2008 Emission, Compliance, and Market Analyses
Download the Emission, Compliance, and Market Analyses Report:
(pdf, 1.1 MB)
Download Graphics and Source Data
At a Glance: ARP Results in 2008
SO2 Emissions: 7.6 million tons
SO2 Compliance: 100%
SO2 Allowances: Banked allowances increased by almost 2 million from 2007 levels
SO2 Allowance Prices: Since July 2008, allowance prices have fallen sharply, with a monthly average nominal price in May 2009 of $71/ton
NOx Emissions: 3.0 million tons
NOx Compliance: 100%
The Acid Rain Program (ARP), established under Title IV of the 1990 Clean Air Act Amendments, requires major emission reductions of sulfur dioxide (SO2) and nitrogen oxide (NOx), the primary precursors of acid rain, from the electric power industry. The SO2 program sets a permanent cap on the total amount of SO2 that may be emitted by electric generating units (EGUs) in the contiguous United States. The program is phased in, with the final 2010 SO2 cap set at 8.95 million tons, a level of about one-half of the emissions from the power sector in 1980. NOx reductions under the ARP are achieved through a program that applies to a subset of coal-fired EGUs and is closer to a more traditional, rate-based regulatory system. Since the program began in 1995, the ARP has achieved significant emissions reductions. As Figure 1 shows, these reductions have occurred as electricity generation has increased.
You will need Adobe Acrobat Reader to view some of the files that constitute this report. See EPA's About PDF page to learn more about PDF, and a link to the free Adobe Acrobat Reader.
Over the next several months, EPA will release a series of reports summarizing progress under the ARP. This second report analyzes 2008 data on emission reductions, reviews compliance results, and summarizes market activity. A future report will compare changes in emissions to changes in acid deposition and surface water chemistry. For more information on the ARP, please visit the program's website.
Figure 1: Trends in Electricity Generation, Fossil Energy Use, Prices, and Emissions from the Electric Power Industry, 1990-2008
SO2 Emission Reductions
The SO2 requirements under the ARP apply to EGUs, fossil fuel-fired combustors that serve a generator which provides electricity for sale. The vast majority of ARP SO2 emissions result from coal-fired EGUs, although the program also applies to oil and gas units; in total, there were 3,572 EGUs subject to the SO2 program in 2008. As Figure 2 shows, ARP units have reduced annual SO2 emissions by 56 percent compared with 1980 levels and 52 percent compared with 1990 levels. Sources emitted 7.6 million tons of SO2 in 2008, well below the current annual emission cap of 9.5 million tons, and already below the statutory annual cap set for compliance in 2010 of 8.95 million tons.
Reductions in SO2 emissions from other sources not affected by the ARP (including industrial and commercial boilers and the metals and refining industries) and use of cleaner fuels in residential and commercial burners contributed to a similar overall decline (56 percent) in annual SO2 emissions from all sources since 1980. National SO2 emissions from all sources have fallen from nearly 26 million tons in 1980 to about 11.4 million tons in 2008 (see data available from the National Emissions Inventory).
The states with the highest emitting sources in 1990 have generally seen the greatest SO2 reductions under the ARP. Most of these states are upwind of the areas the ARP was designed to protect, and reductions have resulted in important environmental and health benefits over a large region.
In addition, from 2007 to 2008, reductions in SO2 emissions from ARP units in 38 states totaled about 1.3 million tons, or about 15 percent for the year. Five states (Georgia, Indiana, North Carolina, Ohio, and Pennsylvania) accounted for most of the one-year reductions from 2007 to 2008, ranging from 119,271 to 244,651 tons of SO2 in each of these states.
From 1990 to 2008, annual SO2 emissions in 38 states and the District of Columbia fell by a total of approximately 8.2 million tons. In contrast, annual SO2 emissions increased by a total of 79,309 tons in 10 states from 1990 to 2008. The seven states with the greatest reductions in annual emissions since 1990 include Ohio, which decreased emissions by over 1.5 million tons, and Illinois, Indiana, Kentucky, Missouri, Tennessee, and West Virginia, each of which reduced total emissions during this time period by more than 500,000 tons. To view emission data in an interactive format using Google Earth or a similar three-dimensional platform, use the resources on EPA’s Clean Air Markets Interactive Mapping page.
