The Ångström Exponent and Single-Scattering Albedo of Black Carbon: Effects of Different Coating Materials

Abstract

In this work, the absorption Ångström exponent (AAE), extinction Ångström exponent (EAE), and single-scattering albedo (SSA) of black carbon (BC) with different coating materials are numerically investigated. BC with different coating materials can provide explanations for the small AAE, small EAE ,and large AAE observed in the atmosphere, which is difficult to be explained by bare BC aggregate models. The addition of organic carbon (OC) does not necessarily increase AAE due to the transformation of BC morphologies and the existence of non-absorbing OC. The addition of coating materials does also not necessarily decrease EAE. While the addition of coating materials can increase the total size of BC-containing particles, the effective refractive index can be modified by introducing the coating materials, so increases the EAE. We found that it is not possible to differentiate between thinly- and heavily-coated BC based on EAE or AAE alone. On the other hand, SSA is much less sensitive to the size and can provide much more information for distinguishing heavily-coated BC from thinly-coated BC. For BC with different coating materials and mixing states, AAE, EAE, and SSA show rather different sensitivities to particle size and composition ratios, and their spectral-dependences also exhibit distinct differences. Different AAE and EAE trends with BC/OC ratio were also found for BC with different coating materials and mixing states. Furthermore, we also found empirical fittings for AAE, EAE, SSA, and optical cross-sections, which may be useful for retrieving the size information based on the optical measurements.

Effects of black carbon morphology on brown carbon absorption estimation: from numerical aspects

“…In this work, R c = 50R g and R c = R g were assumed to represent the film and spherical coatings and are named Model B and Model C, respectively. In our previous study (Luo et al, 2019), we have demonstrated that our proposed models can greatly simulate the internally mixed BC morphologies and reproduce the measured absorption as well. For more details about the algorithm to generate the coated BC, please refer to Luo et al (2019), and the typically generated morphologies are shown in Fig.…”

“…Non-spherical aerosol models show more excellent performance on reproducing the measurements even though the Mie theory was commonly used in remote sensing and climate modeling (Bi et al, 2018;He et al, 2016He et al, , 2015Chakrabarty et al, 2007;Luo et al, 2019). In the atmosphere, BC can be mixed with BrC, and the mixing states are commonly divided into externally mixed and internally mixed.…”

“…For the internally mixing particles, the BC-containing morphologies were generated based on the models proposed by Luo et al (2019). Here we simply describe the algorithm to generate the internally mixing particles.…”

“…Then the coating materials were added based on two coating methods. The first coating method identifies the edge dipoles (BC surface) first, and then adds the coating materials based on a parameter q (Luo et al, 2019):…”

“…BC morphologies considered in this work. The internally mixed particles were generated using the models developed byLuo et al (2019).…”