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

Abstract

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 SO42-, NO3-, NH4+, and Ca2+ 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 SO42- and NO3- in the rainfall were fully neutralized by NH4+ and Ca2+, 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 SO42- and 0.26 and 0.11 keq ha-1 yr-1 for NO3-, 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 NH4--N and 3.7 and 1.5 kg N ha-1 yr-1 for NO3--N, respectively, leading to a total nitrogen deposition of 16.4 kg N ha-1 yr-1. Moreover, the annual equivalent ratio of [SO42-]/[NO3-] 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 NH4-N/NO3--N was 2.3, reflecting the important role of agricultural activities in nitrogen deposition in Mianyang.