Huge challenges of improving ozone pollution in China: High regional background ozone concentrations calculated from...

Understanding current regional background ozone concentrations can clarify their ability to meet air quality standards. In this study, the regional background ozone concentration was defined as the stable concentration of ozone caused by large-scale winds. Therefore, on the basis of an experimental equation, the national regional background ozone concentration was calculated via national hourly ozone concentration and wind speed observational data from 2015 to 2023. This study divided China into five regions according to the spatial distributions of climatic zones and regional background ozone concentrations. The regional background ozone concentration in China was low in the northeast (77.3 ± 5.7 μg/m³) and southeast monsoon (79.3 ± 5.2 μg/m³) regions and high in the southwest monsoon region (94.5 ± 8.0 μg/m³) and northern region (95.3 ± 7.1 μg/m³). The regional background concentration of the plateau region was considered high at the national level (99.6 ± 4.8 μg/m³). This study highlights that the regional background ozone concentration in plains is related to soil NO_x, and that in plateaus is related to altitude (stratospheric intrusion and strong radiation). Cross-border transport from Southeast Asia contributed significantly to regional background ozone in Southwest China. Given the differences in regional background ozone concentrations, the ability to meet national air quality standards varies across regions. The plateau, northern, and southwest monsoon regions presented high regional background ozone concentrations, which indicates that these regions are under great pressure to reduce emissions.