[Characteristics and Formation Mechanism of Three Haze Pollution Processes in Chengdu in Winter]

Based on the online monitoring data of gaseous pollutants and components in PM_2.5 from Chengdu super observatory of atmospheric environment, the meteorological factors and component characteristics of three haze pollution process in Chengdu from 2019 to 2020 were analyzed. The CMB model was adopted to simulate the sources and variation trends of PM_2.5 pollution during the study period, and the causes of each pollution process were analyzed. The results showed that all the three pollution processes occurred under adverse meteorological conditions, where the relative humidity and temperature continued to rise and the wind speed and boundary layer height continued to decrease. The average daily relative humidity was greater than 70%, average daily temperature was greater than 8℃, average daily wind speed was less than 0.8 m ·s^-1, and average daily boundary layer height was less than 650 m. During the three events of pollution, the main components were NO₃^-, OC, NH₄⁺, and SO₄^2-. Among them, the mass concentration and proportion of NO₃^- increased by 1.47-2.09 and 0.22-0.35 times, respectively, during the pollution period as compared to those during the clean period. NO₃^- was a key component of PM_2.5 pollution during winter in Chengdu. During the three pollution processes, the mean values of SOR and NOR were 0.40 and 0.27, respectively, and the secondary transformation degree of SO₂ and NO was high. The conversion of SO₂ to SO₄^2- was mainly dominated by heterogeneous oxidation at night, and the conversion of NO to NO₃^- was dominated by heterogeneous hydrolysis. The characteristics of the three processes were slightly different. Process Ⅰ showed evident secondary nitrate-dominated characteristics. During the period of rising PM_2.5 concentration in process Ⅱ, it was mainly affected by coal emissions, but during the periods of high PM_2.5 concentration, it was mainly affected by NO₃^-. Process Ⅲ was also a nitrate-dominated process, but emissions of fossil fuel combustion had increased during certain polluted periods. Secondary nitrate, secondary sulfate, motor vehicles, and coal combustion were the main pollution sources during the study period. The PM_2.5 concentration was positively correlated with the contribution of secondary nitrate and negatively correlated with the contribution of dust source.