Figure 2: SO2 Emissions from Acid Rain Program Sources, 1980-2008
Why SO2 Emissions Decreased in 2008
ARP sources decreased SO2 emissions sharply in 2008, with a 15 percent decline from 2007. A portion of this reduction may be attributable to a decrease in utilization, as heat input in 2008 fell by about 3 percent (See Table 1). The larger cause, however, was the large increase in the number of units that employed flue gas desulfurization (or scrubbers). The number of reported scrubbers climbed by 20 percent, from 246 in 2007 to 295 in 2008. This increase led to a decline in the overall SO2 emission rate for ARP units—0.64 lb/mmBtu in 2007 versus 0.56 lb/mmBtu in 2008. It is of historical interest that the ARP allowance allocations for Phase II in 2000 were based on an SO2 emission rate of 1.2 lb/mmBtu.
With the exception of reductions that might have occurred as a result of enforcement actions, most of the SO2 reductions in 2008 are likely to have resulted from early compliance planning for the Clean Air Interstate Rule (CAIR). Although some states have regulations that will require SO2 reductions in future years (such as North Carolina’s Clean Smokestacks Act in 2009), one reason controls were installed and operated in 2008 was the significant incentive to bank pre-CAIR vintage SO2 allowances, which could be used on a 1:1 basis under the CAIR annual SO2 program. In contrast, ARP allowances with later vintage years would be subject to increased retirement ratios under CAIR (2:1 in 2010 through 2014, and 2.86:1 in 2015 and thereafter). Going forward, the decision by the U.S. Court of Appeals remanding CAIR for further rulemaking by EPA might affect sources’ compliance planning and the observed trends in SO2 emission reductions.
Table 1: SO2, NOx, and Heat Input Trends in Acid Rain Program Units, by Fuel Type
Updating the Human Health Benefits of the Acid Rain Program
In 2005, a peer reviewed journal article, “A Fresh Look at the Benefits and Costs of the US Acid Rain Program,”1 assessed the human health and welfare benefits of ARP implementation for the prospective year 2010. The benefits were estimated using modeled emission reductions and ambient air quality expected to be achieved in 2010 under the ARP. The majority of the monetized benefits of ARP implementation are from the prevention of health-related impacts, such as premature death, due to reductions in ambient concentrations of fine particulate matter (PM2.5) and ground-level ozone. This study used the Pope et al., 2002, concentration response function to estimate incidences of adult premature mortality as a result of PM2.5 exposure.2 The results of this study estimated the U.S. PM2.5 and ozone health-related benefits of the ARP to be $134 billion and $5.5 billion annually, respectively.3
Since publication of this article, the assumptions used to develop human health effects estimates have changed. For example, EPA now also includes concentration response functions derived from the Laden et al., 2006 study4 and an expert elicitation to estimate incidences of adult premature mortality as a result of PM2.5 exposure. Additionally, many underlying modeling assumptions have been updated, including population forecasts and baseline incidence rates. A majority of these updated assumptions are discussed in detail in the recent PM2.5 Regulatory Impact Analysis.5 The revised assessment uses the original modeled ambient air quality6 and updated health benefits assessment assumptions to update the U.S. PM2.5 and ozone related benefits of ARP implementation.
