| The three-dimensional wind field,temperature,aerosol distribution are the main driving factors of the dynamics,thermodynamic and material structure in the planetary boundary layer(PBL).They not only affect the development of the local PBL,but also have an important impact on the transmission and diffusion of pollution.Therefore,the refined PBL structure improves our understanding of the local boundary layer structure and helps us understand the complex mechanism of the PBL on pollution.This study used the Doppler wind lidar,microwave radiometer,ceilometer and pollution data,mainly analyzing the applicability of Doppler wind lidar in polluted areas and also explored the temporal and spatial representation of different wind measurement systems in the urban PBL.This work quantitatively studied the complex wind structure of the mountain-plain wind(MPW)in the Beijing area,and the development characteristics of the local thermal(Ht),dynamic(Hu)and material(Hc)PBL structure under the influence of the MPW.Besides,this study systematically explored the PBL and pollution characteristics in different seasons in Beijing,and focused on the influence mechanism of the PBL in each season on pollution,providing important scientific references and suggestions for future research.The detection of the vertical structure of the urban boundary layer wind field is important for the study of the physical and chemical characteristics of the atmospheric boundary layer.Based on the 325-m meteorological tower,the performance of three different wind measurement systems of Doppler wind lidar,cup anemometer and vane,sonic anemometer were systematically compared.The results showed that the horizontal wind speed measurements of the Doppler wind lidar closely matched those of the cup anemometer and the sonic wind anemometer with high coefficients of determination.Doppler wind lidar can accurately detect the vertical distribution of wind speed in the boundary layer.In view of the different principles of vertical wind detection between Doppler wind lidar and sonic anemometer,there was a certain difference in the consistency between the vertical wind of Doppler wind lidar and the supersonic anemometer of different time scales.This difference reflected the uncertainty and complexity of multi-scale atmospheric turbulence detection.Sonic anemometers had advantages in capturing the vertical motion of small-scale turbulence,while Doppler wind lidar was more suitable for capturing the vertical motion of larger-scale turbulence.Using Doppler lidar,microwave radiometer and ceilometer to comprehensively detected the fine structure of the MPW in Beijing,revealing the development characteristics of the multi-attribute PBL under the action of the MPW.Process detection results showed that the MPW in Beijing was a weak local daily circulation system,and the development of mountain wind(northern wind)and plain wind(southern wind)were not symmetrical,and the intensity of mountain wind was greater than that of plain wind.The development height of mountain breeze extended from near the ground to 500 m,and the wind speed firstly increased and then decreased.The maximum wind speed of mountain breeze appeared at about 350 m,and the maximum wind speed could reach 6.50 m s-1.At the top of mountain breeze development,there was obvious wind shear(wind shear index range: 2.0-2.8),and this wind shear developed over time and presented a band-like evolution structure.The development height of the plain wind was not particularly clear.The wind speed gradually increased from near the ground to the upper air.The overall wind speed ranged from 0.50-5.80 m s-1.During this period,there was no similar band-like wind shear structure.Under the effect of wind shear at the top of mountain wind,the local atmosphere tended to be unstable(Richardson number Ri is less than-0.5).In the process of mountain-plain wind transition,turbulence was obviously strengthened(turbulent kinetic energy TKE could reach 1.08-1.56 m2 s-2).Under the control of the mountain-plain wind,the thermal,dynamic and material structure of the urban atmospheric boundary layer showed different evolution characteristics.Under the control of the afternoon plain wind,the results showed that the height of the material boundary layer(mainly maintained at 1100 m)> the height of the dynamic boundary layer(1050 m)> thermal boundary layer height(550 m).under the control of the mountain wind at night,the three attribute boundary layer heights were relatively consistent,and all were below 300 m.Probing into the evolution law of the boundary layer structure with different attributes had certain research significance and application value for the study of local air pollution diffusion or accumulation mechanism.Beijing's air pollution was the result of the combined effects with various boundary layer elements,and the effects of boundary layer elements in the air pollution process in different seasons were not the same.Preliminary observations show that during the accumulation stage of pollution,it was generally controlled by the