In the present study, 11 size classes of particulate matter were collected from a semi-rural site in the industrial city of Ulsan, South Korea in 2019 to investigate the size distribution and day-night variation of water-soluble inorganic ions (WSIIs). Approximately 70% of the detected WSIIs were found in fine particles, with Na + , Ca 2+ , Cl − , and NO 3 − dominant (~70%) in coarse particles and SO 4 2− , NO 3 − , and NH 4 + (SNA) dominant (~70%) in fine particles. Monthly variation in total WSIIs was observed, with the highest average concentration found in April for both coarse (2.71 μg/m 3 ) and fine particles (5.75 μg/m 3 ). Given that this month is characterized by prevailing southeasterly winds, these high levels may represent the adverse effects of industrial activity.Coefficients of divergence for individual WSIIs exhibited large variability, which is indicative of significant day-night differences in meteorology, chemistry, and source contributions. The size distributions of Na + , K + , F − , Cl − , NO 3 − , and SO 4 2− were bimodal, while Mg 2+ , Ca 2+ , and NH 4 + were unimodal. Peaks were generally found at 1.8-5.6 μm and 0.18-0.56 μm for coarse and fine particles, respectively. By plotting the linear relationship between the mole charge ratios of SNA, we found that coarse NO 3 − particles were mainly the result of heterogeneous reactions between HNO 3 and sea salts or crustal species, while the homogeneous reaction of HNO 3 and NH 3 played a crucial role in the formation of fine NO 3 − particles. In conclusion, though day-night variation in WSIIs was apparent, especially in SNA, the formation pathways for both coarse and fine particles were similar for day and night. Based on the verification of the formation mechanisms for WSIIs at this semirural site, the influence of industrial activity on the secondary formation of PM in urban and industrial areas can be investigated further.
