Effect of morphology on the optical properties of soot aggregated with spheroidal monomers

- Optical properties of soot aerosol aggregated by spheroidal monomers are calculated.
- Optical differences caused by the morphologies of soot monomers are significant.
- Relative deviations reached up to 15% on cross sections, 10% on SSA and −25% on ASY.
- More nonspherical morphology of monomers leads to larger optical relative deviation.
- MAC=7.5 m2/g for spheroidal monomers approaches the measurements of 7.5±1.2 m2/g.

The monomers of fractal aggregated soot particles are usually considered to be standard spheres in simulations, but a number of less regular shapes may be found in some burning conditions. In this paper, we simulated and investigated the optical properties of fresh dry soot particles as the aggregations of spheroidal monomers with different aspect ratios. Their optical properties were calculated using the numerically exact discrete dipole approximation (DDA) method. The simulated results indicated that the optical properties of soot aggregates composed of spheroidal monomers with highly nonspherical morphologies were considerably different from those composed of spherical monomers. The soot aggregates composed of the oblate spheroids with larger aspect ratios or the prolate spheroids with smaller aspect ratios may have led to larger cross sections of extinction, absorption and scattering. In extreme cases with Ra/Rb=3 and Ra/Rb=1/3 for the soot spheroidal monomers, the relative deviations compared to spherical monomers models reached up to 15% for the absorption cross sections, 10% for the single scattering albedo (SSA) and −25% for the asymmetry parameter (ASY). Moreover, by assuming a soot refractive index of 1.95+0.79i, a mass density of 1.8 g/cm3 and a mean volume-equivalent spherical monomer radius of 0.02 µm, the estimated mass absorption cross sections (MAC) of soot aggregates composed of the oblate spheroidal monomers with large aspect ratios (Ra/Rb=3) reached up to 7.5 m2/g, which was closer to the measurements of 7.5±1.2 m2/g than the ~6.5 m2/g determined by the spherical monomers models. For future research with this type of small aggregated aerosol particles, it would be valuable to consider the monomer morphologies used in this paper for their optical simulations.