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Progress Report

Emission Reductions

SO₂ Trends

Last updated: 05/2016

Related Figures

SO₂ Emissions from CAIR SO₂ Annual Program and ARP Sources, 1980–2013
State-by-State Annual SO₂ Emission Levels for CAIR and ARP Sources, 1990–2013
Comparison of SO₂ Emissions and Heat Input for CAIR and ARP Sources, 2000–2013
CAIR and ARP Annual SO₂ Trends

Key Points

SO₂ Emission Trends

  • ARP: Units in the ARP emitted 3.1 million tons of SO₂ in 2014, well below the ARP's statutory annual cap of 8.95 million tons. ARP sources reduced emissions by 12.6 million tons (80 percent) from 1990 levels and 14.1 million tons (82 percent) from 1980 levels.
  • CAIR and ARP: In 2014, the fourth year of operation of the CAIR SO₂ program, sources in both the CAIR SO₂ program and the ARP together reduced SO₂ emissions by 12.6 million tons (80 percent) from 1990 levels (before implementation of the ARP), 8.1 million tons (72 percent) from 2000 levels (ARP Phase II), and 7.1 million tons (69 percent) from 2005 levels (before implementation of CAIR). All ARP and CAIR sources together emitted a total of 3.2 million tons of SO₂ in 2014.
  • CAIR: Annual SO₂ emissions from sources in the CAIR SO₂ program alone fell from 9.1 million tons in 2005 to 2.7 million tons in 2014, a 71 percent reduction. Between 2013 and 2014, SO₂ emissions fell 48,000 tons (2 percent) and were about 970,000 tons below the regional CAIR emission budget.

SO₂ State-by-State Emissions

  • CAIR and ARP: From 1990 to 2014, annual SO₂ emissions in the ARP and the CAIR SO₂ program dropped in 43 states (42 states plus Washington, D.C.) by a total of approximately 12.6 million tons. In contrast, annual SO₂ emissions increased in five states (Arkansas, Idaho, Nebraska, Oregon, and Vermont) by a combined total of 20,000 tons from 1990 to 2014.
  • CAIR: In 2014, seventeen states (16 states plus Washington, D.C.) had emissions below their CAIR allowance budgets, collectively by about 1.1 million tons. Another six states exceeded their 2014 budgets by a combined total of about 140,000 tons, indicating that, on an aggregate basis, sources within those states covered a portion of their emissions with allowances banked from earlier years, transferred from an out-of-state account, or purchased from the market.

SO₂ Emission Rates

  • In 2014, the average SO₂ emission rate for units in the ARP and CAIR SO₂ program fell to 0.25 lb/mmBtu. This indicates a 71 percent reduction from 2000 rates, with the majority of reductions coming from coal-fired units.
  • Although heat input has remained steady over the past 14 years, emissions have decreased dramatically since 2000, 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 fossil fuels at power plants. In addition to contributing to the formation of fine particle pollution (PM₂.₅), SO₂ is 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. Most of these states are located in the Ohio River Valley and are upwind of the areas the ARP and CAIR were designed to protect. Reductions under the ARP and CAIR have provided important environmental and health benefits over a large region.

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