Table 2: Estimated PM2.5 Health Benefits Due to ARP Implementation in 2010
|Health Effect||Incidences Avoided||Monetized Value (millions; 2008$)|
|Adult Mortality7 from PM2.5|
|Pope et al., 2002||20,000||$160,000|
|Laden et al., 2006||50,000||$400,000|
|Range of Expert Elicitation||7,000 to 66,000||$58,000 to $520,000|
|Infant Mortality from PM2.5|
|Woodruff et al.||82||$710|
|Morbidity from PM2.5|
|Acute Myocardial Infarction||30,000||$3,500|
|Acute Respiratory Symptoms||12,000,000||$790|
|Emergency Room Visits; Respiratory||18,000||$7.2|
|Hospital Admissions; Cardiovascular||10,000||$300|
|Hospital Admissions; Respiratory||4,800||$72|
|Lower Respiratory Symptoms||290,000||$5.6|
|Upper Respiratory Symptoms||220,000||$6.8|
|Work Loss Days||2,500,000||$640|
|Pope et al.||$170,000|
|Laden et al.||$410,000|
Table 3: Estimated Ozone Health Benefits Due to ARP Implementation in 2010
|Health Effect||Incidences Avoided||Monetized Value (millions; 2008$)|
|Mortality, Non-Accidental (Ito et al., 2005)||1,900||$17,000|
|Mortality, Non-Accidental (Schwartz, 2005)||660||$5,700|
|Mortality, Non-Accidental (Bell et al., 2004)||430||$3,700|
|Mortality, All Cause (Levy et al., 2005)||2,000||$17,000|
|Mortality, All Cause (Bell et al., 2005)||1,400||$12,000|
|Mortality, Cardiopulmonary (Huang et al., 2005)||720||$6,200|
|Hospital Admissions, Respiratory (age 65 and up)||3,000||$75|
|Hospital Admissions, Respiratory (age 0-2)||2,500||$26|
|Emergency Room Visits, Respiratory||1,900||$0.74|
|School Loss Days||910,000||$87|
|Acute Respiratory Symptoms||2,600,000||$170|
|Total Value Range||$4,100 - $17,000|
The results of the revised assessment show an increase in the estimated value of U.S. PM2.5 and ozone health benefits expected from ARP implementation in 2010. Depending on whether the Pope or Laden function is used as the primary estimate of incidences of adult mortality avoided, the monetized PM2.5 benefit increase ranges from 25 to 204 percent more than was estimated in the 2005 article noted above (see Table 2). Using updated methods to assess ground-level ozone benefits results in total benefits ranging from 75 to 319 percent of those previously predicted (see Table 3). These updated benefits do not include human welfare benefits due to better ecological conditions, such as improved visibility and reduced acidification of lakes and streams.
SO2 Program Compliance
In 2008, all ARP facilities complied with the requirement to hold enough allowances to cover SO2 emissions. EPA allocated 9.5 million SO2 allowances under the ARP for 2008. Together with 6.7 million unused allowances carried over (or banked) from prior years, there were 16.2 million allowances available for use in 2008 (see Figure 3). ARP sources emitted approximately 7.6 million tons of SO2 in 2008, less than the allowances allocated for the year, and far less than the total allowances available (see Figure 3). As a result, between 2007 and 2008 the bank increased by nearly two million allowances to 8.6 million, a 28 percent increase. In 2010, the total number of Title IV allowances allocated annually will drop to 8.95 million and remain statutorily fixed at that annual level.
How Are Allowances Traded and Tracked?
Once allowances have been auctioned and allocated, utilities can buy, sell, trade, or save them to meet their compliance needs. Along with the utilities that hold allowances for compliance purposes, other actors such as brokers, environmental groups, and private citizens maintain accounts in EPA’s AMS. The AMS database records account balances and transaction records, and allows public access to the trading history of each allowance until it is finally retired. EPA does not maintain any sensitive business data, such as the price associated with allowance transfers. Allowance brokers and other market participants generally maintain a market price index (MPI) to track trends in prices over time and provide market signals similar to other commodity markets.
Most allowance transactions take place in the over-the-counter market, where prices are determined by each day’s bids and offers, and immediate settlement cash trades are enacted bilaterally or through brokers. Once trading parties agree on a price, they generally complete the transaction using standard contracts developed by trade associations or other market players (see, for example, the sample contract available at Environmental Markets Association). EPA provides a list of brokers and environmental groups that may be interested in facilitating trades or in helping parties retire allowances voluntarily (see EPA's Clean Air Markets Buying Allowances page). At some point after a transaction is complete, the account representative of the transferring or selling party will usually register the transfer of allowances with EPA. The representative can submit a paper form or transfer the allowances online using the CAMD Business System (see EPA's Clean Air Markets Transfer Allowances page).
Outside of the spot market, utilities and investors have developed a range of more sophisticated structures to manage risk, including forward settlements, options, and swaps. However, these allowance trading strategies generally apply to market participants who have significant interests in allowance holdings or who are active investors in the market; such strategies are not necessary for transferring allowances for basic compliance needs.
Figure 3: SO2 Emissions and the Allowance Bank, 1995-2008
2008 SO2 Allowance Market
Figure 4 shows the cumulative volume of SO2 allowances transferred under the ARP. The figure differentiates between allowances transferred in private transactions and those annually allocated and transferred to source accounts by EPA.
