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

A large population in China has been exposed to both severe ozone (O-3) pollution and extreme heat under global warming. Here, the spatiotemporal characteristics of coupled extremes in surface O-3 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 degrees N; 114-120 degrees E) is identified as a hot spot of OPCs, where more than half of the O-3 pollution days are accompanied by high temperature extremes. OPCs over NCP exceeded 40 d during 2014-2019, exhibiting an increasing trend. Both O-3 concentrations and temperatures are elevated during OPCs compared with O-3 pollution days occurring individually (OPIs). Therefore, OPCs impose more severe health impacts to humans 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 conditions under anomalous weather patterns primarily contributes to the exacerbated O-3 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 temperatures will play an increasingly important role in modulating O-3 pollution in a warming climate.

Automated summary

The North China Plain is identified as a hot spot of coupled extremes in surface ozone and heat. The number of days with both ozone pollution and high temperature extremes is increasing in this region. These coupled extremes have more severe health impacts on humans compared to ozone pollution days occurring individually, and the higher temperatures play a major role in driving these health effects.