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Abstract. Atmospheric aerosols scatter or absorb a fraction of the incoming solar radiation to cool or warm the atmosphere, decreasing surface temperature and altering atmospheric stability to further affect the dispersion of air pollutants in the planetary boundary layer (PBL). In the present study, simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the North China Plain (NCP) were performed using the Weather Research and Forecasting model with Chemistry (WRF-Chem) to comprehensively quantify contributions of aerosol shortwave radiative feedback (ARF) to near-surface (around 15 m above the ground surface) PM2.5 mass concentrations. The WRF-Chem model generally performs well in simulating the temporal variations and spatial distributions of air pollutants concentrations compared to observations at ambient monitoring sites in the NCP, and the simulated diurnal variations of aerosol species are also consistent with the measurements in Beijing. Additionally, the model simulates the aerosol radiative properties, the downward shortwave flux, and the PBL height against observations in the NCP well. During the episode, ARF deteriorates the haze pollution, increasing the near-surface PM2.5 concentrations in the NCP by 10.2 µg m−3 or with a contribution of 7.8 % on average. Sensitivity studies have revealed that high loadings of PM2.5 attenuate the incoming solar radiation reaching the surface to cool the low-level atmosphere, suppressing the development of the PBL, decreasing the surface wind speed, further hindering the PM2.5 dispersion, and consequently exacerbating the haze pollution in the NCP. Furthermore, when the near-surface PM2.5 mass concentration increases from around 50 to several hundred µg m−3, ARF contributes to the near-surface PM2.5 by more than 20 % during daytime in the NCP, substantially aggravating the heavy haze formation. However, when the near-surface PM2.5 concentration is less than around 50 µg m−3, ARF generally reduces the near-surface PM2.5 concentration due to the consequent perturbation of atmospheric dynamic fields.

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Abstract. Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM2.5] is not significant, being generally within 1.0 µg m−3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation.