Changes in sulfate deposition in eastern USA following implementation of Phase I of Title IV of the Clean Air Act Amendments of 1990

Abstract

Phase I of Title IV of the Clean Air Act Amendments of 1990 (CAAA) was implemented on 1 January 1995. On that date limitations were imposed on sulfur dioxide (SO 2) emissions from 110 coal-fired electric utility plants (263 units) in 21 states of which 17 are east of the Mississippi River. Sulfur dioxide emissions at these plants dropped an average 2.86 million tons (38%) from 1995 through 1997, when compared to mean 1993–1994 emissions from these plants. As a result of these reductions, wet sulfate (SO 4 2−) deposition decreased an average of 2.4–4.0 kg ha −1 (10–25%) over a large area of the eastern USA. Based on more than 15 years of data, the decrease in wet deposition was unprecedented in magnitude and spatial extent. The largest decrease occurred across the Mid-Appalachian and Northeast regions of the country, the regions specifically targeted by Phase I for reductions in acidic deposition. Although the reductions in wet SO 4 2−deposition were generally consistent with observed reductions in SO 4 2− concentrations, deviations in precipitation volumes between the pre- and post-Phase I periods did result in some sites recording significantly ( p<0.05) lower post-Phase I concentrations but higher wet depositions. Deviations in precipitation within and between regions also influenced the magnitude and spatial distribution of the observed changes in wet SO 4 2− deposition. Clearly, deviations in the amount and distribution of precipitation need to be considered along with concentration changes in evaluating the CAAA effectiveness in reducing acidic deposition. This is particularly important in the eastern USA, since many of the most acid-sensitive ecosystems are located in regions, mostly mountainous and forested, that are subject to considerable fluctuations in precipitation volumes and subsequently acidic wet deposition.