Acid rain was first recognized in the 1970s in North America and Europe as an atmospheric pollutant that was causing harm to ecosystems. In response, the U.S. Congress enacted Title IV of the Clean Air Act Amendments (CAA) in 1990 to reduce sulfur and nitrogen emissions from fossil fuel burning power plants. This study reports trends in wet-precipitation chemistry in response to emissions reductions implemented as part of the CAA. Trends were calculated for sulfate (SO₄), nitrate (NO₃) and ammonium (NH₄) from 1985 to 2017 at 168 stations operated by the National Atmospheric Deposition Program (NADP); stations were divided into 9 regions across the United States. Trend analyses were conducted for three time periods: Period 1 (1985–1999), Period 2 (2000–2017), and the entire study period (1985–2017). Seasonal and regional Kendall trend analyses reveal significant decreasing trends in mean wet-precipitation SO₄ concentrations in all 9 regions during the entire study period. The largest decreasing trends in monthly mean SO₄ precipitation-weighted concentrations were measured in the Mid-Atlantic (−1.29 μeq/l/yr), Midwest (−1.15 μeq/l/yr), and Northeast regions (−1.10 μeq/l/yr). The trends in monthly mean NO₃ concentrations were not as strong as those for SO₄, but all of the regions had significant decreasing trends in NO₃ and again the Mid-Atlantic (−0.53 μeq/l/yr), Midwest (−0.44 μeq/l/yr), and Northeast regions (−0.50 μeq/l/yr) had the strongest trends. Trends were steepest during Period 2 for SO₄ and NO₃, in fact for NO₃ 86% of the stations had significant decreasing trends during Period 2 while only 8% of the stations had significant decreasing trends during Period 1. The stations with the highest concentrations of SO₄ and NO₃ at the beginning of the study had the strongest decreasing trends and the relations were stronger during Period 2 than Period 1. For NH₄, 22% of the stations had statistically significant increasing trends in concentration during Period 1. The largest increasing trends in wet-precipitation NH₄ concentration occurred in the North-Central region during Period 1, Period 2 and throughout the entire study. By comparison, NH₄ trends in the Rocky-North and Rocky-South regions were about half as steep and trends in the South-Central and Midwest regions were about one-third as steep.

We compared trends in SO₄ and NO₃ concentrations from NADP stations to emissions of sulfur dioxide and nitrogen oxides, respectively to determine whether there was a relation between emissions and wet-precipitation concentration trends within proximity to NADP stations. There was a statistically significant relation (r² = 0.62–0.69, p < 0.01) between the trend in SO₄ concentrations at individual NADP stations and total and mean sulfur dioxide (SO₂) emissions from power plants within a range of 750 km and 1000 km from each station. There were also significant relations between NO₃ concentration trends at NADP stations and power plant emissions of nitrogen oxides, but they were not nearly as strong (r² = 0.18–0.36, p < 0.01) as those for SO₄ and were strongest for emissions within a range of 1000 km and 1500 km from each NADP station. Decreases in wet-precipitation SO₄ concentrations were more consistent across regions and through time than decreases in NO₃ and SO₄ trends were more closely linked to stationary emissions sources than NO₃ trends. There were statistically significant increases in NH₄ wet-precipitation concentrations, as have been reported in previous studies, but this study found that those increases were strongest during Period 1 and were not consistent across the United States. During the first 3 years of the study period, wet-precipitation acidity was dominated by SO₄ in 8 of the 9 regions; by 2017 NO₃ dominated the acidity of wet-precipitation in 7 of the 9 regions. There has also been a downward shift in the NO₃:NH₄ ratio of wet-precipitation as the emissions of nitrogen oxides have declined while ammonia emissions have remained essentially constant. This shift has resulted in an increase in wet-precipitation total nitrogen concentrations in 7 of the 9 regions and indicate that efforts to control NH₃ emissions will become increasingly important as emissions of nitrogen oxides continue to decline.