysis of transport over a broad range of episode conditions. In particular, the current monitoring network is primarily focused on the larger metropolitan areas, whereas spatial coverage in less populated areas or along boundaries of political jurisdictions is quite limited. The current monitoring network is also primarily geared towards monitoring ozone attainment status and trends. Measurements of ozone aloft or of ozone precursors are quite limited. Such measurements are needed both for transport assessments and to better evaluate the performance of photochemical models used to estimate control strategy impacts.

Summary

Written Report Available

Internet Document*

1. Spatial and Temporal Pattern of Ozone and Precursors

Spatial Pattern of Daily Maximum Ozone Over the OTAG Region (Husar, 1996a)

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Pattern of 8-Hour Daily Maximum Ozone Over the OTAG Domain (Husar, 1996b)

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Weekly Pattern of Ozone Over the OTAG Region (Husar, 1996c)

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Y

Spectral Decomposition of Ozone Time Series (Porter et al., 1996)

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Y

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Seasonal Pattern of Ozone Over the OTAG Region (Husar, 1997)

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Historical Perspective on the Climatological Potential for "Local" Pollution Episodes (Poirot and Wishinski, 1997)

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Ozone Exceedances Data Analysis: Representativeness of 1995 (Chinkin et al, 1996)

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Ozone Measurement Trends Studies in the Northeast
(Morris, 1996)

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SOS Nashville/Middle Tennessee Ozone Study (Meagher, 1996)

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2. Transport of Ozone and Precursors

Air Trajectory Analysis of Long-Term Ozone Climatology (Poirot and Wishinski, 1996a, b, c; Wishinski and Poirot, 1996)

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Source Regions of Influence for High and Low Ozone Conditions in the Eastern U.S. (Schichtel and Husar, 1997, 1996)

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Ozone/Tracer/NOy Relationships at Three SOS/SCION Sites (Edgerton and Hartsell, 1996)

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Analysis of Ozone, NOy, and Tracer Data from a Site in South-Central Pennsylvania (Edgerton, 1997a)

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Analysis of Low-Level Jets Using NARSTO-NE Data
(Ray et al., 1997)

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Intra-Annual and High Frequency Variations at SOS/SCION Sites (Vukovich, 1996)

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Transport and Mixing Phenomena Related to Ozone Exceedances in the Northeast U.S. (Blumenthal et al., 1997)

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Classification of Ozone Episodes for Four Southern Cities According to Transport Characteristics (Hudischewskyj and Douglas, 1997)

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Y

3. Model Performance Evaluation

A Comparison of Modeled and Measured Ozone, NOy, and CO at Nine Regional Monitoring Stations During the 1995 OTAG Episode (Hartsell and Edgerton, 1996)

Y

Y

Y

Ambient Monitoring Sites for OTAG (Time Series) Model Evaluation (Poirot, 1996)

Y

Y

Y

Comparison of OTAG UAM-V/BEIS2 Modeling Results With Ambient Isoprene Observations (Edgerton, 1997b; Morris et al., 1997)

Y

Y

Y

Comparison of SOS Nashville Data to OTAG 1995 Base Model (Imhoff, 1996)

Y

N

N

Evaluation of the UAM-V Model Performance in the Northeast Region for OTAG Episodes (Lurmann et al., 1997)

N

Y

Y

4. Episode Representativeness

Episode "Representativeness" (Preliminary View from Backward Trajectory Perspective) (Poirot and Schichtel, 1996)

N

N

N

Comparison of Ozone Distributions During OTAG-Modeled Episodes With Climatological Distributions (Husar, 1997b)

N

N

N

* A "Y" in this column indicates documentation of the study is available on the AQA Workgroup Web page at the following address: http://capita.wustl.edu/OTAG; an "N" denotes that documentation is not availabl e as of the publication date.

4. References

Blumenthal, D.L., F.W. Lurmann, N. Kumar, T.S. Dye, and M.E. Korc, 1997. "Assessment of Transport and Mixing and OTAG Model Performance for Northeast U.S. Ozone Episodes: Summary of Results," STI-996133-1710/1716-S, March 1997, Sonom a Technology, Inc., Santa Rosa, California. (http://capita.wustl.edu/otag/Reports/otagrept/otagsum3.html).

Cox, W.M., and S.-H. Chu, 1993. "Meteorologically Adjusted Ozone Trends in Urban Areas: A Probabilistic Approach," Atmospheric Environment, 27B(4), 425–434.

Cox, W.M., and S.-H. Chu. "Assessment of Interannual Ozone Variation in Urban Areas from a Climatological Perspective," Accepted for publication in Atmospheric Environment, 1995.

Edgerton, E.S., 1997a. "Analysis of Ozone, NOy and Tracer Data from a Site in South-Central Pennsylvania." (http://capita.wustl.edu/otag/Reports/Reports.html).

Edgerton, E.S., 1997b."Comparison of Modeled versus Observed Isoprene Concentrations at Rural and Suburban Sites Across the Eastern U.S."(http://capita.wustl.edu/OTAG/Reports/ Isoprene/ISOPRENE.html).

Edgerton, E.S., and B.E. Hartsell, 1996. "Ozone/Tracer/NOy Relationships at Three SOS/SCIONSites." (http://capita.wustl.edu/otag/Reports/Reports.html).

EPA, 1994. "National Air Pollutant Emission Trends, 1900–1994." EPA 454/R-95-007.

EPA, 1995. "National Air Quality and Emissions Trends Report, 1994." EPA 454/R-96-005, October 1995.

Falke, S.R., and R.B. Husar, 1996. "Uncertainty in the Spatial Interpolation of Ozone Monitoring Data," May 1996. (http//capita.wustl.edu/capita/capitareports/03interp/03interp.html).

Gillani, N.V. and J.E. Pleim, 1996. "Sub-Grid-Scale Features of Anthropogenic Emissions of NOx and VOC in the Context of Regional Eulerian Models," Atmospheric Environment 30, 2043–2059.

Hartsell, B.E., and E.S. Edgerton, 1996. "A Comparison of Modeled and Measured Ozone, NOy, and CO at Nine Regional Monitoring Stations During the 1995 OTAG Episode," November 24, 1996. (http://capita.wustl.edu /otag/Reports).

Hudischewskyj, A.B., and S.G. Douglas, 1997. "Classification of Ozone Episodes for Four Southern Cities According to Transport Characteristics," SYSAPP-97/03, March 1997. Systems Applications International, San Rafael, California.

Husar, J.D., and R.B. Husar, 1996. "Preparation of Ozone Files for Data Analysis and Model Comparison," September 20, 1996. (http://capita.wustl.edu/otag/Reports/dataanalysis/Analysis.html).

Husar, R., and B. Schichtel, 1996. "OTAG Model–Data Comparison Files and Animations." (http://capita.wustl.edu/otag/data/ModO3Cmp/ModelDataComp.html).

Husar, R.B., 1996a. "Spatial Pattern of Daily Maximum Ozone Over the OTAG Region," September 16, 1996. (http://capita.wustl.edu/otag/Reports/otagspat/otagspat.html).

Husar, R.B., 1996b. "Pattern of 8-Hour Daily Maximum Ozone and Comparison with the 1-Hour Standard, " September 16, 1996. (http://capita.wustl.edu/otag/Reports/).

Husar, R.B., 1996c. "Weekly Pattern of Ozone Over the OTAG Region," September 16, 1996. (http://capita.wustl.edu/otag/Reports/otagweek/otagweek.html).

Husar, R.B., 1997. "Seasonal Pattern of Ozone Over the OTAG Region," February 16, 1997. (http://capita.wustl.edu/otag/Reports/otagseas/otagseas.html).

Imhoff, R.E., 1996. "Comparison of SOS Nashville Data to OTAG 1995 Base Model," August 1996. Presentation to the OTAG Air Quality Analysis Workgroup.

Levy, H., P.S. Kasibhatla, W.J. Moxim, A.A. Klonecki, A.I. Hirsch, S.J. Oltmans, and W.L. Chameides, 1997. "The Global Impact of Human Activity on Tropospheric Ozone," Geophysical Research Letters, Vol. 24, No. 7, pp. 791– ;794.

Lurmann, F.W., N. Kumar, R. Londergan, and G. Moore, 1997. "Evaluation of the UAM-V Model Performance in the Northeast Region for OTAG Episodes," STI-996133-1716, March 1997. (http://capita.wustl.edu/otag/Reports/sonom a/report2.1.html).

Meagher, 1996. Slides from presentation given by Jim Meagher of the Tennessee Valley Authority to the OTAG Air Quality Analysis Workgroup, Washington, D.C., July 1996.

Morris, 1996. "Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States," March 1996. ENVIRON International Corporation, Novato, California.

Morris, R.E., K. Lee, and G. Yarwood, 1997. "Phase I Comparison of OTAG UAM-V/BEIS2 Modeling Results with Ambient Isoprene and Other Related Species Concentrations," January 1997. ENVIRON International Corporation, Novato, California. (http://capita.wustl.edu/OTAG/Reports/morris/EXECSUM.html).

NRC, 1991. "Rethinking the Ozone Problem in Urban and Regional Air Pollution," 1991. National Research Council, National Academy Press, Washington, D.C.

OTAG, 1996. "Evaluation of the UAM-V Model Performance in OTAG Simulations: Summary of Performance Against Surface Observations," October 25, 1996, Draft Report. Ozone Transport Assessment G roup.

OTC, 1994. "The Long-Range Transport of Ozone in the Ozone Transport Region," January 1994. Ozone Transport Commission, Washington, D.C.

Poirot, R., and P. Wishinski, 1996a. "VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to the OTAG Air Quality Analysis Work group," December 3, 1996. (http://capita.wustl.edu/otag/reports/Status_Dec96/Status_Dec96.html).

Poirot, R., and P. Wishinski, 1996b. "VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to OTAG Air Quality Analysis Workgroup," November 7, 1996. (http://capita.wustl.edu/otag/Reports/VTTRAJ5/StatusReport1.html).

Poirot, R., and P. Wishinski, 1996c. "VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology. Status Report to OTAG Air Quality Analysis Workgroup," August 15, 1996. (h ttp://capita.wustl.edu/otag/Reports/vtdecair/vtdecair.html).

Poirot, R., and P. Wishinski, 1997. "Historical Perspective on the Climatological Potential for "Local" Pollution Episodes," February 3, 1997. Vermont Department of Environmental Conservation. (http://capita.wustl.edu/ota g/Reports).

Rao, S.T., E. Zalewsky, and I.G. Zurbenko, 1995. "Determining Temporal and Spatial Variations in Ozone Air Quality," Journal of Air and Waste Management Association, Vol. 45, pp. 57–61.

Ray, S.E., M.E. Korc, M. Byars, T.S. Dye, C.G. Lindsey, T. Haste, and P.T. Roberts, 1997. "Analysis of Nocturnal Low-Level Jets in the Northeastern United States During the Summer of 1995," STI-996440-1706-DFR, January 31, 1997. Sonoma Techn ology, Inc., Santa Rosa, California.

Schichtel, B., and P. Wishinski, 1996. "HY-SPLIT and CAPITA Monte Carlo Model Back Trajectory Comparison," July 1, 1996, Draft Summary. (http://capita.wustl.edu/otag/Reports/Trajcomp/trajcomp.ht ml).

Schichtel, B., and R.B. Husar, 1996. "Source Regions of Influence for High and Low Ozone Conditions in the Eastern U.S.," August 8, 1996, Draft Summary. (http://capita.wustl.edu/otag/Report s/Reports.html).

Schichtel, B., and R.B. Husar, 1997. "Update on the Characterization of Transport Over the Eastern U.S.," February 1, 1997. (http://capita.wustl.e du/otag/Reports/Reports.html).

Schichtel, B.A., and R.B. Husar. "Regional Simulation of Atmospheric Pollutants with the CAPITA Monte Carlo Model," Journal of Air and Waste Management Association, Accepted for publication, 1995.

SOS, 1995. "The State of the Southern Oxidants Study: Policy-Relevant Findings in Ozone Pollution Research, 1988-1994," April 1995. Southern Oxidants Study, College of Forest Resources, North Carolina State University, Raleigh.

Trainer, M. et al., 1993. "Correlation of O₃ with NOy in Photochemically Aged Air," Journal of Geophysical Research, Vol. 98, pp. 2917–2925.

Vukovich, F.M., 1996. "A Study of the Intra-Annual and High Frequency Variations of Ozone and Its Precursors in the Southeastern United States," Synopsis of the OTAG Air Quality Analysis Workgroup presentation, August, 1996. Science Applications International Corporation, Raleigh, North Carolina.

Wishinski, P., and R. Poirot, 1996. "Air Trajectory Residence Time Analysis Investigation of Ozone Transport Pathways: 1989–95," February 20, 1996, Draft Summary. (http://capita.wustl.edu/otag/Reports/Restime/Restime.html).

Wolff, G.T., 1993. "On a NOx-Focused Control Strategy to Reduce O₃," AWMA Association 44 (December), 1593.

Wolff, G.T., and P.E. Korsog, 1994. "Statistical Trends in Ozone in the Eastern U.S.," May 1994. Presented at AWMA Specialty Conference on Tropospheric Ozone, Orlando, Florida.

Zalewsky, E., S.T. Rao, and I.G. Zerbenko, 1994. "Relating Changes in Ozone Concentrations to Changes in Emissions in the Presence of Meteorological Fluctuations," May 1994. Transactions of the AWMA Specialty Conference on Tropospheric Ozon e: Critical Issues in the Regulatory Process, Orlando, Florida.

1. Summaries of Individual AQA Workgroup Analyses

Brief summaries and critical reviews were prepared for most of the AQA Workgroup analyses identified in Table 4-1 in Part II: Summary and Integration of Results. These summaries provide a description of the purpose, methodology, and major findings of each study and are intended to serve as a resource for quick reference in lieu of turning to the full study report(s). These summaries also include brief discussions of each study’s limitations and the scientific and policy implications for the issu es being addressed by OTAG. The discussions of findings, limitations, and implications were used as the basis for the integrated assessment presented in Part II. Summaries are presented on the following pages in the order in which projects are listed in Table 4-1.

Spatial Pattern of Daily Maximum Ozone Over the OTAG Region

Participants. Rudy Husar, CAPITA, Washington University, St. Louis, Missouri.

References. Husar, R.B., 1996a. "Spatial Pattern of Daily Maximum Ozone Over the OTAG Region," September 16, 1996. (http://capita.wustl.edu/otag/Reports/otagspat/otagspat.html).

