[Source and Cause Analysis of High Concentration of Inorganic Aerosol During Two Typical Pollution Processes in Winter over Tianjin].

Inorganic aerosol is the main component of haze days in winter over Tianjin. In this study, two typical high concentrations of secondary inorganic aerosol (SIA) processes, defined as CASE1 and CASE2, were selected during polluted days in January 2020 over Tianjin, and the effects of meteorological factors, regional transport, and chemical processes were comprehensively investigated combined with observations and numerical models (WRF-NAQPMS). The average SIA concentrations in CASE1 and CASE2 were 76.8 μg·m and 66.0 μg·m, respectively, and the nitrate concentration was higher than that of sulfate and ammonium, which were typical nitrate-dominated pollution processes. Meteorological conditions played a role in inorganic aerosol formation. The temperature of approximately -6-0℃ and 2-4℃ and the relative humidity of 50%-60% and 80%-100% would be suitable conditions for the high SIA concentration (>80 μg·m) in CASE1, whereas the temperature of approximately 2-4℃ and the relative humidity of 60%-70% would be suitable in CASE2. The average contribution rates of external sources to SIA in the CASE1 and CASE2 processes were 62.3% and 22.1%, which were regional transport-dominant processes and local emission-dominant processes, respectively. The contribution of the local emission of CASE1 to nitrate and sulfate was 16.2 μg·m and 8.2 μg·m, respectively, higher than that of external sources (31.7 μg·m and 8.8 μg·m). the local contribution of CASE2 to nitrate and sulfate was 29.3 μg·m and 25.1 μg·m, respectively, whereas the contribution from external sources was 8.1 μg·m and 9.4 μg·m, respectively. The quantitative result indicated that local formation and regional transport resulted in higher nitrate concentration than sulfate in CASE1, in contrast to only local sources in CASE2. The gas phase reaction was the main source of inorganic aerosol formation, contributing 48.9% and 57.8% in CASE1 and CASE2, respectively, whereas the heterogeneous reactions were also important processes, with contribution rates of 48.1% and 42.2% to SIA. The effect of aqueous phase reaction was negligible.