[Characteristics and Meteorological Factors of PM_2.5-O₃ Compound Pollution in Tianjin]

The characteristics, pollutant concentration distribution, and key meteorological factors of PM_2.5-O₃ compound pollution in Tianjin were analyzed based on the high-resolution online monitoring data of PM_2.5, O_3,and meteorological data observed in Tianjin from 2013 to 2019. Total PM_2.5-O₃ compound pollution was 94 days and showed a decreasing trend by year; a significant decreasing trend of PM_2.5-O₃ compound pollution days were observed in the early stage, with a decline rate of 52.2% from 2013 to 2015. By contrast, in the later period from 2016 to 2019, a fluctuating increasing trend of PM_2.5-O₃ compound pollution days of 16.7% was observed. PM_2.5-O₃ compound pollution days mainly occurred from March to September each year with substantial variation by year, mainly occurring in June to August from 2013 to 2016 and in April and September from 2017 to 2019. The peak value of ρ(O₃) (301-326 μg·m^-3) appeared when ρ(PM_2.5) ranged from 75 μg·m^-3 to 85 μg·m^-3. PM_2.5-O₃ compound pollution days accounted for 34.4% of total O₃ pollution events in Tianjin, which showed a decreasing trend by year. The peak O₃ concentration and average O₃ concentration during PM_2.5-O₃ compound pollution were higher than those during simplex O₃ pollution, and the number of days with PM_2.5 and O₃ as the primary pollutant decreased and increased in compound pollution days by year, respectively. The weather situation of PM_2.5-O₃ compound pollution was categorized into five weather types, namely low pressure, weak high pressure, rear of high pressure, front of cold front, and equalized pressure. The low pressure, front of cold front, and weak high pressure were observed most frequently, accounting for 92.5% of the total weather situation. The occurrence of PM_2.5-O₃ compound pollution was most probable when the dominant wind direction was the southwest and south, the average wind speed was less than 2 m·s^-1, the temperature was between 20-35℃, and the humidity was between 40%-60%.