The Effects of Monomer Size Distribution on the Radiative Properties of Black Carbon Aggregates

Abstract

Black carbon (BC), normally existing as aggregates, significantly affects the Earth radiative forcing, energy balance, and climate by scattering and absorbing both solar radiation and terrestrial emission. The BC particles are usually treated as fractal aggregates with same-sized monomers. However, experimental studies show that monomer diameters of BC normally obey a lognormal distribution ranging from 10 nm to over 100 nm. This study investigates the effects of monomer size distribution on the radiative properties of BC particles. The fractal aggregates are generated by a cluster-cluster aggregation (CCA) algorithm, and the Multiple Sphere T-Matrix (MSTM) method is used to simulate the radiative properties of randomly oriented aggregates. The integral radiative properties of aggregates with different-sized monomers have normal distributions with large standard deviations, and it requires to average radiative properties of over 60 aggregate realizations to represent their ensemble-averaged properties. The aggregates with different-sized monomers exhibit much stronger scattering and absorption than the aggregates with same-sized monomers and the geometric mean diameter, whereas the absorption cross section becomes comparable to that given by aggregates with same-sized monomer and the equivalent volume diameter. Similar phase matrix elements are obtained for the aggregates with different-sized and same-sized monomers. Furthermore, the Rayleigh-Debye-Gans (RDG) approach is significantly challenged for approximating the absorption and scattering cross sections of the aggregates with different-sized monomers, whereas it performs quite accurately for the phase matrix elements.

The optical properties of dust: the effects of composition, size, and structure

“…10 shows the effect of building an aggregate out of monomers with different radii. A similar study was already undertaken by Liu et al (2015) in which soot aggregates composed of 50 to 500 monomers with a 0 ∼ 33(45) ± 1.3(1.1) nm with , Q sca (shape, λ), κ(shape, λ), and the ratio of the latter to κ(sphere, λ) for a compact spherical grain with same mass. D f = 1.8 were considered.…”

“…10 presents the case where there are only four monomers but of larger sizes: a 1 = 0.05 µm and a 2 = 0.1 or 0.5 µm. In the a 2 /a 1 = 10 case, the smallest monomers always represent less than 1% of the total aggregate volume for all the structures considered and in the a 2 /a 1 = 2 case, they represent 4 to 27% of the volume, proportions close to those in Köhler et al (2012) and Liu et al (2015). As in the two previous studies, both Q abs and Q sca are increased and the increase is larger when more small monomers are incorporated into the aggregate which can be likened to an increase of the monomer surface irregularity.…”

Numerical investigation on the Ångström exponent of black carbon aerosol

“…It can obtain the T‐matrix of multiple spheres from the T‐matrices of individual spheres by employing the addition theorem of vector spherical wave functions to account for mutual interaction among the system [ Mackowski and Mishchenko , ], and all information required for particle scattering and absorption properties can be derived from its T‐matrix. The popular and robust MSTM code developed and published by Mackowski and Mishchenko [] has been widely applied to obtain the optical properties of fractal aggregates [ Liu and Mishchenko , , ; Smith and Grainger , ; Liu et al ., ] and is used in this study. Meanwhile, the RDG assumes the monomers to be Rayleigh scatterers and ignores the intercluster multiple scattering [ Sorensen , ; Liu et al ., ].…”

“…Thus, more and more sophisticated geometric models are developed to study the optical properties of those nonspherical and inhomogeneous BC particles [ Chung et al ., ; Dong et al ., ]. For homogeneous cases, the aggregates with overlapping or dispersal monomers have been considered [ Bescond et al ., ; Liu et al ., ]. To account for the coating effects, a few complicated models with different mixing structures were built [ Liu et al ., ; Scarnato et al ., ; Dong et al ., ].…”

Gaussian process and design of experiments for surrogate modeling of optical properties of fractal aggregates

“…Liu et. al [30] proposed two different adjustments for the aggregates with particle size distribution, and found that the one using effective diameter, d ef f , is superior to the other. The scattering and absorption cross sections of aggregates with different-sized monomers are defined accordingly as…”