The underlying mechanisms of PM and O synergistic pollution in East China: Photochemical and heterogeneous interactions.

The rapid development of Chinese cities is accompanied by air pollution. Although the implementation of air pollution control strategies in recent years has alleviated PM pollution, O pollution and the synergistic pollution of PM and O have become more serious. To understand the underlying chemical interaction mechanisms between PM and O, we applied the modified Weather Research and Forecasting model with Chemistry (WRF-Chem) to study the effects of aerosol-photolysis feedback and heterogeneous reactions on the two pollutants and revealed the contribution of different mechanisms in different seasons and regions in Yangtze River Delta (YRD) in eastern China. We found that, through the aerosol-photolysis feedback, PM decreased the surface photolysis rates J and J, resulting in a decrease in O concentration in the VOC-sensitive area and a slight increase in the NO-sensitive area. The heterogeneous reactions reduced O concentration in the YRD in spring, autumn and winter by consuming HO. While in summer, the heterogeneous absorption of NO decreased O in the NO-sensitive areas and increased O in the VOC-sensitive areas. Heterogeneous reactions also promoted the secondary formation of fine sulfate and nitrate aerosols, especially in winter. Through the combined effect of two chemical processes, PM can lead to a decrease in O concentration of -3.3 ppb (-7.6 %), -2.2 ppb (-4.0 %), -2.9 ppb (-6.3 %), and - 5.9 ppb (-18.7 %), in spring, summer, autumn and winter in YRD. Therefore, if the PM concentration decreases, the weakening effect of PM on the ozone concentration will be reduced, resulting in the aggravation of ozone pollution. This study is important for understanding the synergistic pollution mechanism and provides a scientific basis for the coordinated control of urban air pollution.