Figure 4: Cumulative SO2 Allowances Transferred Under the ARP (through 2008)
Private transactions are indicative of both market interest and use of allowances as a compliance strategy. Of the nearly 379 million allowances transferred since 1994, about 68 percent were traded in private transactions. In December 2001, parties began to use a system developed by EPA to allow online allowance transfers. In 2008, account holders registered over 99 percent of all private allowance transfers through EPA’s online transfer system. Allowance transfers are posted and updated daily on EPA's Clean Air Markets page.
In 2008, 3,236 private allowance transfers involving approximately 13.9 million allowances of past, current, and future vintages were recorded in EPA’s Allowance Management System (AMS). About 5.9 million allowances (42 percent) were transferred in economically significant transactions (i.e., between economically unrelated parties). Transfers between economically unrelated parties are “arm’s length” transactions and are considered a better indicator of an active, functioning market than are transactions among the various facility and general accounts associated with a given company. In the majority of all private transfers, allowances were acquired by power companies. Figure 5 shows the annual volume of SO2 allowances transferred under the ARP (excluding allocations, retirements, and other transfers by EPA) since official recording of transfers began in 1994.
Figure 5: SO2 Allowances Transferred under the ARP
Over the first decade of the Acid Rain Program, allowance prices were stable and significantly lower than projected. When CAIR was proposed in late 2003, allowance prices were influenced by the more stringent CAIR SO2 cap and new compliance deadline. After CAIR was finalized in March 2005, allowance prices continued to trend upward. CAIR was the most significant driver of the price adjustment that began in 2004 and culminated with prices in the $1,600 range for a short time in December 2005. The Acid Rain SO2 market essentially became the CAIR SO2 market.
In 2008, the SO2 allowance market experienced a 65 percent price decline; the monthly average fell from $509 per ton in January to $179 per ton by December.9 That decline has continued in 2009, with the allowance price falling to an average of $71 per ton by May. Together with the price decline, the volume of significant transactions fell sharply in 2008. Market observers should not confuse temporary high prices in the market response to major regulatory changes (more or less regulation), where buyers and sellers are searching for a new equilibrium based on available information they have from consultants and various services, with price volatility. EPA and market analysts have identified these regulatory forces — the CAIR emission caps and compliance deadlines, followed by the rule changes resulting from the July 2008 CAIR court decision — as the primary factor affecting current market conditions in the period from 2004 to 2008 and not inherent volatility in cap and trade programs due to shifts in other variables that influence the market. For further analysis see EPA's Allowance Markets Assessment White Paper.
Table 4: SO2 Allowance Market in Brief (close of 2008)
|Total Value of the SO2 Allowance Market||$2.9 billion*|
|Average Nominal Price||$179 per ton|
|Total Allowance Volume (Allowable Emissions)||16,227,082|
|2008 Private Transactions||3,236 transactions moving 13.9 million allowances|
42 percent of allowances transferred between economically unrelated parties
SO2 Allowance Auction
EPA’s 2009 annual ARP allowance auction was held on March 24. The annual auction provides an opportunity for power plants, brokers, and private citizens to buy and sell allowances. Title IV mandates that a limited number of allowances allocable to existing sources be withheld and auctioned. The auctions help ensure that new electric generating plants have a source of allowances beyond those allocated initially to existing units. Proceeds from the auctions are returned to sources in proportion to the allowances withheld.
The auction includes two “vintages” of allowances. Vintage describes the earliest year an allowance may be applied against SO2 emissions. In addition to year 2009 allowances, the Clean Air Act mandated that EPA auction additional allowances seven years in advance to help provide stability in planning for capital investment. These advance allowances will be usable first in 2016. Complete results of the annual SO2 Allowance Auction are available at EPA's Clean Air Markets Annual Auction page.
NOx Emission Reductions
Title IV requires NOx emission reductions for certain coal-fired EGUs by limiting the NOx emission rate (expressed in lb/mmBtu). Congress applied these rate-based emission limits based on a unit’s boiler type. The goal of the NOx program is to limit NOx emission levels from the affected coal-fired boilers so that their emissions are at least 2 million tons less than the projected level for the year 2000 without implementation of Title IV.
