[Evolution of PM_2.5 and O₃ Pollution in the Qinhuangdao City During Sustained Air Quality Improvement from 2018 to 2022].

In order to understand the characteristics of PM_2.5 and O₃ composite pollution in the Qinhuangdao City, analyses were conducted based on ambient air pollutant concentration, meteorological, and PM_2.5 chemical component data in the Qinhuangdao City from 2018 to 2022. The results revealed that the annual mean, seasonal mean, number of polluted days, and maximum daily mean of PM_2.5 showed an overall decreasing trend from 2018 to 2022. The annual and spring-autumn averages fell by 24.32%, 31.65%, and 21.05%, respectively. The average PM_2.5 mass concentration decreased by 10.39% on PM_2.5-polluted days. According to the meteorological elements, the prevailing wind direction was westerly in spring and autumn and during all pollution periods. During the PO composite pollution period, the wind direction of the high concentration pollutants was dominated by the southwest wind, and the wind speed was generally less than 2 m·s^-1 during each period. Temperatures at the time of moderate PM_2.5 pollution were close to the mean spring and autumn temperatures, and temperatures at PO3 and PO4 were higher than the autumn mean and lower than those at the time of O. Among the water-soluble ions in spring and autumn, the mass concentration of WSIIs decreased by 34.99% during spring, and the mass concentration of WSIIs was similar during autumn from 2019 to 2022. Among the carbon fractions, OC mass concentrations were similar during autumn, while EC, spring OC and EC mass concentrations decreased by 41.11%, 28.18%, and 40.29%, respectively. SNA mass concentrations in water-soluble ions accounted for more than 90% of WSIIs in all pollution periods, and SNA mass concentrations were higher in PO than in O during spring and autumn. For the same types of pollution, Cl^- mass concentrations were higher during spring than during autumn. Among the carbon fractions, OC mass concentrations were highest during the periods of moderate PM_2.5 pollution （P3 and P4）, and SOC mass concentrations were higher during PO than during the P and O periods during spring and autumn. The values of OC/EC and EC/TC showed that the carbon component was mainly obtained from fossil fuel combustion in the P and PO periods and from biomass combustion and fossil fuel combustion in the O period.