| The problem of air pollution has a serious impact on the production and life of the public.Based on the L-band sounding data of anqing,Fuyang,Nanjing and Xuzhou stations in January,April,July and October from 2016 to 2017,the statistical characteristics characteristics of low-level jet in the four stations were studied.Combined with the observation data and simulation data from December 16-17,2016 in Shouxian county,Anhui Province,the influence of a nighttime low-level boundary layer jet on PM2.5diffusion was analyzed.The main conclusions are as follows:(1)The seasonal variation of low-level jet stream over Nanjing and Anqing is similar to that of inversion layer,and the occurrence frequency of low-level jet stream and inversion is high in autumn and winter.The seasonal variation of low level jet stream and inversion layer is similar in Fuyang and Xuzhou.The frequency of low-level jet stream and inversion layer is high in spring and winter.Low-level jet streams occurring in the four regions are mainly10-15m/s eastward,and the occurrence of low-level jet streams is highly concentrated between 200-800m.(2)The diurnal variation of a typical southeast low-level jet stream in Nanjing and Shouxian is similar and accompanied by surface radiation inversion.(3)For different heights,the impact of the low-level jet stream on the diffusion of pollutants is obviously different.From ground to the low-level jet axis,PM2.5overall 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 PM2.5significant reduction.The peak value of net mass flux can reach-103·10-3?g/(m~2·s).The horizontal transportation of the low-level jet stream can bring cleaner air mass upwind,and also reduce PM2.5concentration in this layer.However,compared with turbulence,its effect is small,and the net mass flux is only-2.9·10-3?g/(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 PM2.5concentration in this layer,the net mass flux is about 11·10-3?g/(m~2·s).Between the low-level jet axis and wind direction transition height,PM2.5overall increase.This is because the turbulence transports high concentration pollutants in the low layer to this layer,making PM2.5concentration increase.The net mass flux is about23.9·10-3?g/(m~2·s).Horizontal transportation makes this layer PM2.5concentration increased slightly,and the net mass flux was about 2.3·10-3?g/(m~2·s).The vertical transportation brings clean air mass at high altitude and reduces PM2.5concentration,the net mass flux is about-6.6·10-3?g/(m~2·s).Between wind direction transition height and the low-level jet minimum wind speed height,PM2.5overall increase.Turbulence is still dominant,and the net mass flux is about17.8·10-3?g/(m~2·s).The vertical transport has a slight contribution,and the net mass flux is about 1.4·10-3?g/(m~2·s).The horizontal transport plays a reducing role,and the net mass flux is about-3.7·10-3?g/(m~2·s). |
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