[Effects of Meteorological Conditions in Summer and Early Autumn on PM2.5 and O3 Pollution in Beijing-Tianjin-Hebei and Surrounding Areas].

PM_2.5 pollution remains prominent in autumn, whereas O₃ pollution gradually manifests in summer. To understand the dual high characteristics and meteorological effects of PM_2.5 and O₃ in the summer and early autumn of 2021 in the Beijing-Tianjin-Hebei and surrounding areas, the spatiotemporal distribution characteristics of PM_2.5 and O₃ concentrations, as well as meteorological conditions, subtropical high index, and weather situation in the Beijing-Tianjin-Hebei and surrounding areas were analyzed. The results showed that PM_2.5 concentration and DPO₃ （O₃ daily maximum 8h mean minus O₃ concentration at 06：00） from June to September 2021 decreased compared with those in the same period in 2020 and 2022, and high concentrations were mainly occurring in June. The overall PM_2.5 concentration and DPO₃ showed a gradual decrease from the middle to the north and south, with synchronous spatiotemporal changes. The hourly value of PM_2.5 concentration presented a multimodal distribution, reaching the peak at 07：00-10：00 and 22：00-24：00. O₃ concentration showed an opposite trend of change with PM_2.5 concentration, reaching their peak from 14：00-16：00. When DPO₃ and the concentration of PM_2.5 were high, the characteristics of near-surface meteorological elements were characterized by temperatures ranging from 24.0-28.0℃, relative humidity concentrated at 58.4%-76.3%, and wind speeds ranging from 1.5-3 m·s^-1. There was a high lag correlation between the subtropical high index and DPO₃. When the subtropical high was farther and stronger from the Beijing-Tianjin-Hebei and surrounding areas, DPO₃ was higher. The major weather patterns with both high PM_2.5 and O₃ concentrations in the study area were near surface low-pressure front, high-pressure type, and frontal type. The high altitude was controlled by high-pressure ridges, and the subtropical high ridge line was stable between 21°-28°N.