Simulation Study Of Depolarization Ratio Spectral Characteristics Of Dust And Smoke Aerosols

In recent decades,polarization measurements of lidar have been widely used to identify and retrieve aerosols.Depolarization ratio is an important parameter reflecting the non-sphericity of aerosol particles,and defined as the ratio of perpendicular to parallel backscattering intensity with respect to the polarization plane of the emitted laser.However,due to the complex morphology and structure of aerosols,and the impact of other microphysical properties,including the effective radius and complex refractive index,depolarization ratio at a single wavelength has become increasingly difficult to meet the current demand for fine detection.Dust and smoke are the main types of aerosols.Dust aerosol account for 75% of the global aerosol load.The greenhouse effect brought by smoke aerosol is second only to carbon dioxide in the atmosphere.Thus they both have a significant impact on the global climate.In order to better understand the polarization characteristics of aerosols and improve the detection ability of lidar,the following studies have been conducted on dust and smoke in this paper.This study is based on the ellipsoidal model,and simulated depolarization ratio,lidar ratio,and other optical properties of dust and smoke aerosol at typical laser wavelengths using a numerically accurate T-matrix method.The effects of microphysical properties such as aspect ratio,effective radius,and complex refractive index on their spectral characteristics are discussed.The results show that depolarization ratio of dust and smoke aerosol have significant spectral dependence,which is helpful for identifying aerosol types.We define a parameter,the ratio of depolarization ratios(RDR),to express this characteristic.For depolarization ratio at a single wavelength,the effects of effective radius and aspect ratio are not linear,and the imaginary part of the complex refractive index only has a significant impact when the numerical value is large.In contrast,RDR has the following advantages: it also reveals the non-sphericity of particles,and has a clear linear fitting relationship with absorption characteristics.We compared simulation and observation results of parameters such as depolarization ratio under different channels,and found that depolarization ratio,color ratio,and lidar ratio had a good logarithmic fitting relationship in dual channel detection at 532 nm and 1064 nm.We propose a lidar aerosol recognition algorithm,“1β+1α+2δ”,that based on this point.We can use backscatter coefficients(β),extinction coefficient(α),depolarization ratio(δ)at dual channels to improve the retrieval accuracy and range of lidar.On this basis,this article established a relevant calculation plan for aerosol microphysical properties and analyzed two dust events.In the first dust event,we validated the conclusion that RDR of aerosols and the absorption coefficient were highly fitted,and compared the simulated and observed results of the absorption coefficient.In the second dust event,we use “1β+1α+2δ”algorithm to invert the optical parameters at other channels.The results show that“1β+1α+2δ” algorithm can extent invert absorption and backscattering optical properties of aerosols,improving the detection ability of lidar.However,there is still a significant error between the inversion results and observation data,and there is a lack of sufficient quantitative analysis,which requires the combination of more lidar observation data.