Optical properties of mixed black and brown carbon aerosols

Abstract

Based on actual atmospheric observations of internal mixing of light-absorbing brown carbon (BrC)-coated black carbon (BC), the optical properties of mixed black and brown carbon aerosols (BBC) were calculated using four mixing models (external mixing, core–shell, Bruggeman, and Maxwell-Garnett models), and changes in their optical properties with wavelength were compared and analyzed. Under the assumption of different volumetric mixing ratios (VR=VBC/VBBC), there is little difference in volumetric absorptive coefficient (Kab) of BBC in these models in the ultraviolet band where both BC and BrC have strong absorption, particularly in the ultraviolet A band. In visible and near-infrared bands, the three internal mixing models significantly reduce the single scattering albedo compared to the external mixing scenario. In addition, the widely used core–shell model was used to evaluate the effects of BrC shell thickness and environmental relative humidity (RH) on the optical properties of BBC. The impacts of these factors are mainly seen in ultraviolet and visible bands. The volumetric extinction coefficient (Kex) decreases with BrC shell thickness under a fixed BC core radius (0.12 µm) in these bands. This is because the radiation reaching the surface of BC particles is reduced under the absorption of less efficient BrC shells (known as the blocking effect), which is different from the BC and light-scattering aerosols internal mixing scenario. Moreover, the Kex and Kab of BBC decrease with RH, which is mainly due to both the increasing thickness of the BrC shell and the change in the complex refractive index of the BrC shell. Based on the assumptions of unchanged BC core parameters and actual observations, the extinction ability increases when BrC particles are more light-scattering in the ultraviolet and visible bands. The increase in extinction is mainly from scattering, rather than absorption. However, the situation is the opposite in the near-infrared band.