Characteristics and comparative of typical haze weather during autumn and winter in the Yangtze River Delta

To summarize the correlation influential factors, characteristics, and commonness under haze weather conditions, eight cities in the Yangtze River Delta and seven typical haze pollution processes were selected that occurred during the winter and autumn from 2016 to 2019. Based on the Air Quality Index (AQI) of the three most representative processes, PM2.5 concentrations, meteorological factors, synoptic weather situation, atmospheric boundary layer characteristics, and pollution sources for the three haze processes were analyzed and found contrast with one another. The results showed that the unfavourable meteorological condition and the stagnant weather patterns due to the configuration of high and low altitude resulted in the formation of extreme haze condition. The peak values of the AQI in three representative processes were 247, 306 and 272, respectively, which were consistent with the change of PM2.5 concentration. There was an obvious negative correlation between PM2.5 concentration and visibility, the valley values of visibility were observed as low as 50m in the second and third haze processes. Generally, the pollution processes occur with low visibility. High relative humidity, stable temperature, and static wind were closely related to the formation of the haze process. In general, the AQI illustrated a negative correlation with mixed layer height. A lower mixed layer height affected the level of vertical convection of air contributed to the regional accumulation of substances at low altitude, and the minimum heights of the mixed layer in the three pollution processes were less than 100m. The inversion layer was conducive to high concentrations of pollution during haze pollution processes. On the one hand, the pollutants were trapped in the surface by the ground inversion layer, where the intensity of the ground inversion layer was up to 8.2℃ in the first process. On the other hand, the untouched ground inversion layer inhibited the vertical dispersion of pollutants in the atmospheric boundary layer; the second process was dominated by the untouched ground inversion for which the intensity reached to 4.8℃. Aerosol sources were mostly from the dust, polluted continental, polluted dust, and smoke. Air pollution was affected by the joint influence of local emission, regional transport, and long-range transport. The fine particles due to anthropogenic factors carried by the air mass were a major contributor to the pollution. © 2021, Editorial Board of China Environmental Science. All right reserved.