Purpose. To identify spatial patterns in daily maximum 1-hour ozone concentrations over the OTAG domain.

Methodology. A 5-year (1991–1995) data set of June–August daily maximum 1-hour ozone concentrations for the eastern United States was obtained by combining data from AIRS, CASTNet, and several other monitoring networks. All sites wit h at least 25-percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season). The final data consisted of 715 monitoring sites. Six of these sites were discarded as a result of quality control procedures. Average concentrations and various percentiles of the concentration distribution were calculated for each monitoring site, and contour plots were prepared for each summary statistic. Specific emphasis was placed on the spatial distr ibution of the mean, 10th, 50th, and 90th percentiles.

Findings. The following principal findings of this analysis were cited:

1. Mean daily maximum values range from a value close to 40 ppb (which is within the range of northern hemisphere tropospheric background values) in the "corners" of the OTAG domain (south Florida, south Texas Gulf coast, upper midwest/northern plains, and northern Maine), up to approximately 80 ppb in the vicinity of some major urban centers. Mean values along the southern portion of the western boundary in Texas, Oklahoma, and Nebraska are about 10 ppb higher than those in the corners of the domain.

2. Mean daily maximum concentrations are highest in the Washington, D.C.-New York urban corridor, with elevated concentration "hot spots" also observed in the vicinity of several other (but, surprisingly, not all) major urban centers.

3. A large region of elevated (65–70 ppb) mean daily maximum concentrations is evident along a band stretching across southern Indiana and Ohio, just north of the Ohio River.

4. East-west sections across the center of the domain show concentrations increasing consistently from west to east.

5. In the industrial midwest, from Missouri through Illinois, Indiana, Ohio, Kentucky, West Virginia, and Virginia, low (10th percentile) concentrations range from 42 ppb to 47 ppb, about 15 ppb above values observed along the edges of the OTAG domain . However, in contrast to seasonal mean values, 10th percentile concentrations in urban areas are indistinguishable from their surroundings.

6. Ninetieth percentile concentrations are highest in urban areas, including those urban areas that did not show mean concentrations elevated above regional background values. Elevated 90th percentile values are also found over a large region coverin g southern Indiana and southern Ohio.

7. Differences between 90th and 10th percentiles show that daily maximum ozone concentrations are most variable in urban areas and least variable in the corners of the OTAG domain. This finding is consistent with the notion that monitored values in t he corners of the domain represent broad-scale background ozone levels.

Limitations. These results are dependent on the spatial distributions of monitors; the spatial interpolation methods used; and the accuracy, precision, and completeness of the data. Quantitative estimates of spatial interpolation errors applica ble to the AIRS/CASTNet data subset of this study are discussed by Falke (1996), but the impact of these errors are not discussed in connection with the principal findings of this study. A complete spatial analysis of interpolation errors for the integra ted data set used in the spatial analysis would be needed to confirm some of the findings noted above. In particular, because urban, near-urban, and rural sites were weighted equally in the interpolation procedure, it is possible that the interpolation f rom urban or near urban sites (which make up most of the network) produced a bias in the results in some locations. Also, because extreme concentrations (e.g., 90th percentiles) exhibit larger spatial gradients than mean or median values, the interpolati on procedure may give a less accurate picture of the spatial pattern of extreme events as compared to average events. It should also be noted that the author used a 25-percent data completeness criteria for including monitors in the analysis rather than the more commonly used 75-percent criteria. No analysis of the effect of the less stringent completeness criteria on uncertainties in computed summary statistics is provided. This is particularly of concern for the more extreme (i.e., 10th and 90th) pe rcentiles and the exceedance statistics. Application of the completeness criteria to the integrated 5-year data set rather than year by year is also a potential concern because missing values are unlikely to be randomly distributed over the 5-year period and individual years are known to differ markedly in mean and peak ozone concentrations due to variations in meteorological conditions. Thus, blocks of missing values at key groups of monitoring sites could produce a bias in the results in unknown ways. Finally, it should also be noted that many or all of the days contributing to the low 10th percentile concentrations observed in many of the urban areas may have been affected by NOx titration under conditions leading to high NOx concentrations and limi ted photochemical ozone production. This observation is in contrast to the author’s conclusion that the low urban 10th percentile values reflect well ventilated conditions under which urban and surrounding rural concentrations are similar in magnitu de. Additional analysis of the conditions contributing to the 10th percentile values will be needed to resolve this question.

Scientific Implications. Any realistic model of ozone/precursor relationships in the OTAG domain should be able to reproduce (or at least be consistent with) the principal features of the monitoring data described in this analysis. Of particul ar importance are the tropospheric background levels found in the corners of the domain and the apparent narrowing and shift to the right (i.e., toward higher concentrations) in the distribution of daily maximum values in southern Ohio and Indiana.

Policy Implications. Human activities must be responsible for some or most of the ozone excess in the central portions of the eastern United States over hemispheric background levels. Furthermore, these results suggest that precursor sources (p resumably major NOx point sources) along the Ohio River valley are associated with a broad region of elevated ozone levels not associated with any specific major urban centers. These results by themselves, however, do not indicate whether or not this bro ad region of elevated ozone concentrations is of concern from the standpoint of limiting exceedances of the ambient ozone standard. Also, these results by themselves do not indicate whether or not control of the major NOx sources geographically associate d with this region will have a beneficial effect on reducing exceedances in the OTAG domain.

Pattern of 8-Hour Daily Maximum Ozone Over the OTAG Domain

Participants. Rudy Husar, Washington University, St. Louis, Missouri.

References. Husar, R.B., 1996b. "Pattern of 8-Hour Daily Maximum Ozone and Comparison with the 1-Hour Standard," September 16, 1996. (http://capita.wustl.edu/otag/Reports/).

Purpose. To compare and contrast the spatial patterns of daily maximum 8-hour and 1-hour ozone concentrations over the OTAG region.

Methodology. A 5-year (1991–1995) data set of June–August hourly average ozone concentrations for the eastern United States was obtained by combining data from AIRS, CASTNet, and several other monitoring networks. All sites with at l east 25-percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season). The final data consisted of 715 monitoring sites. Six of these sites were discarded as a result of quality contr ol procedures. Daily maximum 1-hour and 8-hour averages were extracted for each day with sufficient data, and means, 10th, 50th, 90th percentiles, and exceedance counts for various 1-hour and 8-hour thresholds were computed.

Findings. Comparisons of daily maximum 1-hour values to 8-hour values for the same day indicate that the 8-hour to 1-hour ratios are lowest in urban areas where high peak 1-hour ozone concentrations are observed and approach 1:1 at rural remot e sites.¹ For example, the average ratio at Greenwich, Connecticut (adjacent to New York City) is 0.82, with ratios corresponding to daily maximum 1-hour values above 100 ppb averaging about 0.75. Other locations exhibit ratios between these extremes. For the OTAG region as a whole, the average ratio is 0.86, with slightly higher values in the northwestern portion of the region and slightly lower values in the southwestern portion. Exceedances of a 1-hour, 120-ppb level are most common in the northeastern urban corridor and near other major urban centers. The spatial pattern of 8-hour exceedances of 102 ppb is similar. Moving to a 1-hour, 94-ppb or an 8-hour, 80-ppb level broadens the geographic extent of the exceedances to include a ban d of eight or more exceedance days per year just north of the Ohio River stretching from Illinois to Ohio and into western Pennsylvania. Although the overall spatial patterns of 1-hour and 8-hour exceedances are similar, examination of a difference plot (8-hour exceedances of 80 ppb minus 1-hour exceedances of 94 ppb) indicates that the 8-hour exceedances are relatively more frequent within those broad areas north of the line separating Kentucky and Virginia from Tennessee and North Carolina, which are a way from the direct influence of a major urban area. With the exception of North Carolina, differences in exceedances are much less pronounced south of this line. Interestingly, Dallas and Houston, Texas, show quite different responses, with 8-hour exce edances relatively more common in Dallas and less common in Houston.

Maps of 90th and 10th percentile 8-hour daily maximum ozone concentrations are nearly identical to corresponding maps for the 1-hour daily maxima. Similarly, 8-hour and 1-hour maps of the difference between 90th and 10th percentiles are also very simi lar.

Limitations. Limitations noted for Husar’s analysis of 1-hour daily maxima also apply to this study, with the exception that an appropriately more stringent data completeness criterion was applied to the analysis of 8-hour exceedances, thu s reducing the potential for distortions resulting from missing data in these results. This study was done prior to EPA’s announcement of plans to replace the current 1-hour ozone NAAQS of 0.12 ppm with an 8-hour, 0.08-ppm standard and, therefore, d id not directly compare areas of violation of the current and proposed standards, although exceedance maps for these two concentration levels are provided. Such area of violation maps have, however, been produced by EPA and others.

Scientific Implications. One-hour and 8-hour daily maximum concentrations are highly correlated both temporally and spatially. The spatial patterns of threshold exceedances and percentiles for 1-hour and 8-hour daily maxima are very similar. Comparisons of the magnitude of same-day 1-hour and 8-hour peaks discussed above indicate that, under ozone episode conditions, concentrations at high ozone urban sites tend to exhibit relatively sharp 1-hour peaks. As a result, on a same-day basis daily maximum 8-hour averages at such sites are relatively small compared to 1-hour maxima. In contrast, rural remote sites are characterized by low amplitude diurnal cycles in 1-hour values and therefore maximum 8-hour values tend to be only slightly less tha n corresponding 1-hour maxima.

Maps presented in this study show that multiple exceedances per year of an 8-hour, 80-ppb level covered a much broader area between 1991 and 1995 as compared to multiple exceedances per year of a 1-hour, 120-ppb level. The latter are confined to the i mmediate vicinity of a few large urban areas while the former cover much of the OTAG domain with the exception of the extreme northwest, northern Maine, and the deep south, indicating the broad spatial extent of such "mid-level" ozone concentrat ions. The threshold levels for which the frequency of 8-hour exceedances is comparable to the frequency of 1-hour exceedances of 120 ppb for the OTAG domain as a whole is approximately 102 ppb.

Policy Implications. Results presented in this study indicate that daily maximum 1-hour and daily maximum 8-hour concentrations are very closely related both spatially and temporally. Differences in the areas over which 1-hour versus 8-hour s tandard violations occur depend on the level and form of the 8-hour standard and were not explicitly addressed here. However, the above results do indicate that at locations experiencing the highest 1-hour concentrations in the OTAG domain (e.g., downwin d of major urban areas such as in southern Connecticut and along the eastern shore of Lake Michigan), the 1-hour peak exceeds the 8-hour peak by a greater amount than in other areas. Thus, all else being equal, an 8-hour standard may be relatively less stringent in these locations.

¹In some cases at these sites where ozone concentrations increase overnight, the ratio exceeds one. This is due to EPA âs proposed method of assigning a given 8-hour average to the day on which the first hour of the 8-hour window falls.

Weekly Pattern of Ozone Over the OTAG Region

Participants. Rudy Husar, CAPITA, Washington University, St. Louis, Missouri.

References. Husar, R.B., 1996c. "Weekly Pattern of Ozone Over the OTAG Region," September 16, 1996. (http://capita.wustl.edu/otag/Reports/otagweek/otagweek.html).

Purpose. To analyze the influence of weekly cycles in anthropogenic emissions on daily maximum ozone concentrations.

Methodology. A 5-year (1991–1995) data set of June–August daily maximum 1-hour ozone concentrations for the eastern United States was obtained by combining data from AIRS, CASTNet, and several other monitoring networks. All sites wit h at least 25 percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season). The final data consisted of 715 monitoring sites. Six of these sites were discarded as a result of quality control procedures. After disagregation by day of week, average concentrations and various percentiles of the concentration distribution were calculated for each monitoring site, and contour plots were prepared for each summary statistic. Specific emph asis was placed on the spatial distribution of the mean, 10th, 50th, and 90th percentiles and on exceedances of 120 ppb.

Findings. For the OTAG region as a whole, seasonal peaks (90th and 95th percentiles) of daily maximum ozone concentrations were found to be about 7-percent lower on Sundays than during the week. After correcting for a constant 40-ppb backgroun d, the weekday to Sunday difference was found to represent about 33 percent of the OTAG region ozone excess above background. This "Sunday effect" is most pronounced in the 90th percentiles around the major urban areas where peak concentrations are highest but is notably absent (actually reversed with slightly higher values on Sundays than on Fridays) along a broad band stretching northeastward from western Tennessee through Kentucky, West Virginia near the Ohio border, Pittsburgh, and then nort hwestward to Cleveland. With the exception of Pittsburgh, this band overlaps the region of relatively high 10th percentile ozone concentrations noted by Husar (1996a). This "negative" Sunday effect is also evident at a few other isolated locat ions. No significant weekly cycle is evident in average or 50th percentile concentrations.

Weekly patterns in the frequency of exceedance of 120 ppb are similar to those for the 95th percentile of daily maximum concentrations, with one-third as many exceedances on Sundays as on Fridays. The number of exceedances increases nearly linearly fr om Monday through Friday. In most cases, the largest positive weekday/weekend differences in exceedances (i.e., more exceedances on Fridays than on Sundays) are over the urban areas with smaller differences elsewhere. In some urban areas, however, excee dances are actually somewhat more common on Sundays than on Fridays. Most of these urban areas fall within the geographic region noted above in which 90th percentile concentrations on Sundays are slightly higher than on Fridays. Several other isolated a reas also exhibit a weak "negative" Sunday effect. The weekly cycle noted at the 120-ppb exceedance threshold is progressively damped at lower thresholds (100, 80, and 60 ppb). Almost no weekend effect is evident at 60 ppb.

Scientific Implications. Results of this study indicate that the changes in average emissions between weekdays and weekends are sufficient to significantly reduce peak ozone concentrations on weekends while mean concentrations are largely unaff ected. Unfortunately, because the nature (magnitude of emissions reductions, relative source mix, diurnal pattern) of the differences in emissions on weekends as compared to weekdays have not been quantified, the mechanism resulting in lower peak ozone l evels on weekends in urban areas is not yet fully understood. The gradual increase in upper percentiles of the concentration distribution and in the numbers of exceedances of high concentration thresholds (e.g., 120 ppb) during the course of the work week is indicative of the influence of carryover of ozone and precursors from one day to the next. This carryover keeps Saturday concentrations higher than they would otherwise be, but by Sunday the full influence of the weekend emissions changes are evident . The weekend effect is most pronounced at the upper end of the concentration distribution and, therefore, changes in urban areas, where high concentrations occur most frequently, tend to be larger than in rural areas. In some urban areas, however, peak concentrations on Sundays are actually slightly higher than on Fridays (e.g., Pittsburgh, Pennsylvania). The reasons for this anomalous behavior are not known but may be related to geographic variations in combinations of several factors (mix of source types, activity patterns, VOC/NOx ratios). Geographically, the overlap between areas of relatively high 10th percentile concentrations and areas of negative weekend effect in the central portion of the OTAG domain may be related to the impacts of elevated NOx sources associated with base-load utility boilers characterized by relatively small weekly variations in emissions. However, the temporal pattern of emissions in this region has not been examined, and the extent of any weekday/weekend differ ences are unknown.

