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.