Simultaneous Retrieval of Dust Aerosol Optical Depth and Particle Size Using Spaceborne Lidar

Spaceborne lidar such as the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite provides global observation of aerosol backscatter properties. We develop a retrieval algorithm to simultaneously infer the dust aerosol optical depth and particle size from CALIOP layer-integrated depolarization ratio (IDR) and layer-integrated attenuated total color ratio (ICR) data. The IDR is the ratio of attenuated backscatter in two orthogonal polarization directions, and the ICR is the ratio of attenuated backscatter in 1064 nm and 532 nm wavelengths. Particle morphology, size and the aerosol optical depth determine the IDR and ICR. Light scattering and radiative transfer computations show that in the size range of dust aerosol, both the IDR and ICR are sensitive to the effective particle size and optical depth. Therefore, a look-up table (LUT) can be built to map the IDR and ICR onto the effective particle size and optical depth. The LUT is computed by a fast Monte Carlo lidar simulator with the input of dust aerosol single-scattering properties, optical thickness, and atmospheric profile. A hexahedron ensemble model is assumed for dust shape. The two-dimensional cubic spline interpolation and an iteration method are utilized to find an optimal solution in the LUT by matching CALIOP IDR and ICR data with the simulated counterparts. Furthermore, the present retrieved dust aerosol optical depth and effective particle size are compared with other independent retrieval products and measurements.