Characteristics and source analysis of water-soluble ions of PM_2.5 during spring in Mang City, Yunnan Province

To understand the chemical characteristics of water-soluble ions in PM_2.5 in spring in Mang City, Yunnan Province, PM_2.5 samples were collected at three monitoring stations in Mang City from March 11 to April 1, 2019 and 9 water-soluble ions (SO_4^2-、NO_3^-、Cl^-、F^-、NH_4^+、K⁺、Na⁺、Ca²⁺、Mg²⁺) were analyzed. At the same time, the main sources of water-soluble ions in PM_2.5 were discussed by principal component analysis (PCA). The results showed that: The daily mean value of PM_2.5 was 25.5-84.1 μg/m³ during the sampling period in spring, most of which less than the level II standard limit of Ambient Air Quality Standards (GB 3095-2012). SNA (collectively referred to SO_4^2-、NO_3^- and NH_4^+) accounted for 84.1% of the total water-soluble ions, indicating that SNA dominated the main fraction of PM_2.5. The proportion of SO_4^2- in total water-soluble ions was the highest, about 52.8%, and the largest contribution to secondary ions. SOR (sulfur oxidation rate) was 0.58, indicating that there was an obvious secondary conversion process from SO₂ to SO_4^2- in the atmosphere. SO_4^2-/NO_3^->1 indicated that the contribution of stationary source to water-soluble ions was greater than that of mobile source. The correlation analysis of water-soluble ions showed that Cl ^- had a good correlation with Ca²⁺ and Mg²⁺, indicating that Ca²⁺、Mg²⁺ may exist in the forms of CaCl₂ and MgCl₂ in atmospheric particulate matter. The correlation coefficients between K⁺ and NO_3^- was 0.81, which indicated that K⁺ may exist as KNO₃. NH_4^+, NO_3^- and SO_4^2- mainly existed in the form of (NH₄)₂SO₄ and NH₄NO₃. The anion to cation micro-equivalent ratio of PM_2.5 was 0.82, indicating that PM_2.5 was weakly alkaline. The PCA showed that the main sources of water-soluble ions in PM_2.5 in spring in Mang City were dust source, biomass combustion source and secondary inorganic aerosol.