Progress Report - Emissions Reductions
Emissions Reductions
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Last updated on September 18, 2023
Data current through 2022
The Acid Rain Program (ARP) and Cross-State Air Pollution Rule (CSAPR) programs have significantly reduced sulfur dioxide (SO2), annual nitrogen oxides (NOX), and ozone season NOX emissions from power plants over the last several decades. The Mercury and Air Toxics Standards (MATS) have reduced hazardous air pollutant emissions since 2010. This section covers changes in emissions at power plants affected by CSAPR, ARP, and MATS between 2022 and previous years.
Compared to 2021, the 2022 annual emissions data from power plants in the lower 48 states show:
- 10 percent decrease in SO2 emissions
- 4 percent decrease in NOX emissions
- 1 percent decrease in CO2 emissions
- 3 percent decrease in Hg emissions
During this time period, electric generation from these power plants increased by 2 percent. Additionally, ozone season NOX emissions decreased by 10% nationwide and 21% in the Revised CSAPR Update states in 2022 compared to 2021.
From 1995-2022, annual emissions of SO2 from power plants fell by 93 percent and annual emissions of NOX from power plants fell by 87 percent. In 2022, sources in both the CSAPR SO2 annual program and the ARP together emitted 0.85 million tons, a reduction of 11 million tons from 1995 levels. In 2022, sources in both the Cross-State Air Pollution Rule (CSAPR) NOX annual program and the ARP together emitted 0.75 million tons, a reduction of 5.1 million tons from 1995 levels.
Additional data reports for all programs can be found in our Latest Emission Comparisons & Pollution Controls. A complete dataset for all programs is available in Clean Air Markets Program Data (CAMPD). More information about power plants is available on our homepage.
In the graph below, hover over the points on the graph to see the percentage increase or decrease.
For the following facts and figures, please select a pollutant:
Sulfur Dioxide (SO2)
SO₂ is a highly reactive gas that is generated primarily from coal-fired power plants. In addition to contributing to the formation of acid rain fine particle (PM₂.₅) pollution, SO₂ emissions are linked with a number of adverse effects to human health and ecosystems.
The states with the highest emitting sources in 1990 have generally seen the greatest SO₂ emission reductions under the ARP, and this trend continued under CAIR and CSAPR. Most of these states are in the Ohio River Valley and are upwind of the areas the ARP and CSAPR were designed to protect. Reductions under these programs have provided important environmental and health benefits over a large region.
- Under the ARP, CAIR, and CSAPR, power plants have significantly lowered SO₂ emissions while electricity generation from power plants in these programs has remained relatively stable since 2000.
- These emission reductions are a result of an overall increase in the environmental effectiveness at affected sources as electric generators installed controls, switched to lower emitting fuels, or otherwise reduced their SO₂ emissions. These trends are discussed further in the Program Basics section.
SO₂ Emission Trends
- ARP: Units in the ARP emitted 845,000 tons of SO2 in 2022, well below the ARP's statutory annual cap of 8.95 million tons. The ARP sources reduced emissions by 14.9 million tons (95 percent) from 1990 levels and 16.4 million tons (95 percent) from 1980 levels.
- CSAPR and ARP: In 2022, the eighth year of operation of the CSAPR SO2 programs, sources in both the CSAPR SO2 annual programs and the ARP together reduced SO2 emissions by 14.9 million tons (95 percent) from 1990 levels (before implementation of the ARP), 10.4 million tons (92 percent) from 2000 levels (ARP Phase II), and 9.4 million tons (92 percent) from 2005 levels (before implementation of the CAIR and the CSAPR). All ARP and CSAPR sources together emitted a total of 852,000 tons of SO2 in 2022.
- CSAPR: Annual SO2 emissions from sources in the CSAPR SO2 programs fell from 7.7 million tons in 2005 to 538,000 in 2022 (93 percent). In 2022, SO2 emissions were about 1.4 million tons below the regional CSAPR emission budgets (0.91 million in Group 1 and 0.53 million in Group 2); the CSAPR SO2 annual programs' 2022 regional budgets are 1,372,631 and 597,579 tons for Group 1 and Group 2, respectively.
SO₂ State-by-State Emissions
- CSAPR and ARP: From 1990 to 2022, annual SO2 emissions from sources in the ARP and the CSAPR SO2 program dropped in 46 states plus Washington, D.C. by a total of 14.9 million tons. In contrast, annual SO2 emissions increased in two states (Idaho and Vermont) by a total of 11 tons from 1990 to 2022.
