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摘要
An understanding of soot aggregate optical properties is needed for the interpretation of optical measurements of soot. A modified cluster-cluster aggregation (MCCA) was used to numerically generate fractal aggregates containing 15-600 primary particles of d(p) = 30nm diameter. For each size, the aggregates with fractal dimension D-f = 1.78 and 2.1 (with k(f) = 2.24 and 2.26, respectively) were selected, and the scattering and absorption cross sections were calculated using the discrete dipole approximation (DDA). For uncoated aggregates, the optical properties of these numerically generated aggregates were compared with the Rayleigh-Debye-Gans (RDG) approximation. It was found that the differences in the optical cross sections obtained from the two methods were small. In addition, it was also found that the aggregates with similar fractal dimension and fractal prefactor exhibit small variations in the absorption and scattering cross sections. For aggregates with transparent coatings, DDA predicted larger absorption than the core-shell Mie model for volume-equivalent spherical particles. The coating-induced absorption enhancement increases with coating thickness from 10% for a 5-nm layer to about 50%-75% absorption enhancement for a coating thickness of 20nm. The presence of the coating on the aggregates also results in an increase in the discrepancy between the scattering predicted by the DDSCAT and the equivalent core-shell Mie model with the scattering discrepancies become larger as the coating thickness increases (the scattering discrepancies increases from 65% to 75% and from 20% to 25% for aggregates with N = 200 and 15, respectively).
- 出版日期2013-3-1
