North China Plain as a hot spot of ozone pollution exacerbated by extreme high temperatures.

A large population in China has been increasingly exposed to both severe ozone (O₃) pollution and extreme heat under global warming. Here, the spatiotemporal characteristics of coupled extremes in surface O₃ and heat (OPCs) over China are investigated using surface observations, a process-based chemical transport model (GEOS-Chem), and multi-model simulations from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). North China Plain (NCP, 37-41°N; 114-120°E) is identified as a hot spot of OPCs, where more than half of the O₃ pollution days are accompanied by high temperature extremes. OPCs over NCP exceed 40 days during 2014-2019, exhibiting an increasing trend. Both O₃ concentrations and temperatures are elevated during OPCs compared to O₃ pollution days occurring individually (OPIs). Therefore, OPCs impose more severe health impacts to human than OPIs, but the stronger health effects are mainly driven by the higher temperatures. GEOS-Chem simulations further reveal that enhanced chemical production resulting from hot and stable atmospheric condition under anomalous weather pattern primarily contributes to the exacerbated O₃ levels during OPCs. In the future, CMIP6 projections suggest increased occurrences of OPCs over NCP in the middle of this century, but by the end of this century, OPCs may decrease or increase depending on the pollutant emission scenarios. However, for all future scenarios, extreme high temperature will play an increasingly important role in modulating O₃ pollution in a warming climate. [ABSTRACT FROM AUTHOR]