Pinpointing drivers and health effects of exceptional summertime ozone pollution in the Sichuan basin of China in 2022.

Tropospheric ozone (O) is a ubiquitous pollutant that is detrimental to human health and ecosystems. The Sichuan Basin (SCB), one of the most populous city clusters in China, has experienced more-intense O pollution episodes and longer O season with more-frequent stagnant conditions over the past decade. In 2022, the prolonged O season featured extremely high levels of O and region-wide O events were observed, posing significant threats to public health. However, it remains unclear to what extent meteorological fields could contribute to O anomaly and to the adverse health impacts from extreme O season. Here, we investigate the drivers of extreme summer O pollution in 2022 over the SCB using a high-resolution Community Multiscale Air Quality (CMAQ) model in conjunction with surface air quality measurements. Further, the health effects of exposure to high levels of O are quantified using the Environmental Benefits Mapping and Analysis Program (BenMAP). Both meteorological reanalysis data and the Weather Research and Forecasting (WRF) modeling revealed extreme heat featured by persistent heatwaves in the study period, which significantly perturbed daytime photochemical reactions and primed the landscape for elevating O. Sensitivity experiments with fixed anthropogenic emissions indicate that unfavorable meteorology and subsequent enhancements in biogenic emissions substantially contributed to O anomaly. Importantly, this unprecedented O season resulted in 48285 all-cause deaths due to long-term exposure, which is 8064 higher than the same period in 2019 and significantly overtake previous recognition. CMAQ simulations point to that O elevation could be partially offset by concurrent 50 % emission reductions on nitrogen oxides (NO) and volatile organic compounds (VOCs), leading to avoided deaths of 2660. This work highlights the underestimated O-related mortality burden and pinpoints the necessity of stringent emission regulations toward O mitigation in basin topography.