Progress Report - Emissions Reductions

Sulfur Dioxide (SO₂)

SO₂ is a highly reactive gas generated primarily by coal-fired power plants. SO₂ contributes to the formation of acid rain and fine particle (PM₂.₅) pollution and are linked to 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 programs, power plants have significantly lowered SO₂ emissions while electricity generation from these power plants remained relatively stable from 2000 through 2023.
• 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 647,000 tons of SO₂ in 2023, well below the ARP's statutory annual cap of 8.95 million tons. The ARP sources reduced emissions by 15.1million tons (96 percent) from 1990 levels and 16.6 million tons (96 percent) from 1980 levels.
• CSAPR Programs: Annual SO₂ emissions from sources in the CSAPR SO₂ programs fell from 7.7 million tons in 2005 to 393,000 in 2023 (95 percent). In 2023, SO₂ emissions were about 1.6 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' 2023 regional budgets are 1,372,631 and 597,579 tons for Group 1 and Group 2, respectively.
• Both CSAPR and ARP: In 2023, the ninth year of operation of the CSAPR SO₂ programs, sources in both the CSAPR SO₂ annual programs and the ARP together reduced SO₂ emissions by 15.1 million tons (96 percent) from 1990 levels (before implementation of the ARP), 10.6 million tons (94 percent) from 2000 levels (ARP Phase II), and 9.6 million tons (94 percent) from 2005 levels (before implementation of CAIR and CSAPR). All ARP and CSAPR sources together emitted a total of 650,000 tons of SO₂ in 2023.

Figure 2: Annual Sulfur Dioxide Emissions from Power Plants

SO₂ State-by-State Emissions

• 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 1.6 million tons.
• Both CSAPR and ARP: From 1990 to 2023, 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 15.1 million tons. In contrast, annual SO₂ emissions increased in two states (Idaho and Vermont) by a total of 15 tons from 1990 to 2023.

Figure 3: State-by-State SO₂ Emissions

SO₂ Emission Rates

• The average SO₂ emission rate for units in the ARP or CSAPR SO₂ program fell to 0.06 pounds per million British thermal units (lb/mmBtu). This indicates a 92 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.

Figure 4: Comparison of SO₂ Emissions and Generation

See the trends for the other pollutants:

• Annual Nitrogen Oxides
• Ozone Season Nitrogen Oxides
• Carbon Dioxide
• Mercury

Annual Nitrogen Oxides (NO_X)

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 from these power plants remained relatively stable from 2000 through 2023.
• 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 2023.

Annual NOₓ Emissions Trends

• ARP: Units in the ARP NO_X program emitted 627,000 tons of NO_X emissions in 2023. Sources reduced emissions by 7.5 million tons from the projected level in 2000 without the ARP, over three times the program’s NO_X emission reduction objective.
• CSAPR: Emissions from the CSAPR NO_X annual program sources were 337,000 tons in 2023. This is about 1.8 million tons (84 percent) lower than in 2005 and 732,000 tons (68 percent) below the CSAPR NO_X annual program's 2023 regional budget of 1,069,256 tons.
• Both CSAPR and ARP: In 2023, the ninth year of operation of the CSAPR NO_X annual program, sources in both the CSAPR NO_X annual program and the ARP together emitted 639,000 tons, a reduction of 5.8 million tons (90 percent reduction) from 1990 levels, 4.5 million tons (88 percent reduction) from 2000, and 3.0 million tons (82 percent reduction) from 2005 levels.

Figure 5: Annual Nitrogen Oxides Emissions

Annual NOₓ State-by-State Emissions

• CSAPR: All 22 states had emissions below their CSAPR 2023 allowance budgets, collectively by about 0.7 million tons. For more information about Program Compliance, see the Program Compliance chapter.
• Both CSAPR and ARP: From 1990 to 2023, annual NO_X emissions in the ARP and the CSAPR NO_X program dropped in 47 states plus Washington, D.C. by a total of approximately 5.8 million tons. In contrast, annual emissions increased in one state (Idaho) by 567 tons from 1990 to 2023.