Limitations. These results are highly dependent on the spatial distributions of monitors; the spatial interpolation methods used; and the accuracy, precision, and completeness of the data. Quantitative estimates of spatial interpolation errors applicable to the AIRS/CASTNet data subset of this study are discussed by Falke (1996), but the impact of these errors are not discussed in connection with the principal findings of this study. A complete spatial analysis of interpolation errors for the integrated data set used in the spatial analysis would be needed to confirm some the findings noted above. In particular, because urban, near-urban, and rural sites were weighted equally in the interpolation procedure, it is possible that the interpolati on from urban or near-urban sites (which make up most of the network) produced a bias in the results in some locations. Also, because extreme concentrations (e.g., 90th percentiles) exhibit larger spatial gradients than mean or median values, the interpo lation procedure may give a less accurate picture of the spatial pattern of extreme events as compared to average events. It should also be noted that the author uses a 25-percent data completeness criteria for including monitors in the analysis rather t han the more commonly used 75-percent criteria. No analysis of the effect of the less stringent completeness criteria on uncertainties in computed summary statistics is provided. This is particularly of concern for the more extreme (i.e., 10th and 90th ) percentiles and the exceedance statistics. Application of the completeness criteria to the integrated 5-year data set rather than year by year is also a potential concern because missing values are unlikely to be randomly distributed over the 5-year pe riod and individual years are known to differ markedly in mean and peak ozone concentrations due to variations in meteorological conditions. Thus, blocks of missing values at key groups of monitoring sites could produce a bias in the results. Finally, r easons for the "negative" weekend effect found in some areas are not well understood and require further study. In particular, no analysis is presented here of weekday/weekend differences in precursor emissions.

Policy Implications. Results of this analysis imply that a control scenario mimicking weekday to weekend emissions reductions would result in decreases in peak ozone concentrations in most urban areas. For the OTAG region as a whole, exceedan ces of the current NAAQS (120 ppb) would be reduced by roughly a factor of 3. Unfortunately, the exact nature of weekday/weekend emissions differences is not currently known. Not only the magnitude of the emissions reductions on weekends, but also chang es in the relative mix of VOC versus NOx emissions and of low-level versus elevated NOx sources are likely to be important. Weekday/weekend differences in the diurnal pattern of emissions are also likely to be important. Differences in the timing of mob ile source emissions may be as important in reducing ozone concentrations as differences in the total amount of emissions.

Spectral Decomposition of Ozone Time Series

Participants. P.S. Porter, University of Idaho, Idaho Falls, Idaho 83405; S.T. Rao, I. Zurbenko, and E. Zalewsky, State University of New York, Albany, New York 12222; R.F. Henry and J.Y. Ku, New York State Department of Environmental Conservat ion, Albany, New York 12222.

References. Porter, P.S., S.T. Rao, I.G. Zurbenko, E. Zalewsky, R.F. Henry, and J.Y. Ku, 1996. "Statistical Characteristics of Spectrally-Decomposed Ambient Ozone Time Series Data," August 1996. Final Report.

Purpose. To (1) estimate the spatial representativeness of data collected at routine ozone monitoring sites and the typical size of airsheds (zones of influence), and (2) identify principal features of the spatial and temporal characteristics o f daily maximum 1-hour and 8-hour ozone concentrations within three distinct frequency domains: long-term (i.e., annual), seasonal, and short-term (i.e., synoptic variations on time scales of less than about 30 days).

Methodology. Using multiple pass moving average filters (Kolmorgorov-Zurbenko, or KZ filters) described by Rao and Zurbenko (1994), the authors developed a spectral decomposition of the time series of natural logarithms of daily maximum 1-hour and 8-hour average ozone concentrations into long-term (annual), seasonal, and short-term (synoptic scale) components. This was done at all routine ozone monitoring sites in the United States reporting to EPA’s AIRS database (few values were missing for the period 1983–1994). Monitors were segregated into two categories: those that operate year-round and those that operate only during the summer ozone season. Characteristics of each frequency domain were investigated, including temporal vari ance and autocorrelation, spatial patterns in means and variances, spatial correlations, and the influence of meteorological conditions.

Findings. Using two KZ filters—a 15-day average, 5-pass filter and a 365-day, 3-pass filter—the authors found that the time series of log-transformed daily maximum concentrations can be cleanly separated into (1) a long-term component representing the influence of trends in emissions and (possibly) short-term climatic fluctuations, (2) a seasonal component representing the influence of the earth’s rotation about the sun, and (3) a short-term component representing the influence o f fluctuating synoptic meteorological conditions and random processes (noise). An analysis of variance showed that covariances between the different components are generally small (less than 2 percent of the total variance). Long-term component variance s were less than 10 percent of the total at more than 90 percent of all monitoring sites, with median values of 3.6 percent for summer season only monitors. For monitors with year-round data, the seasonal component accounts for up to 73 percent of the to tal variance (median value of 51 percent) while the median contribution of the short-term component is 51 percent. For summer season only monitors, the median seasonal component variance contribution was 12 percent, while the median short-term component was 77 percent. Seasonal components are higher in the northeast than in the south. Contributions of short-term variance to the total are highest in coastal areas and lowest in the midwest. In absolute terms, the highest short-term variances are in the northeast and along the Gulf coast.

An evaluation of the statistical properties of the short-term component, W(t) revealed it to be significantly negatively skewed and therefore not normally distributed at nearly all monitoring sites, while exp(W) was more nearly normally distributed in most locations. Autocorrelations in W(t) at a 1-day lag had a median value of 0.35, indicating some day-to-day dependence resulting from persistence in meteorological conditions. Nevertheless, modeling the short-term component as a random variable norma lly distributed with zero mean, the authors found that it is possible to reproduce the observed median concentrations at nearly all sites (to within the 95 percent confidence interval of the predicted value); 95th percentiles of the daily maximum values w ere less accurately reproduced. Exceedance counts were similarly evaluated, but it is not clear how to interpret these results because most monitors had very few or no exceedances over the 12-year period examined. Rao et al. (1996) provide comparisons o f observed and modeled exceedance counts and 95th percentiles at five locations that experienced multiple exceedances per year (at least prior to 1989) which show generally close agreement with some exceptions.

A trends analysis for the period 1985–1995 was also presented by the authors. Trends were computed for the ozone baseline (long-term plus seasonal component) both before and after adjusting for variations in temperature. Raw (unadjusted) baselin e trends are mixed, with generally negative trends in the southern tier states, Illinois, and the Philadelphia-New York urban corridor but increases at most sites in Indiana, Ohio, West Virginia, western Pennsylvania, and the Washington, D.C., area. Afte r adjusting for temperature fluctuations, negative trends were found at a much larger number of sites.

Spatial correlations in baseline and short-term components of both ozone and surface temperature time series were examined. For the Washington, D.C., area, temperature baseline correlations remain constant and near unity over the maximum distance exam ined (1,280 km or 800 miles), while correlation in the short-term component decreases linearly with distance over this radius, dropping to 0.3 at 1,120 km (700 miles). Ozone baseline spatial correlations drop off more rapidly but also respond approximate ly linearly with distance, at least out to the maximum distance considered (800 km or 500 miles), at which point the correlation coefficient is approximately 0.8. Short-term ozone component spatial correlations decrease exponentially with distance over th is range, dropping to a value of 0.3 at about 720 km (450 miles) along the "direction of transport."² A map of spatial correlation isopleths for the short-term component is presented by the authors for five cities in the OTAG domain (Atlanta, Georgia; Chicago, Illinois; Cincinnati, Ohio; Greenbelt, Maryland; and Pittsburgh, Pennsylvania). In all cases, correlations drop to 0.4 within 560–640 km (350–400 miles) of the selected monitors. It is suggested that these spatial d omains be considered in designing control strategies.

Comparisons of time series of 1-hour average daily maximum ozone to 8-hour average daily maxima indicated that variances are higher for the 8-hour averages, both in an absolute sense, relative to the mean, and for the short-term component only. Lookin g at daily maximum n-hour averages at one site (Cliffside Park, New Jersey), the authors found that variances of the short-term components peak at n=10 hours (õ = 0.38 as compared to õ = 0.27 at n=1 and õ = 0.35 at n=24). The statistical significance of these differences was not discussed.

Finally, using the technique described above of modeling the short-term component, W(t) as white noise, the authors computed the percent reduction needed in the ozone baseline component to achieve attainment of the ozone NAAQS for the 1987–1989 an d 1991–1993 attainment periods at each monitoring site. These required reductions range from 0 percent (for monitors already in attainment), to 50 percent at monitors in high-ozone locations in the northeast. Generally, lower reductions are require d during the latter period due to the progress toward attainment already achieved.

Limitations. From a regulatory perspective, much of the value in these analyses is based on the viewpoint that the baseline component of the ozone time series (i.e., the component remaining after removal of short-term fluctuations using the KZ_15,5 filter) is "deterministic" in the sense that it is controlled entirely by seasonal meteorological fluctuations and year-to-year trends in emissions. The remaining, short-term component, on the other hand, resembles random fluctuat ions. Thus, management of the ozone problem should focus on the effects of control strategies on the baseline rather than being concerned with a highly fluctuating peak statistic (e.g., fourth highest concentration in 3 years) in which much of the varia bility is associated with "noisy" processes that cannot be accurately modeled. To make use of this approach, however, one must be able to accurately estimate the impact of future control strategies on the baseline, and such methods have yet to be successfully demonstrated (i.e., it is not clear how to adapt episodic photochemical models to models of baseline ozone). In addition, one must be able to estimate the impact (or convincingly demonstrate that there is no impact) of control scenarios o n the statistical characteristics of the short-term component. To date, neither of these conditions have been met. Of course, it should also be noted that the ability of episodic photochemical models to accurately predict the impact of future-year contr ol strategies on ozone levels has also yet to be successfully demonstrated.

Although the trends analysis results presented by the authors are in general agreement with results of other studies (see review prepared by Morris, 1996), the fact that nearby sites often exhibit opposing trends is indicative of a complex spatial stru cture in trend statistics that may not be fully resolvable with the current monitoring network. Because of the large number of sites examined, the trend maps presented by the authors are difficult to read, especially in urban areas, which contain many cl osely spaced monitoring sites. In these areas, the green disks (denoting negative trends) overlap red disks (denoting positive trends). Because green disks are always on top, they obscure an unknown number of sites with positive trends. Tables of trend values for each site are available from the authors for review and Rao et al. (1995) present a table of trends for sites in the eastern United States for 1983–1992. Similar trend maps for the northeastern United States showing spatially smoothed tr ends (where the smoothing was done with spatial KZ filters) are presented by some of the same authors (Zurbenko, et al., 1995), and these maps do not suffer from this difficulty. Unfortunately, the spatial smoothing may obscure some important small-scale trend differences. Also, interpretation of trend statistics would be enhanced by further characterization of trends by site type (i.e., rural, remote, suburban, downwind urban peak, urban center, etc.).

Published demonstrations of the independence of short-term components of daily maximum ozone and temperature (Rao and Zurbenko, 1994; Flaum, Rao, and Zurbenko, 1996) are not conclusive because seasonal influences on the correlation were not resolved. Therefore, it has not been demonstrated that the short-term ozone fluctuations during the summer season are completely independent of meteorological effects. Thus, spatial correlations in this component may be partially attributable to spatial correlatio ns in meteorological conditions as well as to emissions forcing and defining a spatial domain for control strategy development on the basis of zero lag time; spatial correlations in the short-term components may not properly address the area of source inf luence. Furthermore, correlations at non-zero lag times (e.g., 1- or 2-day lags) also need to be considered. Spatial scales of correlations in the short-term temperature component appear to be reasonable from a synoptic scale meteorological perspective. The fact that these scales are greater than those for the ozone component may indicate that much of the remaining variance in the ozone component (i.e., that portion not related to temperature) is associated with other meteorological factors that operate on smaller spatial scales.

Science/Policy Implications. Perhaps the most important implication of the above findings is that, when only the summer high ozone season is considered, short-term fluctuations are the dominant source of variability in daily maximum concentrati ons (either 1-hour or 8-hour average). Thus, identifying the impact of control strategies on ozone levels is extremely difficult due to the low signal to noise ratio. Furthermore, areas close to the attainment /nonattainment threshold are likely to move in and out of attainment (where attainment or nonattainment is determined on the basis of 1–3 years of monitoring data), thus complicating management efforts. This study shows that the problem appears to be particularly severe in the northeast— ;precisely where accurate estimates of control strategy impacts are most needed.

When seasonal and long-term trends are removed from the ozone time series, the above-mentioned results show the spatial scale of the remaining short-term fluctuations to be 560–640 km (350–400 miles), suggesting a scale for the coherence of < I>same-day fluctuations in peak ozone, which may be useful for air quality management efforts. However, the degree to which this spatial scale is determined by spatial correlations in meteorological conditions as compared to same-day transport of ozo ne and precursors is unknown.

² This direction (also referred to by the authors as the "major axis ") is oriented in the direction of the prevailing wind at a particular location (e.g., southwest to northeast along the Washington, D.C. - New York urban corridor).

Additional References

Morris, 1996. "Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States," February 21, 1996. ENVIRON International Corporation, Novato, California.

Rao, S.T., and I.G. Zurbenko, 1994. "Detecting and Tracking Changes in Ozone Air Quality," Journal of Air and Waste Management Association, Vol. 44, pp. 1089–1092.

Rao, S.T., I.G. Zurbenko, P.S. Porter, J.Y. Ku, and R.F. Henry, 1996. "Dealing with the Ozone Non-Attainment Problem in the Eastern United States" Environmental Manager, January 1996, pp. 17–31.

Flaum, J.B., S.T. Rao, and I.G. Zurbenko, 1996. "Moderating the Influence of Meteorology on Ambient Ozone Concentrations," Journal of Air and Waste Management Association, Vol. 46, pp. 35–46.

