Acid Rain Reduced in Eastern United States

Concentrations of sulfate (SO42-) and free hydrogen ions (H+) in precipitation decreased from 10% to 25% over a large area of the Eastern United States from 1995 through 1997 as compared to the previous 12-year (1983−1994) reference period. These decreases were unprecedented in magnitude and spatial extent. In contrast, nitrate (NO3-) concentrations generally did not change over this period. The largest decreases in both H+ and SO42- concentrations, which nearly mimicked one another, occurred in and downwind of the Ohio River Valley, the same area where Title IV of the 1990 Clean Air Act Amendments (CAAA) set limitations on sulfur dioxide (SO2) emissions from a large number of utility-owned coal-fired sources. Phase I of the CAAA required that these limitations be met by January 1, 1995. On the basis of our analysis of precipitation chemistry and emissions data, we conclude that significant declines in acid rain occurred in many parts of the Eastern United States from 1995 through 1997 because of large reductions in SO2 emissions in this region and a corresponding reduction in SO42- concentrations in precipitation.