Evolution Of Atmospheric Boundary Layer Over Typical Terrain And Its Impact On Air Pollution

Boundary layer height(BLH)is one of the key meteorological factors that affect the formation and development of atmospheric pollution.Its magnitude determines the vertical diffusion height of pollutants,near-surface concentration and load amount,and it can affect the atmospheric chemical process to some extent by compress water vapor and precursors concentration.However,China has a vast territory and variety of landforms.Due to the lack of real-time observation of the boundary layer with long time series and high spatial and temporal resolution under the same observation standard,it is impossible to accurately understand the evolution characteristics of the boundary layer under different topography in China,which affects the research process of regional pollution formation mechanism.To make up for the deficiencies of previous research,based on the laser radar cloud altimeter,the long time series of high-resolution boundary layer data were obtained.Combined with meteorological sounding and pollutant data,the evolution of the boundary layer and its causes under four different terrain conditions were analyzed.Taking Beijing as an example,the reasons for the formation of the BLH differences in urban and suburban areas and their influence on pollution distribution are further analyzed,and the southwest pollutants transport in residual layer(RL)on the inducing of heavy pollution events in Beijing is explained,which provide efficient and scientific advice for regional pollution control.The main conclusions are as follows:(1)The boundary layer has different evolution characteristics under different terrain conditions.The BLH diurnal variation presents an obvious unimodal structure in mountains,plains and basins.The BLH in mountain site is significantly higher in spring than in other seasons,up to 849 m,while in summer,autumn and winter are 695 m,628 m and 607 m,with a daily range of 491 m.The BLH at the plain site is 696 m and 688 m in spring and summer,and 616 m and 544 m in autumn and winter,respectively,with a daily range of 590 m.Because of unique basin topography,the change of BLH in Chengdu during the four seasons is small and maintained at about 600 m,and the daily range of the boundary layer is only 314 m.The statistical diurnal changes of the BLH in the grassland underlying surface remained basically unchanged in autumn and winter.Through analysis of shear term,buoyancy term and frequency of gradient Richardson number larger than 1,this study found out that the reason for the insignificant BLH change in Chengdu in four seasons is mainly due to the relatively stable degree of atmospheric turbulence.The higher boundary layer in Xinglong spring has a lot to do with the larger wind shear in the mountains.(2)There is no significant difference in the BLH between Beijing urban and suburban areas,both are about 600m.However,there is a significant difference in the daily change of the boundary layer between urban and suburban areas.The maximum BLH in the urban area is 200 meters higher than that in the suburbs during the day and reaches its peak 2 hours earlier.The emission of anthropogenic heat in the urban area can cause large differences in the small-scale between the urban and suburban areas of Beijing,10:00-15:00,the estimated value of urban anthropogenic heat can reach 23 W/m²,43 W/m²,63 W/m²,55 W/m²,44 W/m²,and 13W/m² respectively.The poor development of the suburban boundary layer may lead to the same degree of pollution in the relatively clean suburbs as in the urban areas.The insufficient development of the boundary layer in the suburbs at noon is an important meteorological causing of heavy pollution.(3)The diurnal variation of the RL is similar in spring,summer and autumn,and the RL disappears when the MLH reaches its peak in the afternoon.Due to the weak development of the mixing layer in winter,the residual layer can occur at any time during the day and always maintain a thickness of about 600m.The average RL top height in spring,summer,autumn and winter was1155m,1139m,1036m and 1195m,respectively.There was no significant correlation between the PM_2.5 concentration in the RL and the maximum PM_2.5 concentration of the previous day,indicating that the pollutants in the RL did not come from the accumulation of local pollutants of the previous day,or the residual pollutants of the previous day were less.The south wind prevailing in the RL and the wind speed is 2-3 times of that near the ground,which provides favorable conditions for regional transport in the RL.There is a strong correlation between PM_2.5 concentration in the RL and PM_2.5 concentration near the surface when RL disappears(R=0.84),indicating that the enhanced vertical diffusion ability after RL disappears leads to the downward transport of pollutants and the increase of PM_2.5 concentration near the surface.The southwest transport of pollutants in the residual layer can induce the occurrence of heavy pollution events.Related research in this article has deepened understanding of the evolution of the boundary layer under different topography,clarified the difference of boundary layer between urban and suburban areas and the influence of RL transportation on regional pollution,which is good for providing a good reference value for clarifying the formation mechanism of pollution and formulating scientific pollution control policies.