Optical properties of chain-like soot with water coatings--颗粒学报PARTICUOLOGY

Fan, M., Chen, L., Cheng, L., Xu, B., Tao, J., & Li, S. (2019). Optical properties of chain-like soot with water coatings. Particuology, 47, 94-103. https://doi.org/10.1016/j.partic.2018.09.011

Optical properties of chain-like soot with water coatings

Meng Fan ^a, Liangfu Chen ^a, Liangxiao Cheng ^a, Benben Xu ^{a b}, Jinhua Tao ^a *, Shenshen Li ^a *

a State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China
b College of Geomatics, Shandong University of Science and Technology, Qingdao 266000, China

10.1016/j.partic.2018.09.011

Volume 47, December 2019, Pages 94-103

Received 23 January 2018, Revised 19 September 2018, Accepted 21 September 2018, Available online 18 July 2019, Version of Record 7 December 2019.

E-mail: taojh@radi.ac.cn; liss01@radi.ac.cn

Highlights

• Optical properties calculated for aged soot aggregates/cores with water coatings.

• The looser the chain-like soot inclusion is, the more the SPF is estimated.

• P₂₂(Θ)/P₁₁(Θ) is a potential indicator to internal particle (inclusion) morphology.

• The ageing process can enhance the absorption properties of the entire mixtures.

Abstract

In a moist atmosphere, the ageing process of aerosol can make the agglomerated soot particles compact, and cause them to be covered by a water coating. Based on the cluster‒cluster aggregation (CCA) algorithm, the models of chain-like soot with water coatings (Models A to E) were generated in this study. The superposition T-matrix method was employed to calculate their optical properties at 337, 550, 860, and 1060 nm wavelengths, with a focus on the impact of the soot inclusion morphology and water coating. Our results indicate that for particles with a looser soot-inclusion structure, there is a larger difference in the scattering phase function between them and the corresponding particles with a spherical soot core. The largest relative difference reached 51.8% at 337 nm. Impacted by the size parameter, the extinction cross section (C_ext), absorption cross section (C_abs), scattering cross section (C_sca), and single scattering albedo (SSA) increased as the water coating radius (R_water) increased and incidence wavelength decreased. The traditional assumption of a spherical soot core can cause the C_ext, C_abs, and C_sca to be overestimated, and cause the SSA to be underestimated when the incident wavelength is 337 nm. At 1060 nm, the assumption can cause the C_ext, C_abs, and C_sca to be underestimated, and lead the SSA to be overestimated. When the fractal dimension (D_f) of chain-like soot inclusion increased from 1.8 to 2.6, the SSA of the particles with a R_water of 0.20 μm significantly decreased from 0.784 to 0.764 at 1060 nm. Moreover, the thickness of the water coating had a stronger effect on the particles with chain-like soot inclusion at 337 nm than that at 1060 nm. For the 337 nm wavelength, the difference between the C_ext and C_sca in Model B when R_water = 0.30 and 0.20 μm was 0.588 and 0.587 μm², respectively. The differences were only 0.096 and 0.095 μm², respectively, for the 1060 nm wavelength. Based on the results calculated by the superposition T-matrix method, the ratios of P₂₂(Θ)/P₁₁(Θ) for chain-like soot with water coatings are not absolutely equal to 100%. When the D_f value of aggregated soot inclusion is a constant, P₂₂(Θ)/P₁₁(Θ) decreased as the volume ratio of soot inclusion to the water droplet increased. Therefore, the ratio of P₂₂(Θ)/P₁₁(Θ) can be potentially used as an optical indicator to describe the morphology of non-spherical and/or inhomogeneous particles (or inclusion) for internal aerosol, fog, or cloud particles. Generally, although the thickness of the water coating, to a large extent, dominates the optical properties of the internal mixtures, the morphology of aggregated soot inclusion is a key factor for causing uncertainties in optical parameters. This is especially so when the volume ratio of the soot inclusion and water droplet is large, and the structure of the soot inclusion is loose.

Graphical abstract

Keywords

Soot particle; Water coating; Optical property; Superposition T-matrix

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