Tropospheric ozone (O3) is a critical air pollutant that poses significant risks to human health and ecosystems. While previous studies have primarily focused on O3 changes in Eastern China, limited attention has been given to Northwest China, where fragile but ecologically important systems may be vulnerable to O3 pollution. The temporal evolution and driving mechanisms of surface O3 in this region remain poorly understood. Using the European Centre for Medium-Range Weather Forecasts Reanalysis Version 5 (ERA5) datasets and simulations from the Community Atmosphere Model with Chemistry (CAM-Chem), we identified a significant increase in summer surface O3 concentrations across Northwest China from 1980 to 2020, with the most pronounced rise occurring during 1993–2010. This period accounts for the majority of the long-term upward trend, despite relative declines before and after. The increase in O3 during 1993–2010 is primarily attributed to rising surface temperatures, which reduce hydroperoxyl radical (HO2) concentrations and enhance nitrogen dioxide (NO2) production, leading to elevated nitrogen oxides (NOx) levels and promoting O3 formation. The warming trend is closely associated with a concurrent decrease in low cloud cover, which increases surface shortwave radiation and further contributes to surface warming. Further investigation reveals that warming sea surface temperature (SST) in the North Pacific influence atmospheric circulation through wave train processes, amplifying the regional geopotential height field. These circulation changes reinforce the reduction in low cloud cover and the associated increases in surface temperature and O3 concentrations over Northwest China. The decadal variability of North Pacific SST may therefore serve as an important indicator of long-term surface ozone variability in this region.
Han et al. (Tue,) studied this question.