Rainfall Chemistry and Total Wet and Dry Deposition of Acidity and Nitrogen in Mianyang, Southwest China

Wet and dry deposition samples were collected from September 2020 to August 2021 at a suburban site in Mianyang, Southwest China.Major water-soluble inorganic ions (WSIIs) in daily rainfall samples and integrated dry deposition samples were determined using ion chromatography.In rainfall, the annual volume-weighted mean (VWM) concentrations of SO 4 2-, NO 3 -, NH 4 + , and Ca 2+ were 30.1, 28.7, 64.9, and 31.8 μeq L -1 , respectively.The annual values of acid-neutralizing capacity and fractional acidity were 37.8 μeq L -1 and 0.0035, respectively, indicating that SO 4 2-and NO 3 -in the rainfall were fully neutralized by NH 4 + and Ca 2+ , which was supported by the annual rainfall pH of 6.7.The annual wet and dry deposition fluxes were 0.27 and 0.09 keq ha -1 yr -1 for SO 4 2-and 0.26 and 0.11 keq ha -1 yr -1 for NO 3 -, respectively, resulting in a total acid deposition flux of 0.73 keq ha -1 yr -1 , which approximated the maximum critical load of acidity in Sichuan province.The annual wet and dry deposition fluxes were 8.3 and 2.9 kg N ha -1 yr -1 for NH 4 + -N and 3.7 and 1.5 kg N ha -1 yr -1 for NO 3 --N, respectively, leading to a total nitrogen deposition of 16.4 kg N ha -1 yr -1 .Moreover, the annual equivalent ratio of [SO 4 2-]/[NO 3 -] in rainfall was 1.1, suggesting that the acid deposition in Mianyang was a mixed type caused by sulfuric and nitric acids.The annual mass ratio of NH 4 + -N/NO 3 --N was 2.3, reflecting the important role of agricultural activities in nitrogen deposition in Mianyang.