Study On The Method Of Obtaining Aerosol Extinction Backscatter Ratio Based On AERONET And Mie Scattering Polarization Lidar

The distribution of atmospheric aerosols changes dramatically with space and time.It is necessary to use various methods to monitor aerosols for a long time in order to further understand the impact of aerosols on climate and to verify different aerosol models.The widely used passive remote sensing monitoring,such as MODIS,provides abundant information for the global aerosol spatial distribution.However,passive remote sensing can not describe the vertical distribution of aerosols,while active remote sensing such as lidar provides the vertical distribution of aerosols.Among them,Raman lidar can accurately obtain the profile of aerosol optical properties,but the influence of sunlight on the signal-to-noise ratio(SNR)increases,and the aerosol observation is obviously limited.However,when using Mie scattering lidar data to retrieve aerosol optical profile,it is necessary to assume the extinction/backscattering ratio(lidar ratio)to solve the inversion problem.In the actual inversion process,the parameter is often assumed to be a fixed value.Since the lidar ratio is related to the complex refractive index,particle size distribution and shape of aerosols,the use of fixed values will lead to significant errors in aerosol retrieval.Therefore,in this study,the sensitivity of AERONET multi-wavelength observation to the content of fine-mode particles is used to study the influence of fine mode ratio on lidar ratio.The contribution of coarse and fine mode particles in aerosol mixture is separated by fine particle optical thickness ratio FMF,and the lidar ratio model based on FMF is established.At the same time,the relationship between depolarization ratio and dust ratio is used to determine the lidar ratio.The aerosol mixture is divided into sand dust and non dust components,and a model of lidar ratio profile based on sand dust ratio is established.Finally,the AOD retrieved from the two models is compared with the optical thickness measured by AERONET in Nanjing and Osaka.The results show that:(1)In the lidar ratio model established by FMF,the lidar ratios of coarse and fine mode particles in Nanjing are 44.77 and 73.66 sr respectively,and the values of 40 and 70 sr provided by CALIPSO are used to establish a new model as a reference,the correlation coefficients of AOD and AOD of solar photometer are increased from 0.44 to 0.69 and 0.72 respectively;the lidar ratio of coarse mode and fine mode particles in Osaka is 43.4 and 70.3 sr,and the value provided by CALIPSO is also used as the reference model.Although the correlation coefficient between the AOD derived by lidar ratio models and AERONET AOD is not significantly improved,the AOD errors obtained by inversion are improved from -0.1108 to 0.0578 and 0.0455 respectively,the proportion of errors between -0.05 and + 0.05 is increased from 22.3% to 36.2% and 40%,and the proportion of errors between-0.1and +0.1 is increased from 49.1%to 60% and 63%.The comparative study shows that the method of obtaining lidar ratio based on FMF is effective,and it is feasible to optimize the lidar ratio by using FMF.(2)Based on the relationship between depolarization ratio and dust ratio,the lidar profile in vertical direction based on dust ratio is established.In the model,the lidar ratios of dust and non dust particles in Nanjing are 49.81 and 73.13 sr,respectively,and a new reference model is established by using the values of 40 and 70 sr provided by CALIPSO.The errors between the retrieved AOD and the solar photometer AOD are improved from-0.0555 to-0.0470 and-0.0375,respectively;the lidar ratios of dust and non dust particles in the model established in Osaka are 52.61 and 67.98 sr,respectively.Although the correlation coefficient between inversion AOD and AERONET AOD is not significantly improved,the AOD error obtained by inversion is improved from -0.1108 to 0.0101 and 0.0116 respectively,the proportion of error between -0.1 and 0.1 is increased from 49.62% to 69.73% and 67.82%,and the proportion of error between -0.05 and 0.05 is increased from 22.52% to 38.31%.It can be concluded that the lidar ratio model based on FMF and the lidar specific profile model based on sand dust ratio both improve the quality of aerosol AOD inversion,which proves the accuracy and effectiveness of the model.Therefore,the lidar ratio profile model based on dust ratio improves the quality of aerosol AOD inversion,which proves the accuracy and effectiveness of the model,and also proves that it is feasible to optimize the lidar ratio by using depolarization data.