Day-night variation and size distribution of water-soluble inorganic ions in particulate matter in Ulsan, South Korea

Abstract

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 NO3- dominant (similar to 70%) in coarse particles and SO42-, NO3-, and NH4+ (SNA) dominant (similar to 70%) in fine particles. Monthly variation in total WSIIs was observed, with the highest average concentration found in April for both coarse (2.71 mu g/m(3)) and fine particles (5.75 mu g/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-, NO3-, and SO42- were bimodal, while Mg2+, Ca2+, and NH4+ were unimodal. Peaks were generally found at 1.8-5.6 mu m and 0.18-0.56 mu m for coarse and fine particles, respectively. By plotting the linear relationship between the mole charge ratios of SNA, we found that coarse NO3- particles were mainly the result of heterogeneous reactions between HNO3 and sea salts or crustal species, while the homogeneous reaction of HNO3 and NH3 played a crucial role in the formation of fine NO3- 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 semi-rural site, the influence of industrial activity on the secondary formation of PM in urban and industrial areas can be investigated further.