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Morphology and optical properties of coated aggregates Soewono, Arka


Soot particles are often internally mixed with non-absorbing organic which enhances light scattering and absorption, but the magnitude of this enhancement is highly uncertain. A better characterization of optical properties of coated soot is critical for interpreting optical soot measurements. Simulations using numerically generated aggregates were performed to assess the impact of coatings on the optical properties. A cluster-cluster aggregation algorithm was used to produce aggregates (Df = 1.78 and 2.1) containing 15–600 primary particles with dp = 30 nm. The optical properties were calculated using the discrete-dipole approximation (DDA). For uncoated aggregates, their optical properties were compared with the Rayleigh-Debye-Gans. It was found that the aggregates with similar fractal dimension and prefactor produced by the two methods showed small variations on the optical cross sections. Two coating models were considered: concentric coating and droplet sphere. For the first model, DDA predicted larger absorption than the volume-equivalent core-shell Mie. The coating also caused increase in the discrepancy between the scattering predicted by DDA and the core-shell Mie. For the second model, the aggregates exhibited less absorption and more scattering compared to the concentrically coated aggregates with similar coating fraction, highlighting the importance of coating configurations on the optical properties of coated soot. Laboratory experiments on particles coated with transparent organic were conducted to investigate the changes in the aggregate structure and scattering. Palas particles (dmobility ≈ 118 nm) were employed as proxy for soot and oleyl alcohol was used as the coating. The results of the experiments confirmed that the aggregate-like particles underwent collapsing as the coating was added. The aggregate was fully collapsed (dmobility ≈ 78 nm) when coating mass ratios ≈ 1.75. The coating also enhanced the scattering of coated Palas by a factor of around 2 – 15 depending on the coating amount. The comparative study between the nephelometer scattering from the experiments and the corresponding volume-equivalent Mie were done. The scattering of the uncoated and coated aggregate when aggregate restructuring still occurred were significantly lower compared to the Mie estimations. As the particles fully collapsed, however, their scattering could be accurately predicted from the Mie theory.

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