长江三角洲初冬一次重污染天气成因分析

利用站点气象和PM_2.5资料以及NCEP的全球再分析数据集研究了2015年12月17~28日长江三角洲地区一次重污染天气过程.结果表明：地面弱气压场是此次污染事件发生发展的主要天气背景，而冷空气带来的大风使PM_2.5浓度迅速下降，有效清除了PM_2.5.区域热力因子和动力因子分析发现，此次过程中大气中低层层结稳定、近地面逆温强，有利于PM_2.5和水汽的累积，使其浓度水平升高；对于动力因子来说，较小的通风率和较低的边界层高度不利于污染物扩散，同样使PM_2.5浓度上升.两者相比，热力因子对PM_2.5浓度值的贡献比动力因子大.结合后向轨迹和排放源分布发现，此次污染过程中长江三角洲地区的PM_2.5主要受来自其西北方向的大陆气团（占46%左右）的影响，这些气团途经高污染排放源并把污染物远距离传输至长江三角洲地区.最后利用PSCF和CWT对长江三角洲地区污染物的潜在来源进行了分析，发现PM_2.5的来源主要集中在安徽、河南、山西、山东以及长江三角洲本地，说明此次过程中长江三角洲地区的污染物浓度受到远距离输送和局地过程的共同影响.

Analysis of a heavy air pollution event in early winter in the Yangtze River Delta

Based on the observational atmospheric data, the PM_2.5 data, the GDAS data and the reanalysis data from NCEP, this study investigated a severe air pollution event happening in the Yangtze River Delta during the period from December 17 to December 28 in 2015. It was 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 caused the rapid decrease in the concentration of PM_2.5. Moreover, the roles of thermal and mechanical factors in this pollution were analyzed. In terms of the thermal factors, the strong mid-to-low level stratification stability, together with intense ground (isothermal) inversion, was conducive to the accumulation of PM_2.5 and abundant water vapor. As for the mechanical factors, weak ventilation rate and shallow boundary layer were unfavorable to the pollutant dispersion, leading to the rise in the concentration of PM_2.5. In comparison, the thermal factors played a more important role in the increase in PM_2.5 than the mechanical factors did. With the aids of the backward trajectory and the distributions of the pollutant emission sources, it was found that the continental air mass from the northwest, which accounted for 46% of all the air masses, had a major influence on this event. As the air mass passes the emission sources in its path, it carried the pollutants from upstream to the Yangtze River Delta. Finally, using PSCF and CWT the main sources of the pollution were 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 was contributed by long-distance transports and the local processes.