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Community Multiscale Air Quality Modeling System (CMAQ)

Research Highlights

EPA scientists have primary responsibility for the continual development of CMAQ and its related models. There are currently a number of research projects underway that focus on a range of topics, from exploration of atmospheric science fundamentals to incorporation and evaluation of recent scientific breakthroughs into research versions of the CMAQ model. Eventually, this new knowledge will serve the public by being incorporated in future publicly available CMAQ releases. The research and development team welcome comments and questions about these topics and would be happy to discuss the potential for collaborations.

Research Highlights: Table of Contents


Impacts of Different Characterizations of Large-Scale Background on Simulated Regional-Scale Ozone 

Researcher: Christian Hogrefe     Posted: June 2018

Citation: Hogrefe, C., P. Liu, G. Pouliot, R. Mathur, S. Roselle, J. Flemming, M. Lin, AND R. Park. Impacts of different characterizations of large-scale background on simulated regional-scale ozone over the continental United States. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 18:3839-3864, (2018).  https://doi.org/10.5194/acp-18-3839-2018  Exit

Summary: As the ozone National Ambient Air Quality Standard (NAAQS) has tightened, non-attainment areas continue to exist in a number of U.S. regions. To design efficient and effective NAAQS attainment strategies, it is important to accurately quantify the contributions of local vs. background sources (such as trans-Pacific transport of air pollution) to non-attainment. Doing so requires evaluating and advancing our modeling techniques for representing ozone fluctuations on regional-to-hemispheric scales which in turn necessitates a detailed understanding of the relevant processes (vertical mixing in source and receptor regions, multi-day transport and chemistry in the free troposphere, and stratosphere/troposphere exchange processes) and how they are represented in current-generation global and regional-scale modeling tools.

This recent study analyzed CMAQ-modeled ozone burdens over the U.S. both near the surface and aloft, estimated the contributions of different physical and chemical processes to these burdens, and calculated the sensitivity of the simulated regional-scale ozone burden to large-scale background ozone as specified by lateral boundary conditions derived from different hemispheric or global scale models using a consistent set of anthropogenic emission inputs. The analysis of these regional-scale CMAQ simulations highlighted that differences in ozone boundary conditions resulted in differences of CMAQ-simulated ozone column mass for different vertical layer ranges across the entire modeling domain. In particular, the results strongly suggest that differences in the mid-tropospheric ozone mixing ratios simulated by the large-scale models were the main driver of ozone differences between the corresponding regional-scale CMAQ simulations.

Monthly average time series of observed ozone at 5 ozonesonde sites.Monthly average time series of 500 hPa observed ozone, ozone simulated by large-scale models (solid lines), and ozone simulated by regional CMAQ driven with boundary conditions from different large-scale models (dashed lines) at the Trinidad Head, Edmonton, Boulder, Huntsville, and Wallops Island ozonesonde sites. These time series show the close relationship between free tropospheric ozone simulated by regional CMAQ and the global model providing the lateral boundary conditions.Vertical mixing, including mixing by convective clouds, couple the fluctuations in free tropospheric ozone to ozone in lower layers. Analysis of ozone mixing ratios at the surface confirmed that the characterization of ozone outside the regional-scale modeling domain can have a profound impact on CMAQ-simulated ground-level ozone and can affect the comparison of these simulations with available observations.

The study was published as part of a special issue on “Global and regional assessment of intercontinental transport of air pollution: results from HTAP, AQMEII and MICS” in the journal Atmospheric Chemistry and Physics.

Time series of differences in CMAQ-simulated ozone Time series of differences between CMAQ-simulated and observed daily maximum 8-hr surface ozone mixing ratios at AQS monitors. Four different CMAQ simulations are displayed. For the “BASE” configuration, lateral boundary conditions were obtained from the C-IFS global model. For the three sensitivity simulations, lateral boundary conditions were obtained from hemispheric CMAQ, GEOS-Chem, and AM3. This figure illustrates the impact of lateral boundary conditions on surface ozone simulated by CMAQ, especially outside the summer season.

