[Characteristics of Water-soluble Ions in Atmospheric Particulates in Farmland in the Suburb of Nanjing].

Water-soluble ions are an important component of atmospheric particles. However, there has been limited research on water-soluble inorganic ions in atmospheric particulates in agricultural areas until now. In this study, the characteristics of eight water-soluble inorganic ions （Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, NO₃^-, SO₄^2-, and Cl^-） in different sizes of atmospheric particulates （>9.0, 5.8-9.0, 4.7-5.8, 3.3-4.7, 2.1-3.3, 1.1-2.1, 0.65-1.1, 0.43-0.65, and <0.43 μm） were analyzed by sampling farmland in the suburb of Nanjing from December 2020 to November 2021. The results showed that the annual mass concentrations of total suspended particulate matter （TSP）, < 9.0 μm and < 2.1 μm particulates were 54.73, 49.04, and 27.35 μg·m^-3, respectively, all of which reached the national secondary standards. The mass concentration of < 2.1 μm particulates accounted for 50.0% of TSP, indicating that fine particles were the main components of atmospheric particulates in this area. The concentration of water-soluble inorganic ions in different sizes of particulates was highest in autumn, followed by those in spring/summer and winter. SO₄^2- was the primary contributor among all seasons and particle sizes, accounting for 30.6%-66.5%. The ratio of SO₄^2-/NO₃^- was much higher than 1, indicating that the contribution of fixed-source pollution played a dominant role in the study area. Moreover, SOR （SO₂ gas-particle conversion rate） in different particle sizes was less than 0.1 only at individual times in winter, indicating that SO₄^2- was mainly derived from the secondary oxidation of SO₂ in the study area. Significant positive correlations （P < 0.05） between SOR and temperature were observed except for 4.7-5.8 μm particulates, indicating that the increase in temperature significantly promoted the photochemical formation process of SO₄^2- in particles with different sizes. However, the relationship between SOR and relative humidity was significant only in fine particulates smaller than 2.1 μm （P < 0.05）, indicating that the liquid phase reaction was an important pathway for the formation of SO₄^2- in fine particles. Principal component analysis was used to analyze the sources of water-soluble inorganic ions in particulates. Moreover, it showed that water-soluble inorganic ions in fine particulates mainly came from the secondary transformation of SO₂, while those in the coarse particles mainly came from the soil dust during farmland tillage. The results of this study are of great significance for understanding the characteristics of water-soluble inorganic ion pollution in atmospheric particulate matter in agricultural areas. They provide a reference for understanding the source and formation mechanisms of atmospheric particulate matter pollution in agricultural areas near Nanjing.