Relationship between water-soluble ions in PM2.5 and their precursor gases in Seoul megacity

In this study, fine-particle (PM2.5) ionic composition in the city of Seoul was investigated using short-term measurement data (1 h interval) collected during the year 2010. The partitioning of semi-volatile inorganic species and phase state of aerosols were studied using ISORROPIA II thermodynamic equilibrium model. The analysis of ionic composition identified the relative dominance of NO 3 − , SO 4 2 − , and NH 4 + in the fine particles. The presence of strong correlation between these components indicates similarity of their formation pathways. Seasonal patterns of these ionic components and their gas-phase precursors were somewhat different from each other. The observed SO 4 2 − / SO 2 mass ratio indicated a significant role of the gas-phase oxidation of SO2 (to H2SO4) in the SO 4 2 − formation. Moreover, both SO42− and NO3− were almost completely neutralized by NH4+ to form (NH4)2SO4 and NH4NO3. The formation of NO3− aerosol would be ammonia-sensitive (i.e., deficit ammonia), as evidenced by smaller total ammonia concentration than the sum of total sulfate and nitrate concentrations. The uptake of SO2 on dust particles during Asian dust event is also likely to exert controls on SO42− concentrations.