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Lightning NOX Production in CMAQ and Its Implications for Air Quality

Researcher: Daiwen Kang     Posted: June 2018

Summary:  Lightning-produced nitrogen oxides (LNOXNOX=NO+NO2) in the middle and upper troposphereHelptroposphereThe lowest region of the atmosphere, extending from the earth's surface to a height of about 3.7–6.2 miles (6–10 km), which is the lower boundary of the stratosphere. Because more solar radiation hits the equator than the poles, the troposphere is tallest along the equator and gets shorter as one moves north or south. play an essential role in the production of ozone (O3) and influence the oxidizing capacity of the troposphere. We have implemented a LNOx production scheme based on hourly gridded cloud-to-ground lightning flashes from the National Lightning Detection Network (NLDN) and satellite-based climatological intracloud to cloud-to-ground flash ratios to estimate gridded hourly total LNOx across the contiguous US. After the column total LNOx is calculated, it is distributed vertically through the model layers based on the double-peak algorithm described in Allen et al. (2012). For modeling exercises where the observed lightning strikes are not generally available, such as air quality forecasts, parameterization schemes based on the observed relationship between NLDN lightning strikes and model predicted convective precipitation rates have been employed and evaluated. The updated lightning NOX production algorithms have allowed for more robust trend analysis of the contribution of lightning NOX to the total NOX budget and quantification of the “policy-relevant background” NOX and O3 that can be attributed to lightning. 

Daily lightning flash rate and maximum change in simulated surface ozone (LNOx-Base).The daily lightning flash rate (left) and the maximum difference in hourly surface O3 on July 11, 2011 between the model simulation with LNOx and the base simulation without LNOx (right). The lightning-produced NOx leads to widespread increases in O3 throughout the domain on this day.Top of Page


Impacts of Particulate Matter on Boundary Layer Mixing

Researcher:  Rohit Mathur     Posted: June 2017

Citations:  Wang, J., Xing, J., Mathur, R., Pleim, J.E., Wang. S., Hogrefe, C., Gan, C.-M., Wong, D.C., and Hao, J. (2016) Historical Trends in PM2.5-Related Premature Mortality during 1990-2010 across the Northern Hemisphere.  Environmental Health Perspectives. doi: 10.1289/EHP298EXIT

Xing, J., Wang, J., Mathur, R.,  Pleim, J., Wang, S., Hogrefe. C., Gan, C.-M., Wong, D.C., and Hao, J. (2016). Unexpected Benefits of Reducing Aerosol Cooling Effects. Environ. Sci. Technol, 50(14), 7527–7534. doi: 10.1021/acs.est.6b00767EXIT

Contrasting trends of exposure to PM2.5 in North America and East AsiaContrasting trends in population exposure to ambient PM2.5: Control measures have significantly reduced exposure to high levels of PM2.5 in North America, but uncontrolled growth has resulted in larger populations being exposed to higher PM2.5 in many parts of Asia.Summary:  Two journal articles on comprehensive modeling of 1990-2010 trends in air pollution exposure across the Northern Hemisphere demonstrate the benefits of EPA's fine particulate matter (PM2.5) control measures, including some that were unexpected. Modeling results in Wang et al. (2016) indicate that while PM2.5-related mortalities in East Asia and South Asia increased 21 percent and 85 percent, respectively, they decreased in Europe and North America substantially by 67 percent and 58 percent respectively. A companion article, Xing et al. (2016), shows that while fine particles contributed to cooling effects, partially offsetting greenhouse gas warming, because of reduced atmospheric mixing resulting from the cooling, air pollutants become more concentrated locally, especially in highly polluted and populated regions. Therefore, control measures aimed at reducing PM2.5 -related mortalities, result in unexpected additional benefits, making the controls even more effective than originally projected.

