Chemical characteristics of PM2.5 during spring and fall at two sites in Chungcheongnam-do, South Korea; insight into Fe solubility and SO42− formation

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24-hr integrated PM2.5 were measured at two sites (Seosan and Asan) in Chungcheongnam-do, South Korea, during both spring and fall, with the following three objectives; i) to investigate the major chemical components driving PM2.5 pollution episodes and their formation processes, ii) to identify possible factors controlling aerosol Fe solubility, and iii) to examine the causes of high SO2 and SO42− measured at Asan site. The Seosan site is a place where the city, countryside, and agricultural and industrial complexes are mixed, and the Asan site is a place in which the city and industrial complexes are mixed.PM2.5 pollution episodes (>24-hr Korean PM2.5 of 35 μg/m3) during spring were dominated by local pollution, while during fall they were influenced by regional transport and local pollution. The major chemical components driving spring PM2.5 pollution episodes were NO3− for Seosan and SO42− and NO3− for Asan. On the other hand, in the fall, the increases in concentration of NO3− at both sites resulted in PM2.5 pollution episodes. During the spring and fall pollution episodes, favorable weather conditions such as low wind speed and high RH were likely an important factor for the enhanced formation of secondary inorganic aerosols. The difference in driving chemical composition leading to high PM2.5 between the two sites during pollution periods suggests that the formulation of a different control strategy is required to reduce PM2.5 concentrations at each site.Fe solubility (soluble Fe/total Fe) at Seosan and Asan were 6.2% and 5.9%, respectively, during spring, and they were 2.6% and 2.5%, respectively, during fall. Fe solubility was also enhanced during pollution episodes at both sites. The high correlations (R = 0.74–0.77) between Fe solubility and aerosol acidification [(SO42− + NO3−)/Fe] at both sites during the two seasons indicate that secondary acidic aerosol particles played a potential role in enhancing the solubility of Fe. However, Fe solubility during spring was more influenced by the acidification by SO42− particles than that by NO3− particles. Meanwhile, the opposite result was obtained for the Fe solubility during fall.The substantially increased SO42− concentration measured at the Asan site during spring was strongly associated with high SO2 emissions (∼45 ppb) from the upwind local source region and could be due to both the photochemical reaction in the afternoon and the aqueous-phase reaction between the evening and early morning hours. The results of this study suggest that reductions in SO2 and NOx emissions from local sources at the study region are absolutely required to reduce PM2.5 pollution episodes.