MISR Dark-water Aerosol Retrievals of Mineral Dust Properties

Recently we have made progress in characterizing dust aerosol sources and dust plume evolution during transport over ocean, by combining field observations with satellite instruments operating in UV, visible and NIR. MISR multi-angle retrievals contribute by providing information about aerosol optical depth (AOT), single scattering albedo (SSA), angstrom exponent, fraction of coarse to medium-mode aerosols, and fraction of spherical to non-spherical particles. These are of interest in themselves, and can also be used to quantitatively test model-derived dust transport and deposition. We present an extensive sensitivity study of MISR's ability to retrieve dust shape, SSA, and medium-to-large particle ratio, under favorable but not ideal viewing conditions over dark water. Our study adopts the MISR research aerosol retrieval algorithm that, unlike the MISR Standard Aerosol Retrieval Algorithm designed to automatically process the entire MISR global data stream with a fixed climatology, analyzes individual measured or simulated pixel patches of any size, and has the flexibility to test numerous aerosol component combinations. Our theoretical MISR sensitivity studies demonstrate that over ocean, at wind speeds up to 5m/s, MISR can distinguish between plates, grains and spheroid dust-like components, between medium and large dust modes, and between weakly- and strongly absorbing dust models, provided the total column mid-visible AOT is at least about 0.1, and each component contributes more than 15-20% to the AOT. We show from tests of coincident MISR and AERONET observations of dust plumes over dark water near Cape Verde that retrieved MISR AOTs, angstrom exponents, and size distributions are in good agreement with those obtained by AERONET. In one available case where AERONET ran retrievals assuming only spherical particles, and then including non-spherical (spheroidal) particles, the spherical single-scattering albedo (SSA) was significantly smaller than the non-spherical result, which in turn was in good agreement with the MISR-derived SSA value. We also present our investigation of dust property evolution for cases observed by MISR near the source and progressively down-wind, for events during the PRIDE experiment in April 2001, for which transport model calculations and some AERONET measurements/retrievals are also available, and for several less-well-constrained cases of Saharan dust transport west of the North African coast.