Ultraviolet dust aerosol properties as observed by MARCI

Observations by the Mars Color Imager (MARCI) on board the Mars Reconnaissance Orbiter (MRO) in two ultraviolet (UV, Bands 6 and 7; 258 nm, and 320 nm, respectively) and one visible (Band 1, 436 nm) channels of the 2007 planet encircling dust storm are combined with those made by the two Mars Exploration Rovers (MERs) to better characterize the single scattering albedo (ω0)(ω0) of martian dust aerosols. Exploiting the low contrast of the surface in the UV (and blue) as well as the reduced importance of surface reflectance under very dusty conditions, we utilize the sampling of photometric angles by the MARCI cross-track geometry to synthesize an analog of the classical Emergence Phase Function (EPF). This so-called “pseudo-EPF”, used in conjunction with the “ground-truth” measurements provided by the MERs, is able to effectively isolate the effects of the dust ω0ω0. The motivation for this approach is the elimination of a significant portion of the type of uncertainty involved in many previous radiative transfer analyses. Furthermore, we produce a self-consistent set of complex refractive indices (m=n+ik)(m=n+ik) through our use of an explicit microphysical representation of the aerosol scattering properties. Because of uncertainty in the exact size of the dust particles during the epoch of the observations, we consider two effective particle radii (reff)(reff) to cover the range anticipated from the literature: 1.6 and 1.8 μm. The resulting set of model–data comparisons, ω0ω0, and m   are presented along with an assessment of potential sources of error and uncertainty. Analysis of the Band 1 results is limited to ω0ω0 as a “proof-of-concept” for our approach through a comparison to contemporaneous CRISM EPF results at 440 nm. The derived ω0ω0 are: assuming reff=1.6μm–0.619–0.626,0.648, and 0.765, for Bands 6, 7, and 1, respectively; for reff=1.8μm–0.625–0.635,0.653,0.769, for the same band order. For either reffreff case, the total estimated error is 0.022, 0.019, and 0.010, again for Bands 6, 7, and 1. We briefly discuss our retrievals, including the asymmetry parameter (g) associated with our model phase functions, within the context of previous efforts, with an emphasis on the improved precision of our results compared to those in the literature. We also suggest several applications of our results, including an extension of the dust climatological record using MARCI Band 7 pseudo-EPFs outside of 2007 global dust event. Initial work on this particular application using a sample of 135 pseudo-EPFs near the MERs suggests that optical depth retrievals with a precision in the range 0.2–0.4 may be possible under moderate loading conditions (i.e., τ < 1.5).