A study of urban pollution and haze clouds over northern China during the dusty season based on satellite and surface observations

This paper presents a multi-scale study on formation process of urban pollution and haze clouds as well as their interactions over northern China in spring using integrated satellite and surface observations. Several extreme haze events occurred in Beijing area in March 2013, but primary atmospheric pollutants in the urban pollution exhibited inconsistent variations with the widespread haze clouds observed by satellites. Two typical types of haze event were found in Beijing area. Type-1 haze pollution appeared in stagnant weather conditions, during which PM2.5 was <200 μg m−3 with a short duration within 1-2 days. By contrast, strong northwestern winds prevailed in type-2 haze events with durative and intense temperature inversion near surface. Meanwhile, PM2.5 concentration exceeded ∼400 μg m−3 in type-2 pollution, and the heavy pollution can last 3-5 days. Different from urban pollution, our results show that the thick haze clouds were dominated by mixing of floating dust and anthropogenic pollutants in the middle and upper part, accompanied by hygroscopic growth of fine particles. Elevated coarse dust particles were prevalent over northern China, which accounted for a predominant fraction in the columnar optical volume during all the haze events. Furthermore, comparison between satellite and surface observations indicates that haze clouds above surface had no significant direct contribution to the serious urban pollution. In addition, mixing of dust and anthropogenic pollutants at high altitudes regulates regional aerosol optical properties throughout the whole March.