Estimating aerosol optical properties over the oceans with the multiangle imaging spectroadiometer: some preliminary...

The multiangle imaging spectroradiometer (MISR) scheduled to be flown on the first platform of the Earth Observing System in 1998 provides an opportunity to enhance considerably the accuracy with which aerosol properties over the ocean can be retrieved through passive sensing from Earth orbit. As opposed to most radiometers in space that scan the earth in a plane normal to the subsatellite path, the MISR will scan the earth simultaneously in nine planes and thus provide the radiance exiting the atmosphere over a given pixel in nine different directions and at four wavelengths. We examine the problem of extracting the aerosol optical thickness (τ(a)) over the oceans from MISR data, and we produce two algorithms, a single-band algorithm and a spectral or two-band algorithm, for deriving τ(a). The algorithms are based on the use of realistic aerosol models as candidates on which to base an estimation of the aerosol optical properties. They take into account all orders of multiple scattering. Simulations suggest that for nonabsorbing or mildly absorbing aerosol (single-scattering albedo ω(a) > 0.90) the error in the recovered τ(a) is ≲ 10%, as long as the candidate models adequately cover the size refractive index distribution range of the expected aerosols. In the special case of a strongly absorbing aerosol (ω(a) ≍ 0.75), the error in τ(a) becomes large; however, the combination ω(a)τ(a) (the scattering optical thickness) can still be recovered with an error of ≲ 20%, although it is always underestimated. The reason for this decrease in accuracy is that multiple-scattering effects are a strong function of ω(a). A simple extension of the two-band algorithm permits the retrieval of the aerosol scattering phase function with surprising accuracy.