Aerosol Remote Sensing Using Ground-Based Measurements and POLDER Airborne Sensor above Coastal Waters

We conducted in 1994 and 1995 several airborne campaigns with an aircraft version of the POLDER (POLarization and Directionality of Earth Reflectance) instrument. We flew over the Straits of Dover. The main goal of this activity was to see how well we can characterize the atmospheric aerosols using POLDER data in order to achieve atmospheric corrections for ocean color purposes. We also set up atmospheric measurements (solar extinction, sky radiance, and degree of polarization) on which we tested classical aerosol models (Junge size distribution; Shettle and Fenn, 1979) for retrieval. For standard meteorological visibilities, such models allow correct matching of the measurements. For most turbid days, it is shown that a submicron particles haze can be superposed to the continental aerosol background. But we indicated that the SF models enable a good retrieval of the measurements. They thus appear relevant to describe more complex situations. Classically, we first used POLDER images to select an aerosol model based on the spectral dependence of the aerosol reflectance between the red and the infrared for which the water is dark offshore of the Straits. Compared again to the SF models, the spectral dependence observed differ substantially by large angular variations. A detailed analysis showed that these large dispersions are not related to instrumental noise but to natural variation of the aerosols and to residual errors in the Fresnel reflection correction. By averaging, we can also propose SF models. Both ground-based and POLDER-derived aerosol models are consistent in terms of retrieval of the atmospheric functions required for atmospheric corrections. We also tried less successfully a selection based on the polarized reflectance which is a POLDER capability.