Validation of spaceborne estimates of aerosol loading from Sun photometer measurements with emphasis on polarization

Over land, aerosol products from the Polarization and Directionality of the Earth's Reflectances (POLDER) are derived from the multispectral analysis of multiangular measurements of polarized radiance. A retrieval algorithm, developed on the basis of radiative transfer simulations and airborne observations, will process all measurements during the instrument mission. An evaluation of the algorithm performance and possible improvements is needed before the release of the aerosol products to the scientific user communities. A validation effort has been planned to test the derived products against independent measurements. The validation is achieved against ground-based Sun photometer measurements from a worldwide station network. Although both measurements depend on the optical properties of the aerosols, they differ from the surface contribution and from the different viewing geometry. In this paper, we describe the planned procedure for POLDER aerosol product validation, and we discuss possible sources of uncertainties in comparing ground-based measurements with satellite based ones. The study shows that ground-based aerosol products are less affected by uncertainties than satellite-based ones and therefore constitute a reliable ground truth and a valid source of information in the process of the POLDER algorithm improvement. However, because a Sun photometer cannot observe the larger scattering angles, it may not be adapted to validate the refractive index estimate.