Formation and Evolution of Secondary Particulate Matter During Heavy Haze Pollution Episodes in Northeast China in Winter

Based on the simultaneous observation of fine particulate matter (PM2.5) and its chemical components in four heavy haze pollution episodes at 14 sampling sites in northeast China from 2017 to 2019, the formation and existence of sulfate (SO42-) and nitrate (NO3-) secondary contaminants under different stages of the pollution episodes, and different meteorological and emission conditions were compared. The results yielded three main findings. (1) Organic carbon (OC) was the most important component of PM2.5, followed by NO3-,SO42-,and ammonium (NH4+). Nitrate surpassed sulfate as the most important secondary inorganic component over the study period. (2) The significant increase in atmospheric OC, SO42-, and NO3-concentrations was an important reason for haze formation. Meteorological factors such as wind direction, wind speed, temperature (T), relative humidity (RH), and atmospheric oxidability played an important role in secondary pollutant formation. (3) There were two potential SO42- formation mechanisms. The first was the gas-phase reaction of the hydroxyl radical(OH·) leading to the oxidation of nitrogen dioxide (NO2) and sulfur dioxide (SO2),and high ozone (O3) concentrations. A high atmospheric oxidability and high winter Ts were very important for SO42- formation. The second mechanism occurred under neutral or weakly alkaline conditions when large amounts of SO2 could enter aerosol droplets, and NO2 was more likely to react in the aqueous phase with SO2 to increase the output of SO42-. Nitrate formation was may be mainly due to the homogeneous gas-phase reaction of OH· with NO2, SO2, and ammonia(NH3). The highest NO3 concentration was observed under mild winter Ts, high RH, high atmospheric oxidability (O3 and nitrous acid (HONO)), high NH3 concentrations, and suitable light conditions. The differences in SO42- formation between northeast China and other regions were mainly a result of the suppression of the aqueous reaction of SO42- due to the low T in winter and low-sulfur coal emissions, which resulted in the gas-phase oxidation process with the highest SO42- production capacity becoming an important process. However, the aqueous reaction process was the most common mechanism of SO42- production in northeast China.