The Influence of Water Coating on the Optical Scattering Properties of Fractal Soot Aggregates

摘要

The effect of water coating of constituent monomers on the optical single-scattering properties of fractal soot aggregates is investigated numerically using core-mantle theory and approximations involving two effective medium theories. A cluster-cluster aggregation algorithm is used to numerically generate fractal aggregates, and the core-mantle Generalized Multi-particle Mie (GMM) method is used to compute the exact single-scattering properties of soot aggregates with water-coated monomers. Comparisons are then made with results obtained using approximations that combine either the GMM method or the Rayleigh-Debye-Gans (RDG) method with either the Maxwell-Garnett or the Bruggeman effective medium approximation (a total of four approximation methods). The optical properties calculated are the extinction and absorption cross sections, the single-scattering albedo, and the phase matrix of water-coated fractal aggregates; these calculations are done for two wavelengths, 0.628 μm and 1.1 μm. Water coating of the fractal aggregates is shown to increase the extinction and absorption cross sections, the single-scattering albedo, and forward scattering, but decrease backward scattering. The combination GMM + Maxwell-Garnett gives approximations that are quite good over a range of coating thicknesses and aggregate size. The combination GMM + Bruggeman performs less well, overestimating the extinction and absorption cross sections and underestimating the single-scattering albedo. In the case of RDG, the better combination is with the Bruggeman approximation, but the errors involved are greater than with the GMM + Maxwell-Garnett combination. Results from simple idealized calculations indicate that the differences between results with the Maxwell-Garnett and Bruggeman approximations should be even larger in cases of aerosol cores that are less absorptive than soot.