Progress Report

Emission Reductions

Sulfur Dioxide (SO₂) Figures

Source: EPA, 2019
Last updated: 06/2019

Related Figures

SO₂ Emissions from CSAPR and ARP Sources, 1980–2017
State-by-State SO₂ Emissions from CSAPR and ARP Sources, 1990–2017
Comparison of SO₂ Emissions and Generation for CSAPR and ARP Sources, 2000–2017
CSAPR and ARP SO₂ Emissions Trends


Overall Results

  • Under the ARP, CAIR, and now CSAPR, power plants have significantly lowered SO₂ emissions 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, switched to lower emitting fuels, or otherwise reduced their SO₂ emissions. These trends are discussed further in Chapter 1.

SO₂ Emission Trends

  • ARP: Units in ARP emitted 1.3 million tons of SO₂ in 2017, well below the ARP's statutory annual cap of 8.95 million tons. ARP sources reduced emissions by 14.4 million tons (92 percent) from 1990 levels and 15.9 million tons (92 percent) from 1980 levels.
  • CSAPR and ARP: In 2017, the third year of operation of the CSAPR SO₂ program, sources in both the CSAPR SO₂ annual program and ARP together reduced SO₂ emissions by 14.4 million tons (92 percent) from 1990 levels (before implementation of ARP), 9.9 million tons (88 percent) from 2000 levels (ARP Phase II), and 8.9 million tons (87 percent) from 2005 levels (before implementation of CAIR and CSAPR). All ARP and CSAPR sources together emitted a total of 1.3 million tons of SO₂ in 2017.
  • CSAPR: Annual SO₂ emissions from sources in the CSAPR SO₂ program alone fell from 8.1 million tons in 2005 to 0.8 million tons in 2017, a 91 percent reduction. In 2017, SO₂ emissions were about 1.2 million tons below the regional CSAPR emission budgets (0.7 million in Group 1 and 0.5 million in Group 2); the CSAPR SO₂ annual program's 2017 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 2017, annual SO₂ emissions from sources in ARP and the CSAPR SO₂ program dropped in 46 states plus Washington, D.C. by a total of 14.4 million tons. In contrast, annual SO₂ emissions increased in two states (Idaho and Vermont) by a combined total of 7 tons from 1990 to 2017.
  • 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 1.2 million tons.

SO₂ Emission Rates

  • The average SO₂ emission rate for units in ARP or CSAPR SO₂ program fell to 0.12 pounds per million British thermal units (lb/mmBtu). This indicates an 84 percent reduction from 2005 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 on coal-fired units and increased generation at natural gas-fired units that emit very little SO₂ emissions.

Background Information

SO₂ is a highly reactive gas that is generated primarily from coal-fired 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 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 ARP and CSAPR were designed to protect. Reductions under these programs have provided important environmental and health benefits over a large region.