Information content of near-infrared spaceborne multiangular polarization measurements for aerosol retrievals

The information content of near-infrared spectral multiangular polarization measurements, utilized in the POLDER operational aerosol algorithm, for retrieving aerosol microphysical and optical properties is assessed. It is found that spectral measurements contain just one unique piece of microphysical information, while multiangular measurements add a second and polarization observations permit the constraint of a third microphysical parameter. In addition, it is demonstrated that the relationship between the aerosol single scattering properties and the aerosol microphysical parameters is nonunique. The implications of these findings for aerosol microphysical retrievals are demonstrated through the development of a retrieval for the aerosol optical depth, the index of refraction and a parameter describing the ratio of the number concentration of accumulation mode aerosols to those in the coarse mode. Application of the technique centers on the explicit weighting of individual measurements by rigorous estimates of their uncertainties offering the benefit of a realistic, dynamic retrieval error estimate. The aerosol optical depth, refractive index and number concentration ratio can respectively be estimated to within 15%, 5% and one order of magnitude over oceans. Error estimates are shown to increase by 50% over a mixed forest land surface type. The results further highlight a possible limitation of the measurements at low optical depths below 0.1 where the retrieval errors approach 100% for the optical depth and index of refraction and several orders of magnitude for the number concentration ratio.