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.

酸雨在1970年代首次在北美和欧洲被确认为对生态系统造成危害的大气污染物。作为回应，美国国会于1990年颁布了《清洁空气法修正案》（CAA）的第四章，以减少化石燃料燃烧发电厂的硫和氮排放。这项研究报告了作为CAA一部分实施的减少排放的湿法降水化学趋势。计算了硫酸盐（SO ₄），硝酸盐（NO ₃）和铵盐（NH _{4）的趋势}）从1985年至2017年，由美国国家大气沉积计划（NADP）运营的168个站点；美国各地的车站分为9个地区。进行了三个时间段的趋势分析：第一期（1985-1999年），第二期（2000-2017年）和整个研究期（1985-2017年）。季节性和区域性Kendall趋势分析显示，在整个研究期间，所有9个区域的平均湿沉降SO ₄浓度均呈显着下降趋势。在大西洋中部（-1.29μeq/ l / yr），中西部（-1.15μeq/ l / yr）和东北地区（-1.10μeq/ l）测得的月平均SO ₄降水加权浓度的最大下降趋势/ yr）。月平均NO ₃浓度的趋势不如SO浓度强_{如图4所示}，但所有区域的NO ₃均呈显着下降趋势，中大西洋地区（-0.53μeq/ l / yr），中西部地区（-0.44μeq/ l / yr）和东北地区（-0.50μeq/ l） / yr）的趋势最为强劲。SO ₂和NO _3的变化趋势在第2阶段最明显，实际上，NO ₃的NO ₃下降趋势在第2阶段显着，而第1阶段只有8％的下降趋势明显。 SO浓度₄和NO ₃，在研究开始时具有最强的减小趋势和关系周期2期间比周期1中更强对于NH ₄，有22％的气象站在第1阶段的浓度有统计学上的显着增加趋势。在第1阶段，第2阶段和整个研究中，湿降水NH ₄浓度的最大增长趋势发生在中北部地区。相比之下，北多岩石地区和南多岩石地区的NH ₄趋势陡峭约一半，而中南部和中西部地区的NH ₄趋势陡峭约三分之一。

我们分别比较了NADP站的SO ₄和NO ₃浓度趋势与二氧化硫和氮氧化物的排放量，以确定在NADP站附近的排放量与湿降水浓度趋势之间是否存在关系。在各个NADP站的 SO ₄浓度趋势与在750 km和以下范围内发电厂的总二氧化硫（SO ₂）和平均二氧化硫（SO ₂）排放之间存在统计学上的显着关系（r ² = 0.62-0.69，p <0.01）。每个车站1000公里。NO ₃之间也存在重要关系NADP站的浓度集中趋势和电厂的氮氧化物排放，但它们的强度 不如SO ₄强（r ² = 0.18–0.36，p <0.01），并且在1000 km和1500范围内的排放最强距离每个NADP站公里。在湿沉淀降低SO _4个浓度为在不同区域和通过时间比降低NO更加一致₃和SO _4度的变化趋势更密切相关的固定排放源比NO ₃的趋势。NH ₄有统计学上的显着增加正如先前研究中所报道的那样，湿降水的浓度很高，但是这项研究发现，在第一阶段，这些降水的增加最为明显，而且在美国各地并不一致。在研究期的前三年，在9个地区中的8个地区，湿沉淀酸度以SO ₄为主。到2017年，NO ₃在9个地区中的7个地区占主导地位。由于氮氧化物的排放量下降而氨气的排放量基本保持恒定，湿式沉淀法的NO ₃：NH ₄比率也有下降的趋势。这种变化导致9个地区中有7个地区的降水总氮浓度增加，这表明人们正在努力控制NH₃随着氮氧化物排放量的持续下降，排放量将变得越来越重要。