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Progress Report

Emission Reductions

Sulfur Dioxide (SO₂) Figures

Source: EPA, 2017
Last updated: 05/2017

Related Figures

SO₂ Emissions from CSAPR and ARP Sources, 1980–2015
State-by-State SO₂ Emissions from CSAPR and ARP Sources, 1990–2015
Comparison of SO₂ Emissions and Heat Input for CSAPR and ARP Sources, 2000–2015
CSAPR and ARP SO₂ Emissions Trends

Key Points

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 total 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 2.2 million tons of SO₂ in 2015, well below the ARP's statutory annual cap of 8.95 million tons. ARP sources reduced emissions by 13.5 million tons (86 percent) from 1990 levels and 15.1 million tons (87 percent) from 1980 levels.
  • CSAPR and ARP: In 2015, the first year of operation of CSAPR SO₂ program, sources covered by both the CSAPR SO₂ annual program and the ARP reduced SO₂ emissions by 13.5 million tons (86 percent) from 1990 levels (before implementation of the ARP), 9.0 million tons (80 percent) from 2000 levels (ARP Phase II), and 8.0 million tons (78 percent) from 2005 levels (before implementation of CAIR and CSAPR). All ARP and CSAPR sources together emitted a total of 2.2 million tons of SO₂ in 2015.
  • CSAPR: Annual SO₂ emissions from sources in the CSAPR SO₂ program alone fell from 7.7 million tons in 2005 to 1.8 million tons in 2015, a 77 percent reduction. In 2015, SO₂ emissions were about 1.7 million tons below the regional CSAPR emission budgets (1.3 million in Group 1 and 0.4 million in Group 2).

SO₂ State-by-State Emissions

  • CSAPR and ARP: From 1990 to 2015, annual SO₂ emissions from sources in the ARP and CSAPR SO₂ program dropped in 45 states plus Washington, D.C. by a total of approximately 13.5 million tons. In contrast, annual SO₂ emissions increased in three states (Idaho, Nebraska, and Vermont) by a combined total of only 14,000 tons from 1990 to 2015.
  • CSAPR: In 2015, each of the 23 states (16 states in Group 1 and 7 states in Group 2) had emissions below their CSAPR allowance budgets, collectively by about 1.7 million tons.

SO₂ Emission Rates

  • In 2015, the average SO₂ emission rate for units in the ARP or CSAPR SO₂ program fell to 0.18 lb/mmBtu. This indicates a 76 percent reduction from 2005 rates, with the majority of reductions coming from coal-fired units.
  • Although heat input has decreased slightly over the past 10 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.

Analysis and Background Information

SO₂ is a highly reactive gas that is generated primarily from the burning of certain 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 the ARP and CSAPR have provided important environmental and health benefits over a large region.

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