Sensitivity of multiangle imaging to aerosol optical depth and to pure-particle size distribution and composition over ocean

Multiangle, multispectral remote sensing observations, such as those anticipated from the Earth Observing System (EOS) Multiangle Imaging Spectroradiometer (MISR), can significantly improve our ability to constrain aerosol properties from space. Simulations over cloud-free, calm ocean conditions were studied for pure particles with natural ranges of optical depth, particle size, and indices of refraction. According to the theoretical simulations we can retrieve column optical depth from measurements over calm ocean for all but the darkest particles, with typical size distributions and compositions, to an uncertainty of at most 0.05 or 20%, whichever is larger, even if the particle properties are poorly known. For one common particle type, soot, constraints on the optical depth over dark ocean are very poor. The simulated measurements also allow us to distinguish spherical from nonspherical particles, to separate two to four compositional groups based on indices of refraction, and to identify three to four distinct size groups between 0.1 and 2.0 mu m characteristic radius at most latitudes. The technique is most sensitive to particle microphysical properties in the "accumulation mode" sizes, where particle scattering undergoes the transition from Rayleigh to large-particle regimes for the MISR wavelengths. On the basis of these results we expect to distinguish air masses containing different aerosol types, routinely and globally, with multiangle remote sensing data. Such data complement in situ and field data, which can provide detailed information about aerosol size and composition locally. An extension of this study to mixtures of pure particles is part of continuing work.