Since at least the 1970s, acid rain has been recognized as an important pollutant that harms ecosystems and human health across a wide region of the United States and around the world. Congress enacted Title IV of the Clean Air Act Amendments in 1990 to reduce sulfur and nitrogen emissions from fossil fuel burning power plants. The Environmental Protection Agency has documented a 92% decrease in sulfur dioxide emissions from 15.73 million tons in 1990 to 1.26 million tons in 2018. Nitrogen oxide emissions decreased 84% from 6.40 million tons in 1990 to 1.02 million tons in 2018.

This study used National Atmospheric Deposition Program data from 167 stations across the United States to examine trends in acid deposition from 1985 to 2017, which show how these emission reductions are reflected in atmospheric wet-deposition chemistry in 9 regions across the conterminous US. Seasonal and regional Kendall trend analyses indicate that there were significant decreasing trends in mean sulfate deposition concentrations in all 9 regions during the study period. The largest trends in monthly mean sulfate concentrations were measured in the Mid-Atlantic (-1.29 µeq l^-1 yr^-1), Midwest (-1.15 µeq l^-1 yr^-1), and Northeast regions (-1.10 µeq l^-1 yr^-1). The trends in monthly mean nitrate concentrations were not as strong as those for sulfate, but all of the regions had significant decreasing trends in nitrate and again the Mid-Atlantic (-0.53 µeq l^-1 yr^-1), Midwest (-0.44 µeq l^-1 yr^-1), and Northeast regions (-0.50 µeq l^-1 yr^-1) had the strongest trends. In those regions the stations with the highest concentrations of sulfate and nitrate had the strongest decreasing trends and the relations were stronger during phase II of the regulations (2000 to 2009) than during the initial phase of the legislation (1995 to1999). As the concentrations of sulfate and nitrate decreased, the variability in concentrations across the 3 regions with the highest concentrations also decreased and there was also less annual variability at individual stations. In recent years there appears to be a flattening of the trends in sulfate and nitrate in the most impacted regions that may indicate the current status of emissions controls have reached the limit of their effectiveness. As a result, total nitrogen deposition has begun to be dominated by ammonium which has been increasing during the study period in all regions except the Mid-Atlantic and Northeast.