- CSAPR: All 22 states (16 states in Group 1 and 6 states in Group 2) had emissions below their CSAPR allowance budgets, collectively by about 1.4 million tons.
SO₂ Emission Rates
- The average SO2 emission rate for units in the ARP or CSAPR SO₂ program fell to 0.08 pounds per million British thermal units (lb/mmBtu). This indicates an 89 percent reduction from 2005 rates, with most reductions coming from coal-fired units.
- Emissions have decreased dramatically since 2005, due in large part to greater use of control technology on coal-fired units and increased generation at natural gas-fired units that emit very little SO2 emissions.
See the trends for the other pollutants:
Annual Nitrogen Oxides (NOX)
Nitrogen oxides (NOₓ) are made up of a group of highly reactive gases that are emitted from power plants and motor vehicles, as well as other sources. NOₓ emissions contribute to the formation of ground-level ozone and fine particle pollution, which cause a variety of adverse health effects.
Overall Results
- Annual NOₓ emissions have declined dramatically under the ARP, CAIR, and CSAPR programs, with most reductions coming from coal-fired units. These reductions have occurred while electricity generation has remained relatively stable since 2000.
- These emission reductions are a result of an overall increase in the environmental efficiency at affected sources as power generators installed controls, ran their controls year-round, switched to lower emitting fuels, or otherwise reduced their NOₓ emissions. These trends are discussed further in the Program Basics section.
- Other programs – such as regional and state NOₓ emission control programs – also contributed significantly to the annual NOₓ emission reductions achieved by sources in 2022.
Annual NOₓ Emissions Trends
- ARP: Units in the ARP NOX program emitted 734,000 tons of NOX emissions in 2022. Sources reduced emissions by 7.4 million tons from the projected level in 2000 without the ARP, over three times the program’s NOX emission reduction objective.
- CSAPR and ARP: In 2022, the eighth year of operation of the CSAPR NOX annual program, sources in both the CSAPR NOX annual program and the ARP together emitted 749,000 tons, a reduction of 5.7 million tons (88 percent reduction) from 1990 levels, 4.4 million tons (85 percent reduction) from 2000, and 2.9 million tons (79 percent reduction) from 2005 levels.
- CSAPR: Emissions from the CSAPR NOX annual program sources were 412,000 tons in 2022. This is about 1.8 million tons (81 percent) lower than in 2005 and 657,000 tons (61 percent) below the CSAPR NOX annual program's 2022 regional budget of 1,069,256 tons.
Annual NOₓ State-by-State Emissions
- CSAPR and ARP: From 1990 to 2022, annual NOX emissions in the ARP and the CSAPR NOX program dropped in 47 states plus Washington, D.C. by a total of approximately 5.7 million tons. In contrast, annual emissions increased in one state (Idaho) by 381 tons from 1990 to 2022.
- CSAPR: All 22 states had emissions below their CSAPR 2022 allowance budgets, collectively by about 0.6 million tons. For more information about Program Compliance, see the Program Compliance chapter.
Annual NOₓ Emission Rates
- In 2022, the ARP and CSAPR average annual NOX emission rate was 0.07 lb/mmBtu, a 74 percent reduction from 2005.
- Emissions have decreased dramatically since 2005, due in large part to greater use of control technology, primarily on coal-fired units, and increased generation at natural gas-fired units that emit less NOX emissions per unit of electricity than coal-fired units.
See the trends for the other pollutants:
Ozone Season Nitrogen Oxides (NOX)
The CSAPR NOₓ ozone season program was established to reduce interstate transport of air pollution during the ozone season (May 1 – September 30), the warm summer months when ozone formation is highest, and to help eastern U.S. counties attain the 1997 ozone standard. The CSAPR Update NOₓ ozone season program was similarly established to help eastern U.S. counties attain the 2008 ozone standard. On March 15, 2021, EPA finalized the Revised CSAPR Update to further reduce NOₓ emissions from power plants in 12 states. The rule responded to a September 2019 ruling by the United States Court of Appeals for the D.C. Circuit, Wisconsin v. EPA, which remanded the 2016 CSAPR Update to EPA for failing to fully eliminate significant contribution to nonattainment and interference with maintenance of the 2008 ozone NAAQS from these states by downwind areas’ attainment dates.
Overall Results
- Ozone season NOₓ emissions have declined dramatically under the ARP, NBP, CAIR, and CSAPR programs.
- States with the highest emitting sources of ozone season NOₓ emissions in 2000 have seen the greatest reductions under the CSAPR NOₓ ozone season programs. Most of these states are in the Ohio River Valley and are upwind of the areas CSAPR was designed to protect. Reductions by sources in these states have resulted in important environmental and human health benefits over a large region.
