Optical scattering and absorption by branched chains of aerosols

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Abstract
We utilize the Volume Integral Equation Formulation (VIEF) and the method of moments to calculate the electromagnetic scattering and absorption of aerosol particles with branched-chain structures. Two kinds of polarization of the incident electromagnetic wave were considered: the cross- and end-fire polarizations. The numerical results of internal electric field distribution, absorbed power, and extinction and scattering cross sections, obtained from the VIEF, show excellent agreement with the Mie theory for the special case of spherical particles. Comparison between the results of the VIEF and Iterative Extended Boundary Condition Method for very long oriented (elongated) chains of particles also showed good agreement. After validating the accuracy of the VIEF, the absorption characteristics of three branched-chain structures simulated from microscopic pictures of coagulated smoke aerosol particles were calculated. Results showed that the ratio of absorption in the two polarization cases, Pcross-fire/Pend-fire, for very long oriented chain structures is as high as a factor of 4 at lower frequencies (λ = 10 μm). While in the higher frequency (λ = 0.5-μm) case, the ratio of Pcross-fire/Pend-fire is reduced to 2.0. For branched-chain structures, the ratio of Pcross-fire/Pend-fire decreased with the increase in the number of the side branches. These observations show that the frequency, polarization, and structure factors play important roles in determining the optical characteristics of branched chains of aerosol particles.

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