The 21st-century wetting inhibits growing surface ozone in Northwestern China

Abstract. Previous studies have shown that surface air temperature (SAT) facilitates the formation of surface ozone (O3), while relative humidity (RH) often inhibits ozone generation. However, the degree to which O3 may respond simultaneously to rising SATs and RH due to climate change remains less understood. We conducted extensive atmospheric chemistry model scenario simulations to investigate the impacts of long-term trends of humidification and warming on summer O3 concentrations in Northwestern China (NW) from 1998 to 2017, a period during which this region experienced both warming and the most significant wetting trend in China. We found that the summer mean O3 level in NW increased by 19.9 % during this time. The changes in meteorology led to a reduction in O3 levels, primarily due to humidification in NW, which counteracts the warming-induced O3 increase. We demonstrate that the wetting trend in NW will continue from 2019 to 2030 under a shared socioeconomic pathway (SSP) in conjunction with a representative concentration pathway SSP5-RCP8.5, but will either cease or shift to drier conditions from 2019 to 2060 under the SSP2-RCP4.5. No significant responses of O3 fluctuations to RH variations were observed, partly due to manual intervention under the SSP2-RCP4.5.