SATELLITE RETRIEVAL OF AEROSOL PROPERTIES OVER THE OCEAN USING MEASUREMENTS OF REFLECTED SUNLIGHT - EFFECT OF INSTRUMENTAL ERRORS AND AEROSOL ABSORPTION

A major task of several currently existing and planned satellite instr uments is to provide accurate global monitoring of the distribution an d properties of tropospheric aerosols using radiance and/or polarizati on measurements of the reflected sunlight. We use advanced computer si mulations of radiative transfer in a realistic atmosphere-ocean model at a wavelength of 865 nm to examine the sensitivity of several widely used and recently developed retrieval techniques to aerosol absorptio n and instrumental errors. We assume that nonabsorbing (e.g., sulfate) and strongly absorbing (e.g., soot) aerosol components are mixed exte rnally and that the scattering matrix of the mixture is that of the no nabsorbing component, while the only effect of the absorbing component is to reduce the single-scattering albedo. We show that neither algor ithms using multiple-viewing-angle radiance measurements nor analogous polarization measurements or their combination can retrieve the aeros ol single-scattering albedo with sufficient accuracy. However, accurat e retrievals of the aerosol optical thickness, refractive index, and e ffective radius using polarization measurements do not require a preci se knowledge of the aerosol single-scattering albedo, whereas potentia l uncertainties in the single-scattering albedo can strongly influence the accuracy of aerosol retrievals based on intensity measurements al one. Another important conclusion is that the accuracy of aerosol retr ievals based on intensity and/or polarization measurements of the refl ected sunlight is strongly corrupted by instrumental errors. We show t hat nonzero measurement errors can result in the best fit of model com putations to measurement data being obtained with aerosol parameters f ar different from the actual ones. Our results emphasize the importanc e of accurate and stable instrumental calibration and suggest that the absolute radiometric uncertainty should be constrained to about +/-4% or better and the absolute polarization accuracy should be kept to wi thin +/-0.2%. However, less accurate polarization measurements can sti ll be used to estimate the aerosol refractive index and effective radi us with reasonable accuracy. The results of our previous paper [Mishch enko and Travis, 1997] and this one demonstrate the limited capabiliti es of aerosol retrieval techniques based on intensity measurements alo ne and suggest that high-precision spaceborne polarimetry may potentia lly be the only way of retrieving aerosol characteristics with accurac y needed for long-term monitoring of global climate forcings and feedb acks.