Ability of multiangle remote sensing observations to identify and distinguish mineral dust types: 2. Sensitivity over dark water

We explore the ability of multiangle, multispectral measurements, such as those from the Multiangle Imaging SpectroRadiometer (MISR), to retrieve mineral dust properties. We first investigate MISR sensitivity theoretically, to a selection of medium-mode particle shapes, to single-scattering albedo (SSA), and to the ratio of medium- to large-mode aerosol optical thickness (AOT). Favorable, but not ideal, viewing conditions over dark water are assumed, and new dust optical models are adopted. This study shows sensitivity to differences in the angular spectral signals of medium-mode shapes when column midvisible AOT is greater than $0.15 for components contributing 15–20% or more to the AOT. MISR-like retrievals can also distinguish strongly absorbing (red channel SSA $ 0.94) from less absorbing (SSA $ 0.98–0.99) dust particles. Bimodal size sensitivity results suggest that MISR can distinguish the ratio of medium to larger components in roughly 20% midvisible AOT increments. For five Saharan dust field events over homogeneous, cloud-free dark water near the Cape Verde AERONET station, MISR research retrieval results were dominated by weakly absorbing medium grains. MISR-retrieved spectral AOT was within 0.02 of the Aerosol Robotic Network (AERONET), except for one spatially heterogeneous AOT > 1 case, and one where AERONET retrieved a significant component <0.1 mm in radius. In two cases where AERONET ran retrievals assuming nonspherical (spheroidal) particles, SSA fell within 0.02 of MISR. When included in the MISR Standard Aerosol Retrieval Algorithm, the new dust optical models improve both the coverage of successful retrievals and the spectral AOT agreement with near-coincident AERONET measurements. Large-mode dust models and available field constraint limitations are being addressed in continuing work.