Increased importance of aerosol–cloud interactions for surface PM _2.5 pollution relative to aerosol–radiation interactions in China with the anthropogenic emission reductions

page14360Surface fine particulate matter (PMinline-formula_2.5) pollution can be enhanced by feedback processes induced by aerosol–radiation interactions (ARIs) and aerosol–cloud interactions (ACIs). Many previous studies have reported enhanced PMinline-formula_2.5 concentrations induced by ARIs and ACIs for episodic events in China. However, few studies have examined the changes in the ARI- and ACI-induced PMinline-formula_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 PMinline-formula_2.5 changes for 2013–2021 under different meteorology and emission scenarios using the Weather Research and Forecasting model with Chemistry (WRF-Chem), and we investigate the driving factors behind the changes. Our results show that, in January 2013, when China suffered from the worst PMinline-formula_2.5 pollution, the PMinline-formula_2.5 enhancement induced by ARIs in eastern China (5.59 inline-formulaµg minline-formula⁻³) was larger than that induced by ACIs (3.96 inline-formulaµg minline-formula⁻³). However, the ACI-induced PMinline-formula_2.5 enhancement showed a significantly smaller decrease ratio (51 %) than the ARI-induced enhancement (75 %) for 2013–2021, making ACIs more important for enhancing PMinline-formula_2.5 concentrations in January 2021. Our analyses suggest that the anthropogenic emission reductions played a key role in this shift. Owing to only anthropogenic emission reductions, the ACI-induced PMinline-formula_2.5 enhancement decreased by 43 % in January, which was lower than the decrease ratio of the ARI-induced enhancement (57 %). The relative change in ARI- and ACI-induced PMinline-formula_2.5 enhancement in July was similar to the pattern observed in January, caused by anthropogenic emission reductions. The primary reason for this phenomenon is that the decrease in ambient PMinline-formula_2.5 for 2013–2021 caused a disproportionately small decrease in the liquid water path (LWP) and an increase in the cloud effective radius (Re) under the condition of high PMinline-formula_2.5 concentrations. Therefore, the surface solar radiation attenuation (and, hence, the boundary layer height reduction) caused by ACIs decreased slower than that caused by ARIs. Moreover, the lower decrease ratio of the ACI-induced PMinline-formula_2.5 enhancement was dominated by the lower decrease ratio of ACI-induced secondary PMinline-formula_2.5 component enhancement, which was additionally caused by the smaller decrease ratio of the air temperature reduction and the relative humidity (RH) increase. Our findings indicate that, with the decrease in ambient PMinline-formula_2.5, the ACI-induced PMinline-formula_2.5 enhancement inevitably becomes more important. This needs to be considered in the formulation of control policies to meet the national PMinline-formula_2.5 air quality standard.