2016年冬季寿县一次边界层低空急流对污染物扩散的影响

利用安徽寿县地区2016年12月16~17日的观测资料与模拟资料,分析了一次夜间边界层低空急流对PM_2.5扩散的影响.此过程中,急流分布范围广,强度大,最大风速可达10~12m/s,而且风向随高度有明显转向,高低层风向差可达90°.急流发展过程中,急流轴基本位于200m以下,急流的最小风速高度出现在400~800m之间.通过分析可知,对于不同高度,急流对污染物扩散的影响存在明显差异.地面至急流轴范围内,PM_2.5总体减少.急流的出现使湍流混合明显增强,在湍流作用下污染物向上混合,使该层PM_2.5显著减少,净质量通量的峰值可达-103×10^-3μg/(m²·s).急流的水平输送可带来上风方较为清洁气团,同样减少了该层的PM_2.5浓度.但与湍流作用相比其影响较小,净质量通量仅为-2.9×10^-3μg/(m²·s).急流存在时,还会加强向下的垂直风速,在垂直输送作用下,上层污染物向下输送,增加了该层PM_2.5浓度,净质量通量约为11×10^-3μg/(m²·s).急流轴至风向转变高度之间,PM_2.5总体增加.这是由于湍流作用将低层高浓度污染物输送至该层,使PM_2.5浓度增加,净质量通量约为23.9×10^-3μg/(m²·s);水平输送作用使该层PM_2.5浓度略有增加,净质量通量约为2.3×10^-3μg/(m²·s);而垂直输送作用带来了高处较为清洁的气团,减少了PM_2.5浓度,净质量通量约为-6.6×10^-3μg/(m²·s).风向转变高度至LLJ最小风速高度之间,PM_2.5总体增加.湍流作用仍占主导,净质量通量约为17.8×10^-3μg/(m²·s);垂直输送作用稍有贡献,净质量通量约为1.4×10^-3μg/(m²·s);而水平输送起减少作用,净质量通量约为-3.7×10^-3μg/(m²·s).

Influence of a boundary layer low-level jet on pollutant diffusion in Shouxian,Anhui Province in winter 2016

Based on the observation data and simulation data in Shouxian area of Anhui Province from December 16 to 17, 2016, the effect of a nocturnal boundary layer low-level jet on PM_2.5 diffusion has been analyzed. In this process, the low-level jet has a wide distribution range and high intensity, the maximum wind speed can reach 10~12m/s, the wind direction difference between high and low layers can reach 90℃ during the development of the low-level jet. In the process of the low-level jet development, the jet axis is basically below 200m, and the minimum wind speed height of the low-level jet appears between 400~800m. The analysis shows that there are obvious differences in the impact of the low-level jet on pollutant diffusion at different heights. From ground to the low-level jet axis, PM_2.5 overall decrease. The emergence of the low-level jet flow significantly enhances the turbulent mixing. Under the action of turbulence, the pollutants are mixed upward, making the layer PM_2.5 significant reduction. The peak value of net mass flux can reach -103×10^-3μg/(m²·s).The horizontal transportation of the low-level jet stream can bring cleaner air mass upwind, and also reduce PM_2.5 concentration in this layer. However, compared with turbulence, its effect is small, and the net mass flux is only -2.9×10^-3μg/(m²·s). When the low-level jet exists, the downward vertical wind speed will be strengthened. Under the action of vertical transportation, the pollutants in the upper layer will be transported downward, increasing the PM_2.5 concentration in this layer, the net mass flux is about 11×10^-3μg/(m²·s). Between the low-level jet axis and wind direction transition height, PM_2.5 overall increase. This is because the turbulence transports high concentration pollutants in the low layer to this layer, making PM_2.5 concentration increase. The net mass flux is about 23.9×10^-3μg/(m²·s). Horizontal transportation makes this layer PM_2.5 concentration increased slightly, and the net mass flux was about 2.3×10^-3μg/(m2·s).The vertical transportation brings clean air mass at high altitude and reduces PM_2.5 concentration, the net mass flux is about -6.6×10^-3μg/(m²·s). Between wind direction transition height and LLJ minimum wind speed height, PM_2.5 overall increase. Turbulence is still dominant, and the net mass flux is about 17.8×10^-3μg/(m²·s).The vertical transport has a slight contribution, and the net mass flux is about 1.4×10^-3μg/(m²·s). The horizontal transport plays a reducing role, and the net mass flux is about -3.7×10^-3μg/(m²·s).