Aerosol retrieval over land using a multiband polarimeter and comparison with path radiance method

This study focuses on the development of a new approach to retrieve aerosol properties over land, based on the use of multispectral polarized measurements (0.67–2.2 μm). We use the measurements of the airborne MICROPOL polarimeter during regional aircraft field experiments located in France and dedicated to the study of aerosol (pollutant and mineral dust particles) and surface properties. We have developed a multiband polarization algorithm (MBP) and compared the retrievals with both a path radiance algorithm and Sun photometer data. It is shown that surface polarized reflectance exhibits only a small spectral variation for forward scattering geometries (3% on average, 15–20% for a single view and scattering angle < 110°). The atmospheric contribution at 2.2 μm is small for aerosol optical thicknesses (AOTs) up to 0.15 at 0.67 μm and can be accounted for in the retrieval of surface properties. The 2.2 μm channel therefore enables us to accurately derive the surface polarization in the shorter MICROPOL bands. For the observations we have made, the surface model developed for the analysis of the Polarization and Directionality of the Earth Reflectance (POLDER) measurements overestimates surface polarization from a few to fifty percents. This leads to AOT underestimation by a factor two. For the pollutant aerosol cases (0.065 < AOT < 0.20), the MBP approach retrieves AOT with an accuracy of 0.03, over both natural and urban surfaces. However, this method remains only weakly sensitive to coarse mode particles and fails when dust particles associated with large AOTs (0.5) are considered.