Chemical characteristics of long-term acid rain and its impact on lake water chemistry: A case study in Southwest China.

The chemical composition of acid rain and its impact on lake water chemistry in Chongqing, China, from 2000 to 2020 were studied in this study. The regional acid rain intensity is affected jointly by the acid gas emissions and the neutralization of alkaline substances. The pH of precipitation experienced three stages of fluctuating decline, continuous improvement, and a slight correction. Precipitation pH showed inflection points in 2010, mainly due to the total control actions of SO and NO implemented in 2011. The total ion concentrations in rural areas and urban areas were 489.08 µeq/L and 618.57 µeq/L, respectively. The top four ions were SO, Ca, NH and NO, which accounted for more than 90% of the total ion concentration, indicating the anthropogenic effects. Before 2010, SO fluctuated greatly while NO continued to rise; however, after 2010, both SO and NO began to decline rapidly, with the rates of -12.03 µeq/(L·year) and -4.11 µeq/(L·year). Because the decline rate of SO was 2.91 times that of NO, the regional acid rain has changed from sulfuric acid rain to mixed sulfuric and nitric acid rain. The lake water is weakly acidic, with an average pH of 5.86, and the acidification frequency is 30.00%. Acidification of lake water is jointly affected by acid deposition and acid neutralization capacity of lake water. Acid deposition has a profound impact on water acidification, and nitrogen (N) deposition, especially reduced N deposition, should be the focus of future research.