Mortality changes from aerosol radiative effectsMortality changes resulting from aerosol radiative effects on temperature (left) and combined temperature and PM2.5 (right).Top of Page


Improving Vegetation Processes for WRF/CMAQ Using MODIS Vegetation (LAI and FPAR) Product and a Coupled Photosynthesis and Stomatal Conductance Approach

Researcher: Limei Ran     Posted: June 2017

Image showing improvements made in land surface processes using MODIS vegetationCitations:  The following links exit the site ExitRan, L., Gilliam, R., Binkowski, F.S., Xiu, A., Pleim, J., & Band, L. (2015). Sensitivity of the WRF/CMAQ modeling system to MODIS LAI, FPAR, and albedo. J. Geophys. Res. Atmos., 120(16), 8491-8511. https://doi.org/10.1002/2015JD023424  

Ran, L., Pleim, J., Gilliam, R., Binkowski, F.S., Hogrefe, C., & Band, L. (2016a). Improved meteorology from an updated WRF/CMAQ modeling system with MODIS vegetation and albedo. J. Geophys. Res. Atmos.121, 2393–2415. https://doi.org/10.1002/2015JD024406  

Ran, L., Pleim, J., Song, C., Band, L., Walker, J.T., & Binkowski, F.S. (2016b). A Photosynthesis-based Two-leaf Canopy Stomatal Conductance Model for Meteorology and Air Quality Modeling with WRF/CMAQ PX LSM. J. Geophys. Res. Atmos. https://doi.org/10.1002/2016JD025583.  

Summary:  EPA scientists have been working on improving land surface processes in the Wweather Research and Forecasting (WRF)/CMAQ modeling system with a focus on vegetation. With realistic MODIS vegetation being evaluated and ingested into the system (Ran et al., 2015; Ran et al., 2016a) and a coupled photosynthesis and stomatal conductance approach being evaluated for the system (Ran et al., 2016b), scientists are implementing the photosynthesis-based approach into the system.  Meanwhile, EPA researchers are developing new land cover categories with irrigation information and plant function types which are more suitable for the photosynthesis-based approach. The CMAQ team is also developing and evaluating a simple irrigation scheme in WRF for preparing to use the new land cover categories. Ultimately, the system with improved land surface representation and vegetation processes can be used to assess the coupling effects of air quality, climate, and vegetation productivity directly with CO2 concentration which changes temporally and spatially.  For example, the system will have the ability to assess the effects of air pollutants such as O3 on ecosystem productivity in addition to CO2 influence.

Photosynthesis-based Model EvaluationTop of Page


Air-Land-Water Integrated Assessments Using Coupled FEST-C EPIC, WRF/CMAQ, and SWAT Modeling Systems

Researchers: Ellen Cooter and Limei Ran     Posted: June 2017

Schematic of Agricultural Modeling in CMAQSchematic of modeling agricultural systems in CMAQ.EPA researchers are continuing efforts on improving and advancing an agriculture fertilization and production model (FEST-C EPIC) coupled with WRF/CMAQ and the Soil and Water Assessment Tool (SWAT) for air quality and ecosystem assessments. This coupling is required for bidirectional CMAQ simulations. Improving this coupling leads directly to improved CMAQ estimates of nitrogen (N) deposition and emissions (N removal) and particulate matter concentrations. Currently, the system is used to assess the impacts of agricultural fertilization and management practices on air quality (NH3) and climate (N2O), as well as the impacts of meteorology/climate and air quality (N deposition) on crop yield, soil erosion and water quality at regional scales. EPA scientists are improving the system on the crop and biomass production in comparison with USDA reports.  As climate change is affecting agriculture, crop production under different climate and air quality, like O3HelpO3Ozone (O3) is a colorless gas with a pungent odor. It is found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere, ozone provides a protective layer shielding the Earth from ultraviolet radiation's potentially harmful health effects. At ground level (the troposphere), ozone is a pollutant that affects human health and the environment, and contributes to the formation of smog. damage, scenarios can be assessed using this system to address food security, risk mitigation, and policies with a holistic approach. The CMAQ team is assessing the impacts of climate on fertilization and productions and in turn on water quality through 11-year simulations using the integrated system. The system will be expanded to the globe for the next generation air quality modeling system at the global scale. Ongoing research focuses on other predicted FEST-C EPIC variables such as temporally resolved leaf area index for natural vegetation, like mixed forest, and dust emissions associated with agricultural tillage activities.

