Sensitivity of multi-angle photo-polarimetry to vertical layering and mixing of absorbing aerosols: Quantifying measurement uncertainties

The impact of tropospheric aerosols on climate can vary greatly based upon relatively small variations in aerosol properties, such as composition, shape and size distributions, as well as vertical layering. Polarimetric measurements have been advocated in recent years as an additional tool to better understand and retrieve the aerosol properties needed for improved predictions of aerosol radiative forcing on climate. The goal of this study is to introduce a formal approach to assessing the sensitivity of both intensity and polarization signals to absorbing aerosol layering, explicitly accounting for instrument measurement uncertainties. If ignored, sensitivity to aerosol height can introduce biases in aerosol property retrievals at short (ultraviolet or blue) wavelengths; if properly exploited, it may enable the extraction of some basic information on aerosol profiles. Employing a vector successive-orders-of-scattering (SOS) radiative transfer code, we conducted modeling experiments to determine how the measured Stokes vector elements are affected at 446 nm (blue band) by the vertical distribution, mixing and layering of smoke and dust aerosols under the assumption of a simple Lambertian surface and predefined aerosol microphysical properties. We find that smoke and dust vertical layering, if ignored, can introduce biases in radiometric and polarimetric aerosol property retrievals for aerosol optical depth (AOD) above 0.3 (polarimetric) and AOD above 0.5 (radiometric), and should, therefore, be accounted for in retrievals at high aerosol loadings.