Regional transport of aerosol above boundary layer and its radiation effect trigger severe haze pollution in Beijing

Contribution of the regional transported aerosol above planetary boundary layer (PBL), together with its radiation effect (AR), in triggering severe haze pollution in Beijing were investigated in this work. Both observations and model simulations of a polluted episode during Dec.14-16, 2018 indicated that regional transported aerosol above PBL was downwards transported and resulted in a fast increase in fine particulate matter (diameter < 2.5 mu m, PM2.5) at ground level. Quantitative evaluation by sensitive simulations indicated that regional transported aerosol contributed 60.49 mu g m(-3) (55.1%) in total PM2.5 at ground level, and this contribution became even larger in the beginning stage (e.g., 14:00 on Dec.14) (70.3%). The transported aerosol stabilizes the boundary layer via its AR effect, which further enhances PM2.5 accumulation at ground level. PM2.5 at ground level further increased by 5.07 mu g m(-3) due to the AR effect of transported aerosol, contributing 77.8% in PM2.5 enhancement due to AR effect. Further analyses indicated that the high altitude of transported aerosol and higher fraction of black carbon in beginning stage were two factors that resulted in transported aerosol exerting more important role in PM2.5 enhancement than local aerosol. Hence, regional transported aerosol and its AR effect trigger rapid increase of PM2.5, exaggerate pollution level in Beijing.