Scattering and radiative properties of complex soot and soot-containing aggregate particles

We use the superposition T-matrix method to compute the scattering matrix elements and optical cross-sections for a variety of complex soot and soot-containing aggregate particles in random orientation at a visible wavelength 0.628 μm. It is shown that random variations in the geometrical configuration of monomers in a soot cluster for fixed fractal dimension and prefactor, monomer size, and number of monomers have a rather weak effect on scattering and absorption, at least in the visible part of the spectrum. Thus, the electromagnetic scattering and absorption characteristics of a single cluster realization are sufficient to represent the mean values obtained by averaging over many realizations of the “equivalent” clusters generated for the same fractal parameters. However, the results for the soot clusters differ fundamentally from those calculated for the volume-equivalent soot sphere and for the corresponding external mixture of soot monomers, assuming that there are no electromagnetic interactions between the monomers. We also compute and analyze the scattering and absorption properties of aerosols formed by semi-external aggregation of larger ammonium sulfate, silica, or dust particles with soot clusters as well as semi-external aggregates consisting of several components with different sizes and refractive indices. Depending on its chemical composition and size, the larger particle that is in touch with a soot cluster can strongly influence, or even dominate, the overall optical characteristics of the aggregate. Aggregation can result in stronger extinction, absorption, and scattering cross-sections relative to those computed for the corresponding external mixture. Possibly owing to mutual shadowing, the optical cross-sections of multi-component aggregates are smaller than those of their externally mixed counterparts, but by no more than ∼20%. Implications of our study for analyses of remote sensing observations and atmospheric radiation balance computations are discussed.