Synergy between air pollution and urban meteorological changes through aerosol-radiation-diffusion feedback A case study of Beijing in January 2013

The interactions of aerosol-radiation-stratification-turbulence-cloud processes during a severe haze event in Beijing in January 2013 were studied using a numerical model. For the clear days, solar radiation flux was reduced by approximately 15% and surface temperature was slightly decreased from 0 to 0.5 K throughout the day and night, except for a 1.4 K decrease around sunrise when fog was presented. The longwave radiation cooling was intensified by the fog or drizzle droplets near the top of the fog layer. Thus, in Beijing, both in the daytime and at night, the surface air temperature was decreased by air pollutants. In the presence of the low-level stratus and light precipitation, the modification of meteorology by aerosols was amplified and changed the wind speed and direction much more significantly compared to clear days. The non-linear effect (or positive feedback) of pollutant emission control on the surface air concentration was newly assessed severe air pollution leads to the intensification of stable stratification near the surface at night and delays the evolution of the mixing layer, which in turn causes more severe air pollution. The non-linear effect was not significant for the current emission levels in the current case, approximately 10%. In another word, the mixing ratio of aerosols became higher by 10% due to their radiation effects.