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

PM pollution remains prominent in autumn, whereas O pollution gradually manifests in summer. To understand the dual high characteristics and meteorological effects of PM and O in the summer and early autumn of 2021 in the Beijing-Tianjin-Hebei and surrounding areas, the spatiotemporal distribution characteristics of PM and O 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 concentration and DPO （O daily maximum 8h mean minus O 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 concentration and DPO showed a gradual decrease from the middle to the north and south, with synchronous spatiotemporal changes. The hourly value of PM concentration presented a multimodal distribution, reaching the peak at 07：00-10：00 and 22：00-24：00. O concentration showed an opposite trend of change with PM concentration, reaching their peak from 14：00-16：00. When DPO and the concentration of PM 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. There was a high lag correlation between the subtropical high index and DPO. When the subtropical high was farther and stronger from the Beijing-Tianjin-Hebei and surrounding areas, DPO was higher. The major weather patterns with both high PM and O 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.