With the implementation of the Clean Air Act Amendments of 1990 in the U.S. and similar emission reduction measures in Canada, sulfur dioxide (SO₂) and nitrogen oxides (NO_x = NO + NO₂) emissions in the eastern U.S. and Eastern Canada declined significantly during the period 1989–2016. Correspondingly, air quality over the region improved tremendously and sulfate, nitrate and acidity in precipitation decreased significantly. In this study, we analyzed measurement data in the eastern U.S. and Eastern Canada from the two national long-term wet deposition networks, the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) in the U.S. and the Canadian Air and Precipitation Monitoring Network (CAPMoN) in Canada, to reveal the spatial-temporal trends of precipitation concentrations and wet deposition of sulfate, nitrate, ammonium and acid in the eastern part of North America (broken into 4 different regions) for the period of 1989–2016. Comparing 2014–2016 to 1989–1991, the results show that the acid, sulfate and nitrate wet deposition decreased by 78%, 69% and 46% for 1989–2016, and decreased annually at rates of −2.8~-3.6% yr⁻¹, -2.5~-2.9% yr⁻¹, and -1.6~-2.0% yr⁻¹ over different regions, respectively. It also revealed that there were no statistically significant trends for the annual mean wet concentrations of ammonium (NH₄⁺) in precipitation in the Midwest and the Mid-Atlantic of the U.S. (P>0.4 respectively). However, there were statistically significant increasing trends of annual wet deposition of NH₄⁺ over these latter two regions (P = 0.01–0.13) with an annual rate of increase of 0.6% and 0.4%, respectively. The increasing trends of wet deposition of NH₄⁺ in these regions were mostly due to increasing annual precipitation amounts, as indicated by positive correlation coefficients between them (R = 0.67–0.77), and a statistically significant increasing trend of the precipitation amount during the period. The annual mean wet concentration of chloride ion (Cl⁻) in precipitation in the above two regions had a clear decreasing trend of −1.5% annually (P < 0.001). In 20 years (2010–2014 vs. 1990–1994), the frequency of acidic rain with pH < 5 was reduced from 72% to 49% in the Northeast U.S. and Eastern Canada, 88% to 37% in the Midwest, 97% to 60% in the Mid-Atlantic and 79% to 49% in the Southeast. The imbalance between the measured cations and anions increased over the years from 2009 to 2016 with the anions dropping lower than the cations in all regions. This can be attributed to the increased dissociation of weak organic acids, i.e., the free H⁺ contributed from these weak acids increased as the acidity of the precipitation decreased, but the corresponding conjugate bases were not measured and not included in the calculation of total anions. This pH buffering effect needs to be taken into account in assessing the reductions in acid deposition following the implementation of the Clear Air Act Amendments (CAAA) of 1990 in the U.S. and measures to reduce the emissions of SO₂ and NO_x in Canada.

随着美国1990年《清洁空气法修正案》的实施以及加拿大类似的减排措施的实施，二氧化硫（SO ₂）和氮氧化物（NO _x  = NO + NO ₂1989-2016年期间，美国东部和加拿大东部的排放量显着下降。相应地，该地区的空气质量大大改善，降水中的硫酸盐，硝酸盐和酸度显着下降。在这项研究中，我们分析了美国东部和加拿大东部两个国家长期湿沉降网络，美国国家大气沉积计划/国家趋势网络（NADP / NTN）以及加拿大空气和降水监测的测量数据网络（CAPMoN）在加拿大，以揭示1989-2016年期间北美东部地区（分为4个不同区域）的降水浓度以及硫酸盐，硝酸盐，铵和酸的湿沉降的时空趋势。将2014–2016年与1989–1991年进行比较，结果表明，^-1，-2.5〜^- 2.9％yr ^-1和-1.6〜-2.0％yr ^-1在不同区域。它还显示，美国中西部和大西洋中部地区降水中氨的年平均湿浓度（NH ₄ ⁺）没有统计学上的显着趋势（分别为P > 0.4）。但是，在后两个区域中，NH ₄ ⁺的年湿沉降有统计学上的显着增长趋势（P  = 0.01-0.13），年增长率分别为0.6％和0.4％。NH ₄ ⁺湿沉降的增长趋势这些地区的降雨主要是由于年降水量增加所致，如它们之间的正相关系数所示（R  = 0.67–0.77），以及该期间降水量具有统计学意义的增长趋势。氯离子的年平均湿浓度（CL ^- ）在上述两个区域中沉淀每年具有-1.5％的明确减少的趋势（P  <0.001）。在20年内（2010-2014年与。1990-1994年），pH值小于5的酸性降雨的频率在美国东北部和加拿大东部从72％降至49％，中西部从88％降至37％，在大西洋中部97％至60％东南地区为79％至49％。从2009年到2016年，被测阳离子和阴离子之间的不平衡加剧，阴离子的下降幅度低于所有地区的阳离子。这可以归因于弱有机酸（即游离H ⁺这些弱酸的贡献随着沉淀酸度的降低而增加，但未测量相应的共轭碱，也未计入总阴离子的计算中。在评估美国1990年实施的《空气清洁法修正案》（CAAA）之后的酸沉积减少量以及加拿大减少SO ₂和NO _x排放量的措施时，必须考虑这种pH缓冲作用。