Lidar Observation Of Aerosols In Boundary Layer And Free Troposphere At Wuhan,China

Aerosols are widely distributed in the atmosphere and have a wide range of sources.They play an important role in human health,life and radiation transmission of the earth.With the rapid development of industry and the rapid increase of population,the aerosol generated by human activities keeps increasing,leading to a great increase in aerosol pollution,so it is significant to detect aerosols in the atmosphere.Lidar has the characteristics of high temporal and spatial resolution,and suitable for long-term monitor,so it is widely used around the world to monitor aerosols.This paper studied the aerosol daily cycle of the boundary layer,and the relationship between the height of the boundary layer in Wuhan using the polarization lidar data and the PM_2.5?PM₁₀ data of the surface.The geometric characteristics,optical characteristics and seasonal characteristics of free tropospheric aerosol layers as well as the AOD of boundary layer and free troposphere were obtained by using ground-based lidar and CALIPSO spaceborne lidar,respectively.And the sources of aerosols were analysed using the HYSPLIT model.The main contents are as follows:Based on polarized lidar data and ground air quality data,the dynamic changing procedure of aerosol in sunny day is acquired,while the daily variation of boundary layer height,ground aerosol quality concentration,AOD of boundary layer and AOD of free atmosphere is revealed.With the development of boundary layer height,the aerosol on the ground starts to transmit upward,the AOD_0.36-2 strarts to increase and the ground aerosol mass concentration begins to decrease.With the reduction of boundary layer height,the aerosol mass concentration on the surface starts to rise,and PM_2.5 starts to increase about an hour later than PM₁₀,which is caused by the difference in aerodynamic diameter between coarse mode particles and fine mode particles.However,the AOD of the free atmosphere has no obvious diurnal variation,indicating that the entrainment effect has little influence on the free atmosphere in a short time.Free-tropospheric aerosol layers and their seasonal variation over Wuhan,China,are presented based on a 532-nm polarization lidar measurements on 162 days from January through December 2013.Using the aerosol layer selection criterions,a total of402 free-tropospheric aerosol layer events were identified.Two examples are presented in detail:an optically thick dust layer reduced the solar radiation directly,thus inhibiting the height of the boundary layer,and increased the pollution of the ground through sedimentation,and drastically reduced the water vapor content due to the effect of hygroscopic growth;under the action of dust particles,the cloud droplets formed to ice crystals by heterogeneous nucleation at higher temperature(-3°C).The bottom height of layers less than 2km accounts for 68%of the total,the top height within 1?4km accounts for 76%,and the thickness of layers less than 1km accounts for 61%.The optical depth of layers less than 0.1 account for 67%,87%of layer's mean backscattering coefficient was between 0.16?4 Mm^-1sr^-1.46%of the layer's mean particle depolarization ratio are below 0.1,29%are between 0.1 and 0.2,and 25%are distributed between 0.2 and 0.35.The aerosol layer in the free atmosphere show obvious seasonal characteristics,the mean thickness of the aerosol layers in spring is the largest(1.2km),the mean thickness of the layers in autumn is the smallest(0.7km),and in summer and winter is 0.93km and 1km,respectively.Dust layers and mixed dust layers mainly appear in spring,autumn and winter,but seldom found in summer due to the influence of monsoon.According to the results of HYSPLIT backward trajectories analysis,41%of the air masses at the height of 2km come from northwest of China,31%come from adjacent areas,and 27%come from south of China.The vertical distribution and seasonal characteristics of aerosol layers in Wuhan and its surrounding areas were obtained based on CALIPSO spaceborne lidar data from2013 to 2019.82%of the free atmospheric aerosol layers are under 4km.55%of free atmospheric aerosol are optically-thin layers less than 0.1,while only 9%of layers are optically-thick layers greater than 0.4.Dust is the main source of aerosol in Wuhan and its surrounding areas,especially in spring and winter,the polluted dust and desert dust account for 77%and 71%in spring and winter,and 39%and 54%in summer and autumn,respectively.Secondly,the somke aerosol layer is also an important source of pollutants,accounting for 35%and 25%respectively in summer and autumn.According to the MODIS fire map,the somke in summer and autumn mainly comes from straw burning in the central and eastern China.The monthly mean optical depth of the boundary layer and free atmosphere,as well as their ratios to the optical depth of the troposphere,are obtained based on the ground-based lidar data from 2013 to 2019.The monthly mean ratio of AOD_2-7 to AOD_0-7 was 11?28%.The AOD_0-7 in spring,summer,autumn and winter were 0.50,0.38,0.48 and 0.53,respectively.The optical depth of AOD_2-7 in spring,summer,autumn and winter were 0.13,0.06,0.067 and 0.07,respectively.The ratio of AOD_2-7to AOD_0-7 in spring,summer,autumn and winter were 27%,16%,14%and 13%,respectively.More dust layers occur in spring led a higher ratio of AOD_2-7 to AOD_0-7,and the big AOD values of the boundary layer led a lower ratio of AOD_2-7 to AOD_0-7.