An Analysis Of A Heavy Air Pollution Event In Early Winter In The Yangtze River Delta

Based on the observational atamospheric data,the PM2.5 data,the GDAS data and the reanalysis data from NCEP,this study investigates a severe air pollution event happening in the Yangtze River Delta during the period from December 17 to December 28 in 2015.It is found that the low pressure at the earth surface provides favorable environment for the occurrence and development of this air pollution event,whereas the strong wind and temperature fall brought by the cold air cause the rapid decrease in the concentration of PM2.5,both rising and horizontal movements are conducive to the proliferation of PM2.5.Moreover,the roles of thermal and mechanical factors in this pollution are analyzed.In terms of the thermal fators,the strong mid-to-low level stratification stability,together with intense ground(isothermal)inversion,is conducive to the accumulation of PM2.5 and abundant water vapor.As for the mechanical factors,weak ventilation rate and shallow boundary layer are unfavourable to the pollutant dispersion,leading to the rise in the concentration of PM2.5.In comparison,the thermal factors play a more important role in the increase in PM2.5 than the mechanical factors do.With the aids of the backward trajectory and the distributions of the pollutant emission sources,it is found that the continental air mass from the northwest,which accounts for 46%of all the air masses,has a major influence on this event.As the air mass passes the emission sources in its path,it carries the pollutants from upstream to the Yangtze River Delta.Finally,using PSCF and CWT the main sources of the pollution are found to be concentrated in Anhui,Henan,Shanxi,Shandong,as well as the Yangtze River Delta.That is to say,the air pollution in the Yangtze River Delta is contributed by long-distance transports and the local processes.