Impacts of compound extreme weather events on summer ozone in the Beijing-Tianjin-Hebei region

The Beijing-Tianjin-Hebei (BTH) region faces severe summer ozone (O3) pollution. In this study, we conducted a comprehensive analysis of O3 concentration variations during compound extreme (CE) weather events that occur simultaneously with atmospheric stagnation and heat wave during the summer period from 2015 to 2021. Our results showed that during CE events, the rate of O3 exceedance was nearly three times higher compared to non-CE days. O3 concentrations remained relatively stable during CE events lasting five days or less in regions other than Beijing, but when CE events extended beyond five days, both O3 concentration and the exceedance rate increased rapidly. Under CE conditions, daily O3 pollution persisted for a longer duration with 70.0% of daily O3 peaks surpassing 200 mu g m- 3, especially between 7 a.m. and 12 p.m., the time when CE events had a significant impact on O3 concentration characterized by a rapid rise in O3 concentration. The movement of the western Pacific subtropical high towards the east and its northward shift may contribute to the occurrence of favorable conditions for CE events in the BTH region, but meteorologically these results can be attributed to anticyclonic and high-pressure system anomalies during CE events in the BTH region, which are characterized by low total cloud cover, high temperature, low relative humidity, and high radiation. Positive anomalies in vertical velocity further contribute to favorable conditions for photochemical O3 formation, accumulation, and persistent O3 exceedances.