Acid rain reduced in Eastern United States

Concentrations of sulfate (SO42-) and free hydrogen ions (H+) in precipitat ion decreased from 10% to 25% over a large area of the Eastern United State s from 1995 through 1997 as compared to the previous 12-year (1983-1994) re ference period. These decreases were unprecedented in magnitude and spatial extent. In contrast, nitrate (NO3-) concentrations generally did not chang e over this period. The largest decreases in both H+ and SO42- concentratio ns, 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 Amend ments (CAAA) set limitations on sulfur dioxide (SO2) emissions from a large number of utility-owned coal-fired sources. Phase I of the CAAA required t hat these limitations be met by January 1, 1995. On the basis of our analys is of precipitation chemistry and emissions data, we conclude that signific ant declines in acid rain occurred in many parts of the Eastern United Stat es from 1995 through 1997 because of large reductions in SO2 emissions in t his region and a corresponding reduction in SO42- concentrations in precipi tation.