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Acid Rain Program 2007 Progress Report

Published January 2009

Other progress reports

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The Acid Rain Program (ARP) was created under Title IV of the 1990 Clean Air Act (CAA) Amendments to reduce the adverse effects of acid deposition through reductions in annual emissions of SO2 and NOx primarily from fossil-fuel burning electricity generation. The Act calls for SO2 reductions from all sources of 10 million tons from 1980 emission levels, largely achieved through a market-based cap and trade program which imposes a permanent emission cap on SO2 emissions from electric generating units (EGUs) at power plants. NOx reductions under the ARP are achieved through a program that applies to a subset of coal-fired EGUs and is closer to a more traditional, rate-based regulatory system. The goal of the NOx program is to limit NOx emission levels from the affected coal-fired boilers so that their emissions are at least 2 million tons less than the projected level for the year 2000 without implementation of Title IV.


In 2007, for the first time, SO2 emissions were below the ARP's long term emission cap of 8.95 million tons -- three years before the 2010 statutory deadline. Total SO2 emissions in 2007 were 8.9 million tons from over 3,500 affected EGUs. The ARP has reduced SO2 emissions by about 6.8 million tons since 1990 (43 percent). Sources emitted just below the 8.95 million tons in 2007, well below the current emission cap of 9.5 million tons, and already below the statutory cap set for compliance in 2010.

NOx emissions coal-fired EGUs also continued a steady decline in 2007, decreasing by about 121,000 tons (3.5 percent) from 2006 levels to about 3.0 million tons. Total NOx emissions from all ARP units were 3.3 million tons in 2007.

Comparisons between the 1989-1991 and 2005-2007 observation periods show wet sulfate deposition decreased 35 percent in the Northeast and 33 percent in the Midwest. Wet nitrogen deposition also decreased between these periods with a decrease in the Northeast by 21 percent and in the Midwest by 7 percent. These reductions in sulfur and nitrogen deposition have resulted in positive changes in environmental indicators, including improved water quality in lakes and streams.

However, as evidenced by long-term monitoring and assessment data analyses, the relationship of emission reductions to ecological and air quality improvements is complex and not entirely commensurate with the level of ARP emission reductions, suggesting extenuating circumstances attributable to other source sectors or factors such as soil type and atmospheric chemistry.

The ARP achieved full compliance in 2007 with the SO2 allowance holding requirements and NOx emission limits. Estimated public health benefits from ARP emission reductions - over $120 billion annually at full implementation in 2010 -- exceed program costs by a margin of more than 40:1.

Bar chart showing SO2 emissions declining from about 17.3 million tons in 1980 to 9.5 million tons in 2007
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Please see the report for more detailed information or higher resolution images



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