Retrieving Dust Aerosol Aspect Ratio with POLDER/PARASOL Measurements

Multi-spectral multi-directional polarimetry instruments, such as the POLarization and Directionality of the Earth's Reflectances (POLDER) on the Polarization & Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL), provide an unprecedented opportunity to retrieve the optical and microphysical properties of aerosol in the atmosphere. We show the deficiency of spherical aerosol model that is widely adopted in retrieving dust aerosol properties, compared with the results based on the spheroidal aerosol model. Although the non-sphericity effect is not prominent for fine mode, the results from the spheroidal models always display better fit not only to the multiangular radiance measurements but also to the multiangular polarization measurements of POLDER in the case of coarse mode regardless of the parameters chosen for the spherical model. Errors due to the spherical model can be quite large in backscattering and some other directions. Moreover, the best-fit aspect ratio of spheroids is retrieved from the global observations by POLDER. The spheroidal model with the best-fit aspect ratio tends to give very accurate prediction of polarization for almost all dusty conditions. This new model would be potentially useful for retrieving dust aerosol optical depth and particle size globally. We also explore an algorithm to retrieve dust aerosol shape, optical depth and size parameter, taking advantage of the multi-spectral multi-angular radiometry and polarimetry instrument, when dust aerosol optical depth is large.