佛山地区干季边界层垂直风温结构对空气质量的影响

利用佛山地区2013年12月大气边界层观测试验得到的垂直风温资料和相应逐日AQI资料、逐时PM_(2.5)浓度资料,研究了佛山地区大气边界层垂直风温结构对空气质量的影响.结果表明:佛山地区干季持续存在的逆温结构是导致PM_(2.5)污染较重的重要原因.干季污染日近60%的最低逆温层高度低于1000 m,而非污染日低于1000 m的最低逆温层仅占36%,污染日佛山贴地逆温频率高达31.2%.逆温层出现高度较低,将污染物压缩积累在贴地层大气中导致污染较重.在大陆冷高压控制下,佛山地区的边界层结构演化非常典型,最大边界层高度和最大边界层通风量表现出了显著相关,污染日日平均边界层高度始终维持在较低的水平,多数时候不足500 m,最大边界层高度则大部分小于1000 m,日平均边界层通风量主要分布在500~1500 m~2·s~(-1)之间,在极端情况下甚至不足300 m~2·s~(-1),最大边界层通风量大部分处于1500~5000 m~2·s~(-1)之间,导致污染物始终聚集在较低的大气边界层内,使得PM_(2.5)浓度长时间维持在较高的污染水平.佛山地区风场存在显著的3层结构,较小的底层风速意味着大气的输送和扩散能力较弱,高度较低的中层使得污染物进一步被压缩累积在大气底层,垂直风场的不稳定性使得污染日佛山地区局地回流活跃,回流(RF)指数极小值多分布在0.2~0.4之间,污染日RF指数普遍小于非污染日,垂直风场的有效输送能力被显著削弱.

Influence of the structure of vertical wind and temperature in atmospheric boundary layer on air pollution in Foshan area during dry season

Based on the sounding data obtained from the boundary layer observation experiment in Foshan area during December 2013, the corresponding daily AQI data, and hourly PM_2.5 data, the influences of vertical wind and temperature on air quality in Foshan were analyzed.It was found that the persistent inversion layer occurred in dry season was a very important factor to PM_2.5 air pollution.There are 60% pollution days whose heights of inversion layer were lower than 1000 m, whereas the non-pollution days were only 36%.Especially, the frequency of surface inversion was up to 31.2% in pollution days. The lower inversion layer could help pollutants accumulated near the surface and result in heavy air pollution. Controlled by the cold high, the evolution of atmospheric boundary layer was quite typical in Foshan area, there is a strong correlation between maximum atmospheric boundary layer height (ABLH) and ventilation index (VI).During pollution days, the daily mean ABLH always maintained at a low level about 500 m, and most of the peak ABLH was lower than 1000 m, the daily mean VI was about 500 to 1500 m²·s^-1 with the minimum value even smaller than 300 m²·s^-1 in some extreme cases, and most of the peak VI was between 1500 to 5000 m²·s^-1, these conditions caused air pollutants accumulated in atmospheric boundary layer and resulted in a high concentration of PM_2.5.The vertical wind structure of Foshan could be divided into three layers, which means that the transportation and diffusion were very weak under the low layer with calm wind, and pollutants may be further compression into surface layer influenced by the reduced middle layer.During pollution days, the local recirculation was active because of the unstable vertical wind fields; most of the recirculation factors were about 0.2 to 0.4, which means the efficient transportation capacity was weakened significantly.