A comparison of solutions for light scattering and absorption by agglomerated or arbitrarily-shaped particles

Three approximate solutions for light scattering and absorption by agglomerated or arbitrarily-shaped particles have been investigated by comparing both their solution formulas and numerical results. Ranked according to the soundness of their derivation and resulting level of accuracy, the Iskander-Chen-Penner (I-C-P) solution is the best, followed in order by Purcell-Pennypacker (P-P) and Jones. The Jones solution is in general unreliable due to its inaccurate accounting for multiple-scattering effects. The P-P solution is almost identical to the I-C-P but without the self-interaction term, which has a significant effect on accuracy, especially for nonabsorbing particles. For the I-C-P solution, the range of validity is x < 0.8|(m2 + 5)/(2m2 + 1)| (x = cell size parameter) for a < 10% error on cross sections and near-forward scattering. This range of validity could be extended by 40% for highly absorbing particles with |m| < 2, but reduced by 20% for those with larger |m|. The Maxwell-Garnett relation has been shown to be an accurate optical mixing rule for light scattering by inhomogeneous particles. It is found that randomly-oriented, chain-like agglomerates of small particles, such as flame soot particles and other colloidal aggregates, have unique scattering characteristics. The extinction is roughly the same, whereas total and near-forward angular scattering are N times higher when comparing an agglomerate with the same number of individual spheres. Even randomly oriented, these chain-like agglomerates cannot be modeled by equivalent spheres, and they yield significant depolarized scattering.