Satellite observation of abnormal yellow haze clouds over East China during summer agricultural burning season

Durative haze clouds with unusual yellow color appeared in East China in agricultural burning period during June 8–12 in 2012, causing extreme air pollution in densely populated regions including Jiangsu, Hubei, and the Yangtze River Delta. The spatial variation, vertical structure, optical properties, as well as formation process, were investigated using combined multiple satellite observations, ground measurements, and meteorological data. Different from previous studies, our analysis reveals that the yellow haze clouds were caused by mixing and interaction among airborne dust, fire emissions, and urban pollution under humid conditions. The pollution layers were 3–5 km thick, and their vertical structures were very inhomogeneous, with dust mostly distributed in the upper part and mixing of fires smoke and urban haze concentrated near surface. Compared with fire smoke, the dust-like haze clouds exhibited different optical properties with higher volume depolarization ratio and notable increase in coarse mode aerosols. Although fire emissions and urban pollution may play a more important role in surface pollution, we conclude that dust transport and high humidity were the main reason that the haze pollution was much heavier than that in previous years. In addition, regional concentrated fires only occurred in several days, and fire count was inconsistent with regional average aerosol loading. The long-range transport of fire emissions can be overestimated. In order to avoid such regional pollution event, our results also suggest that the strict measures in fire management should be extended from special periods to normal season.