Retrieval of aerosol single scattering albedo with high resolution polarimetric O2 A-band observations

This paper investigates a satellite based method for retrieving aerosol single scattering albedo (SSA) using high spectral resolution polarimetric measurements in the O$_{2}$ A-band. It is shown that both intensity and the degree of linear polarization are sensitive to aerosol SSA and this sensitivity varies across the O$_{2}$ A-band spectrum and therefore according the O$_{2}$ optical depth. This variation provides a possible way to separate the dependence of the measurements on SSA providing a basis for its retrieval. In addition, since the reflected radiation deep within absorption lines is scattered primarily from the upper atmosphere whereas reflected radiation in the continuum is scattered primarily from the lower atmosphere, some information on the aerosol absorption profile is possible. Polarimetric measurements can help further constrain SSA retrievals do to the differing sensitivities of unpolarized and polarized measurements to SSA and the dependence of the degree of linear polarization on the vertical distribution of aerosol scattering. It is shown that the sensitivities presented are highly dependent on the instrument's spectral resolution, i.e. its capability to resolve a large enough range of O$_{2}$ optical depths. An information content analysis performed within an optimal estimation framework combined with an experimental retrieval using simulated observations reveals the strict but realistic noise and spectral resolution requirements for a successful retrieval of aerosol SSA. Finally, it shown that by using a synergistic approach with a multispectral/multiangle polarimeter and an O$_{2}$ A-band spectrometer a complete retrieval of aerosol optical thickness along with scattering and absorption properties is possible and appropriate for the NASA Glory (scheduled) and OCO (replacement pending) A-Train missions.