[Seasonal Variation of Water-soluble Ions in PM_2.5 in Xi'an]

To explore the seasonal variations and sources of water-soluble ions, PM_2.5 samples were collected from 2017 to 2018. Water-soluble ions including SO₄^2-, 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^-]/[SO₄^2-] ratio were explored. The results showed that dust, coal combustion, biomass burning, and secondary aerosols were the dominant contributors to water-soluble ions. Ca²⁺, SO₄^2-, NH₄⁺, and NO₃^- were the main water-soluble ions in PM_2.5 in Xi'an. Correlation analysis results showed that NH₄⁺ could not completely neutralize SO₄^2- in spring; unneutralized SO₄^2- 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^-]/[SO₄^2-] 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.