Increased importance of aerosol–cloud interactions for surface PM 2.5 pollution relative to aerosol–radiation interactions in China with the anthropogenic emission reductions

Surface fine particulate matter (PM2.5) pollution can be enhanced by feedback processes induced by aerosol-radiation interactions (ARI) and aerosol-cloud interactions (ACI). Many previous studies have reported enhanced PM2.5 concentration induced by ARI and ACI for episodic events in China. However, few studies have examined the changes in the ARI- and ACI-induced PM2.5 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 PM2.5 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 PM2.5 pollution, the PM2.5 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 PM2.5 enhancement shows a significantly smaller decrease ratio (51 %) than the ARI-induced enhancement (75 %) for 2013–2021, making ACI more important for enhancing PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 enhancement is dominated by the lower decrease ratio of ACI-induced secondary PM2.5 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 PM2.5, the ACI-induced PM2.5 enhancement inevitably becomes more important, which needs to be considered in the formulation of control policies to meet the national PM2.5 air quality standard.