Light absorption and scattering of core-shell aerosol particles

Many atmospheric aerosol particles have inhomogeneous chemical compositions, resulting in pronounced differences in their optical properties compared with those of homogeneous particles. It has been proposed to approximate the optical properties of inhomogeneous particles by those of spherical core-shell particles because this includes aspects of the particle inhomogeneities while still being computationally feasible to be implemented in radiative forcing models. Core-shell particles are also receiving a lot of interest to better understand processes creating inhomogeneous particles, such as liquid-liquid phase transition, coagulation, and condensation. However, there are still no systematic studies of the influence of volume and the refractive index of core and shell on scattering and absorption of light by core-shell particles. It also remains unclear under which conditions absorption and scattering by core-shell particles can be approximated by (equivalent) homogeneous particles. In this study, we present systematic simulation results for absorption and scattering of light by core-shell particles. Absorption cross-sections of core-shell particles depend mainly on the volumes and the imaginary part of the refractive index of core and shell, while the real part of the refractive index has only a moderate influence on absorption. Scattering cross-sections of core-shell particles depend strongly on all particle properties and vary substantially. The study highlights general trends found for the absorption and scattering cross-section as a function of core and shell properties. General conditions are formulated that allow one to assess when core-shell particles have optical properties similar to those of homogeneous particles. The results also provide a guideline for using optical properties of core-shell particles in the analysis of light scattering measurements and for radiative forcing and climate models.