Effects of brown coatings on the absorption enhancement of black carbon: a numerical investigation

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Abstract

Abstract. Using the numerically exact multiple sphere T-matrix (MSTM) method, we explored the effects of brown coatings on absorption enhancement (Eabs) of black carbon (BC) at different wavelengths (λ). In addition, the ratio of the absorption of BC coated by brown carbon (BrC) to an external mixture of BrC and BC (Eabs_internal) is also investigated. In this work, thinly coated BC is defined as that with a BC volume fraction over 20%, and other BC is considered to be thickly coated. Eabs increases with the absorption of coatings, while an opposite trend is observed for Eabs_internal. A much wider range of Eabs is observed for BC with brown coatings compared to that with non-absorbing coatings. As the mass ratio of BrC to BC (MR) is over 13.9, Eabs can exceed 5.4 for BC with brown coatings at λ=0.35µm under a typical size distribution. Specifically, as MR increases to approximately 13.9, Eabs values of larger than 3.96 can be observed at 0.532µm, which is a little higher than the commonly measured Eabs of 1.05–3.5 at this wavelength. Previous studies have focused on the lensing effects of coatings but neglected the blocking effects of absorbing coatings. Eabs_internal can be below 1 at an ultraviolet spectral region for BC with brown coatings, which indicates that the absorption of internally mixed BC is less than that of an external mixture of BrC and BC due to the blocking effects of outer coatings, and we named the blocking effect of absorbing coatings the “sunglasses effect”. In addition, the applicability of a core–shell sphere model is also evaluated for BC with brown coatings. The absorption cross section (Cabs) of thickly coated BC is underestimated by the core–shell sphere model for all wavelengths while the underestimation becomes negligible as the imaginary part of the refractive index of brown carbon (kBrC) becomes very large. The lensing effect and the sunglasses effect are clearly defined. Moreover, the effects of composition ratios and the size distribution are explored at different wavelengths. Our findings can improve the understanding of the absorption enhancement of BC with brown coatings.