A new Monte Carlo treatment of multiple scattering of light by black carbon aggregates with varying levels of compactness

Abstract. We use a specially designed Monte Carlo (MC) model that includes individual Mueller matrix selection in each scattering event to conduct a comprehensive theoretical investigation of the intensity and polarization of light multiply scattered by an atmospheric layer embedded with black carbon (BC) particles with varying compactness. We find that the spatial distributions of the degree of linear and circular polarization are sensitive to BC particle shape, and that depolarization is more dominant for spherical BC particles than for fractal aggregates. We also find that the layer transmittance is highest for the more spherical BC particle shapes and lowest for the extended fractal aggregates, with an overall difference of approximately 10 % relative to the incident intensity between the highest and lowest values of transmittance. We find that the reflectance exhibits a similar tendency, and that correspondingly, the layer absorptance is lowest for the more spherical BC shapes and highest for the extended fractal aggregates. In addition, we compare the results obtained from our MC simulations with those obtained with the delta-Eddington approximation for the same optical thickness, single scattering albedo, asymmetry factor, and incident zenith angle. We find that there is a relatively small difference between our results and the delta-Eddington results with respect to the layer transmittance and the layer absorbance, but that the layer reflectance obtained with our MC simulations is up to 50 % lower than that obtained with the delta-Eddington approximation. These results could have important implications for radiative forcing estimations, climate modeling, and remote sensing implementations.