Through synthesizing bulk precipitation chemistry in forest sites (n = 128) from three monitoring networks, (NADP in Northern America, EMEP in Europe, and EANET in East Asia), this study quantifies the temporal changes of precipitation acidity and its dominant acidifying agents over the last two decades. Results show distinct declines of sulfate and nitrate depositions and increases of precipitation pH in northeast America and central and east Europe, but not in Asia during 1999 and 2018. The decreases of sulfate and nitrate depositions likely reflect the long-term effort of pollutant emission controls. The temporal pattern of sulfate (SO42−)/nitrate (NO3−) and ammonium nitrogen (NH4-N)/nitrate nitrogen (NO3-N) equivalent ratios indicate that acid rain in the NADP and EMEP have transitioned from sulfate-dominated to nitrate-dominated, and the DIN deposition has shifted from nitrate-dominated to ammonium-dominated in recent years, owing to reductions of sulfur dioxides (SO2) and nitrogen oxides (NOx) emissions. In contrast, sulfate still plays a dominant role on the acidity of precipitation than nitrate in Asia, and NH4-N deposition also has a significant contribution in N flux due to increasing trends of ammonia emissions in Southeast Asia.
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