Estimate of the aerosol properties over the ocean with POLDER

The wide field of view imaging spectroradiometer Polarization and Direction ality of the Earth's Reflectance (POLDER) developed by Centre National d'Et udes Spatiales and operated aboard the Japanese heliosynchronous platform A dvanced Earth Observation Satellite (ADEOS) from October 30, 1996, to June 30, 1997, provided the first global systematic measurements of the spectral , directional, and polarized characteristics of the solar radiation reflect ed by the Earth/atmosphere system. These original observational capabilitie s offer an opportunity to enhance the characterization of several component s of the global environment, especially the oceanic and terrestrial vegetal primary production, the aerosol physical and optical properties, and the t ridimensional structure and microphysics of clouds. Here we examine the rem ote sensing of aerosols over the oceans. In a first step the aerosol optica l thickness and Angstrom exponent are derived from the radiance measurement s. In a second step the polarization measurements are used for the retrieva l of the aerosol refractive index. The inversion algorithm assumes spherica l, nonabsorbing particles with monomodal lognormal size distribution. The a dequacy of this modeling is discussed for a representative set of aerosol o bservations. Successful retrieval is generally achieved in the presence of small aerosols with Angstrom exponent larger than similar to 1.0. For such particles, polarization may provide information on the particle refractive index. As the Angstrom exponent of the particle decreases, the data fitting residual errors increase, especially in polarized light, which prevents th e retrieval of the aerosol refractive index. The trends of the discrepancie s point out two shortcomings of the aerosol modeling. The theoretical resul ts systematically underestimate the contribution of small polarizing partic les in the polarization measurements for side-scattering angles ranging fro m 80 degrees to 120 degrees. This indicates very probably that aerosol mode ls have to follow bimodal size distribution. On the other hand, the systema tic trend of the directional behavior of the upward radiance and the lack o f significant rainbow effect in the measurements result probably from nonsp hericity of some large aerosols. Confirmation of these points requires impr oved analysis of the POLDER data.