Characterization of optically effective complex refractive index of black carbon composite aerosols

Abstract Black carbon aerosols, always existing as complicated mixtures of various species, have important impacts on the global and regional climate, whereas their effective aerosol complex refractive index (ACRI) is a must for modeling their radiative effects. Retrieval of optically effective ACRI from accurately calculated scattering and absorption properties of an internal-mixed aerosol model with BC and dust coated with sulfate, roughly representing ambient aged multicomponent BC particles, are performed. The sensitivities of retrieved optically effective ACRI to particle size distribution and composition ratio are explored, and the popular volume weighted average (VWA) method and Bruggeman effective medium (BEM) theory are also considered for comparison. The optically effective ACRIs depicts dramatic variation for monodisperse size parameter larger than π, as opposed to almost a constant for size parameter less than π, whereas the VWA and BEM can hardly provide acceptable effective ACRIs for coarse BC composite particles. The retrieved optically effective ACRI are sensitive to both size distribution and composition ratio, and becomes more sensitive to particle size distribution with aerosols containing more BC content. As BC volume content reaches ∼4%, the VWA overestimates the imaginary part of effective ACRI with significant bias when compared with the optically effective ACRI, indicating overestimated aerosol absorption in most atmospheric environments in aerosol-climate models employing the VWA. We suggest that the optically effective ACRI, rather than the ACRI given by the VWA, should be applied to account for coarse BC-containing particles in the state-of-the-art aerosol-climate models.