Pollution characteristics of PM2.5 during high concentration periods in summer and winter in Ulsan, the largest industrial city in South Korea

Abstract

High PM2.5 episodes frequently occur in Northeast Asia, and the source−receptor relationship for PM2.5 in megacities is a critical issue. As the largest industrial city in South Korea, Ulsan suffers from frequent high PM2.5 episodes. However, studies on the long-range atmospheric transport (LRAT), local emissions, and secondary formation of PM2.5 in Ulsan have been limited. In this study, the characteristics of high PM2.5 episodes in Ulsan were interpreted using hourly data for PM2.5 components. The periods with the highest PM2.5 concentrations in winter 2014 (February 24–26; 99.3 ± 18.6 μg/m3) and summer 2014 (June 24–27; 49.9 ± 12.3 μg/m3) were designated as Pollution Periods 1 and 2, respectively. In general, secondary inorganic ions (SO42−, NO3−, and NH4+; SNA) were generated by the liquid phase reaction of water-soluble materials during winter, and sulfate and secondary organic aerosols were mainly formed via photochemical reactions during summer. During Pollution Period 1, the concentrations of sulfate, organic carbon, and elemental carbon sharply increased, and three major sources were identified: (1) LRAT from fossil fuel and biomass burning in eastern China and North Korea, (2) the influence of petrochemical and non-ferrous industrial facilities in Ulsan, and (3) enhanced secondary formation of ammonium sulfate and organic aerosols due to air stagnation. During Pollution Period 2, the concentration of SNA and heavy metals sharply increased, and three pollution sources were identified: (1) the influence of local industrial facilities and ship emissions, (2) external inflow from thermal power stations and national industrial facilities in southern coastal cities, and (3) secondary organic and inorganic formation. In this study, the reasons for the high winter and summertime PM2.5 events in Ulsan were more clearly understood, which can be the basis for the establishment of PM2.5 management policies that consider LRAT, local primary emissions, and secondary formation.