Divergent impacts of summertime extreme heat on O₃ and PM_2.5 levels in the Beijing-Tianjin-Hebei and Yangtze River Delta regions: Driving chemical and physical processes

As global warming increases the frequency of summertime extreme heat events in China, understanding their impact on secondary pollutants is essential for air quality management. This study investigates the influence of extreme heat (defined as a daily maximum temperature exceeding 35 °C) on surface ozone (O₃) and fine particulate matter (PM_2.5) in the Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD) regions. We analyze observational data from 2013 to 2022 and perform GEOS-Chem model simulations for 2013–2020. During extreme heat, enhanced chemical production at 850–950 hPa strongly promotes O₃ formation. For each 1 °C rise in the daily maximum temperature, the maximum daily 8-h average (MDA8) O₃ concentration increases by 11.79 μg m−3 in BTH and by 5.08 μg m−3 in YRD. In contrast, the PM_2.5 response to extreme heat diverges notably between the regions. In BTH, observed PM_2.5 decreases by 2.48 μg m−3 °C−1 under the typical dry extreme heat (mean RH = 50.13%), which favors the thermal dissociation of ammonium and nitrate. Conversely, in YRD, PM_2.5 increases by 0.98 μg m−3 °C−1. Influenced by the Western Pacific Subtropical High, the YRD experiences higher humidity (mean RH = 69.71%) under extreme heat, facilitating secondary inorganic aerosol formation. Analysis reveals relative humidity thresholds of approximately 58% in BTH and 60% in YRD, above which the negative chemical contributions to nitrate and ammonium decline linearly and eventually turn positive. These findings clarify the chemical mechanisms driving air pollutant behavior during extreme heat and highlight important implications for the health risks posed by compounded heat and pollution events.