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Acid Rain Program Results

The Acid Rain Program (ARP) has delivered significant 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. Together with more recent power sector regulations, including the Cross-State Air Pollution Rule (CSAPR), and a rapidly changing energy sector, the ARP has helped deliver annual SO2 reductions of over 93% and annual NOX emissions reductions of over 86%. The Power Plant Emissions Trends page has maps and data highlighting these emissions reductions, and the Progress Reports provide an annual overview of program features and results, from compliance to air quality impacts.

Hover left to right over the maps to see the changes before and after.


These emissions reductions have led to major decreases in acid rain nationwide. Wet sulfate deposition – a common indicator of acid rain – dropped by 70% between 1989-1991 and 2016-2018. Additional data and maps detailing deposition and ambient air pollution are available on the Clean Air Status and Trends Network (CASTNET) website.

Hover left to right over the map to see the changes before and after.

The Long Term Monitoring program (LTM), another monitoring program within EPA, observes surface waters to determine the effects of acid rain on aquatic ecosystems.  During the time period in which wet sulfate deposition decreased, LTM data show 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 goal of this long-term program is to track whether the Clean Air Act Amendments (Plain English Guide to the Clean Air Act) have been effective in reducing the acidity of surface waters.

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