Effective refractive indices of water and sulfate drops containing absorbing inclusions

In this study the range of visible refractive indices and single scattering albedos that can be obtained using various mean field formulations for mixtures of two components, where one component absorbs in the visible, is explored. For calculations of global radiative forcing and climate response, such formulations provide a convenient way to take into account internal mixing of aerosol components without the computational burden of a more exact theory, provided that the limits of their applicatibility are kept in mind. Within those limits, the results show that for water and sulfate drops containing dust and soot inclusions, the degree of interactions between the electric fields of neighboring inclusions is not important. However, the shape and size of the inclusions can cause a variation in the effective imaginary refractive index of up to a factor of 3.4 and a variation in the single scattering albedo of up to 0.18 in absolute value. This variation has important consequences for calculating the forcing that clouds comprised of mixtures of water drops and aerosol particles and aerosol layers comprised of internal mixtures of absorbing and non-absorbing components impose on the climate within the context of a general circulation or global climate model.