Zurbenko, I.G., S.T. Rao, and R.F. Henry, 1995. "Mapping Ozone in the Eastern United States," Environmental Manager, January 1995, Vol. 1, pp. 24–30.

Ozone Exceedances Data Analysis: Representativeness of 1995

Participants. Lyle Chinkin, Richard Reiss, Douglas Eisenger, Timothy Dye, and Christopher Jones, Sonoma Technology, Inc., Santa Rosa, California.

References. Chinkin, L. et al., 1996. "Ozone Exceedances Data Analysis: Representativeness of 1995." Sonoma Technology, Inc.

Purpose. To assess the representativeness of the 1995 ozone season in the northeast in the context of the past 15 ozone seasons. The motivation for this study was the realization that the summer of 1995 was one of the hottest on record for the northeastern United States, and that the 1995 summer season was therefore a strong candidate to experience high ozone concentrations.

Methodology. The authors undertook an investigation of whether the above-normal temperatures during the summer of 1995 were associated with above normal ozone concentrations. The authors utilized air quality and meteorological data from seven metropolitan areas within the northeast’s ozone transport region, as well as the OTR overall. The metropolitan areas studied include Baltimore, Boston, Hartford, New York, Philadelphia, Providence, and Washington, D.C. Numerous regional and subregio nal trends in 1-hour average ozone concentrations were evaluated for the 1980s and 1990s to determine to what extent 1995 was consistent with these trends. Year-to-year fluctuations in the frequency and severity of meteorological conditions conducive to ozone formation were accounted for using several different methods, including (1) an examination of year-to-year differences in the relationship between the frequency or severity of high temperature days (e.g., the number of days above 90°F) and the frequency or severity of ozone episodes (e.g., number of exceedance days), (2) the Poison regression model developed by Cox and Chu (1995), and (3) the synoptic typing scheme of Comrie and Yarnal (1992).

Findings. The authors found that despite near record-breaking temperatures, the 1995 ozone season was no worse than the average ozone season experienced during the 1990s. In fact, 1995 is consistent with overall trends showing declining ozone concentrations in the northeast. The major findings cited by the authors are as follows:

• The number of days per year above the national ozone air quality standard has declined by about 70 percent from 1981 to 1995. The summer of 1995, despite being one of the hottest on record, is consistent with that trend.

• The severity of individual ozone events is declining, and 1995 is consistent with that trend. For example, on days above the national standard, the average concentration by which the standard was exceeded has declined by about 70 percent from 1981 to 1995.

• The extent of the geographic area experiencing days above the ozone standard is declining.

• During the 1990s, ozone concentrations above the national standard occurred at higher temperatures than in previous years; the most plausible explanation is that ozone precursor emissions have decreased.

• Between 1985 and 1994, VOC emissions in the northeast (EPA Regions I, II, and III) decreased 13 percent, while NOx emissions decreased only 5 percent.

Limitations. As with most other trends studies, the emphasis here is on peak concentrations in urban areas in the northeast. However, the study did include a comparison of trends at sites designated as urban or suburban in the AIRS database wi th sites determined to be in rural locations based on a review of their map coordinates. This analysis indicated no discernable difference between urban and rural trends. It should also be noted that uncertainties in the emissions inventory trends (espe cially in the NOx trends) were not evaluated. This is particularly important given the lack of attention to ambient NOx trends, which could be used to corroborate the inventory trend (assuming sufficient high quality NOx data are available). Uncertainti es in the correction for meteorological conditions must also be taken into consideration. In particular, the Cox and Chu Poison regression method assumes underlying (i.e., emissions-related) trends are linear in time—this is a rough approximation at best. In addition, year-to-year changes in the ratio of the number of days in a given season above a temperature threshold to the number of ozone exceedance days are very sensitive to the temperature and concentration thresholds used and are subject to numerous factors not related to emissions. These ratios thus provide only a very rough indication of actual precursor emissions trends.

Scientific Implications. Results of this study are generally consistent with those of other recent trends analyses that show concurrent declines in peak ozone levels and VOC emissions. Data on ambient precursor concentration trends are needed to verify estimated trends in emissions.

Policy Implications. Although not entirely definitive for the reasons noted above, this study supports other independent analyses of northeastern ozone and emissions trends, strongly suggesting that recent VOC emissions reductions resulted in d ecreases in peak 1-hour ozone levels over the routine monitoring network. Concurrent but relatively small NOx emissions reductions may have contributed to the ozone reductions. Of course, these studies do not provide any direct information on the relati ve effectiveness of future VOC controls in the region.

Additional References

Comrie, A.C., and B. Yarnal, 1992. "Relationships Between Synoptic-Scale Atmospheric Circulation and Ozone Concentrations in Metropolitan Pittsburgh, Pennsylvania," Atmospheric Environment, 26B, 301–312.

Cox, W.M., and S.H. Chu, 1996. "Assessment of Interannual Ozone Variations in Urban Areas From a Climatological Perspective," Atmospheric Environment, 30(14): 2615.

Ozone Measurement Trends Studies in the Northeast

Participants. Ralph Morris, ENVIRON International Corporation, 101 Rowland Way, Novato, California (reviewing work performed by U.S. EPA (1994 Air Quality Trends Report)); S. T. Rao et al., New York State Department of Environmental Conservatio n; George Wolff and Patricia Korsog, General Motors Corporation; William Cox and Shao-Hang Chu, OAQPS, USEPA; Kay Jones, Zephyr Consulting.

References. Morris, R.E., 1996. "Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States," March 1996. Novato, California, ENVIRON International Corporation, and references therein.

Purpose. To review and prepare an integrated summary of recent major northeastern United States air quality and emissions trends analyses.

Methodology. Several researchers have performed trends analysis of ozone in the eastern United States in recent years. These analyses differ primarily in the techniques used to account for the impact of meteorological variations in the ozone trends. A brief summary of methodologies and results were prepared for each analysis. These summaries were then used as the basis for an integrated assessment of recent ozone and emission trends in the northeast.

Findings. Summaries of methods and results for individual studies follow:

EPA National Trends Reports. EPA’s 1994 annual trends report (EPA, 1994) examined both longer term (1984–1993) and shorter term (1991–1993) trends in ozone using national composite averages (EPA’s 1995 trends reports were no t available). Between 1984 and 1993, the composite average exceedances over the 532 monitoring sites examined decreased 60 percent, a statistically significant amount. The EPA national trends report also examined the trends in ozone concentrations after the effects of meteorological variations had been removed using the parametric regression technique developed by Cox and Chu (1993). The meteorological-adjusted ozone concentrations illustrated a definite downward trend in ozone during the period studie d. EPA’s 1995 emissions trends report (EPA, 1995) provides trends in national ozone precursor emissions for the 1900–1994 time period, with particular focus on the last 10-year period of 1985–1994. During this 10-year period, there was a d istinct downward trend in VOC emissions (14-percent reduction) and a slight increase in NOx emissions (6-percent increase), suggesting that the downward trend in ozone is likely due to the reductions in VOC emissions.

Accounting for Meteorological Variations in Ozone Trends by the New York State Department of Environmental Conservation (NYSDEC) and the State University of New York (SUNY) at Albany. Researchers at NYSDEC and SUNY developed a statistical technique to eliminate the high-frequency variations in ozone trends due to meteorological fluctuations (Rao and Zurbenko, 1994). Rao and co-workers applied this technique to analyze the trends in meteorological-adjusted ozone concentrations for the peri od of 1980–1992. They found a definite downward trend in ozone for the northeast corridor stretching from Wilmington, Delaware, to Boston, Massachusetts, and essentially no change in the meteorological-adjusted ozone in the Baltimore-Washington, D.C ., and southern Maine regions. Zalewsky and co-workers (1994) examined the trends in VOC and NOx emissions in the northeast corridor during this period and found a downward trend in VOC emissions and very little change in NOx emissions. This finding sugg ests that the decrease in ozone concentrations are attributable to the VOC emissions reductions.

General Motors Corporation Studies on Ozone Trends. A series of studies performed by researchers at General Motors Corporation (GMC) analyzed ozone trends at several cities in the eastern United States for the time period of 1980 through 1993, as well as trends in morning VOC/NOx ratios and the Reid vapor pressure (RVP) of gasoline as a surrogate for VOC emissions (Wolff, 1993; Wolff and Korsog, 1994). Although the General Motors researchers did not perform adjustments to the ozone trends to a ccount for meteorological variations, trends were calculated for several different ozone parameters, including 8-hour averages and number of days the maximum 1-hour and 8-hour ozone exceeds threshold levels. They also calculated confidence intervals to d etermine whether trends were statistically significant. Researchers noted statistically significant downward trends in many of the ozone summary statistics in New York, Newark, Milwaukee, and Chicago, and downward trends in maximum and second-highest 1-h our ozone (although not always statistically significant) in all cities studied except Boston and Grand Rapids. It is interesting to note that Atlanta and Philadelphia (and to a lesser extent, Baltimore) showed downward trends in peak 1-hour ozone concent rations, but upward trends in the number of days in which the 8-hour ozone concentration exceeded 60 ppb. These results suggest that the distribution in 1-hour ozone concentrations in these cities is becoming narrower and urban peaks are declining, but m id-level (baseline) ozone concentrations are increasing. The researchers’ analysis of the trends in VOC/NOx ratios also suggest a downward trend in the cities studied from a value of approximately 10 in 1986 to approximately 5–6 in 1991. These results suggest that urban photochemistry is becoming more VOC limited in these cities (i.e., further VOC emission reductions will be effective in lowering ozone concentrations, while NOx emissions reductions may result in local increases in ozone concen trations).

Trends in Meteorological-Adjusted Ozone Concentrations Using the Cox and Chu Method. Cox and Chu (1993, 1995) used a probability model to examine meteorological-adjusted ozone trends with statistical confidence estimates. For the 1981–199 0 time period, statistically significant declines in ozone concentrations were calculated in New York, Philadelphia, Pittsburgh, and Baltimore, and small, but not statistically significant, declines were noted in Washington, D.C., and Boston. Cox and Chu also used their probability model to estimate the relative potential for the occurrence of meteorological conditions conducive to ozone formation in each urban area for the 40-year record from 1953 to 1993. For northeastern urban regions, the rank order statistics suggest that the year 1988 had the most conducive meteorological conditions for ozone formation in the 40-year record, followed by 1953, 1955, 1983, 1991, and 1993. The return time of the extremely adverse conditions seen in 1988 was estimate d to be a 1-in-40–year event in most northeastern cities.

Meteorological-Adjusted Trends in Ozone Concentrations Calculated by Kay Jones. Kay Jones of Zephyr Consulting performed several analyses of trends in yearly maximum, second highest, and 3-year design value (fourth highest ozone value in 3 year s) ozone concentrations. Year-to-year variability in meteorology were accounted for by the number of days per year the temperature in Philadelphia exceeded 90° F. A steep decline in meteorological-adjusted ozone concentrations in all northeastern c ities was estimated. Using more recent ozone data to calculate design values suggests fewer ozone nonattainment areas than the 3-year period that was used to classify them in the Clean Air Act Amendments of 1990 (which included 1988).

Integrated Results. According to the author’s review, the measurement studies demonstrate that (1) meteorological conditions play an important role in the occurrence of ozone exceedances, and (2) after accounting for meteorological variati ons, there has been a definite downward trend in ozone concentrations over the last 10 years in the eastern United States. Because VOC emissions have been reduced during this period while NOx emissions have remained fairly stable, this air quality improv ement can be attributed to reductions in VOC emissions. Although most of the studies focused on 1-hour ozone concentrations, one study also considered 8-hour average ozone concentrations. This study found that while peak 1-hour concentrations decreased, 8-hour concentrations increased. This finding may indicate either that the diurnal variation in ozone is getting narrower or that the VOC reductions are effectively reducing the urban peak 1-hour ozone concentrations, but they have less effect on region al ozone (which is more closely represented by the 8-hour ozone concentration).

Limitations. The principal focus of this review was on modeling studies; therefore, the review of measurement studies was not comprehensive. Furthermore, most measurement programs were focused on nonattainment areas, thus the trends were biase d towards urban areas; trends in rural areas were not specifically analyzed. In addition, uncertainties in the emissions inventory trends (especially in the NOx trends) were not evaluated; this fact is particularly important given the lack of attention t o ambient NOx trends which could be used to corroborate the inventory trend (assuming sufficient high quality NOx data are available).

Scientific Implications. Recent declines in (predominantly urban) peak 1-hour ozone levels in the northeastern United States (after at least partially accounting for variations in meteorological conditions) are coincident with decreases in VOC emissions. In contrast, inventory NOx emissions are relatively stable during this period. In at least one case, opposing trends in 8-hour versus 1-hour concentrations have been noted, implying either that VOC controls may be decreasing peak concentratio ns or that diurnal variations in ozone are getting narrower. Data on ambient precursor concentration trends are needed to verify estimated trends in emissions. Additional information on rural area trends is also needed.

Policy Implications. Although not entirely definitive for the reasons noted above, these independent analyses of northeastern ozone and emissions trends strongly suggest that recent VOC emissions reductions resulted in statistically significant decreases in peak 1-hour ozone levels throughout the routine monitoring network. Of course, these studies do not provide direct information on the relative effectiveness of future VOC controls in the region.

SOS Nashville/Middle Tennessee Ozone Study

Participants. Jim Meagher, Tennessee Valley Authority, Knoxville, Tennessee.

References. Meagher, J., 1996. Slides from presentation given to the OTAG AQA Workgroup meeting, Washington D.C., July 1996.

Purpose. To analyze ambient data from the 1995 Nashville SOS study to investigate ozone formation in the Nashville area and in major point source plumes released into a southern environment.

Methodology. The SOS Nashville database includes enhanced measurements of air quality and meteorological parameters (e.g., surface/aloft VOC, CO, sulfur dioxide (SO₂), NOx and NOy concentration data; airborne LIDAR ozone data; ozone sondes; and Radar/Radar acoustic sounding systems(RASS) atmospheric profilers) for 6 weeks in June and July 1995. More than 200 scientists were involved in the Nashville 1995 study, which was one of the largest air quality studies ever conducted in the United States. This period overlaps with the July 9–18, 1995, OTAG episode. The study also covered a 4-week period in 1994, but aircraft measurements were much more limited in 1994 than in 1995. For 1995, surface and aircraft ozone and NOy data we re analyzed for ozone productivity, defined as the ratio of O₃ to NOz (where NOz = NOy – NOx), which may be considered as the number of ozone molecules produced per NOx molecule lost (ozone productivity analyses are also discussed in the review of presentations by Bob Imhoff of the Tennessee Valley Authority). Surface and aloft ozone data were analyzed to characterize the spatial extent (horizontal and vertical) of the Nashville ozone episode that occurred July 11–15, 1995. Aircraft data collected in several large point source NOx plumes were analyzed to characterize the spatial/temporal scales for conversion of NOx to NOz and the concomitant ozone production.

