Aerosol-radiation feedback deteriorates the wintertime haze in the North China Plain

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 15m above the ground surface) PM_2:5 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 PM_2:5 concentrations in the NCP by 10.2 μgm^-3 or with a contribution of 7.8% on average. Sensitivity studies have revealed that high loadings of PM_2:5 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 PM_2:5 dispersion, and consequently exacerbating the haze pollution in the NCP. Furthermore, when the near-surface PM_2:5 mass concentration increases from around 50 to several hundred μgm^-3, ARF contributes to the near-surface PM_2:5 by more than 20% during daytime in the NCP, substantially aggravating the heavy haze formation. However, when the near-surface PM_2:5 concentration is less than around 50 μgm^-3, ARF generally reduces the nearsurface PM_2:5 concentration due to the consequent perturbation of atmospheric dynamic fields.