A simplified model to predict the effects of aggregation on the absorption properties of soot particles

An exact electrostatics formulation for sphere clusters is used to predict the Rayleigh-limit radiative absorption properties of soot aggregates. In the near to mid IR wavelengths, it is shown that aggregation can result in absorption cross sections that are significantly larger than that predicted by an independent-sphere (Rayleigh-Gans) model. The relative increase in absorption increases with the number of spheres in the aggregate, and reaches an asymptote for aggregates containing 100-200 spheres. A simplified correlation is developed to predict the aggregate absorption cross section as a function of number of spheres and refractive index. Implications of the effects of aggregation on absorption and emission of thermal radiation by soot in flame and atmospheric environments are discussed.