Findings. The Nashville ozone episode that occurred July 11–15, 1995, appears to be "homegrown," i.e., the area of high ozone was quite localized around Nashville and therefore appears to have been related to Nashville emissions. Ozone concentrations in the mixed layer around Nashville appear to have been elevated by up to about 80 ppb above the regional ozone concentration of 60–70 ppb. The mixed layer depth extended up to about 2 km, and ozone concentrations above the mi xed layer were about 50 ppb (aircraft data).

The author also compared O₃/NOz relationships in urban and power plant plumes using two approaches. In one method, O₃/NOz relationships measured at three ground stations were compared for periods of high and low SO₂ le vels, where the SO₂ levels were used to indicate whether the airmass was associated with power plant or urban emissions. The ozone production efficiency was 70-percent higher for air masses with low SO₂ levels, possibly indicating t hat ozone production is more efficient (in terms of NOx utilization) in urban plumes than in power plant plumes. The relationships between O₃ and NOz were also evaluated from aircraft data obtained in urban and power plant plumes. On one day, July 16, the Tennessee Valley Authority helicopter made measurements in both a power plant plume and an urban plume and observed different ozone production efficiencies, with the urban plume approximately 65 percent more efficient than the power plant pl ume.

Plumes from the Cumberland (approximately 350 tons per day of NOx) and Johnsonville (approximately 80 tons per day of NOx) power plants in Tennessee were followed by helicopter flights from close to the source out to about 150 km downwind. At this dis tance, NOx was observed to be essentially totally converted to NOz, and NOx levels had dropped to a point where ozone production was essentially over (this situation is characterized as one where the NOx reactivity is exhausted). For the Johnsonville plu me on June 23, ozone levels were enhanced by up to approximately 25 ppb over a background of approximately 75 ppb at the point where NOx reactivity was exhausted. This plume filled the mixed layer and plume conditions were sampled at an altitude of about 1,200 m above mean sea level (msl). Based on the plumes followed in this field study, the authors concluded that NOx reactivity is exhausted (and ozone formation is over) within 30–100 km downwind.

Limitations. The SOS Nashville database appears to be rich and robust; therefore, limitations of these findings are likely to be related the number of events captured during the study (4 weeks in 1994 and 6 weeks in 1995). The July 11–15 , 1995, Nashville ozone episode is only one event and may not be representative of all Nashville ozone episodes, thus not all Nashville episodes may be homegrown. The conclusions regarding the lifetime of NOx reactivity in power plant plumes are for dayt ime meteorological conditions typical of the Nashville area in the summer, and may not be applicable to other regions and/or times of year. Also, power plant plumes released at night are generally subject to less turbulent mixing and may remain intact lo nger. These NOx plumes may therefore travel relatively long distances before taking part in photochemical reactions the following day.

Scientific Implications. For the July 11–15, 1995, Nashville ozone episode, enhancement of ozone above background due to local emissions appeared to dominate regional ozone enhancement. Of course, the characteristics of this episode may d iffer from other episodes in the region. The reactivity of NOx in power plant plumes released into the southern, daytime summer boundary layer appears to be rather shortlived (30–100 km downwind). Also, these observations suggest that ozone product ion is more efficient (in terms of NOx utilization) in urban plumes than in power plant plumes.

Policy Implications. If Nashville is representative of other urban areas in the south, and if the July 11–15, 1995, episode, which was the focus of the results described here, is typical of ozone episodes in these areas, then the results s uggest that effective ozone control measures should focus on reducing local urban emissions. The short-lived reactivity of NOx in power plant plumes found in this study suggests that during the day, the ozone formation generated by the plume is completed within about 100 km of the source. On the other hand, plumes released at night may travel relatively longer distances before taking part in ozone generating reactions the following day.

Air Trajectory Analysis of Long-Term Ozone Climatology

Participants. Rich Poirot and Paul Wishinski, Vermont Department of Environmental Conservation, Waterbury, Vermont.

References. Poirot, R., and P. Wishinski, 1996a. "VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to the OTAG Air Quality Analysis Workgroup," December 3, 1996. (http://capita.wustl.edu/otag/repo rts/Status_Dec96/Status_Dec96.html).

Poirot, R., and P. Wishinski, 1996b. " VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to the OTAG Air Quality Analysis Workgroup," November 7, 1996. (http://capita.wustl.edu/otag/Reports/VTTRAJ5/StatusRep ort1.html).

Poirot, R., and P. Wishinski, 1996c. "VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to the OTAG Air Quality Analysis Workgroup," August 15, 1996. (http://capita.wustl.edu/otag/Reports/vtdecair/vtdecair.html).

Poirot, R., and P. Wishinski, 1996. "Air Trajectory Residence Time Analysis Investigation of Ozone Transport Pathways: 1989–95," February 20, 1996. Draft Summary. (http://capita.wustl.edu/otag/Reports/Restime/Restime.html).

Purpose. To (1) perform a back-trajectory–based analysis of relationships between high regional ozone levels and transport pathways, (2) develop trajectory climatologies for regionally representative receptor locations throughout the OTAG domain, and (3) use trajectory sorting techniques to identify predominant synoptic-scale transport pathways when ozone is high at the receptors.

Methodology. The basis for the approach was the calculation of trajectories running backwards in time from a selected receptor/time. Multiple trajectories for the same receptor but different end-times (e.g., 9 a.m. on all summer days) were as sembled into families that define a trajectory climatology. The trajectory climatology suggests where air arriving at the receptor within the selected family of end times had previously resided. A technique known as "residence time an alysis" was used to display integrated results for large numbers of back trajectories. The approach was to grid the trajectory domain, keep track of the time each trajectory spends over each grid square along its path, and integrate the residence ti mes for all trajectories over all grid squares. The resulting grid of residence times were displayed as a gridded density plot called a "residence time plot."

Trajectories ending at a given receptor can be sorted and grouped by various criteria. For concentration-based sorting, trajectories are sorted on the basis of the ozone concentrations occurring at the time of their arrival at the receptor (e.g ., above average, above 60 ppb). The resulting residence time plot addresses the following question: "If the concentration at this site was high (or low or very high), from where did the air come?" Residence time probabilities are comp uted for each grid square as the ratio of the total residence time in the square from a group of trajectories (e.g., those corresponding to above-average ozone at their end points) to the total residence time over all grid squares for the trajectory group . Such probabilities tend to be highest in the grid squares closest to the receptor because all (or nearly all) trajectories must pass through these grid squares regardless of the end point ozone concentration. However, by subtracting the residence time probability for a selected group of trajectories from the residence time probability for all trajectories, a gridded density plot of the incremental residence time probability is obtained. Incremental residence time probability plots indicate the areas that are relatively more likely to be upwind under the selected set of conditions and thus eliminate the "near field" bias.

A different type of sorting is location-based sorting. With this approach, an average (or median) ozone level is calculated for each grid square as the weighted average concentration at the end points of all trajectories that pass through the g rid square. Trajectories are weighted according to their residence times in the grid square. Resulting plots display average concentrations at the receptor for prior airmass locations on the grid and address the following question: If the air had previously been here (or there), what is the average concentration at the receptor? Typically, these average concentrations are expressed in terms of normalized deviations from the receptor mean concentration. Location-based sorting was used to draw maps showing the locations of grid squares where the residence time weighted average end point concentrations were above or below the average receptor concentration by varying amounts.

Back-trajectories were calculated using the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectories (HY-Split) model with archived wind fields from the nested grid model (NGM) weather forecasting (Draxler, 1992; Phillips, 1975). Trajectory end times examined were 3 a.m., 9 a.m., 3 p.m., and 9 p.m. for June–August 1989–95. Results were presented for six high-elevation sites in the Appalachian Mountain chain from Look Rock, Tennessee, to Whiteface Mountain, New York. Results are also presented for 19 low-elevation sites in an area ranging (approximately) from the Gulf coast to the Canadian border and the Mississippi River to the Atlantic coast.

In most of the analyses, raw ozone data were transformed to "Z-scores," based on geometric mean and standard deviations—computed separately for each site and for each hour of the day. All resultant Z-score distributions have a mean of 0 and standard deviation of 1. This standardization technique was used to allow data from different hours of the day and from different sites to be merged and sorted into consistent subsets of the statistical distribution (i.e., upper 50th, 10th or 5th pe rcentiles). Z-scores were used to define various subsets of trajectories and were also employed in a variety of inter-site correlations, time-lagged auto-correlations, and time-lagged inter-regional correlations for comparison to the trajectory results. For the inter-regional analyses, six subregions within the OTAG domain (New England, mid-Atlantic, northwest, midwest, southeast, and southwest) were defined on the basis of groups of monitoring sites in close geographic proximity with high inter-site co rrelations.

Findings. For the complete trajectory ensemble (i.e., unsorted), eastern receptors such as Whiteface Mountain and Look Rock had 2–4 day back trajectories covering almost the entire OTAG domain, suggesting the potential for transport of air parcels to these high elevation sites from across the entire OTAG region. Residence time analyses of trajectories ending at both the high- and low-elevation sites sorted for high-end point ozone levels (based on normalized deviations from the receptor s ite means) suggest high ozone is generally associated with transport from the heart of the OTAG domain rather than from the edges. Similarly, location-based sorting of trajectories indicated that trajectories that traversed areas within the central porti on of the OTAG domain produced higher end point ozone concentrations on average, regardless of the location of the receptor. The incremental probability plots for many receptors or groups of receptors showed that, as the analysis is increasingly restrict ed to the highest ozone concentrations, the area of highest incremental probability becomes increasingly focused on an area around the Ohio River valley. Within the southern portion of the OTAG domain, trajectories associated with above-average ozone tend to extend a smaller distance from the receptor than in the northern portion of the domain, indicating a relatively greater influence of stagnation in the south.

The finding of relatively more stagnation in the south is supported by an analysis of the autocorrelation in ozone concentrations (mean 3:00 p.m. Z-scores) within each of the six subregions within the OTAG domain; autocorrelations at 1-day lag times we re above 0.4 in all subregions, while autocorrelations remained well above 0.2 even at a 3-day lagtime in the southwest and southeast subregions. In contrast, autocorrelations dropped below 0.1 at 2-day lagtimes in the New England, mid-Atlantic, and nort hwest subregions.

Limitations. Residence time analysis of trajectory climatologies can suggest regional airflow patterns associated with high ozone regimes at a receptor, but without information on the age of the elevated ozone reaching the receptor, inferring the location (i.e., distance upwind) of the emissions that caused the elevated ozone is problematic; the trajectory results do not account for chemical transformation or deposition processes. These problems apply to all trajectories but become particular ly acute for trajectories extending back more than approximately 12 hours. Location-based sorting seems to address this issue, but understanding how to interpret the plots of location-sorted trajectories is difficult. One difficulty is that location plot s sorted for high and low ozone frequently show no contribution from grid squares immediately upwind of the receptor. These squares tend to be classified as average because many trajectories of both high and low ozone pass through them. The effects of th is "farsighted" bias in location-based sorting have not been estimated. These limitations imply that relationships between the patterns of trajectory residence times and end-point ozone concentrations do not necessarily indicate a cause-and-eff ect relationship between emissions in a given area and end-point ozone concentrations; these relationships may simply be an expression of the prevailing meteorological conditions under which high ozone concentrations at the end point receptor are favored.

Because all the analyses are limited to regionally representative receptors, the high ozone levels observed are often below the ozone NAAQS. This study fulfills the objective of identifying transport conditions for high regional ozone levels, but it d oes not specifically address how such conditions relate to transport conditions during ozone NAAQS exceedances. These analyses are also limited by uncertainties and/or biases in the back-trajectory calculations, and these uncertainties can be expected to grow as the trajectories are computed further back in time. Trajectories computed from NGM wind fields with relatively coarse vertical resolution are not likely to reflect local or mesoscale flow features such as low-level nocturnal jets, sea breezes, a nd other small-scale flow features likely to be important to ozone and precursor transport under ozone episode conditions. Some trajectory uncertainties have been investigated by Schichtel and Wishinski (1996) by comparing alternate trajectory models, an d these uncertainties do not appear to exert a major influence on the results presented here. Although some trajectories are calculated for as long as 4 days (at which time the uncertainties are relatively large), most trajectories are truncated at 2 0;3 days for various reasons. Furthermore, the authors indicated that they are unaware of any sources of bias in the analyses (nor were any biases found by Schichtel and Wishinski) and therefore that results averaged over a large number of trajectories c alculated under a particular set of conditions should provide a reliable indication of the mean trajectory pattern under those conditions. Ultimately, statements about potential uncertainty or bias should be related to the spatial scales of ozone transpo rt patterns inferred from the results.

Scientific Implications. Results of both location-based and concentration-based sorting of trajectories reveal the locations favored by trajectories associated with above-average ozone concentrations at the trajectory end points (receptors). T he central portion of the OTAG domain is such a favored location for receptors in many different locations throughout the eastern United States. As noted above, however, these results do not indicate the relative contributions of different source regions to ozone levels at the receptor site.

Policy Implications. Emissions controls in the areas suggested by the trajectory calculations to be consistently upwind of receptor sites experiencing high regional ozone levels may be effective in reducing regional ozone levels. For ma ny of the receptor sites examined throughout the eastern United States, the area implicated is centered on the Ohio River valley. However, this conclusion cannot be confirmed solely on the basis of these studies given the limitations of the methodology d iscussed above.

Additional References

Draxler, R.R., 1992. "Hybrid Single-Particle Lagrangian Integrated Trajectories (HY-SPLIT): Version 3.0 - User’s Guide and Model Description," NOAA Technical Memorandum ERL ARL–195. Air Resources Laboratory, Silver Spring, Maryla nd.

Phillips, 1975. NOAA Technical Report NWS-22.

Rolph, 1992. "NGM Archive," NCDC Report TD-6140, July 1992. National Climatic Data Center.

Schichtel, B., and P. Wishinski, 1996. "HY-SPLIT and CAPITA Monte Carlo Model Back Trajectory Comparison," July 1, 1996. Draft Summary. (http://capita.wustl.edu/otag/Reports/Trajcomp/trajcomp.html).

Source Regions of Influence for High- and Low-Ozone Conditions in the Eastern U.S.

Participants. Bret Schichtel and Rudolf Husar, Washington University, St. Louis, Missouri.

