Optical properties of chain-like soot with water coatings

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 (Cext), absorption cross section (Cabs), scattering cross section (Csca), and single scattering albedo (SSA) increased as the water coating radius (Rwater) increased and incidence wavelength decreased. The traditional assumption of a spherical soot core can cause the Cext, Cabs, and Csca 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 Cext, Cabs, and Csca to be underestimated, and lead the SSA to be overestimated. When the fractal dimension (Df) of chain-like soot inclusion increased from 1.8 to 2.6, the SSA of the particles with a Rwater 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 Cext and Csca in Model B when Rwater = 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 Df 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.