Wavelength dependence of dust aerosol single scattering albedo as observed by the Compact Reconnaissance Imaging Spectrometer

Observations by the Compact Reconnaissance Imaging Spectrometer (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) over the range 440-2920 nm of the very dusty Martian atmosphere of the 2007 planet-encircling dust event are combined with those made by both Mars Exploration Rovers (MERs) to better characterize the single scattering albedo (omega(0)) of Martian dust aerosols. Using the diagnostic geometry of the CRISM emission phase function (EPF) sequences and the ground truth'' connection provided at both MER locations allows one to more effectively isolate the single scattering albedo (omega(0)). This approach eliminates a significant portion of the type of uncertainty involved in many of the earlier radiative transfer analyses. Furthermore, the use of a first principles'' or microphysical representation of the aerosol scattering properties offers a direct path to produce a set of complex refractive indices (m = n + ik) that are consistent with the retrieved omega(0) values. We consider a family of effective particle radii: 1.2, 1.4, 1.6, and 1.8 mu m. The resulting set of model data comparisons, omega(0), and m are presented along with an assessment of potential sources of error and uncertainty. We discuss our results within the context of previous work, including the apparent dichotomy of the literature values: dark'' (solar band omega(0) = 0.89-0.90) and bright'' (solar band omega(0) = 0.92-0.94). Previous work suggests that a mean radius of 1.8 mu m is representative for the conditions sampled by the CRISM observations. Using the m for this case and a smaller effective particle radius more appropriate for diffuse dust conditions (1.4 mu m), we examine EPF-derived optical depths relative to the MER 880 nm optical depths. Finally, we explore the potential impact of the resulting brighter solar band omega(0) of 0.94 to atmospheric temperatures in the planetary boundary layer.