References. Schichtel, B., and R.B. Husar, 1997. "Update on the Characterization of Transport Over the Eastern U.S.," February 1, 1997. (http://capita.wustl.edu/otag/Reports/Reports.html).

Schichtel, B., and R.B. Husar, 1996. "Source Regions of Influence for High and Low Ozone Conditions in the Eastern U.S.," August 8, 1996. Draft Summary.

Purpose. To (1) develop a plume-based method for investigating the spatial impact of ozone and ozone precursors from selected source areas (the source region of influence) and (2) apply the technique to the eastern United States to characterize potential source regions of influence and transport patterns during the summers (June–August) of 1991–1995 for both average conditions and periods of high and low ozone concentrations, as well as during the 1991, 1993, and 1995 OTAG episodes.

Methodology. Forward particle plumes were calculated using the CAPITA Monte Carlo Model (Schichtel, 1995) with archived wind fields from the NGM weather forecasting model. The forward plumes represent simulations of air parcel transport away from a source, showing the direction and speed of the transport. Ranges of influence downwind ("source regions of influence") were estimated by imposing 1- or 2-day lifetimes on the released particles. By aggregating trajectories for multiple days, the authors obtained climatological transport directions, air parcel residence times, and regions of influence. Naturally, the region of influence was larger for an assumed 2-day lifetime than for a 1-day lifetime. Two methods were used for visual izing the results. In the first method, six sites were selected throughout the OTAG domain, and the source region of influence around each site was plotted. The region of influence was elongated along the predominant transport direction(s) and, viewed t ogether, the six regions of influence conveyed a sense of the main transport directions and the 1- and 2-day regions of influence. In the second method, 504 sources were evenly distributed on a grid covering most of North America, and 1-day regions of in fluence were calculated around each source. A vector was plotted at each source showing the characteristic (i.e., predominant) transport direction where the length of the vector was a measure of the persistence of transport in the direction of the vector . The resulting plot had the appearance of a gridded wind field plot. In addition, residence times were computed as the inverse of the characteristic transport speed. The characteristic transport speed was in turn estimated by dividing the radius of ci rcle with area equal to the source region of influence by the assumed pollutant lifetime.

Findings. The source regions of influence are highly dependent upon the pollutant lifetime, and because the lifetime of ozone is difficult to characterize, it is not possible to interpret the source regions of influence in a quantitative manner . The results can be used to qualitatively interpret relative changes in the size of regions of influence under different conditions. Results aggregated over all days (June–August, 1991–1995) suggest two characteristic transport regimes over t he eastern United States: One regime is from Texas north, through the midwest, and then east through New England to the Atlantic; the other is in the southeast with shorter transport distances. These regimes are consistent with summer mean wind fields ov er the eastern United States, which are dominated by the Bermuda high pressure system. Mean residence times increase approximately 50 percent from the northern half of the OTAG domain (approximately 0.2 s/m) to the southern half (approximately 0.3 s/m).

Aggregating results for days with the highest and lowest 10 percent average daily maximum ozone at each grid point showed that high ozone in the southeast is associated with longer residence times, indicating relatively stagnant air masses with net tra nsport from the north. Low ozone is associated with shorter residence times and more persistent transport from the Gulf of Mexico and the Atlantic Ocean. In the midwest, high ozone conditions are also associated with relatively long residence times, ind icating stagnant conditions under which a high pressure system is located over the midwest. Low ozone in the midwest is associated with shorter residence times and more persistent, rapid flow from the west. In contrast, high-ozone episodes in New Engla nd are associated with persistent, relatively rapid transport (relatively short residence times) from the west-southwest, whereas low ozone is associated with transport primarily from the northwest. Overall, residence times are lower than average during the low-ozone events, with transport from outside of the OTAG domain inward. High-ozone events have longer-than-average residence times with transport from the center of the OTAG domain outward.

Calculations of residence times and transport vectors during each of the 1991, 1993, and 1995 OTAG episodes show different spatial patterns with some similarities between the 1991 and 1995 episodes. The 1993 episode is characterized by long residence times in the south (as was the case for the other two episodes) but also by shorter-than-average residence times in much of the rest of the domain.

Limitations. The techniques developed are quite effective at illustrating climatological transport patterns, but less effective at showing regions of influence because it is unclear what lifetime to assume. The analyses do not shed light on th e extent to which transport actually was important under different conditions; they can only address the potential for transport. The vector plots focus on the predominant transport direction, but the region of influence plots show significant potential for transport in different, even opposing directions. Therefore, as noted by the authors, the vector plots may obscure important details of the transport patterns. For instance, based on the study findings, a plausible scenario for significant transport from the midwest to New England might be a stagnation event in the midwest followed by the onset of organized transport from the midwest to New England, but it is unclear whether the analyses performed would detect such an interaction. The potential effe cts of trajectory uncertainty and bias should be discussed in the context of the study conclusions.

Science/Policy Implications. This study suggests the potential for transport from the Gulf coast to the midwest and from the midwest to the northeast, but it does not provide information on whether such transport actually occurs. High-ozone ev ents in the southeast and midwest appear to be associated with relatively stagnant conditions, which suggests that ozone levels in theses areas are less strongly influenced by inter-regional transport. However, the net transport under these conditions is from the midwest, suggesting potential contributions to high ozone in the south.

Ozone/Tracer/NOy Relationships at Three SOS/SCION Sites

Participants. Eric S. Edgerton and Benjamin E. Hartsell, ESE Environmental Inc. (Currently affiliated with Atmospheric Research and Analysis, Inc.), Raleigh, North Carolina.

References. Edgerton, E.S., and B.E. Hartsell, 1996. Slides from presentation given to the OTAG–AQA meeting, Washington D.C., July 1996 and written summary posted to the OTAG-AQA Web site on November 4, 1996.

Purpose. To (1) identify the sources contributing to elevated ozone at SCION sites in the south by using CO and SO₂ as tracers to distinguish between contributions from urban areas and major point sources and (2) investigate the spa tial extent of elevated ozone events by observing the coincidence of elevated ozone at different sites.

Methodology. Data for the 1995 ozone season (May to October) from three sites in the SCION network in the rural south were analyzed. The sites were Oak Grove (about 120 km northwest of Mobile, Alabama), Centreville (about 85 km southwest of B irmingham, Alabama), and Yorkville (about 65 km northwest of Atlanta, Georgia). The measurements were for O₃, NOy, CO, and SO₂ and were for 1-minute averaging times. Elevated ozone days were defined as those having 8-hour average o zone greater than 80 ppb. These days were classified as having high ozone due to urban emissions, major point source emissions, or mixed (i.e., periods of both urban and major point source impacts within the same day) by examining CO/NOy and SO₂/NOy ratios. Periods with relatively high SO₂ and low CO were attributed to major point sources, whereas periods with relatively low SO₂ and high CO were attributed to urban emissions. An 80-ppb background for CO was subtracted b efore calculating the CO/NOy ratios.

Findings. All three sites observed elevated ozone days in the 1995 season, but fewer were observed at Oak Grove (5) than at Centreville (19) or Yorkville (29). Elevated ozone was observed at one or more site(s) on 39 days, of which 13 involve d two sites and one involved all three sites. Therefore, about two-thirds (25 out of 39) of the elevated ozone days involved only a single site, suggesting that on these days the spatial scale of elevated ozone was less than the distance between any two s ites (250–300 km).

A strong relationship between O₃ and NOy was observed at all sites on clear sunny days. The relationships involved some scatter, and there were instances of departure from the predominant relationship. Non-linearity was evident in the rela tionship, with a lower slope of O₃/NOy curve at higher values of NOy (5–10 ppb) than at lower values of NOy (1–5 ppb).

Periods of urban and major point source plume impacts were distinguishable at all three sites. Centreville and Yorkville showed elevated ozone days classified as urban, major point, and mixed, but all 5 elevated ozone days were classified as mixed at O ak Grove. Across the network, most impacts were identified as urban, less impacts showed mixed (both urban and point source) signatures, and fewer were identified as being due to major point sources.

Urban and point source plumes could be distinguished at these sites because key tracer species had high degrees of covariance. This finding suggests that the plumes were sufficiently recent in origin to retain their physical identity, i.e., they had n ot yet been dispersed into the regional airmass. The age of the plumes cannot be estimated directly from these data; however, the locations of the monitoring sites relative to nearby major urban areas suggest that the urban plumes must have been transpor ted at least 65–120 km to influence these sites.

Limitations. The implications of the results are limited in the number of sites (three), the geographic distribution of sites (rural southeast), and the number of years investigated (1995). The unfortunate paucity of data required for this ty pe of analysis prevents generalization to other parts of the OTAG domain. On the other hand, the data considered appear very rich mainly by virtue of the high temporal resolution (1-minute), which permit robust inter-species correlation analyses. The hig h degrees of covariance between species during elevated ozone periods may suggest that the plumes were of recent origin, but there is no way to estimate how recent or therefore what range of influence is implied.

The selection of elevated ozone days was based on an 8-hour period, which may tend to focus away from days that had short-term impacts. Point source plumes might be expected to result in shorter term impacts than urban plumes by virtue of their smalle r (narrower) spatial scale, and this finding is born out by some of the data presented by the authors. Therefore, the basis for selecting elevated ozone days may influence the relative numbers of days classified as being urban, point, or mixed influence.

Because the Oak Grove site is much closer to the Gulf coast than the other two sites, it can be expected to have a quite different ozone climatology due to the influence of marine air. Therefore, looking at the simultaneous occurrence of elevated ozon e at all three sites may not be a good way of examining the regionality of high-ozone episodes in south. Looking for concurrent events at Centreville and Yorkville may provide a somewhat more reasonable estimate of frequency of periods of widespread high ozone in the south. Better still would be to use the more extensive multi-year database of routine ozone measurements.

Scientific Implications. This study provided a good example of how tracer SO₂/CO/NOy analyses can be used to distinguish point and urban source signatures in relation to ozone formation. Much of the power of the tracer analyses ste ms from the high temporal resolution of the SCION data. The data strongly suggest that both urban emissions and point sources can cause elevated ozone levels at these three sites. Furthermore, urban emissions played a more dominant role than major point source emissions for the kind of days selected (8-hour ozone concentrations exceeding 80 ppb) at these three sites.

Policy Implications. For the three southern sites considered, the majority of days with 8-hour ozone exceeding 80 ppb were associated with urban signatures, sometimes in combination with point source signatures. Also, some days with 8-hour oz one concentrations exceeding 80 ppb were associated with predominantly point source signatures. These findings suggest that controls on urban area emissions may be more effective at reducing the frequency of such exceedances than controls on point source emissions for this set of sites and this type of ozone episode.

Analysis of Ozone, NOy, and Tracer Data From a Site in South-Central Pennsylvania

Participants. Eric S. Edgerton and Benjamin E. Hartsell, ESE Environmental Inc. (Currently affiliated with Atmospheric Research and Analysis, Inc.), Raleigh, North Carolina.

References. Edgerton, E.S., and B.E. Hartsell, 1997. Written summary posted to the OTAG-AQA Web site on January 6, 1997.

Purpose. To identify types of sources contributing to elevated ozone at a rural site toward the west of the OTR by using CO and SO₂ as tracers to distinguish between contributions from urban areas and major point sources.

Methodology. The Arendtsville, Pennsylvania, site is located about 90 km northwest of Baltimore in a rural area toward the west of the OTR. The site is located at the top of a small hill; no major point source of SO₂ or NOx is withi n 40 km of the site, and no secondary road is within 200 m of the site. The data include continuous measurements of O₃, NO, NOy, CO, and SO₂ reported at 1-, 15- or 60-minute averaging times recorded between June 17 and September 30, 1995. Elevated ozone days were defined as those having 8-hour average ozone greater than 80 ppb. These days were classified as having high ozone due to urban emissions, major point source emissions, or mixed (i.e., periods of both urban and major point source impacts within the same day) by examining CO/NOy and SO₂/NOy ratios. Periods with relatively high SO₂ and low CO were attributed to major point sources, whereas periods with relatively low SO₂ and high CO were att ributed to urban emissions. An 80-ppb background for CO was subtracted before calculating the CO/NOy ratios. The cut-points for assigning episode types were the following:

These cut-points were used to determine if one type of source was probably the dominant contributor, meaning that the source type could have provided at least 75 percent of the NOy. This analysis was similar to one performe d by the same authors for three southern sites and described in the summary entitled Ozone/Tracer/NOy Relationships at Three SOS/SCION Sites.

Findings. Of the 101 days in the 1995 field season, 21 had 8-hour average ozone concentrations greater than 80 ppb. As in the analysis of data from the SOS/SCION sites, the authors found that ozone was correlated with NOy levels on high-ozone days, and that there were correlations between NOy and CO and/or SO₂. The high temporal resolution in the concentration data greatly enhance the ability to discern such correlations. However, compared to the SOS/SCION sites, fewer periods wer e clearly dominated by one type of source in the Arendtsville data. Most of the days were diagnosed as having "mixed" influence (i.e., both urban and major point source emissions contributed to the high ozone), a few (6) were diagnosed as urban dominated, and none were diagnosed as dominated by major point sources. Thus, even for a site at the western edge of the OTR, urban emissions were associated with all 8-hour averages exceeding 80 ppb.

Limitations. The implications of the results are limited because only one site in the western OTR was evaluated. Caution should be used in extrapolating the results for Arendtsville to the western OTR in general. The selection of "high- ozone days" was based on an 8-hour period and a threshold of 80 ppb. An analysis for ozone events that are of shorter duration but higher concentration (e.g., 1-hour, 120-ppb) might reflect a different mix of sources.

Six of the 21 high-ozone days were classified as likely urban dominated using a criterion of SO₂/NOy ratios less than 0.5, but 5 of these 6 days had SO₂/NOy ratios that were between 0.4 and 0.5. Thus, a relatively small change in this classification criterion to SO₂/NOy ratios less than 0.4 would result in only 1 of 21 days being classified as urban with the remainder all classified as mixed. Similarly, several of the "mixed" days exhibited SO₂/NOy ratios of approximately 0.6 and CO/NOy ratios greater than 10 such that, if the criterion for SO₂/NOy ratios were slightly higher, a few more days would be classified as urban.

It appears that high-ozone days at Arendtsville usually are influenced by a mix of urban and point sources. The authors investigated several approaches to estimating the relative contributions of these source types by analyzing ratios of pollutant con centrations, but the analyses are complicated by the need to account for the effects of atmospheric removal on the CO, SO_2, and NOy concentrations measured at Arendtsville. These analyses contain significant uncertainties because of the age of emissions affecting Arendtsville, and the atmospheric lifetimes of NOy, SO₂, and (to a lesser extent) CO in the surrounding environment are hard to quantify.

