Abstract
Biomass burning aerosols mainly consist of black carbon (BC) and organic aerosols (OAs), and some of OAs are brown carbon (BrC). This study simulates the colors of BrC, BC and their mixture with scattering OAs in the ambient atmosphere by using a combination of light scattering simulations, a two-stream radiative transfer model and a RGB (Red, Green, Blue) color model. We find that both BCs and tar balls (a class of BrC) appear brownish at small particle sizes and blackish at large sizes. This is because the aerosol absorption Ångström exponent (AAE) largely controls the color and larger particles give smaller AAE values. At realistic size distributions, BCs look more blackish than tar balls, but still exhibit some brown color. However, when the absorptance of aerosol layer at green wavelength becomes larger than approximately 0.8, all biomass burning aerosols look blackish. The colors for mixture of purely scattering and absorptive carbonaceous aerosol layers in the atmosphere are also investigated. We suggest that the brownishness of biomass burning aerosols indicates the amount of BC/BrC as well as the ratio of BC to BrC.
Highlights
Biomass burning aerosols mainly consist of black carbon (BC) and organic aerosols (OAs), and some of OAs are brown carbon (BrC)
The refractive indices of the tar balls (TBs)-C6, TB-H7 and TB-A8 are obtained from the corresponding literatures, and those reported by Chang and Charalampopoulos[15] are used for the BC
The biomass burning aerosols in the atmosphere always exist in the form of mixtures with different components such as purely-scattering OA, BrC, and BC, and we investigate the effects of aerosol mixing on their colors
Summary
Biomass burning aerosols mainly consist of black carbon (BC) and organic aerosols (OAs), and some of OAs are brown carbon (BrC). This study simulates the colors of BrC, BC and their mixture with scattering OAs in the ambient atmosphere by using a combination of light scattering simulations, a two-stream radiative transfer model and a RGB (Red, Green, Blue) color model We find that both BCs and tar balls (a class of BrC) appear brownish at small particle sizes and blackish at large sizes. Aerosols from vegetation fires, mostly comprising BC and OAs, display a mixture of black, brown, grey and white colors, and this color variation, from blackish to whitish, has been attributed to the relative ratio of BC to BrC and OA2,12 This common thinking is an untested hypothesis, and contradicts a well-known optics theory that absorptive aerosols of any color in the atmosphere become blackish if the layer is thick and dense enough.
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