southerly wind from the plains.The high-concentration pollutants from the North China Plain were transported to Beijing.There were certain differences in the profile structure and intensity of the southerly wind in different seasons.The wind speed in autumn and winter(1.2-7.30 m s-1)was the strongest,and the maximum wind speed occurred at 900 m;the wind speed in spring was at an intermediate level(1.0-6.20 m s-1),and the maximum wind speed occurred at 1000 m;the wind speed in summer was the weakest(0.80-5.70 m s-1),the maximum wind speed appeared at 750 m.As the concentration of pollutants rose,the temperature inversion structure of the boundary layer was formed,forming a stable atmosphere stratification.In spring,the temperature inversion structure generally occurred at night and Ri tended to about0.6.The area where the temperature inversion occurred ranges from near the ground to 1000 m;The temperature inversion structure in autumn and winter had always existed in the whole process of pollution(Ri: 0.6-0.9),and the area where the temperature inversion was mainly concentrated within 100 m;the temperature inversion structure in summer was not very obvious,and the atmosphere was stratified with the overall Ri remained at-0.3.TKE was generally low during the accumulation process,and the development of atmospheric turbulence was weak.Turbulence in spring and winter was relatively strong(TKE: 1.22-1.58 m2 s-2),mainly concentrated between 500-700 m;turbulence in summer was relatively weak(TKE: 1.14-1.32 m2 s-2),mainly concentrated at 100-500 m;it was at an intermediate level in autumn(TKE: 1.16-1.44 m2 s-2),mainly concentrated at 0-200 m.The height of the boundary layer continued to decrease,which restrained the spread of pollution.The height of the boundary layer was generally higher in spring,autumn and winter between 500-800 m;in summer,the height of the boundary layer was generally lower between 150-500 m.As the boundary layer declined,atmospheric humidity increased rapidly,which was conducive to the hygroscopic growth of aerosols and the formation of secondary aerosols,and the accumulation of atmospheric pollution was strengthened.The absolute humidity in summer was the highest,which was basically above 10.0 g m-3;the absolute humidity in spring was second,with a range of 5.0-8.0 g m-3;the absolute humidity in autumn and winter was the lowest and the range was 1.0-3.0 g m-3.During the outbreak phase of pollution,weak mountain winds were generally controlled at night and early morning.Under the cleaning effect of the northerly mountain breeze,pollution decreased to a certain extent.The wind speed was the strongest in winter(1.10-6.40 m s-1),and the maximum wind speed occurred at 300 m;followed by spring and autumn mountain breeze(1.10-5.60 m s-1),the maximum wind speed occurred at 450 m;the weakest in summer(1.10-4.70 m s-1),the maximum wind speed appeared at 700 m.Under the control of weak mountain winds,the atmospheric boundary layer tended to stabilize,and the accumulation of atmospheric pollution accelerated.The height of the boundary layer in summer,autumn and winter was relatively low,and the overall height was between 100-500m;in spring,the boundary layer was relatively high,and the overall height was about 500 m.Under the action of mountain wind,pollution decreased,but it remained at a relatively high level.After that,the wind field turned into plain wind again with the transportation of pollutants in the south,and the accumulation of atmospheric pollution in Beijing increased.In the dissipating stage of pollution,the continuous rise of pollutants was generally broken by the strong systemic north wind.The north wind speed in winter was the strongest(4.60-12.20 m s-1),and the maximum wind speed in the boundary layer was 1000 m;Followed by spring wind speed(3.60-10.60 m s-1),the highest wind speed appeared at a height of 1200 m;the summer was relatively weak(2.40-8.70 m s-1),the highest wind speed appeared at a height of 800 m;the pollution in autumn was sometimes affected by precipitation and wet deposition The wind speed in the boundary layer is relatively small(2.60-4.50 m s-1),and the maximum wind speed appeared at about 500 m.Under the control of strong northerly winds,the local stable boundary layer and temperature inversion structure were destroyed,and the atmosphere was in an unstable state in winter,spring,and summer,and Ri was maintained at-0.6-1.0;in autumn,the atmosphere was affected by precipitation,and Ri was maintained at about-0.3.The height of the boundary layer rose rapidly and the diffusion of pollution is strengthened.The height of the boundary layer in spring still maintained the highest value of 500-1200 m;the heights of the boundary layer in summer,autumn and winter were 100-1000 m.TKE was also strengthened in the dissipation stage.The TKEs of summer and winter were 2.50-4.00 m2 s-2;TKE of spring was 2.00-2.50 m2 s-2;TKE of autumn was 1.00-1.50 m2 s-2,and the boundary layer turbulence was accelerated the diffusion of local pollutants. |
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