Scientific Implications. The analyses performed in this study provide a good example of how tracer SO₂/CO/NOy analyses can be used to investigate the contributions of point and urban sources to ozone formation at a site where high t emporal resolution data are available. Data for Arendtsville indicate that a mix of urban and point source emissions causes elevated ozone levels. Urban emissions appear to be involved in all episodes, while point source emissions appear to be involved i n the majority of episodes. The O₃/NOy ratio measured at Arendtsville was approximately 4, which suggests that ozone was formed under conditions that were VOC limited or intermediate between VOC and NOx limited. Ratios at Arendtsville should be compared to other sites in the western OTR to investigate whether the relatively low values measured are representative of air masses transported into the OTR or whether local emissions may have influenced the Arendtsville site.

Policy Implications. For Arendtsville, almost all days with 8-hour ozone exceeding 80 ppb were associated with mixed urban and point source signatures, manifested by characteristic ratios of CO and SO₂ to NOy. This finding suggests that controls on urban or point source emissions have the potential to reduce elevated ozone levels at Arendtsville. Because of the rural nature of this site, as well as its location in the western OTR, it is reasonable to expect that the urban characte r of episodes would be more dominant in most other parts of the OTR.

A Comparison of Modeled and Measured Ozone, NOy, and CO at Nine Regional Monitoring Stations During the 1995 OTAG Episode

Participants. Benjamin E. Hartsell and Eric S. Edgerton, ESE Environmental Inc. (Currently affiliated with Atmospheric Research and Analysis, Inc.), Raleigh, North Carolina.

References. Hartsell, B.E., and E.S. Edgerton, 1996. Written summary posted to the OTAG-AQA Web site on November 24, 1996.

Purpose. To use data from nine rural monitoring stations to evaluate the OTAG UAM-V model performance for the 1995 episode. The study addressed how the model chemistry performs (in terms of O₃/NOy relationships) and how well the mod el predicts observed O₃, NOy, and CO levels. Additional analyses currently in progress will include additional measurements and will examine some issues in more detail.

Methodology. Air quality data were assembled for the 1995 episode (July 7–18) from four southern sites in the SOS/SCION network (sites in Mississippi, Alabama, Georgia, and Tennessee) and five northern sites from the NARSTO-NE study (site s in Pennsylvania, New York, and Maine). All stations measured O₃ and NOy and four measured CO. All sites were described as being located in "large clearings with excellent fetch and were well removed from the influence of local sources.& quot; Model predicted concentrations for the site locations were from the OTAG D2 basecase. One-hour averages of observed and modeled data were compared.

Findings. Scatter plots of O₃ against NOy across all sites and all hours between 1100 and 1900 local standard time were prepared for the observations and model predictions. The plots showed significant variation in the relationship for both the observations and model predictions; however, the general appearance of the plots was similar. When the data were sorted by ozone concentration and put into 10 bins containing equal numbers of points, linear relationships between O₃ and NOy emerged for both observed and modeled values. Within each bin, the distribution of NOy values tails toward high values; therefore this analysis tends to emphasize the lower NOy values that likely represent cases where ozone is most strongly NOx limited. As a result, the analysis effectively compares the modeled and observed O₃/NOy relationships (i.e., the ozone yield per NOx) under NOx-limited conditions, finding good general agreement between the model (slope 8.7) and observations (slope 9.2). Scatter was more evident in the modeled O₃/NOy relationship than in the observed relationship, which is consistent with the model overpredicting the frequency of fresh NOx emission impacts at these sites.

Plots were prepared of bias (modeled/measured) against modeled values with points averaged into 10 bins of equal numbers of points. These plots are difficult to interpret without scatter plots of the underlying data. For instance, the plots of ozone residuals against modeled ozone and ozone residuals against measured ozone are quite different; one plot suggests model overpredictions of about 20 ppb for the highest bin while the other plot suggests model underpredictions of about 5 ppb f or the highest bin. A clearer picture is obtained from analyses of ozone values exceeding thresholds of 80 ppb, 100 ppb, and 120 ppb. For these rural sites, the modeled and observed ozone exceeded 120 ppb in only a few instances and did not exceed 100 p pb at all sites. This result shows that ozone levels higher than 100 ppb represent the upper tail of the distribution of ozone values for these sites. The ratios of modeled to observed hours with peak ozone levels above 80 ppb and 100 ppb were 342/259 ( i.e., 1.3) and 102/59 (i.e., 1.7), respectively. The ratios of modeled to observed days with peak ozone values exceeding 80 and 100 ppb were 47/39 (i.e., 1.2) and 19/11 (i.e., 1.7), respectively. This finding shows a tendency for the model to overestima te the frequency and duration of high ozone values at these sites.

The time series for CO showed a tendency for overprediction of CO concentrations for two southern sites in Georgia and Tennessee. CO levels were generally predicted better for the southern site in Mississippi (close to the Gulf of Mexico) and for the one northern site in Pennsylvania. The time series for NOy showed generally good agreement, with some tendency for the model to predict higher maximum NOy values than were observed for the northern sites.

Limitations. The paper acknowledges several limitations that apply generally to model performance evaluations of this type:

1. Observations at single points are compared to model predictions that are volume averages over large grid cells. Grid cells were about 12 km by 12 km by 50 m at most sites, but 36 km by 36 km by 100 m for the Mississippi site.

2. The number of sites and days available for comparison is limited. Relative to other analyses of model performance for OTAG episodes, this limitation is more severe for ozone than the other species because a large number (hundreds) of other ozone si tes are available, but only a small number of NOy and CO sites are available.

The modeled NOy does not include organic nitrates (defined in the model as species NTR), because NTR concentrations were not saved in the OTAG model runs. Thus, modeled NOy concentrations are always biased low. However, the magnitude of this bias is not well known and likely varies in time and space depending upon the age of NOy (the bias might be as high as 20 percent for aged air masses). Accounting for the missing contribution of NTR to modeled NOy would increase the model under estimation of the O₃/NOy relationship (perhaps by as much as 20 percent).

The statistical analyses averaged data across all sites so that there is no indication whether significant differences existed for individual sites or between groups of sites (e.g., north versus south).

Scientific Implications. The comparison of modeled and observed O₃/NOy relationships revealed good agreement (modeled slope was 8.7, and observed slope was 9.2). These values can be interpreted as estimates of the ozone yield per N Ox under NOx-limited conditions. However, the model value is biased high by an uncertain amount because organic nitrates were not included in the model NOy values. If possible, the magnitude of this bias should be investigated to determine if it signifi cantly influences this finding. Also, the comparison of modeled to measured O₃/NOy relationships suggests that the model may be overpredicting the frequency with which fresh NOx emissions affect these sites.

For these nine rural sites, the model had a tendency to overestimate the frequency and duration of high ozone values; this tendency should be investigated for other rural sites to determine whether it is indicative of a more general model performance p roblem. The apparent tendency for the model to overpredict CO at two sites in Georgia and Tennessee may be indicative of emissions or atmospheric mixing problems in this region, and should be investigated.

Policy Implications. Analyses for nine rural sites in the south and the northeast indicate that the model has a tendency to overpredict the frequency and duration of ozone levels in 80–100 ppb range. If confirmed by more extensive analyse s, this finding would indicate a potential problem with using OTAG results to provide boundary conditions for urban scale modeling. The good performance of model chemistry for O₃/NOy relationships under NOx-limited conditions suggests that ove rall no gross problem exists with the model chemistry.

Ambient Monitoring Sites for OTAG (Time Series) Model Evaluation

Participants. Rich Poirot, Vermont Department of Environmental Conservation, Vermont Air Program, Building 3 South, 103 S. Main Street, Waterbury, Vermont 05676.

References. Poirot, R., 1996. Memorandum entitled "Ambient Monitoring Sites for OTAG (time series) Model Evaluation," January 11, 1996, from Rich Poirot, Vermont Department of Environmental Conservation, to S.T. Rao and Rudy Husar. (http//capita.wustl.edu/OTAGActivities/AQADocuments/OTAGdiscdocs/ModelCompSites.html).

Purpose. To recommend ozone monitoring sites in the northeast to be used for evaluation of the OTAG basecase regional photochemical model simulations for the July 1988, 1991, 1993, and 1995 episodes.

Methodology. Four ideal objectives were identified for site selection: (1) sites should be spatially representative of approximately 12-km by 12-km model grid squares; (2) data should be available for all OTAG model episodes; (3) data should i nclude hourly measurements of ozone, NOx, and speciated hydrocarbons and carbonyls; and (4) spatial coverage of the modeling domain should be sufficient. The author noted that these four criteria for site selection conflict with each other to some degree . For example, the PAMS data sites have more complete chemical species measured but are more urban oriented (i.e., aligned along the northeast corridor) and primarily have data available only for the 1995 episode (although some data are available for the 1993 episode as well).

Findings. Given the conflicting objectives noted above, the author developed a list of 21 ozone sites in the OTR that meet objectives 1, 2, and 4 (with some NOx measurements) and an additional set of 13 PAMS/NARSTO sites where more complete ch emical species data are available but which primarily operated only during the 1995 episode. (Some data for 1993 are possibly available at Whiteface Mountain, New York; and Wye River and Shenandoah National Park, Virginia). The author suggests that if t his list is too long, priority should be given to: (1) sites above 500 meters, (2) sites with NOx data, and (3) rural PAMS sites.

Limitations. Although the site selection approach used in this study provides a means of selecting optimal sites for evaluating the rural/regional model predictions, the emphasis on rural sites with broad spatial representativeness (e.g., sites above 500 m or rural PAMS sites) tends to preclude recommendation of sites in nonattainment areas. The fundamental purpose of OTAG was to identify options for reducing the transport of ozone and precursors into nonattainment areas. Therefore, the organi zation was also concerned with model performance on the edges of and within nonattainment areas because poor performance in these areas would bring into question the accuracy of model transport estimates.

Scientific Implications. Statistics summarizing model performance are likely to be highly dependent on the choice of sites to include in the analysis. The author was able to identify a number of predominantly rural sites that met the criteria. However, as noted above, OTAG was also concerned with model performance in and near nonattainment areas.

Policy Implications. Meeting OTAG’s goals required development of photochemical model simulations that met acceptable performance criteria. Appropriate selection of data sets to be compared with model predictions was critical to the calcu lation of model performance statistics relevant to OTAG’s goals. This study identified monitoring sites in the northeastern portion of the OTAG domain that could be used to summarize model performance in rural and remote areas. Selection of sites in nonattainment areas and in other portions of the OTAG domain were not addressed here.

Comparison of OTAG UAM-V/BEIS2 Modeling Results With Ambient Isoprene Observations

Participants. Eric Edgerton and Ben Hartsell, Atmospheric Research and Analysis, Inc., Raleigh, North Carolina; Ralph Morris, Keane Lee, and Greg Yarwood, ENVIRON International Corporation, Novato, California.

References. Edgerton, E.S., 1997b. "Comparison of Modeled versus Observed Isoprene Concentrations at Rural and Suburban Sites Across the Eastern U.S." (http://capita.wustl.edu/OTAG/Reports/Isoprene/ISOPRENE.html).

Morris, R.E., K. Lee, and G. Yarwood, 1997. "Phase I Comparison of OTAG UAM-V/BEIS2 Modeling Results with Ambient Isoprene and Other Related Species Concentrations," January 1997. ENVIRON International Corporation, Novato, Ca lifornia. (http://capita.wustl.edu/OTAG/Reports/morris/EXECSUM.html).

Purpose. Estimates of biogenic emissions used in the OTAG UAM-V modeling were based on the BEIS2 emissions model. BEIS2 isoprene emissions are significantly higher than those generated by its predecessor model, BEIS, resulting in a biogenic em issions estimate (of which isoprene is the largest single component) that constitutes 70–80 percent of the total VOC emissions in the OTAG modeling domain. Thus, control strategy impact estimates obtained from the OTAG modeling depend critically on the accuracy of the BEIS2 biogenic emissions estimates. Two independent analyses of the available ambient data were conducted to evaluate the UAM-V/BEIS2 isoprene concentration predictions.

Method. UAM-V/BEIS2 predictions of ambient isoprene concentrations in surface layer grid cells (roughly 12 km by 12 km horizontally by 50 m vertically) corresponding to ambient VOC monitor locations were compared with observed afternoon isopren e concentrations for the July, 1995 OTAG episode. Isoprene observations were obtained from ambient speciated hydrocarbon measurements made at PAMS network monitoring sites as well as sites operated as part of the NARSTO-NE and SOS-Nashville intensive fie ld studies. These sites are primarily concentrated in the northeast and in the Tennessee-Kentucky areas where the two field studies were conducted in 1995. Additional sites are located in Virginia (one); Georgia (one); Baton Rouge, Louisianna (two); and western Pennsylvania (one). Other portions of the OTAG domain (notably the midwest, mid-Atlantic region, Gulf coast outside of Louisiana, southern Florida, and Texas) are not well represented by these databases. Edgerton’s analysis concentrated pr imarily on 16 rural and suburban sites, while Morris et al. focused on 15 urban/suburban (or urban-influenced) sites. The two studies have six sites in common. At some sites (primarily PAMS and NARSTO-NE), observations were made every hour by automated gas chromatographs (auto-GCs), while at others, integrated canister samples were collected at set times. For the auto-GC data, comparisons with model predictions were based on 1100–1700 LST averages (1100–1900 LST in the Morris et al. study); a t the canister sites, afternoon integrated canister samples were compared with model predictions for the corresponding time intervals. In this way, comparisons were restricted to the period of maximum isoprene emissions and turbulent diffusion when compa risons of surface-based observations (5–15 m above ground level) with grid cell (volume average) predictions are most meaningful. All comparisons were done on a site-by-site (spatially matched) basis.

Table 4-2. Summary of Bias and Gross Error Results

Analyses

Within ±25%

Within ±50%

Underprediction greater than 50%

Overprediction greater than 50%

No. of Sites

Percent of Sites

No. of Sites

Percent of Sites

No. of Sites

Percent of Sites

No. of Sites

Percent of Sites

Morris et al.

2

13%

8

53%

1

7%

6

40%

Edgerton

2

12%

6

38%

1

6%

9

56%

¹In the Morris et al. study, bias is defined as the average of the differences (predicted minus observed values) expressed as a fraction of the mean observed value; in the Edgerton study, bias is defined as the mean percent difference.

