The Acid Rain Program: A Success Story

By Christopher Grundler

This May, as EPA celebrates “Improving the Nation’s Air” Month, we salute the resounding success story of the Acid Rain Program (ARP). Since its inception in 1995, the ARP has earned widespread acclaim due to dramatic reductions of sulfur dioxide (SO2) and nitrogen oxides (NOX) emissions from fossil fuel-fired power plants, extensive environmental and human health benefits, and far lower-than-expected costs. The ARP’s SO2 cap and trade program, the first nationwide experiment in emissions trading, has been a victory for policy innovation, stakeholder collaboration, and human health and the environment.

Congress created the ARP in Title IV of the 1990 Clean Air Act Amendments, in response to deterioration of ecosystem health in lakes, streams, and forests across the United States and Canada, particularly in northeastern North America. To achieve this goal, the SO2 program set a permanent cap on the total amount of SO2 that can be emitted by power plants in the contiguous U.S. and allows emissions trading so sources can choose their preferred method of compliance. The final cap for SO2 emissions was set at a very ambitious target in 2010: 8.95 million tons, or about half of the 1980 level of 17.26 million tons. The ARP also required NOx emission reductions under a more traditional rate-based regulatory program, representing about a 27% reduction from 1990 levels. Just five years into ARP implementation in 2000, SO2 emissions were already down to 11.2 million tons and NOx emissions reductions had exceeded their target.

Bean Pond, a Long-Term Monitoring site in Somerset Co., Maine, is one of twenty-six lakes in the New England region that shows improving sulfate trends from 1990-2018.

Bean Pond, a Long-Term Monitoring site in Somerset Co., Maine, is one of twenty-six lakes in the New England region that shows improving sulfate trends from 1990-2018.

EPA is proud of the successes of the ARP and its subsequent interstate emission reduction programs and the marked progress those programs have achieved in cleaning up SO2 and NOx. In 2019, annual SO2 emissions measured only 0.97 million tons, a 94% reduction from 1990 levels. Annual NOX emissions measured 0.88 million tons, a reduction of 86% from 1990 levels. While market forces in the power sector – including significant increases in the availability of low-cost natural gas – have put downward pressure on emissions, by 2019, 82% of coal-fired power plants had installed advanced SO2 controls and 68% had installed advanced NOX controls.

For those of you who don’t have memories of the 1980s, allow the data to explain how the ecosystem and air quality have improved over the last 40 years. The national average of SO2 annual ambient concentrations decreased 93% between 1980 and 2018. Wet sulfate deposition – a common indicator of acid rain – decreased 86% reduction from 2000-2002 to 2016-2018. During that same time period, data from EPA’s Long-Term Monitoring program showed an 81% improvement in the number of monitored streams and lakes that experienced critical load exceedances, an indicator that reveals when acid deposition levels are causing harmful effects.

The human health benefits have been just as significant. A 2011 analysis of the benefits and costs of the 1990 Clean Air Act estimated that adult mortality risk decreased significantly due to the improved air quality, with up to 230,000 premature deaths avoided in 2020 as a result of lowered SO2 and NOx pollution levels.

Emissions trading programs have evolved over time to address changing industry and environmental challenges. These programs have been successful, producing near-perfect compliance, along with emissions and operations data at an unprecedented level of accuracy and detail. Annual Progress Reports and numerous tools enable anyone from power plant operators to students to access and analyze data to provide insights from the national level to our own backyards. EPA just posted the latest data, for the first quarter of 2020.

The core principles of accountability, transparency, and results have characterized every iteration of our regulatory efforts since the ARP started it all. By using ARP as a model, these foundational elements should and will guide EPA’s programs as they continue to fulfill the Agency’s primary mission of protecting human health and the environment.


About the author: Christopher Grundler is the Director of the Office of Atmospheric Programs. He has held multiple senior leadership positions during his forty years of service with the agency, including his recent tenure as the Director of the Office of Transportation and Air Quality.

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