Invariance of polarized reflectance measured at the top of atmosphere by PARASOL satellite instrument in the visible range with marine constituents in open ocean waters.

The influence of oceanic constituents on the polarized reflectance measured at the top of atmosphere (TOA) over open ocean waters in one visible band is investigated. First, radiative transfer modelling is used to quantify the effects of biomass concentration on the TOA polarized signal for a wide range of observation geometries. The results showed that the TOA polarized reflectance remains insensitive to variations in the chlorophyll a concentration whatever the geometrical conditions in oligotrophic and mesotrophic waters, which represent about 90% of the global ocean. The invariance of the polarized signal with water content is explained by the prevailing influence of both atmospheric effects and skylight reflections at the sea surface on the polarization state of the radiation reaching the top of atmosphere level. The simulations also revealed that multidirectional and polarized TOA reflectances obtained in the visible spectrum are powerful tools for the discrimination between the aerosol optical properties. In the second part of the paper, the theoretical results are rigorously validated using original multiangle and polarized measurements acquired by PARASOL satellite sensor, which is used for the first time for ocean color purposes. First, a statistical analysis of the geometrical features of PARASOL instrument showed that the property of invariance of the TOA polarized reflectance is technically verified for more than 85% of viewed targets, and thus, indicating the feasibility of separating between the atmospheric and oceanic parameters from space remotely sensed polarized data. Second, PARASOL measurements acquired at regional and global scales nicely corroborated the simulations. This study also highlighted that the radiometric performance of the polarized visible wavelength of PARASOL satellite sensor can be used either for the aerosol detection or for atmospheric correction algorithms over open ocean waters regardless of the biomass concentration.