Summer ozone pollution in China affected by the intensity of Asian monsoon systems

Ozone in the troposphere is harmful to human health and ecosystems. It has become the most severe air pollutant in China. Here, based on global atmospheric chemistry model simulations during 1981-2019 and nation-wide surface observations, the impacts of interannual variations in Asian summer monsoon (ASM), including East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM), on surface O₃ concentrations during June-July-August (JJA) in China are investigated. EASM intensity has a significant positive correlation with the surface O₃ concentration in south-central China (97.5°-117.5°E, 20°-35°N) with a correlation coefficient of 0.6. Relative to the weak EASM years, O₃ concentrations in strong EASM years increased by up to 5 ppb (10 % relative to the average) due to the weakened transboundary transport of O₃ resulting from the decrease in prevailing southwesterlies. SASM can be divided into two components. The one near East Asia has a similar relation with O₃ in southern China (100°-117.5°E, 22°-32°N) as that of EASM. The other component of SASM is negatively correlated with surface O₃ concentration in eastern China (110°-117.5°E, 22°-34°N) and the maximum difference in O₃ concentrations exceeded 5 ppb (10 %) between the strong and weak monsoon years, which can be explained by the O₃ divergence caused by the anomalous southerlies blowing pollutants away from the northern boundary of eastern China. This study shows that the ASM has an important impact on the O₃ concentrations in China, primarily through changing transboundary transport related to the variability of large-scale circulations, which has great implications for air pollution prevention and mitigation in China. Future projections of ASM suggests that the sustainable and medium development scenarios are the perfect pathways that can help to mitigate O₃ pollution, while high social vulnerability and radiative forcing scenarios could enhance future O₃ pollution in China.