Seasonal Variation of Water-soluble Ions in PM2.5 in Xi'an

Abstract

To explore the seasonal variations and sources of water-soluble ions, PM2.5 samples were collected from 2017 to 2018. Water-soluble ions including SO42-, NO3-, Cl-, F-, Na+, Mg2+, NH4+, K+, and Ca2+ were determined via ion chromatography. Furthermore, the existing form of NH4+, nitrogen oxidation rate (NOR), sulfur oxidation rate (SOR), and [NO3-]/[SO42-] ratio were explored. The results showed that dust, coal combustion, biomass burning, and secondary aerosols were the dominant contributors to water-soluble ions. Ca2+, SO42-, NH4+, and NO3- were the main water-soluble ions in PM2.5 in Xi'an. Correlation analysis results showed that NH4+ could not completely neutralize SO42- in spring; unneutralized SO42- could be mainly combined with K+ and Ca2+. NH4+ mainly existed in the form of ① NH4HSO4 and (NH4)2SO4 in summer; ② NH4HSO4 and NH4NO3 in autumn; and ③ (NH4)2SO4 and NH4NO3 in winter. The yearly mean values of SOR and NOR were 0.35 and 0.16, respectively, indicating a high secondary aerosol transformation rate during the study period. The [NO3-]/[SO42-] ratio showed Xi'an was mainly affected by stationary sources in spring and summer, while the contribution of mobile sources in autumn and winter was greater than stationary sources.