  We derive water vapor column abundances and aerosol properties from Mars
Science Laboratory (MSL) ChemCam passive mode observations of scattered sky
light.
  Each ChemCam passive sky observation acquires spectra at two different
elevation angles. We fit these spectra with a discrete-ordinates multiple
scattering radiative transfer model, using the correlated-k approximation for
gas absorption bands. The retrieval proceeds by first fitting the continuum of
the ratio of the two elevation angles to solve for aerosol properties, and then
fitting the continuum-removed ratio to solve for gas abundances. The final step
of the retrieval makes use of the observed CO2 absorptions and the known CO2
abundance to correct the retrieved water vapor abundance for the effects of the
vertical distribution of scattering aerosols and to derive an aerosol scale
height parameter.
  The ChemCam-retrieved water abundances show, with only a few exceptions, the
same seasonal behavior and the same timing of seasonal minima and maxima as the
TES, CRISM, and REMS-H data sets that we compare them to. However
ChemCam-retrieved water abundances are generally lower than zonal and regional
scale from-orbit water vapor data, while at the same time being significantly
larger than pre-dawn REMS-H abundances. Pending further analysis of REMS-H
volume mixing ratio uncertainties, the differences between ChemCam and REMS-H
pre-dawn mixing ratios appear to be much too large to be explained by large
scale circulations and thus they tend to support the hypothesis of substantial
diurnal interactions of water vapor with the surface. Our preliminary aerosol
results, meanwhile, show the expected seasonal pattern in dust particle size
but also indicate a surprising inter-annual increase in water-ice cloud
opacities.
