Using the numerically exact multiple sphere T-matrix (MSTM) method, we explored the effects of brown coatings on absorption enhancement (E_abs) 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 (E_{abs_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. E_abs increases with the absorption of coatings, while an opposite trend is observed for E_{abs_internal}. A much wider range of E_abs is observed for BC with brown coatings compared to that with non-absorbing coatings. As the mass ratio of BrC to BC (M_R) is over 13.9, E_abs can exceed 5.4 for BC with brown coatings at λ=0.35 µm under a typical size distribution. Specifically, as M_R increases to approximately 13.9, E_abs values of larger than 3.96 can be observed at 0.532 µm, which is a little higher than the commonly measured E_abs 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. E_{abs_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 (C_abs) 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 (k_BrC) 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.

In this work, an assembly of coating structures is generated for coated black carbon (BC) based on two different coating methods with limited tunable parameters. By defining typical parameters, the proposed coating structures show great agreement with the detailed morphologies of coated BC particles in the atmosphere. Our models can accurately reproduce the measured linear backscattering depolarization ratio (LDR). We noticed that BC with spherical coatings (Configuration E) has difficulty estimating large LDRs, while the ability to reproduce a large LDR is improved by making the coating structure slightly nonspherical (Configuration C). Our models also show a better performance in reproducing the laboratory‐measured absorption enhancement (E_abs), mass absorption/scattering cross sections, and absorption Ångström exponent than the simplified models. In addition, sizable uncertainties in the optical properties of coated BC with different coating configurations are found. The uncertainties in the LDR, E_abs, mass absorption cross section, and absorption Ångström exponent caused by the coating configurations can reach approximately 220%, 35%, 90%, and 20%, respectively. Therefore, complex coating structures should be carefully considered. In this work, various coating structures are represented by limited tunable parameters, which is beneficial to the parameterization of the optical properties of BC with a complex coating structure. Our models can also provide tools for exploring the agreement between calculations and measurements.