Assessing Superspheroids in Modeling the Scattering Matrices of Dust Aerosols

Abstract

Atmospheric dust particles are known to have diverse and irregular morphologies. In order to account for nonsphericity, the spheroidal model with an aspect ratio distribution has been extensively used for modeling the optical properties of dust. The spheroidal model is superior to the spherical shape assumption, but it requires further improvement. In this study, superspheroids' modeling capabilities were systematically examined by comprehensively comparing the spheroid's and superspheroid's scattering matrices. Superspheroids have one more degree of freedom than spheroids and can be nonspherical at an aspect ratio of unity. The invariant imbedding T‐matrix and the improved geometrical optics methods were employed to compute superspheroids' single‐scattering properties with a wide distribution of aspect ratios and a number of roundness parameters. We then assessed the spheroidal and superspheroidal models' applicability for simulating the scattering matrices of 25 dust samples from the Amsterdam‐Granada Light Scattering Database. It was found that extreme aspect ratios for spheroids in reproducing the measurements were unnecessary if superspheroids were used. Even with equi‐probable aspect ratio distribution, superspheroids with constrained roundness parameters (from 2.4 to 3.0) could achieve better performances in concurrently matching six nonzero scattering matrix elements from the laboratory measurements. Moreover, superspheroids demonstrated better performances than spheroids in achieving spectral consistency for modeling dust scattering matrices. Therefore, superspheroids appear to be highly promising for atmospheric radiative transfer and remote sensing applications.

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

“…The incident wavelength is 633 nm. els (Merikallio et al, 2011;Dubovik et al, 2006;Lin et al, 2018). We also compare the spectral performance for the red clay and quartz scattering matrices, and similar results (the same optimal RI at the two incident wavelengths) are obtained.…”

“…11, the most notable feature is that the two optimal cases with different variables are highly consistent; the reproductions of the matrix elements (excluding P 22 ) are quite successful, and the simulated results of P 22 /P 11 for the two optimal cases both deviate from the measurement with scattering angles between 60 • and 140 • . For feldspar, i.e., the most extensively studied dust sample in the AGLSD (Bi et al, 2009;Dubovik et al, 2006;Liu et al, 2013;Lin et al, 2018;Merikallio et al, 2011;Volten et al, 2001), multiple models with appropriate combinations of the particle RI and geometry all result in close agreement with measurements. Reasonable results can be obtained by merely changing the RI even if the geometry is relatively different from reality and vice versa.…”

“…The importance of particle geometry in modeling dust optical properties has been well studied Osborne et al, 2011;Lin et al, 2018), and Sect. 3 indicates the clear role of the RI.…”

“…Although it is difficult but possible to mathematically define the exact shape of an actual dust particle in numerical studies (Kahnert et al, 2014;Lindqvist et al, 2014), the use of a simplified but optically equivalent model is more convenient and easier to process (Nousiainen and Kandler, 2015;Liu et al, 2013). By using the measured particle size information and the assumed refractive index (RI), most studies treat the geometry as an unknown variable and look for geometric parameters that result in simulated optical properties that are consistent with measurements Dubovik et al, 2006;Liu et al, 2013;Lin et al, 2018;Mishchenko et al, 1997;Osborne et al, 2011). 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).…”

“…Liu et al (2014) found that the surface roughness and irregularity also share an optical equivalence. Lin et al (2018) revealed that the scattering matrix elements of different types of dust particles can be achieved by changing only two parameters to specify the geometry of the superspheroids. Such "optical equivalences" are important for practical applications using dust optical properties because we can use those from a relatively simple numerical model, instead those based on the exact and actual dust particles, for downstream remote sensing and radiative transfer applications.…”