Nonlinear ozone response to extreme high temperature in a subtropical megacity basin: Integrated observation and...

The unprecedented 2022 summer extreme high temperature (EHT) (>20 days exceeding 40 °C) in China's Sichuan Basin induced complex ozone pollution patterns requiring systematic analysis. Combining surface observations, WRF-Chem simulations, and OMI (Ozone Monitoring Instrument) satellite data, this study investigated the EHT-O₃ relationship and underlying meteorological mechanisms. The daily max 8-h average ozone peaked at 227.5 μg m^-3, showing a southwest-northeast decreasing gradient. Nonlinear relationship between EHT and ozone pollution was identified that O₃ increased by 22.5 % per °C below 39.4 °C (promotion phase) but declined by 4.7 % per °C (suppression phase) above this threshold at Chengdu station. EHT events modulated ozone formation via cascading effects: horizontal wind shifted from chaotic to stable northeastern flow, enhancing O₃ transport; weakened vertical velocity, increased relative humidity in the mid-humidity range and cloudiness changes boosted photolysis, driving O₃ accumulation. In the suppression phase, elevated temperatures weakened northeastern winds and intensified vertical mixing, suppressing O₃. EHT also shifted ozone sensitivity from VOC-limited to transitional regimes (notably in Chengdu and Chongqing), reversing during suppression with enhanced VOC limitation. This study establishes a process-based framework for resolving ozone-climate interactions in subtropical megacity basin, with implications for pollution control strategies under global warming scenarios.