Study On Discrimination Of Aerosols And Clouds And Aerosols Absorption Properties By Using Dual-wavelength Polarization Lidar Observation

Polarization lidar has exhibited a unique advantage of classifying atmospheric aerosols and clouds over the past several decades.To better study characterization of atmospheric aerosols and clouds,we developed a ground-based dual-polarization lidar system that can simultaneously detect polarization measurements at wavelengths of355 nm and 532 nm.The results show that the depolarization ratios（DRs）at 355 nm and 532 nm markedly differ for typical types of aerosols and clouds in the atmosphere.The depolarization ratio（DR）of dust aerosol is high at 532nm but lower at 355nm.For anthropogenic aerosol,the depolarization ratio at 532nm is less than that at355nm.The depolarization ratio of ice cloud at 532nm is larger than those of dust aerosol.However,for water cloud the depolarization ratio at 355 nm is higher than those at 532 nm.After calculating the ratio of depolarization ratio at 532nm to that at 355nm(?₅₃₂/?₃₅₅),we found that for non-spherical particles the ratio of DRs at 532 nm and355 nm were 2.87±1.35（ice cloud）and 1.51±0.29（dust aerosols）,respectively.However,for spherical particles the ratios were 0.43±0.26（water cloud）and0.56±0.05（air pollutants）.Consequently,we proposed a simple reliable method for classifying atmospheric aerosols and clouds from polarization measurements observed by the developed lidar system.The proposed method first distinguishes clouds from aerosols using a combination of the color ratio（CR,532 nm/355 nm）and attenuated backscattering coefficients（ABC）at 532 nm.Then,subtypes of clouds and aerosols are identified based on the ratio of DRs at 532 nm and 355 nm.The results showed that dual-polarization lidar measurements can remarkably improve the classification of atmospheric aerosols and clouds.Moreover,we found a good relationship between the absorption coefficient of aerosols and the ratio of depolarization ratios at 532 nm and 355 nm for dust aerosols.This study illustrates that more information on atmospheric aerosols and clouds can be obtained from polarization measurements at multiple wavelengths by active remote sensing.Aerosol absorption from polarization measurements may be determined by lidar at the ultraviolet and visible wavelengths.