Optical properties of soot aggregates with different monomer shapes

Abstract

The monomer of soot fractal aggregate is usually considered to be sphere, but the monomer shapes are cube and hexagon by some transmission electron microscope (TEM) and scanning electron microscope (SEM) observation. In this paper, the fractal soot models of different monomer shapes (sphere, cube, ellipsoid, hexagonal prism) were established. And the optical properties of models are calculated by discrete dipole approximation (DDA). After systematically comparing the Muller matrix and optical cross section properties between the models, we find that monomer deviation from sphericity does not necessarily lead to further decline of F22(π)/F11(π) even at shorter wavelengths. In other words, the non-sphericity of monomers does not necessarily affect the non-sphericity of whole soot particle. This can provide some implication for lidar remote sensing observation. However, other light scattering matrix elements can keep good consistency. The maximum deviation of extinction cross section of hexagonal prism model is 11.2%. The more the monomer shape deviates from the sphere, the more the optical integral properties of the non-spherical monomer model deviates from the optical integral properties of sphere monomer model. Hence, the difference in optical properties caused by different monomer shapes cannot be neglected when the monomer deviates significantly from a spherical shape. This work is helpful to evaluate the optical properties of soot aggregates more precisely.