Long-term observations of the chemical composition, fluxes and sources of atmospheric wet deposition at an urban site in Xi’an, Northwest China

Abstract

Atmospheric wet deposition (AWD) is closely related to air quality, and excessive deposition poses risks to ecological systems and human health. Seasonal and interannual variations in acidity, electric conductivity (EC), ionic composition, fluxes, sources, and atmospheric transport of AWD were analyzed at an urban site in Xi'an from 2016 to 2019. The annual volume-weighted mean (VWM) pH and EC values were 6.8 and 40.6 μS cm-1, respectively. NO3- (47%) was the most dominant anion, while Ca2+ (34%) was the most dominant cation. The analysis of fractional acidity (FA) and neutralization factors (NFs) showed that 96% of the acidity was neutralized by alkaline constituents, especially Ca2+ and NH4+. The annual AWD flux of total ions was 125.9kgha-1year-1, and NO3-, NO2-, SO42- and NH4+ fluxes accounted for approximately 70%, indicating considerable sulfur (9.1kgha-1year-1) and nitrogen (22.0kgha-1year-1) deposition. Under dilution by precipitation, the EC and major ion concentrations were lower, while the pH and fluxes were higher, in summer and autumn, and the opposite results were observed in spring and winter. The source apportionment via by positive matrix factorization (PMF) revealed that the six sources of major ions were confirmed as follows: vehicular emissions (38.1%), agriculture (22.3%), fossil fuel combustion (13.8%), crust (12.9%), marine (9.6%), and biomass burning (3.3%). And on the basis of back trajectory analysis, the air masses of precipitation were primarily from the northwest in spring and winter, from the southeast in summer, and from various directions in autumn, and they transported different natural and anthropogenic pollutants along their paths, thereby affecting the chemical composition and fluxes of AWD.