SATELLITE RETRIEVAL OF AEROSOL PROPERTIES OVER THE OCEAN USING POLARIZATION AS WELL AS INTENSITY OF REFLECTED SUNLIGHT

Most current and proposed satellite remote sensing of tropospheric aer osols relies upon radiance measurements that are interpreted using alg orithms that determine best fits to precalculated scattered sunlight f or one or more ''standard'' aerosol models. However, the number of dif ferent types of aerosol and the substantial space and time variations typically encountered can pose a severe uniqueness problem even for th e multiple constraints provided by multispectral radiances of a scene at a number of observation zenith angles. Experience with polarimetry remote sensing on planetary missions has demonstrated that the measure ment of polarization as well as the radiance can resolve such uniquene ss problems. We use numerically accurate solutions of the vector radia tive transfer equation for a realistic atmosphere-ocean model to theor etically simulate; several types of satellite aerosol retrievals over the ocean utilizing radiance measurements alone, polarization measurem ents alone, and radiance and polarization measurements combined. We ha ve restricted all simulations to a single near-infrared wavelength of 0.865 mu m and assumed that aerosols are spherical, monomodal, and non absorbing. These simplifications permit a study of practical scope tha t tests the retrieval algorithms under exactly the same conditions, th us clearly demonstrating their relative capabilities. in agreement wit h previous analyses, we have found that radiance-only algorithms using multiple-viewing-angle observations perform far better than those bas ed on single-viewing-angle measurements. However, even multiple-viewin g-angle radiance measurements taken at a single wavelength are not alw ays sufficient to determine the aerosol optical thickness, effective r adius, and refractive index with high enough accuracy. In contrast, hi gh-accuracy, single-wavelength, multiple-viewing-angle polarimetry alo ne is capable of uniquely retrieving all three aerosol characteristics with extremely high accuracy (+/-0.015 in aerosol optical thickness, +/-0.03 mu m in effective radius, and +/-0.01 in refractive index). Fu rthermore, the accuracy of the optical thickness retrieval can be slig htly improved by simultaneously using radiance measurements. Our analy sis demonstrates that algorithms utilizing high-accuracy polarization as well as radiance measurements are much less dependent on the availa bility and use of a priori information and can be expected to provide a physically based retrieval of aerosol characteristics (optical thick ness, refractive index, and size) with accuracy needed for long-term m onitoring of global climate forcings and feedbacks.