Figure 6 shows that NOx emissions from all ARP sources were 3.0 million tons in 2008. This level is 5.1 million tons less than the projected level in 2000 without the ARP, or more than double the Title IV NOx emission reduction objective. While the ARP was responsible for a large portion of these annual NOx reductions, programs such as the NOx Budget Program under EPA’s NOx State Implementation Plan (SIP) Call and other regional and state NOx emission control programs also contributed significantly to the NOx reductions achieved by sources in 2008.
Figure 6: NOx Emission Trends for All Acid Rain Program Units, 1990-2008
From 1995 to 2008, annual NOx emissions from ARP units dropped by about 3.1 million tons, a net decrease of 51 percent. During this period, forty-two states and the District of Columbia reduced NOx emissions, while six other states accounted for only about 15,600 tons of increased NOx emissions.
Seasonal NOx Control Programs
States subject to EPA’s 1998 NOx SIP Call have achieved significant reductions in ozone season NOx emissions since the baseline years 1990 and 2000. All of these states have achieved reductions since 1990 as a result of programs implemented under the 1990 Clean Air Act Amendments, with many of them reducing their emissions by more than half since 1990. A significant portion of these decreases in NOx emissions has been achieved since 2000, largely as a result of reductions under ozone season NOx trading programs implemented by the OTC, from 1999-2002, and under the NOx SIP Call, from 2003-2008. For reports about these programs, see see EPA's Clean Air Markets Progress Reports page.
Even with these other programs, further reductions in annual NOx emissions were achieved in 2008. Possible reasons for this decline could include:
- 8 new selective catalytic reduction systems (SCR) and 7 new selective noncatalytic reduction systems (SNCR) were installed in 2008.
- Sources might have taken advantage of incentives for generation of early action (compliance supplement pool) allowances intended for use under the CAIR annual NOx program that began in January 2009.
The ARP NOx Program does not impose a cap on NOx emissions and does not rely on allowance trading. The ARP NOx Program, however, provides compliance flexibility achieved through an approach that is designed to maximize NOx emission reductions and ensure that those reductions are sustained. Under the ARP, EPA allows affected sources to comply by either meeting a unit-specific emission rate or including two or more units in an emission rate averaging plan. These options provide affected sources with the flexibility to meet the NOx emission reduction requirements in the most cost-effective manner. In 2008, all 969 units that were subject to the ARP NOx Program achieved compliance.
Sources Achieved 100 Percent NOx Compliance in 2008, Using a Variety of NOx Compliance Plan Options
Standard Limitation. A unit with a standard limit meets the applicable individual NOx limit prescribed for its boiler type under 40 CFR Parts 76.5, 76.6, or 76.7 (290 units used this option in 2008).
Alternative Emission Limit (AEL). A utility can petition for a less stringent AEL if it properly installs and operates the NOx emission reduction technology prescribed for that boiler, but is unable to meet its standard limit. EPA determines whether an AEL is warranted based on analyses of emission data and information about the NOx control equipment (six units used this option in 2008).
Emissions Averaging. Many companies meet their NOx emission reduction requirements by choosing to become subject to a group NOx limit, rather than by meeting individual NOx limits for each unit. The group limit is established at the end of each calendar year. The group rate must be less than or equal to the Btu-weighted group rate units would have had if each had emitted at their standard limit rate (673 units used this option in 2008).
Note: Unit counts do not include those with a retired unit exemption.
Emission Monitoring and Reporting
The ARP requires regulated sources to measure, record, and report emissions using continuous emission monitoring systems (CEMS) or an approved alternative measurement method. The vast majority of emissions are monitored with CEMS, while the alternatives provide an efficient means of monitoring emissions from the large universe of units with lower overall mass emissions. Table 5 shows the number of units with and without SO2 CEMS for various fuel types, as well as the amount of SO2 emissions monitored using CEMS. Although only 32 percent of units use CEMS, 99 percent of all SO2 emissions from ARP sources are monitored in this fashion.
CEMS and approved alternatives are a cornerstone of the ARP’s accountability and transparency. Since the program’s inception in 1995, affected sources have met stringent monitoring quality assurance and control requirements, and have reported hourly emission data in quarterly electronic reports to EPA. Using automated software audits, EPA rigorously checks the completeness, quality, and integrity of these data. All emission data are available to the public on the Data and Maps web site maintained by EPA’s Clean Air Markets Division (CAMD). The site also provides access to other data associated with emission trading programs, including reports, queries, maps, charts, and file downloads covering source information, emissions, allowances, program compliance, and air quality.
