[Spatiotemporal Evolution Characteristics of PM_2.5-O₃ Compound Pollution in Chinese Cities from 2015 to 2020]

Based on the monitoring data of PM_2.5 and O₃ concentrations in 333 cities in China from 2015 to 2020, using spatial clustering, trend analysis, and the geographical gravity model, this study quantitatively analyzed the characteristics of PM_2.5-O₃ compound pollution concentrations and its spatiotemporal dynamic evolution pattern in major cities in China. The results showed that:① there was a synergistic change in PM_2.5 and O₃ concentrations. When ρ(PM_2.5_mean) ≤ 85 μg·m^-3, for every 10 μg·m^-3 increase in ρ(PM_2.5_mean), the peak of the mean value of ρ(O₃_perc90) increased by 9.98 μg·m^-3. When ρ(PM_2.5_mean) exceeded the national Grade II standards of (35±10) μg·m^-3, the peak of the mean value of ρ(O₃_perc90) increased the fastest, with an average growth rate of 11.81%. In the past six years, on average, 74.97% of Chinese cities with compound pollution had a ρ(PM_2.5_mean) in the range of 45 to 85 μg·m^-3. When ρ(PM_2.5_mean)>85 μg·m^-3, the mean value of ρ(O₃_perc90) showed a significant decreased trend. ② The spatial clustering pattern of PM_2.5 and O₃ concentrations in Chinese cities was similar, and hot spots of the six-year mean values of ρ(PM_2.5_mean) and ρ(O₃_perc90) were distributed in the Beijing-Tianjin-Hebei urban agglomeration and other cities in the Shanxi, Henan, and Anhui provinces. ③ The number of cities with PM_2.5-O₃ compound pollution showed an interannual variation trend of increasing first (2015-2018) and then decreasing (2018-2020) and a seasonal trend of gradually decreasing from spring to winter. Further, the compound pollution phenomenon mainly occurred in the warm season (April to October). ④ The spatial distribution of PM_2.5-O₃ compound polluted cities was changing from dispersion to aggregation. From 2015 to 2017, the compound polluted areas spread from the eastern coastal areas to the central and western regions of China, and by 2017, a large-scale polluted area centered on the Beijing-Tianjin-Hebei urban agglomeration, the Central Plains urban agglomeration, and surrounding areas was formed. ⑤ The migration directions of PM_2.5 and O₃ concentration centers were similar, and there were obvious trends of moving westward and northward. The problem of high-concentration compound pollution was concentrated and highlighted in cities in central and northern China. In addition, since 2017, the distance between the centers of gravity of PM_2.5 and O₃ concentrations in the compound polluted areas had been significantly reduced, with a reduction of nearly 50%.