² Gross error is defined as the average absolute difference (predicted minus observed values) in both studies.

³In this comparison, bias is defined as the average of the differences (predicted minus observed) expressed as a fraction of the mean observed value in both studies.

Findings. Afternoon observations included in the analysis showed a wide range of concentrations from near zero to more than 60 parts per billion based on carbon content); mean values across the network ranged from 2 ppbC to nearly 30 ppbC. Compar isons with predictions matched in time showed a wide range of prediction errors. Bias and gross error results are summarized in Table 4-2. Results are quite similar for both studies with maximum positive biases (overpredictions) being larger in an absolu te sense than the maximum negative biases (underpredictions).

Bias was within ±50 percent at just 6 of the 16 sites examined by Edgerton, with all but one of the remaining sites (Brookhaven, New York) exhibiting overpredictions. While bias was within ±25 percent at just two sites in each study, slight ly better agreement overall was found by Morris et al.; slightly more than half of the 15 sites examined showed biases within ±50 percent. One site (Ware, Massachusetts) was underpredicted by more than 50 percent, while overpredictions of more than 50 percent were exhibited at the remaining six sites. Thus, more than three-fourths of the sites examined in these studies exhibited biases greater than ±25 percent, and nearly half the sites exhibited mean biases of greater than ±50 percent of mean observed values. Nearly all the biases greater than ±50 percent were over predictions.⁴ Edgerton found no discernable geographic pattern of bias. Edgerton’s analysis of residuals (mean differences as a function of mean predict ed values indicates a fairly strong linear relationship, (correlation coefficient, r², of 0.78) with a positive slope: of the eight sites with mean predicted concentrations below 10 ppbC, the model underpredicted on average at three sites; for the eight sites with mean predictions above 10 ppbC, all but one exhibited over predictions on average. While this relationship is partially a reflection of the lack of correlation between observed and predicted isoprene concentrations, the strength of the correlation between the differences and predicted values in this case suggests that some additional factor is contributing to the relationship. Thus, in an absolute sense, model performance is worse at sites with high mean isoprene concentrations (e. g., in the southeast) than at sites with low mean concentrations. Morris et al. did not conduct a similar analysis of residuals.

Edgerton examined two potential sources of bias between observations made at a fixed point roughly 10 m above the surface and model predictions made for a 50 m deep grid cell:

1. The decrease in isoprene concentrations with height (which would cause predicted values to be less than observed).

2. The use of temperature measurements made at roughly 1.5 m above ground level to estimate isoprene emissions from forest canopies located further above the ground where cooler temperatures can be expected as a result of near adiabatic or super adiaba tic lapse rates typical of sunny summer afternoons (which would cause predicted isoprene values to be greater than observed).

In some cases, forest canopy temperatures may also be lower than the near-surface observations because of enhanced evapotranspiration. Edgerton’s review of the isoprene profile estimates of Andronache and Chameides (1994) indicates that observati ons taken at 10 m should be only about 2–6 percent higher than the 10–50 m layer average. As for the temperature effect, Edgerton states that daytime vertical temperature gradients in rural eastern U.S. environments as measured at CASTNet meteo rological monitoring sites are typically -0.5°C to -1.0°C between 2 m and 9 m. If this is assumed to be the same order of magnitude as the temperature difference between 1.5 m and the vegetation canopy, it could cause a possible overestimation of emissions on the order of 7–15 percent. Thus, the influence of vertical gradients in isoprene and temperature appear to be negligible compared to the relatively large biases between observed and predicted isoprene concentrations found in these st udies. Of course, this fact does not rule out the possibility of other temperature-related problems such as biases in the 1.5 m measurements used to generate gridded wind fields for the model.

Limitations. Comparison of observed and predicted isoprene concentrations are primarily limited by the spatial representativeness of the monitoring data as well as the ability of the model to characterize vegetation within a grid cell. Isopren e emissions exhibit strong spatial and temporal variations due to differences in the amount of plant material and species mix and the response of plants to changes in environmental conditions. In addition, isoprene is highly reactive and undergoes rapid transformation once released. Thus, measurements made at fixed locations with varying degrees of exposure to isoprene sources can provide only an approximate indication of the volume average concentration represented by the model predictions. Additional analysis of both the ambient measurements and the land use and temperature data input to the model are needed to better diagnose the factors contributing to the prediction errors noted in these two studies and to rectify the apparent inconsistencies betw een these results and the emission flux measurements used to support development of BEIS2 (see Guenther et al., 1996 and references therein). If the underlying formulation of BEIS2 is sound as suggested by the earlier studies, then the OTAG overpredictio ns point to a potential problem with the procedures used to calculate the OTAG biogenic inventory. It should also be noted that isoprene represents only one component of the total biogenic VOC inventory, although it is the dominant species. Similar comp arisons for other biogenic species are not included in these studies (and may be difficult or impossible to do for some species that are not unique to biogenic sources or are difficult to measure accurately).

Scientific Implications. Because isoprene is so abundant in the OTAG domain and it is highly reactive, significant errors in isoprene predictions have the potential to introduce significant errors into predictions of the sensitivity of ozone to anthropogenic VOC and NOx controls. Isoprene is a highly reactive primary species with emissions that vary strongly as a function of solar insolation, temperature, and land cover. As a result, one might expect relatively large differences between volum e average predictions made for a 12 km by 12 km by 50 m deep grid cell and measurements made at a single fixed location within that cell. Measurement errors would also contribute to these differences. Results of these studies bear out this expectation, showing much larger differences between UAM-V/BEIS2 isoprene predictions and corresponding observations than is the case for ozone, even when the comparison is limited to the afternoon hours when high emissions rates and strong turbulent diffusion should act to minimize the expected differences. This expectation not withstanding, the magnitude of the prediction errors and the tendency towards over predictions indicates a potentially serious problem with the UAM-V/BEIS2 modeling system. The fact that ove rpredictions occur at many of the rural sites examined by Edgerton is especially troublesome; overpredictions at some urban sites could be explained by the fact that the corresponding modeling grid cells include vegetation cover beyond the urban boundarie s. Isoprene emissions rate biases for tall forest canopies introduced by the use of inappropriately high near-surface temperatures in BEIS2 do not appear to be large enough to explain the observed overpredictions, although other potential sources of error in the temperatures used to drive the model have yet to be explored.

Policy Implications. Incorrect representation of biogenic VOC emissions in the OTAG

UAM-V/BEIS2 modeling system would result in errors in the model’s predictions of ozone concentrations and the model’s responses to anthropogenic emission control strategies. If isoprene emissions in the OTAG inventory are significantly overe stimated (for whatever reason), the overrepresentation of biogenic VOCs would cause the model to underestimate the expected response to anthropogenic VOC controls. In addition, the model would tend to be overly sensitive to NOx controls in all but the la rgest urban areas.

Additional References

Andronache, C., and W.L. Chameides, 1994. "Vertical Distribution of Isoprene in the Lower Boundary Layer of the Rural and Urban Southern United States," Journal of Geophysical Research, Vol. 99, pp. 16989–16999.

Guenther, A., P. Zimmerman, L. Klinger, J. Greenberg, C. Ennis, K. Davis, W. Pollack,

H. Westberg, G. Allwine, and C. Geron, 1996. "Estimates of Regional Natural Organic Compound Fluxes From Enclosure and Ambient Measurements," Journal of Geophysical Research, Vol. 101, p. 1345.

⁴The large underprediction at the Brookhaven National Laboratory site in Long Island noted by Edgerton is likely due to the relatively large portion of the corresponding modeling grid cell that is over water where there are no isoprene emission s.

Comparison of SOS Nashville Data to OTAG 1995 Base Model

Participants. Robert E. Imhoff, Tennessee Valley Authority, Knoxville, Tennessee.

References. Imhoff, R.E., 1996. Summary of, and slides from, presentation given to the OTAG AQA Workgroup meeting at Cherry Hill, New Jersey, August 1996.

Purpose. To use ambient data from the 1995 Nashville SOS study to evaluate how well the OTAG air quality modeling for July 9–18, 1995, simulates atmospheric chemistry and physics in a southern environment.

Methodology. UAM-V results for the July 9–18, 1995, OTAG D2 basecase were obtained from the New York State Department of Environmental Quality. Model results were compared to three ambient data sets. The first data set was morning surfac e CO, NOy, and VOC concentration data for an urban site in Nashville for June 19 to July 28, 1995. The measured species ratios were compared to their relative proportions in the emissions inventory. The second data set was for ozone, NOx and NOy at a su burban and a rural site. Ozone productivity (i.e., the relationship of O₃ to NOz where NOz = NOy – NOx and where O₃/NOz is the number of ozone molecules produced per NOx molecule lost) was assessed at these two sites by compari ng measurements and model results during the 1100 to 1800 daytime period when the atmosphere was well mixed. The third data set was for vertical profiles of ozone over a suburban site near Nashville. The accuracy of vertical mixing processes in the mode l was assessed by comparing measured and modeled ozone profiles. The ozone sonde data were collected by Georgia Institute of Technology, the remainder of the data were collected by the Tennessee Valley Authority.

Findings. For morning surface concentration data at the Nashville urban site, measured CO/NOy slopes were 7.7 for June 19 to July 28 and 10.6 for July 9–18. The modeled slope for July 9–18 was 12.6, and the emissions inventory slopes were 8.2 for mobile sources and 7.5 for all sources combined. The apparent background of CO in the ambient data was 150–200 ppb, compared to approximately 100 ppb in the model. The measured VOC/NOy slope for July 9–18 was 2.5, compared to a m odeled slope of 2.9. The apparent background of VOC in the ambient data was quite close to zero, compared to approximately 90 ppb in the model. Agreement between the modeled and ambient CO/NOy and VOC/NOy slopes was reasonably good. Modeled CO/NOy slop es would be expected to be higher than inventory slopes because VOC oxidation is a secondary source of CO, and because NOy is deposited more rapidly than CO.

Comparing observed and modeled ozone concentrations at the suburban and rural sites revealed a fair amount of variability between the model and observed ozone on a day-to-day basis, but no indication of bias overall. At the rural site, the model estim ated the ozone productivity very well, with least squares regressions for both model and observations showing O₃/NOz slopes of about 6, which was consistent with NOx-limited ozone formation. At the suburban site, there was more variability in both observed and, in particular, modeled O₃/NOz ratios. The observed ozone productivity (least squares regression slope about 3.5) was lower than at the rural site, which was consistent with ozone formation being less NOx limited at the subur ban site than at the rural site. The model ozone productivity at the suburban site was about 4.3 on July 9–17, which is similar to the observed ozone productivity (approximately 3.5). However, on July 18 the model O₃ concentrations were much lower than on July 9–17 resulting in lower O₃/NOz ratios.

Further investigation of the model performance for the suburban site (Youth) on July 18 showed that the model predicted high concentrations of NOx (10–15 ppb) between 1100 and 1800, which was contrary to the observations. In other words, the mode l predicted that Youth was affected during the middle of the day by a substantial source of fresh NOx that was not observed on July 18 or July 9–17. As a result of this error, the model predicted ozone formation at the Youth site to be VOC limited o n July 18, whereas the observations showed the site to be more NOx limited. Therefore, the model sensitivity to VOC versus NOx emissions control around the Youth site would likely be opposite to that predicted by the observations for this day.

The comparison of observed and modeled vertical profiles for ozone showed that the model consistently overestimated the surface and Layer 4 ozone concentrations (measured Layers 2 and 3 were not available for comparison because the resolution of the so nde was too coarse in these layers). The model consistently estimated a slight increase in ozone from Layer 1 through Layer 4, with a sharp drop above Layer 5. The top of the model’s mixed layer was in Layer 5 during this period. The model’s estimated ozone concentration in Layers 6 and 7 were consistently too low. One possible cause is that the previous day’s mixed layer in the model was too low and thus material was not mixed to Layers 6 and 7. Another possibility is that the model do es not handle localized mixing due to convective cells which were common during the meteorological conditions prevalent during this period. These cells can inject significant amounts of air from near the surface above the mixed layer. Once above the mix ed layer, the material can have long residence times. Another possibility is that the "Topcon" concentration used at the upper boundary in the model (35 ppb) is too low.

Limitations. The modeled NOy used in these comparisons does not include organic nitrates (Carbon Bond species NTR) because NTR concentrations were not saved in the OTAG model runs. Therefore, UAM-V NOy concentrations are always biased low. Ho wever, the magnitude of bias is not well known and likely varies in time and space depending on the age of NOy (the bias might be as high as 20 percent for aged airmasses). Accounting for the missing contribution of NTR to modeled NOy may (1) further imp rove agreement between the observed and modeled CO/NOy and VOC/NOy ratios for the urban site, (2) further improve agreement between observed and modeled ozone productivity at the suburban site, but (3) degrade agreement between observed and modeled ozone productivity at the rural site.

Scientific Implications. The relatively good agreement between modeled and ambient CO/NOy and VOC/NOy ratios supports the overall validity of these parameters in the emissions inventory. However, the discrepancy between the apparent background for VOC of 90 ppb in the model compared to near zero in the ambient data possibly indicates a source of VOC in the model that is not also a source of CO. Further investigation is needed to identify the source of this discrepancy. The modeled and observed ozone productivities were consistent at the rural site, and they were consistent at the suburban site on all but one day. The generally good agreement in ozone productivity suggests that the model chemistry performs well for rural and suburban sites in the Nashville area. However, the inability of the model to correctly predict the ozone productivity at the suburban site on July 18 cautions that OTAG model performance is subject to important day-to-day variabilities that have the potential to influence emissions control decisions. The discrepancy in the vertical ozone profile data may suggest that the model is not properly distributing mass vertically or that the top boundary concentration for ozone is too low. Underestimating vertical mixing may ten d to overestimate concentrations at the surface, even though the chemistry and emissions inventory are accurate. Underestimating the top boundary concentration for ozone might have a small impact of biasing surface ozone concentrations low. While no pre diction bias for surface ozone was noted at the sites examined in this study, it is interesting to note that, overall, the model was found to overpredict surface ozone in the southeastern United States (OTAG, 1996).

Policy Implications. Although not conclusive, this study raises the possibility that the UAM-V model runs do not treat concentrations in the upper layers of the modeling domain properly, thereby introducing errors in the estimated impact of tr ansport on downwind areas. The inability of the model to correctly predict the influence of NOx emissions on ozone formation at the suburban site on 1 day sends a cautionary message for the use of OTAG model results at suburban, and presumably urban sites. Further technical analyses are necessary to resolve these issues.