The single scattering albedo is a significant parameter obtaining the magnitude and sign of radiative forcing of aerosols, whereas our understanding of single scattering albedo Angstrom exponent (SSAAE) is still limited, especially for black carbon (BC) particles with brown coating. This study employs the accurate multiple-sphere T-matrix method to numerically evaluate the influences of aerosol microphysics on the SSAAE of coated BC aggregates. The SSAAE of coated BC shows strong dependences on absorbing volume fraction of coating, shell/core ratio, and size distribution, whilst it generally exhibits weak sensitivities to BC fractal dimension, BC position inside coating, and coated volume fraction of BC. Higher SSAAE values are seen for coated BC with less absorbing volume fraction of coating, lower shell/core ratio, or smaller particle size. BC particles coated by brown carbon for various size distributions have a wide variation of SSAAE between 350 nm and 550 nm with a range of -0.95-0.48, while the SSAAE at larger wavelengths (between 550 nm and 700 nm) shows higher values. Our study reveals that BC with non-absorbing coating can show negative SSAAE for thick coating or large size, and BC with brown coating may exhibit positive SSAAE for thin coating or small-sized coated BC, indicating the limitation of separating brown carbon from black carbon with a criterion of negative SSAAE. (C) 2020 Elsevier Ltd. All rights reserved.

The estimates of radiative forcing of black carbon (BC) remain great uncertainty, largely due to variations in the absorption enhancement of BC by mixing with organic and inorganic coatings in ambient aerosols. We applied a two-step solvent treatment method that experimentally removed coating materials in aerosol samples to determine the BC absorption enhancement. Aerosol samples were collected at Mt. Tai and a severely polluted urban area (Jinan) in North China Plain (NCP). The mass absorption cross- (MAC) of BC aerosols was determined before and after the coating removal. Three thermal-optical protocols, NIOSH, EUSAAR and IMPROVE, were tested for determining of BC mass and MAC. The EUSAAR protocol gave the optimal values of BC mass concentrations and MAC. The MAC for decoated BC was 3.8 +/- 0.9 and 3.8 +/- 0.1 m(2) g(-1) (Average and 1SD) at 678 nm wavelength at the urban area and Mt. Tai, respectively, and it was consistent with the theoretical calculation for pure BC. The MAC for ambient aerosol samples was enhanced to 7.4 +/- 2.6 and 7.8 +/- 2.7 m(2) g(-1) at Jinan and Mt. Tai respectively. Non - BC coatings could enhance the MAC (E-MAC) by a factor of 2 at both the polluted urban area and mountain summit. The light absorption of BC may be rapidly enhanced from air pollution in severely polluted area, and then it remains relatively constant for aged aerosols at Mt. Tai. Climate model is recommended for amplifying BC absorption by a factor of 2 in East Asia and other areas with intense industrialization and urbanization. (C) 2018 Elsevier B.V. All rights reserved.

Soot particles positively influence radiative forcing due to their strong absorption. Because of their chain-like structure, aggregated soot particles become more compact with the aging process, and the monomers or particles are always covered by water coatings. The optical parameters of two typical soot-water mixtures (i.e., an aggregate with core-shell monomers and a soot aggregate inside a water droplet) at 550 nm were investigated using the superposition T-matrix method, with a focus on the impact of the morphology and water coating of soot aggregates. For the soot aggregate with core-shell monomers, a relationship among the fractal dimension, relative humidity (RH) and monomer number was established and used to calculate optical parameters. The intensity of forward scattering declined with the increasing RH. The Cext, Csca, Cabs and SSA are much more insensitive to RH under higher RH conditions (RH > 90%) than at a lower RH level. In addition, hygroscopic shrinkage and the thickness of water coating have stronger effects on the optical properties of larger aggregated soot at higher RH than lower RH. For another mixing state, the soot aggregate inside a water droplet, the morphology of the soot core plays an important role in the optical properties when the thickness of the water shell is small. When the diameter ratio of the water droplet to the aggregated soot (D_ratio) changes from 1.2 to 2.8, Cext difference increases from 0.23 mu m(2) to 2.36 mu m(2) for particles with N = 100 and 500, whereas the SSA difference decreases from 0.12 to 0.01. If the agglomerated structure of the soot core is not considered, the Cext, Csca and SSA will be underestimated for a relatively small D_ratio of 1.2. Ignoring the soot core in the water droplet could introduce large errors into the calculation of the optical parameters, and ignoring the structure of the aggregated soot core could enlarge the errors.

The effects of coating on black carbon (BC) optical properties and global climate forcing are revisited with more realistic approaches. We use the Generalized Multiparticle Mie method along with a realistic size range of monomers and clusters to compute the optical properties of uncoated BC clusters. Mie scattering is used to compute the optical properties of BC coated by scattering material. When integrated over the size distribution, we find the coating to increase BC absorption by up to a factor of 1.9 (1.8-2.1). We also find the coating can significantly increase or decrease BC backscattering depending on shell size and how shell material would be distributed if BC is uncoated. The effect of coating on BC forcing is computed by the Monte-Carlo Aerosol Cloud Radiation model with observed clouds and realistic BC spatial distributions. If we assume all the BC particles to be coated, the coating increases global BC forcing by a factor of 1.4 from the 1.9 x absorption increase alone. Conversely, the coating can decrease the forcing by up to 60% or increase it by up to 40% by only the BC backscattering changes. Thus, the combined effects generally, but not necessarily, amplify BC forcing.