Modeling and ground-based observations of light absorbing aerosols and their effect on the degree of linear polarization

The impact of tropospheric aerosols on climate can vary greatly based upon small variations in aerosol properties, especially light absorption. Polarimetric measurements have been advocated in recent years as an additional tool to better understand and retrieve the complex set of aerosol properties which are needed for improved predictions of aerosol radiative forcing on climate. The goal of this study is to investigate to what extent polarimetric measurements can be useful in studies of absorbing aerosols such as black carbon and its mixtures with other aerosol species. Employing a newly developed radiative transfer code with polarization PORTAL (POlarimetric Radiative Transfer with Aerosol Loadings), modeling experiments have been conducted to determine how the degree of linear polarization (LP) is affected by varying size distribution, refractive index, and vertical distribution of atmospheric aerosols representative of different urban environments. A special focus was on the effect of the mixing state (external vs. internal mixtures) of black carbon. The results of modeling studies were utilized to interpret LP measurements conducted at selected AERONET sites using a CIMEL sun-sky photometer/polarimeter as well as our measurements conducted in Atlanta in spring of 2005. Results from AERONET sites, along with polarized radiative transfer sensitivity analysis will be presented with a focus on the potential of polarization measurements for characterizing the properties of light absorbing aerosols.