Figure 6: State-by-State NO_X Emissions

Annual NOₓ Emission Rates

• In 2023, the ARP and CSAPR average annual NO_X emission rate was 0.06 lb/mmBtu, a 77 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 NO_X emissions per unit of electricity than coal-fired units.

Figure 7: Comparison of NO_X Emissions and Generation

See the trends for the other pollutants:

• Sulfur Dioxide
• Ozone Season Nitrogen Oxides
• Carbon Dioxide
• Mercury

Ozone Season Nitrogen Oxides (NO_X)

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. In 2023, the “Good Neighbor” plan was finalized to secure significant reductions in ozone-forming emissions of nitrogen oxides (NO_X) from power plants and industrial facilities. On June 27, 2024, the Supreme Court of the United States granted emergency applications seeking a stay of the Good Neighbor Plan pending judicial review. Since this occurred after the 2023 ozone season was fully processed, this impact will be noticed in next year's report.

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 from these power plants remained relatively stable from 2000 through 2023. 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 2023.

Ozone Season NOₓ Emissions Trends

• ARP: Units in the ARP program emitted 287,000 tons of ozone season NO_X emissions in 2023. Sources reduced emissions by 1.9 million tons (87 percent) from the 2000 ozone season and 980,000 tons (77 percent) from the 2005 ozone season.
• CSAPR: In 2023, units covered under the CSAPR NO_X ozone season programs (Group 1, 2, 2E, and 3) emitted 196,000 tons, a reduction of 260,000 (57%) since 2015.
  • In 2023, the CSAPR NO_X ozone season program emissions were 27 percent below the regional emission budget of 253,675 tons.

Figure 8: Ozone Season Nitrogen Oxides Emissions

Ozone Season NOₓ State-by-State Emissions

• Between 2005 and 2023, ozone season NO_X emissions from the CSAPR sources fell in every state participating in a CSAPR NO_X ozone season program.
• 20 states had emissions below their CSAPR 2023 allowance budgets, collectively by about 67,000 tons. Three state (Maryland, Ohio, and Oklahoma) exceeded their 2023 state level budgets by about 4,100 tons total.

Figure 9: State-by-State Ozone Season NO_X Emissions

Ozone Season NOₓ Emission Rates

• In 2023, 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 69 and 71 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.

Figure 10: Comparison of Ozone Season NO_X Emissions and Generation

See the trends for the other pollutants:

• Sulfur Dioxide
• Annual Nitrogen Oxides
• Carbon Dioxide
• Mercury

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 50% of CO₂ emissions from the sector, it represented only 16% of the electricity generated in the United States in 2023. Natural gas use accounted for 49% of CO₂ emissions and 43% of electricity generation in 2023.

In April of 2024,  EPA issued final carbon pollution standards for power plants that set carbon dioxide (CO₂)  limits for new gas-fired combustion turbines and CO₂ emission guidelines for existing coal, oil and gas-fired steam generating units, securing important climate benefits and protecting public health.. These rules will significantly reduce greenhouse gas emissions from existing coal-fired power plants and from new natural gas turbines, ensuring that all long-term coal-fired plants and base load new gas-fired plants control 90% of their carbon pollution. 

Learn more about greenhouse gas emissions and what EPA is doing on the Greenhouse Gas Standards and Guidelines for Fossil Fuel-Fired Power Plants. Find more information about what EPA is doing about climate change.

Figure 11: Annual Carbon Dioxide Emissions

Figure 12: State-by-State CO₂ Emissions

See the trends for the other pollutants:

• Sulfur Dioxide
• Annual Nitrogen Oxides
• Ozone Season Nitrogen Oxides
• Mercury

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 the Mercury and Air Toxics Standards (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. On April 25, 2024, the EPA announced requirements to strengthen and update MATS for power plants, based on an evaluation of the residual risk and technology review (RTR).

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.

Figure 14: Mercury Emissions

Compared to 2010, units covered under MATS in 2023 emitted 27 fewer tons of mercury (91% reduction).

Figure 15: State-by-State Mercury Emissions

See the trends for the other pollutants:

• Sulfur Dioxide
• Annual Nitrogen Oxides
• Ozone Season Nitrogen Oxides
• Carbon Dioxide