Atmospheric sulfate formation in the Seoul Metropolitan Area during spring/summer: Effect of trace metal ions.

ABSTRACT

Despite the effort to control SO2 emission, sulfate is still one of the major inorganic components of PM2.5 in urban area. Moreover, there is still a lack of understanding of the sulfate formation mechanism via SO2 oxidation under various ambient conditions. In this study, we focus on sulfate formation during a haze pollution episode in the spring/summertime of 2016 in Seoul Metropolitan Area (SMA). During the pollution episode, PM2.5 mass concentration exceeded over 60 µg m-3, and sulfate accounted for about 25% of the total PM2.5 mass concentration. A sharp increase of sulfur oxidation ratio (SOR) values along with aerosol liquid water content (AWC) under humid conditions could be ascribed to an apparent contribution of aqueous-phase oxidation of SO2 of sulfate formation during the pollution period. Comparisons of SOR values with four representative oxidants for the aqueous-phase oxidation (i.e., NO2, H2O2, O3, and TMIs) indicated that TMIs concentration, especially for Mn (II), showed the best positive correlation. Furthermore, for calculating the sulfate production rate, the contribution of TMIs concentration was found to be dominant within the pH range in SMA (2.1-3.0), which was determined by the chemical composition and derived AWC. These results imply that not only the SO2 emission but also other chemical components (e.g., TMI and nitrate) would play a critical combined role in sulfate formation under urban haze condition.