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 Qsca, absorption Qabs, extinction Qext, backscattering Qbk, 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 Qbk 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 Qsca and S11 especially when high carbon contents are considered. Contrarily, EMTs also tend to overestimate the values of the asymmetry parameter and Qabs.