- These reductions have occurred while electricity generation has remained relatively stable since 2000. These trends are discussed further in the Program Basics section.
- Other programs – such as regional and state NOₓ emission control programs – also contributed significantly to the ozone season NOₓ emission reductions achieved by sources in 2022.
Ozone Season NOₓ Emissions Trends
- ARP: Units in the ARP program emitted 316,000 tons of ozone season NOX emissions in 2022. Sources reduced emissions by 1.9 million tons (86 percent) from the 2000 ozone season and 950,000 tons (75 percent) from the 2005 ozone season.
- CSAPR: In 2022, units covered under the CSAPR NOX ozone season programs (Group 1, 2, and 3) emitted 212,000 tons, a reduction of 240,000 (53%) since 2015.
- In 2022, the CSAPR NOX ozone season program emissions were 27 percent below the regional emission budget of 288,049 tons (13,211 tons for Group 1, and 143,408 tons for Group 2, and 131,430 tons for Group 3).
Ozone Season NOₓ State-by-State Emissions
- Between 2005 and 2022, ozone season NOX emissions from the CSAPR sources fell in every state participating in the CSAPR NOX ozone season program.
- 21 states had emissions below their CSAPR 2022 allowance budgets, collectively by about 77,000 tons. One state (Mississippi) exceeded their 2022 state level budgets by about 500 tons total.
Ozone Season NOₓ Emission Rates
- In 2022, the average NOₓ ozone season emission rate fell to 0.06 lb/mmBtu for the CSAPR ozone season program states and 0.06 lb/mmBtu nationally. This represents a 68 and 69 percent reduction, respectively, from 2005 emission rates, with the majority of reductions coming from coal-fired units.
- Emissions have decreased dramatically since 2005, due in large part to greater use of control technology, primarily on coal-fired units, and increased generation at natural gas-fired units, which emit less NOₓ emissions per unit of electricity than coal-fired units.
See the trends for the other pollutants:
Carbon Dioxide (CO2)
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle–both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests and soils, to remove and store CO2 from the atmosphere. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution. Learn more about the impacts of CO2 emissions on the climate on EPA’s Climate Change website.
Coal combustion is more carbon-intensive than burning natural gas or petroleum for electric power production. Although coal use accounted for 56% of CO2 emissions from the sector, it represented only 20% of the electricity generated in the United States in 2022. Natural gas use accounted for 43% of CO2 emissions and 40% of electricity generation in 2022.
In May of 2023, EPA proposed Clean Air Act emissions limits and guidelines for carbon dioxide (CO2) from fossil-fuel fired power plants based on cost-effective and available control technologies. The proposals would set limits for new gas-fired combustion turbines, existing coal, oil, and gas-fired steam generating units, and certain existing gas-fired combustion turbines.
Learn more about greenhouse gas emissions and what EPA is doing on the Greenhouse Gas Emissions and Removals website.
See the trends for the other pollutants:
Mercury
Hazardous air pollutants (HAPs) emitted by power plants include mercury, acid gases (e.g., hydrochloric acid, hydrofluoric acid), non-mercury metallic toxics (e.g., arsenic, nickel, and chromium) and organic HAPs (e.g., formaldehyde, dioxin/furan). Exposure to these pollutants at certain concentrations and durations can increase chances of neurological and developmental effects, cancer, and reproductive, respiratory, and other health problems.
In 2011, EPA issued MATS, establishing national emission standards for mercury and other hazardous air pollutants for new and existing coal- and oil-fired power plants. The standards were finalized under section 112 of the Clean Air Act. The MATS emission standards were established using data from a 2010 information collection request (ICR) that was sent to selected coal and oil burning power plants.
Overall Results
- Mercury and other hazardous air pollutant (HAP) emissions have declined significantly since 2010 estimates. These emission reductions were driven by the installation of new pollution controls and enhancements of existing pollution controls that reduce multiple pollutants. Emissions have also decreased due to operational changes, such as fuel switching and increased generation at natural gas-fired units that emit very little mercury and other HAPs. These trends are discussed further in the Program Basics section.
- Other programs – such as regional and state SO₂ and NOₓ emission control programs – also contributed to the mercury and other HAP emission reductions achieved by covered sources in 2022.
Mercury and Hazardous Air Pollutant Emission Trends
Compared to 2010, units covered under MATS in 2022 emitted 24 fewer tons of mercury (89% reduction).
See the trends for the other pollutants: