Impact of mixing states on aerosol radiative effects and feedback during winter haze episodes over the North China Plain

Abstract

An online-coupled regional climate-chemistry-aerosol model (RIEMS-Chem) was developed and applied to investigate the impact of the aerosol mixing state on aerosol-radiation-meteorology feedback during the winter haze episode of 8–13 February 2020 over the North China Plain (NCP). Model validation demonstrates the overall good ability of the model in reproducing the meteorological variables, PM_2.5 and its components, and aerosol optical properties. Aerosol optical depth (AOD) and single scattering albedo (SSA) simulated with the Maxwell-Garnett mixing assumption are closest to the AERONET observations, whereas external mixing tends to predict lower AOD and higher SSA, in contrast to core–shell mixing and homogeneous mixing, which predict lower SSA. The direct radiative effects (DREs) under various aerosol mixing states differ largely, with the percentage differences of 24% and 40% and a factor of three at the top of the atmosphere (TOA), at the surface and in the atmosphere, respectively, averaged over the haze episode. The sensitivity of the aerosol optical properties and DRE to the black carbon size distribution, coating fraction, and hygroscopic growth under Maxwell-Garnett mixing is also investigated, showing the remarkable effect of hygroscopic growth. During the most severe haze day in Beijing, the changes in air temperature and relative humidity at 2 m (T2, RH2) and wind speed at 10 m (WS10) induced by aerosol radiative feedback vary by −1.2–1.4 °C, 5.5–6.2%, and −0.28–0.37 m s⁻¹, respectively, and the feedback-induced increase in PM_2.5 concentration varies considerably from 40.3 µg m⁻³ to 57.6 µg m⁻³ across different mixing states. The radiative feedback leads to an increase in PM_2.5 concentration by 25%, 30%, 30% and 36% under external mixing, Maxwell-Garnett mixing, core–shell mixing and homogeneous mixing states, respectively. It is noteworthy that the aerosol mixing state not only affects the magnitude but also the direction of near-surface air temperature change, depending on the relative magnitude of aerosol-induced atmospheric heating and DRE-induced cooling. The strongest atmospheric heating rate (AHR) under homogeneous mixing leads to an increase in T2 and planetary boundary layer height (PBLH) in portions of southern NCP, which tends to reduce the PM_2.5 concentration. This study demonstrates the important role of radiative feedback in exacerbating air pollution and the significant impacts of aerosol mixing state on DRE, AHR and radiative feedback during the haze episode.