Progress Report

Emission Reductions

Sulfur Dioxide (SO₂) Figures

Source: EPA, 2018
Last updated: 05/2018

Related Figures

SO₂ Emissions from CSAPR and ARP Sources, 1980–2016
State-by-State SO₂ Emissions from CSAPR and ARP Sources, 1990–2016
Comparison of SO₂ Emissions and Heat Input for CSAPR and ARP Sources, 2000–2016
CSAPR and ARP SO₂ Emissions Trends


Overall Results

  • Under the ARP, CAIR, and now CSAPR, power plants have significantly lowered SO₂ emissions while electricity demand (measured as heat input) remained relatively stable, indicating that the emission reductions were not driven by decreased electric generation.
  • These emission reductions are a result of an overall increase in the environmental efficiency at affected sources as power generators installed controls, switched to lower emitting fuels, or otherwise reduced their SO₂ emissions while meeting relatively steady electricity demand.

SO₂ Emission Trends

  • ARP: Units in the ARP emitted 1.5 million tons of SO₂ in 2016, well below the ARP's statutory annual cap of 8.95 million tons. ARP sources reduced emissions by 14.3 million tons (91 percent) from 1990 levels and 15.8 million tons (91 percent) from 1980 levels.
  • CSAPR and ARP: In 2016, the second year of operation of the CSAPR SO₂ program, sources in both the CSAPR SO₂ annual program and the ARP together reduced SO₂ emissions by 14.2 million tons (91 percent) from 1990 levels (before implementation of the ARP), 9.7 million tons (87 percent) from 2000 levels (ARP Phase II), and 8.8 million tons (85 percent) from 2005 levels (before implementation of CAIR and CSAPR). All ARP and CSAPR sources together emitted a total of 1.5 million tons of SO₂ in 2016.
  • CSAPR: Annual SO₂ emissions from sources in the CSAPR SO₂ program alone fell from 8.8 million tons in 2005 to 1.2 million tons in 2016, a 87 percent reduction. In 2016, SO₂ emissions were about 2.3 million tons below the regional CSAPR emission budgets (1.8 million in Group 1 and 0.5 million in Group 2); the CSAPR SO₂ annual program's 2016 regional budget are 2,551,802 and 917,787 tons for Group 1 and Group 2, respectively.

SO₂ State-by-State Emissions

  • CSAPR and ARP: From 1990 to 2016, annual SO₂ emissions from sources in the ARP and the CSAPR SO₂ program dropped in 45 states plus Washington, D.C. by a total of approximately 14.2 million tons. In contrast, annual SO₂ emissions increased in three states (Idaho, Nebraska, and Vermont) by a combined total of 550 tons from 1990 to 2016.
  • CSAPR: All 23 states (16 states in Group 1 and 7 states in Group 2) had emissions below their CSAPR allowance budgets, collectively by about 2.3 million tons.

SO₂ Emission Rates

  • The average SO₂ emission rate for units in the ARP or CSAPR SO₂ program fell to 0.13 lb/mmBtu. This indicates an 81 percent reduction from 2005 rates, with the majority of reductions coming from coal-fired units.
  • Although heat input has decreased slightly over the past 11 years, emissions have decreased dramatically since 2005, indicating an improvement in emission rate at the sources. This is 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.

Analysis and Background Information

SO₂ is a highly reactive gas that is generated primarily from the burning of fossil fuels at power plants. In addition to contributing to the formation of fine particle pollution (PM₂.₅), 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 located 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.

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