Modelling radiometric properties of inhomogeneous mineral dust particles: Applicability and limitations of effective medium theories

The effect of inhomogeneous mineralogical composition on the optical properties of mineral dust particles is investigated. More specifically, spheres composed of a non-absorbing mineral with multiple spherical hematite inclusions are considered. The size of the particles, the number of inclusions, and the hematite volume fraction are varied, and the differential and integral optical properties are compared to those computed for homogeneous spheres. The effective refractive index of the homogeneous spheres is obtained (i) by use of four conventional effective medium approximations; and (ii) by freely varying the real and imaginary parts of the refractive index until a best-fit of the scattering matrix elements is achieved for all scattering angles and particle sizes. Among the integral radiometric observables, the single scattering albedo is most sensitive to particle inhomogeneity, while the extinction and scattering efficiency and the asymmetry parameter are rather insensitive. The phase function, the degree of linear polarisation, the linear depolarisation, and, indeed, all elements of the scattering matrix are strongly modulated by particle inhomogeneity. None of the effective medium approaches, not even the best-fit method, are able to reproduce the single scattering albedo and the scattering matrix elements over the entire range of particle sizes.