Changes in CMAQ representation of model processes due to land use changesThe One-Biosphere system includes CMAQ and FEST-C

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Understanding Pathways to Organic Aerosol

Researcher: Havala Pye     Posted: June 2017

For organic compounds of high solubility or low vapor pressure, the particle phase is an intermediate in their life cycle in the atmosphere. Due to thousands of compounds being present in organic aerosol, multiple approaches have been used to represent the formation of organic aerosol. Improvements in the representation of secondary organic aerosol as well as emitted organic aerosol are still needed to improve the CMAQ team’s confidence in how organic aerosol can be expected to respond to changes in emissions.

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Improving Representation of Sources and Sinks of Atmospheric Particles

Researcher: Ben Murphy     Posted: June 2017

Particles are introduced to the atmosphere via both direct emissions (primary) and transformation of low-volatility vapors (secondary). In order to understand and quantify the importance of both of these pathways, we have enhanced the representation of the particle size distribution in CMAQ to treat four modes, Nucleation, Aitken, Accumulation and Coarse. We are currently investigating the incorporation of state-of-the-art parameterizatoins for new particle formation from sulfuric acid, ammonia, amines, and organic compounds. Meanwhile, we are also investigating the effect of representing particle direct emissions with source-specific size distributions from, for example, automobiles, fires and power plants.

Studies show that partitioning of organic compounds is important for combustion sources of primary particulate matter to the atmosphere. Example combustion sources include vehicles, power generation, residential wood burning, prescribed and wild fires, construction equipment, and yard appliances among others. New parameterizations exist for modeling the volatility distribution of these compounds. However, there is a potential disconnect between the experimental conditions at which these parameterizations were developed and the conditions at which emission factors were obtained for input to the National Emissions Inventory and SPECIATE databases, which are used to inform anthropogenic emissions in models. Current efforts will determine how to adjust available speciation profiles to ensure consistency among these data sources and provide photochemical models with accurate combustion emissions.

Understanding the removal processes of atmospheric pollutants is many times just as important as understanding their sources. Thus, we are currently improving the ability CMAQ to represent both in-cloud and below-cloud scavenging of particles at local and regional scales. When employed, these improvements to the particle representation will allow CMAQ to predict particle size distributions with enhanced accuracy and lend confidence to applications that use CMAQ data to interpret human health impacts of pollution, for example.

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Novel Atmospheric Chemistry Pathways

Researcher: Golam Sarwar     Posted: June 2017

Iodine and Bromine Chemistry

Iodine and bromine, emitted from seawater, can participate in atmospheric chemistry and affect air quality. Atmospheric chemistry of iodine and bromine speciesHelpspeciesAn individual molecule or chemical compound. has been included in the CMAQv5.2 model. Studies also suggest that iodine and bromine species can participate in heterogeneous reactionsHelpheterogeneous reactionsHeterogeneous reactions occur between reactants in two or more phases (e.g., solid and gas, solid and liquid) or in with participation of an interface (e.g., on the surface of a solid catalyst).. These heterogeneous reactions are being identified and incorporated into the CMAQ model. EPA scientists will perform model simulations to examine the impacts of the updated iodine and bromine chemistry on air quality.

Sulfur Chemistry

Current air quality models fail to capture elevated observed sulfate levels during winter. Recent field and chamber studies suggest new chemical pathways that can produce sulfate in the atmosphere. In this study, EPA scientists are identifying and implementing relevant chemical reactions into CMAQ.

Dimethylsulfide Chemistry

Sulfate aerosol forms from the gas- and aqueous-phase oxidation of sulfur dioxide. Dimethylsulfide (DMS), present in seawater, can be emitted into the atmosphere where it can react with atmospheric oxidants to produce sulfur dioxide, leading to sulfate formation. EPA scientists are incorporating DMS emissions and its atmospheric chemistry into the CMAQ model. The CMAQ team will perform model simulations to examine the impact of DMS chemistry on sulfate. 

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Heterogeneous Chemistry on the Surface of Dust

Researcher: Hosein Foroutan     Posted: June 2017

Many studies have suggested that dust particles can act as a platform for heterogeneous chemistry that can affect important species concentrations such as ozone and sulfate. Following the implementation of a new windblown dust emission model in CMAQ, the heterogeneous reactions that can occur on dust particles are being identified and incorporated into the CMAQ model. EPA scientists are performing model simulations to examine the impacts of the dust chemistry on pollutant concentrations and air quality. 

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