[Characterization of PM

Based on hourly concentration data of PM2.5 and ozone （O3） from 2015 to 2022, as well as meteorological reanalysis data, this study categorized complex pollution days into light, moderate, and severe based on PM2.5 concentration in the presence of O3 pollution. It examined the temporal evolution and meteorological characteristics of different levels of complex pollution in the Beijing-Tianjin-Hebei （BTH） Region. The results indicated that the number of light, moderate, and severe complex pollution days were 149, 120, and 52, respectively, primarily concentrated in April and June. Changes in meteorological factors can affect pollutant concentrations, similar to O3 formation conditions, high temperatures, strong solar radiation, and low humidity-facilitated complex pollution as well. Additionally, extensive and intense downdrafts further exacerbated complex pollution. The ventilation coefficient for severe complex pollution was higher than that for moderate pollution, suggesting that the transportation of pollutants may be a key factor in exacerbating complex pollution. The intensification of complex pollution was accompanied by a substantial increase in the contrabution rate of secondary PM2.5.