Optical Modeling Of Nonspherical Dust Aerosol

Accurate determination of dust aerosol optical properties is fundamental to atmospheric radiative transfer and quantitative aerosol remote sensing.However,it has long been challenging to characterize the nonsphericity of dust aerosol for optical property parameterization in polarized radiative transfer and remote sensing studies.To circumvent the shortcomings of the conventional spheroidal model,a super-spheroidal model that has one more degree of freedom was developed in this dissertation.The optical properties of the super-spheroidal particles were systematically studied by using a combination of the invariant imbedding T-matrix method(II-TM)and the improved geometric optics method(IGOM).An accurate and efficient simulator was developed to calculate the extinction efficiencies of randomly oriented spheroids.To further investigate the potential superiority of the super-spheroidal model and its applicability in active/passive polarized remote sensing,the following works have been carried out:First,the super-spheroidal model was applied to simulate the scattering matrices of 25 selected dust samples from the Amsterdam-Granada Light Scattering Database to determine the optimal range of the morphology parameters.It was found that the convex super-spheroidal particles(with a roundness parameter ranging from 2.4 to 3.0)can achieve favorable performance in concurrently matching the scattering matrices of various dust samples.Compared to the spheroidal model(a special case of superspheroids with the roundness parameter of 1.0),the extreme aspect ratios were found to be unessential and a simple assumption of equi-probable aspect ratio distribution is sufficient in reproducing the laboratory measurements.Besides,the super-spheroidal model demonstrates better performances than the spheroidal model in achieving the spectral consistency when modeling dust scattering matrices at two wavelengths(441.6 and 632.8 nm).Second,the effects of the shape parameters and refractive index of the superspheroidal particle on the depolarization ratio and lidar ratio have been systematically evaluated.The applicability of the super-spheroidal model in lidar detection was assessed by comparing the simulations to the measurements from the triple-wavelength(355,532,and 1064 nm)high spectral resolution lidar(HSRL)reported in the literature.It was found that the lidar ratio substantially increases as the imaginary part gets larger,exhibiting high sensitivity to the imaginary part of the refractive index.The absorbability of the dust aerosol might be overestimated at short wavelengths(for example,an imaginary part of 0.008)in most radiative transfer models.Such a speculation was supported by recent measurements of dust refractive indices by Di Biagio et al.(2019).By comparing the simulated depolarization ratios with the lidar measurements and the AErosol RObotic NETwork(AERONET)retrievals,the superspheroidal particles were found to have enhanced depolarization ratios that are more close to measurements and are relatively larger than those of the spheroidal particles from the ultraviolet to visible bands(about 8% larger).Importantly,the simulated depolarization ratio of the super-spheroidal model and the lidar measurements both have a maximum at 532 nm wavelength,while the depolarization ratio of the spheroidal model monotonously increases as the wavelength gets larger.Lastly,the effects of the super-spheroidal model on the radiance and polarized radiance on the top of the atmosphere have been systematically investigated.A polarized radiative transfer model based on the adding-doubling method was employed for simulation studies of 15 dust events occurred in three main dust source regions(i.e.,North Africa,Middle East,and the Taklimakan dessert).By comparing the measurements from Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar(PARASOL)with the simulations,it was found that super-spheroids with the roundness parameter ranging from 2.6 to 3.0 achieve much better performances than the spheroidal model in fitting the angular distribution of the PARASOL polarized radiance at 865 nm.Note that,the optimal range of the roundness parameter is similar to those obtained from the simulation studies of the laboratory-measured scattering matrices and the backscattering ratios in the lidar measurements,implying that the super-spheroidal model could be physically selfconsistent.The aforementioned findings indicate that the super-spheroidal model is highly promising in atmospheric polarized radiative transfer and quantitative remote sensing of aerosols.