Temporal variation of sulfate concentration in PM2.5 and major factors enhancing sulfate concentration in the atmosphere of Seoul, Korea

Despite frequent observations of high particulate matter (PM) concentrations in Seoul, Korea, the sulfate (SO42-) concentrations in PM with an aerodynamic diameter below or equal to 2.5 mu m (PM2.5) in Seoul have been reduced during the 1990s. However, there has been no decreasing trend during the 2000s. Despite the very low ambient SO2 concentrations, high SO42- concentration episodes frequently occur in Seoul; however, the reasons for this remain poorly understood. In the present study, we sought to determine the major factors that enhance sulfur oxidation in Seoul by analyzing measurement data from an intensive air quality monitoring station in the Seoul metropolitan area acquired in 2015. Additionally, we aimed to distinguish the contributions of local sources and long-range transport to the elevated SO42- concentrations in PM2.5 in Seoul. In some cases in March and October 2015, the molar ratio of SO42- to SO2, which reflects sulfur partitioning between the particle and gas phases, showed an exponential increase with an increase of relative humidity (RH). However, despite the fact that the RH was high during August 2015, no statistically significant relationship was observed between RH and the molar ratio of SO42- to SO2 during this month. In August 2015, the molar ratio of SO42- to SO2 showed an exponential increase with increasing O-3 concentration. Therefore, under sufficient RH, the concentrations of radical oxidants might be the most important factor that underlies the increase in ambient SO42- concentrations. No significant relationship was observed between the molar ratio of SO42- to SO2 and O-3 in October. Furthermore, using Potential Source Density Function (PSDF) and backward trajectory analysis, the high SO42- concentrations in March 2015 were found to be caused by the transportation of air pollutants from China and local sources around Seoul. The high SO42- concentrations in August 2015 are concluded to have been greatly affected by these power plant emissions in the Taean area on the west coast of Korea. The high SO42- concentrations in October 2015 appear to have been caused by the transportation of air pollutants from North Korea. In summary, the high SO42- concentration in Seoul is affected by long-range transport from China and by the emissions from around the Seoul metropolitan area, such as Taean or North Korea.