The emission monitoring requirements for the ARP are found in 40 CFR Part 75. Compliance with these provisions is also required for sources participating in the CAIR trading programs.
Table 5: Units and SO2 Emissions Covered by Monitoring Method for the Acid Rain Program
|Number of Units||Percentage of Units||Percentage of SO2 Emissions|
CAMD recently reengineered the process that the regulated community uses to maintain, evaluate, and submit monitoring plans, quality assurance certifications, and quarterly emission data. An important tool in this effort is the Emissions Collection and Monitoring Plan System (ECMPS). Beginning with reports submitted in April 2009, ECMPS has replaced the processes and multiple software tools used previously for evaluating, submitting, and receiving compliance-related information. Data submitted via ECMPS must meet a basic level of quality. If an evaluation generates a “critical” error, sources are able to submit data to meet the regulatory deadline but must then resolve the errors and resubmit the data. ECMPS also has an expanded set of data validation checks that assist EPA in implementing improved auditing as sources begin to comply with CAIR. ECMPS incorporates the following components:
- A single desktop tool, made available by EPA, for authorized users to import and evaluate their data and submit it to CAMD.
- A new data reporting format based on the flexible Extensible Markup Language (XML) standard.
- A centralized database at CAMD for receiving and maintaining submitted data, which can be accessed directly through the desktop tool.
- Tools and procedures for the quality assurance of data prior to submission, including the consolidation of evaluation results (feedback) into one set.
- The ability to maintain select data outside of the electronic data report.
- New security requirements.
1 Chestnut, L. G., and Mills, D. M. 2005. A fresh look at the benefits and costs of the US Acid Rain Program, Journal of Environmental Management, 77(3): 252-266.
2 Pope C.A., Burnett R.T., Thun M.J., Calle E.E., Krewski D., Ito K., and Thurston G.D. 2002. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution, JAMA, 287(9):1132-1141.
3 Year 2008$ inflated from $108 billion and $4.384 billion 2000$ using CPI inflation calculator.
4 Laden F., Schwartz J., Speizer F.E., Dockery D.W. 2006. Reduction in fine particulate air pollution and mortality: Extended Follow-up of the Harvard Six Cities Study, American Journal of Respiratory and Critical Care Medicine, 173 (2006) 667-672.
5 See EPA’s Regulatory Impact Analysis for the 2006 PM2.5 National Ambient Air Quality Standards, available at: www.epa.gov/ttn/ecas/ria.html.
6 The original air quality estimates were based on a regulatory air quality modeling platform that has been substantially updated since 2005. The updates include (1) major changes to base year and future case emissions and meteorological inputs, (2) a new air quality model with improved chemistry and other scientific features, and (3) new methods for projecting future air quality relative to current measured data. The effects of these improvements on the benefits from the ARP have not been quantified.
7 Valuation includes 3% discount rate for future incidences of premature mortality avoided.
8 O3 Mortality, Non-Accidental
Ito, K., De Leon, S.F., and Lippmann, M. 2005. Associations between ozone and daily mortality: analysis and meta-analysis. Epidemiology 16(4): 446-57.
Schwartz, J. 2005. How sensitive is the association between ozone and daily deaths to control for temperature? American Journal of Respiratory and Critical Care Medicine 171 (6): 627-31.
Bell, M.L., McDermott, A., Zeger, S.L., Samet, J.M., and Dominici, F. 2004. Ozone and short-term mortality in 95 US urban communities, 1987-2000. JAMA 292(19): 2372-8.
O3 Mortality, All Cause
Levy, J.I., Chemerynski, S.M., and Sarnat, J.A. 2005. Ozone exposure and mortality: an empiric bayes metaregression analysis. Epidemiology 16(4): 458-68.
Bell, M.L., Dominici, F., and Samet, J.M. 2005. A meta-analysis of time-series studies of ozone and mortality with comparison to the national morbidity, mortality, and air pollution study. Epidemiology 16 (4): 436-45.O3 Mortality, Cardiopulmonary
Huang, Y., Dominici, F., and Bell, M. L. 2005. Bayesian hierarchical distributed lag models for summer ozone exposure and cardio-respiratory mortality. Environmetrics 16: 547–562.
9 Based on the Market Price Index (MPI) that uses trade, bid, and offer price as tracked by CantorCO2e.