A study of radiative properties of fractal soot aggregates using the superposition T-matrix method

We employ the numerically exact superposition T-matrix method to perform extensive computations of scattering and absorption properties of soot aggregates with varying state of compactness and size. The fractal dimension, of, is used to quantify the geometrical mass dispersion of the clusters. The optical properties of soot aggregates for a given fractal dimension are complex functions of the refractive index of the material in, the number of monomers N-s, and the monomer radius a. It is shown that for smaller values of 0, the absorption cross section tends to be relatively constant when D-f < 2 but increases rapidly when D-f > 2. However, a systematic reduction in light absorption with of is observed for clusters with sufficiently large N-s, m, and a. The scattering cross section and single-scattering albedo increase monotonically as fractals evolve front chain-like to more densely packed morphologies, which is a strong manifestation of the increasing importance of scattering interaction among spherules. Overall, the results for soot fractals differ profoundly from those calculated for the respective volume-equivalent soot spheres as well as for the respective external mixtures of soot monomers under tire assumption that there are no electromagnetic interactions between the monomers. The climate-research implications Of our results are discussed. Published by Elsevier Ltd.