An algorithm that retrieves aerosol properties from dual-wavelength polarized lidar measurements

We developed two types of algorithms, backward and forward, to estimate vertical profiles of extinction coefficient at a wavelength of λ = 532 nm for each aerosol type, using information from three-channel Mie-lidar measurements, i.e., copolarization and cross-polarization components at λ = 532 nm and total component (copolarization and cross polarization) at λ = 1064 nm. The mode radii, standard deviations, and refractive index for each aerosol type are assumed in the algorithms. The algorithms have the following main features: (1) Extinction coefficient of total aerosols at a far end is estimated in the backward algorithm, while the value at a far end is prescribed as the boundary condition in the Fernald method. (2) They determine aerosol types, i.e., water soluble, sea salt, or dust, for each layer. (3) The vertical profiles of microphysical properties of aerosols such as lidar ratio are also estimated. The backward algorithm is first applied to the lidar signals only calibrated for the spectral ratio to the total components to derive the calibration constant and aerosol properties under clear-sky condition. Next, the forward algorithm is used to retrieve aerosol properties under cloud bottom. We performed intensive error analyses. The errors for each aerosol component in the extinction coefficient are found to be smaller than 20% (50%) for the backward (forward) algorithms, respectively, when measurement errors are ±5%. The validation of the algorithms from the comparison against sky radiometer measurements over ocean shows that the optical thickness agrees within 2% (10%) for the backward (forward) algorithms.