2016年冬季寿县一次边界层低空急流对污染物扩散的影响

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

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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·s);垂直输送作用稍有贡献,净质量通量约为1.4×10^(-3)μg/(m^(2)·s);而水平输送起减少作用,净质量通量约为3.7×10^(-3)μg/(m^(2)·s).

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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·s).Horizontal transportation makes this layer PM_(2.5)concentration increased slightly,and the net mass flux was about 2.3×10^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·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^(-3)μg/(m^(2)·s).The vertical transport has a slight contribution,and the net mass flux is about 1.4×10^(-3)μg/(m^(2)·s).The horizontal transport plays a reducing role,and the net mass flux is about-3.7×10^(-3)μg/(m^(2)·s).