Light scattering and absorption by fractal aggregates including soot

This paper addresses how well and under what conditions the Rayleigh-Debye-Gans (RDG) approximation describes scattering and absorption of light by fractal aggregates (FA) including soot. The RDGFA theory, which is the prevailing, first order description of this problem, has two assumptions: the monomers, or primary particles, of the aggregate scatter and absorb in the Rayleigh regime, and the aggregate scatters in the diffraction limit weighted by this Rayleigh scattering and absorbs as a system of independent monomer particles. The aggregates studied here are formed via Diffusion Limited Cluster Aggregation (DLCA) and have a fractal dimension D = 1.78 ± 0.04 and prefactor of k0 = 1.35 ± 0.10. The aggregates are a collection of monodisperse spherical monomers with point contacts. Optical calculations were performed with the multiple sphere T-matrix (MSTM) and DDSCAT codes for incident light polarized perpendicular to the scattering plane. The scattering considered is the forward scattering intensity and the angular scattering as parameterized by the scattering wave vector. The total absorption cross section for aggregates is also calculated. This work stresses the systematic study of the effects of the variables of monomers per aggregate, which ranged from one to 502, two monomer size parameters of 0.157 and 0.314, and a wide range of refractive index real and imaginary parts. It also considers soot refractive indices with three representative dispersions. A summary of results for both scattering and absorption includes deviations from RDGFA theory ranging as large as 35% with positive deviations increasing with the real part of the refractive index and negative deviations growing with the imaginary part. These deviation from the RDG limit are shown to be similar to deviations for spheres.