Angular dependence of aerosol information content in CAPI/TanSat observation over land: Effect of polarization and synergy with A-train satellites

Aerosols affect the radiative transfer in the absorption bands of carbon dioxide (CO2), thereby contributing to the uncertainties in the retrieval of CO2 from space. A Cloud and Aerosol Polarimetric Imager (CAPI) has been designed to fly on the Chinese Carbon Dioxide Observation Satellite (TanSat) and provide aerosol and cloud information to facilitate the measurements of CO2. This study aims to assess the information content about aerosol properties that can be obtained from CAPI's observations of radiance and polarization. We simulate synthetic CAPI observations using the UNified Linearized Vector Radiative Transfer Model (UNL-VRTM), from which the degree of freedom for signal (DFS) and a posteriori error for various aerosol parameters are calculated using optimal estimation theory. The simulation considers different combinations of fine and coarse aerosols and includes detailed treatment for both soil and vegetation surfaces. It is found that CAPI can provide 3 to 4.5 independent pieces of information about aerosol parameters, mainly related to aerosol total volume (or aerosol optical depth), fine mode fraction (fmfv) of aerosol volume, and imaginary part of refractive index for coarse mode aerosols. At directions around back-scattering, aerosol information content is smaller due in part to the large directional surface reflectance. In addition, aerosol DFS also depends on fmfv, aerosol optical depth and solar zenith angle, and such dependence is analyzed for various viewing and azimuth angles. Due to weaker scattering of coarse aerosol, the information content of large particle is relatively less. Therefore, as fmfv decreases, DFS remains large for fine aerosol and increases for coarse aerosol. With larger aerosol optical depth (AOD), more aerosol information content can be obtained, but when AOD increases to a threshold ranging from 0.5 to 1.2, aerosol DFS doesn't increase any more. At larger solar zenith angles (SZA), a longer light path affected by aerosol can slightly increase aerosol information content. Furthermore, the degree of linear polarization (DOLP) is shown to be more sensitive to aerosol properties than reflectance, hence improves CAPI's aerosol retrieval accuracy. The additional information content raised from DOLP measurements ranges from 1 to 1.8 in terms of DFS and reaches the largest in conditions of 0.2<fmfv<0.4 at SZA<60°. The larger AOD and fmfv, the more improvement for characterizing fine aerosol is obtained from polarization due to larger DOLP of fine aerosol scattering (or less for coarse aerosol). If AOD is known a priori (for example, from other A-Train satellites), total DFS for aerosol information content can be improved by 0.8 to 1.6 in most cases, and could exceed 2.0 for conditions of small AOD (<0.2). However, the improvement has little dependence on AOD if AOD is larger than 0.2.