Comparative study on water-soluble inorganic ions in PM_2.5 from two distinct climate regions and air quality

Recently, air quality has significantly improved in developed country, but that issue is of concern in emerging megacity in developing country. In this study, aerosols and their precursor gas were collected by NILU filter pack at two distinct urban sites during the winter and summer in Osaka, Japan and dry and rainy seasons in Ho Chi Minh City (HCMC), Vietnam. The aims are to investigate the contribution of water-soluble inorganic ions (WSIIs) to PM_2.5, thermodynamic characterization and possible formation pathway of secondary inorganic aerosol (SIA). The PM_2.5 concentration in Osaka (15.8 μg/m³) is lower than that in HCMC (23.0 μg/m³), but the concentration of WSIIs in Osaka (9.0 μg/m³) is two times higher than that in HCMC (4.1 μg/m³). Moreover, SIA including NH₄⁺, NO₃^- and SO₄^2- are major components in WSIIs accounting for 90% and 76% (in molar) in Osaka and HCMC, respectively. Thermodynamic models were used to understand the thermodynamic characterization of urban aerosols. Overall, statistical analysis results indicate that very good agreement (R² > 0.8) was found for all species, except for nitrate aerosol in HCMC. We found that when the crustal species present at high amount, those compositions should be included in model calculation (i.e. in the HCMC situation). Finally, we analyzed the characteristics of NH₄⁺- NO₃^-- SO₄^2- system. A possible pathway to produce fine nitrate aerosol in Osaka is via the homogeneous reaction between NH₃ and HNO₃, while non-volatile nitrate aerosols can be formed by the heterogeneous reactions in HCMC.