Increased Importance of Aerosol-Cloud Interaction for Surface PM_2.5 Pollution Relative to Aerosol-Radiation Interaction in China With the Anthropogenic Emission Reduction

Abstract. Surface fine particulate matter (PM_2.5) pollution can be enhanced by feedback processes induced by aerosol-radiation interactions (ARI) and aerosol-cloud interactions (ACI). Many previous studies have reported enhanced PM_2.5 concentration induced by ARI and ACI for episodic events in China. However, few studies have examined the changes in the ARI- and ACI-induced PM_2.5 enhancements over a long period, though the anthropogenic emissions have changed substantially in the last decade. In this study, we quantify the ARI- and ACI-induced PM_2.5 changes for 2013–2021 under different meteorology and emission scenarios using the Weather Research and Forecasting model with Chemistry (WRF-Chem) and investigate the driving factors for the changes. Our results show that in January 2013, when China suffered from the worst PM_2.5 pollution, the PM_2.5 enhancement induced by ARI in eastern China (5.59 μg m⁻³) is larger than that induced by ACI (3.96 μg m⁻³). However, the ACI-induced PM_2.5 enhancement shows a significantly smaller decrease ratio (51 %) than the ARI-induced enhancement (75 %) for 2013–2021, making ACI more important for enhancing PM_2.5 concentrations in January 2021. Our analyses suggest that the anthropogenic emission reduction plays a key role in this shift. Owing to only anthropogenic emission reduction, the ACI-induced PM_2.5 enhancement decreases by 43 % in January and 57 % in July, lower than the decrease ratio of the ARI-induced enhancement (57 % in January and 67 % in July). The primary reason for this phenomenon is that the decrease of ambient PM_2.5 for 2013–2021 causes a disproportionately small decrease of liquid water path (LWP) and increase of cloud effective radius (Re) under the condition of high PM_2.5 concentration, therefore the surface solar radiation attenuation (and hence boundary layer height reduction) caused by ACI decreases slower than that caused by ARI. Moreover, the lower decrease ratio of the ACI-induced PM_2.5 enhancement is dominated by the lower decrease ratio of ACI-induced secondary PM_2.5 component enhancement, which is additionally caused by smaller decrease ratio of the air temperature reduction and relative humidity (RH) increase. Our findings reveal that with the decrease of ambient PM_2.5, the ACI-induced PM_2.5 enhancement inevitably becomes more important, which needs to be considered in the formulation of control policies to meet the national PM_2.5 air quality standard.