The Angstrom exponents of black carbon aerosols with non-absorptive coating: A numerical investigation

Abstract

Black carbon (BC) aerosols exert important effects on regional and global climate, whilst there still are significant uncertainties in their optical properties, particularly the spectral varying Angstrom exponents. This study numerically evaluates the influences of coating morphologies on optical properties of BC with non-absorbing coating, and evaluates the sensitivity of their Angstrom exponents, including extinction Angstrom exponent (EAE), absorption Angstrom exponent (AAE) and single scattering albedo Angstrom exponent (SSAAE), to particle microphysics with the multiple-sphere T-matrix method. Among all microphysical parameters, the shell/core ratio and size distribution play substantial roles in determining the EAE, AAE and SSAAE of coated BC, wherein the dependences of the Angstrom exponents on size distribution are higher. The EAE, AAE and SSAAE of coated BC by non-absorptive materials with different particle size distributions change in ranges of 0.3–1.5, 0.3–2.5 and –0.1–0.9, respectively. The core-shell Mie model can hardly produce accurate results of the Angstrom exponents of coated BC particles, especially the AAE and SSAAE. Our study reveals that BC coated by non-absorbing materials with large size distribution of coated BC may show negative SSAAE values for some coating microphysics, indicating the limitation of separating brown carbon from black carbon with a criterion of negative SSAAE.