Optical behavior of composite carbonaceous aerosols: DDA and EMT approaches

Atmospheric particles may contain a small amount of black carbon (BC) that may affect their optical properties. These optical properties also depend strongly on the particle morphology and internal topology. The efficiency factors for scattering Q_sca, absorption Q_abs, extinction Q_ext, backscattering Q_bk, as well as the asymmetry parameter cos, linear polarization P, and scattering phase functions S₁₁ are analyzed in detail as functions of morphology and internal topology. Backscattering quantities (like Q_bk and P and S₁₁ at large scattering angles) are affected most significantly. The behavior of these quantities depends strongly on the internal mixing of the constituent materials making up the particle. Effective medium theories (EMTs), which assume that the particle is homogeneous and are applied frequently in radiative transfer studies, can underestimate Q_sca and S₁₁ especially when high carbon contents are considered. Contrarily, EMTs also tend to overestimate the values of the asymmetry parameter and Q_abs.