Modeling the scattering properties of mineral aerosols using concave fractal polyhedra

Abstract

The single-scattering properties of concave fractal polyhedra are investigated, with particle size parameters ranging from the Rayleigh to geometric-optics regimes. Two fractal shape parameters, irregularity and aspect ratio, are used to iteratively construct "generations" of irregular fractal particles. The pseudospectral time-domain (PSTD) method and the improved geometric-optics method (IGOM) are combined to compute the single-scattering properties of fractal particles over the range of size parameters. The effects of fractal generation, irregularity, and aspect ratio on the single-scattering properties of fractals are investigated. The extinction efficiency, absorption efficiency, and asymmetry factor, calculated by the PSTD method for fractal particles, with small-to-moderate size parameters, smoothly bridges the gap between those size parameters and size parameters for which solutions given by the IGOM may be used. Somewhat surprisingly, excellent agreement between values of the phase function of randomly oriented fractal particles calculated by the two numerical methods is found, not only for large particles, but in fact extends as far down in equivalent-projected-area size parameters as 25. The agreement in the case of other nonzero phase matrix elements is not as good at so small a size. Furthermore, the numerical results of ensemble-averaged phase matrix elements of a single fractal realization are compared with dust particle measurements, and good agreement is found by using the fractal particle model to represent data from a study of feldspar aerosols.

Assessing Superspheroids in Modeling the Scattering Matrices of Dust Aerosols

“…Merikallio et al (2011) carried out a comprehensive study of spheroids in optical modeling of mineral dusts and recommended a power law shape distribution for reproducing measurements. Apart from spheroids, many other aerosol models, including the Gaussian random particle (Muinonen et al, 1996;Veihelmann et al, 2006), triaxial ellipsoid (Bi et al, 2009), nonsymmetric hexahedra (Bi et al, 2010), agglomerate debris (Zubko et al, 2013), and fractal polyhedral (Jin et al, 2016;Liu et al, 2013) models, have been developed to improve dust optical modeling. Until now, there has not been a standard or a consistent approach for dust optical modeling, and persistent research efforts have been devoted to developing realistic or simplified dust models (see reviews in Nousiainen, 2009, andKahnert et al, 2014).…”

Using depolarization to quantify ice nucleating particle concentrations: a new method

“…These shapes represent generalizations of common ice crystal habits (Bailey and Hallett, 2009). An idealized dust-like particle with fractal facets was used to model aerosols (Liu et al, 2013). These particles are nonspherical and thus will yield different measured depolarization ratios depending on their orientation in the CASPOL.…”

Scattering matrices of mineral dust aerosols: a refinement of the refractive index impact

“…Different nonspherical shapes have been developed and applied, such as spheroids (Mishchenko et al, 1997;Dubovik et al, 2002;Ge et al, 2011;Merikallio et al, 2011), ellipsoids (Bi et al, 2009;Meng et al, 2010;Kemppinen et al, 2015), and superellipsoids (Bi et al, 2018a). Additionally, more complex and irregular particles have also been considered, e.g., spatial Poisson-Voronoi tessellation (Ishimoto et al, 2010), Gaussian random field (GRF) particles (Grynko et al, 2013), Koch-fractal particles (Liu et al, 2013;Jin et al, 2016), and nonsymmetric hexahedra Liu et al, 2014). These "irregular" geometries as well as spheroids can achieve close agreement with measurements by using appropriate shape parameters or combining the results from multiple shapes, which indicates that certain geometries may be optically similar or equivalent with respect to scattering light.…”