Effects of dust particle internal structure on light scattering

Abstract

Abstract. There is a large variety of internal structures inside atmospheric dust particles, making them inherently inhomogeneous. Such structures may have a large effect on ground-level and atmospheric radiation. So far, dust particle internal structures and their effect on the light scattering properties have proved to be hard to quantify, in part due to challenges in obtaining information about these structures. Recently, internal structures of individual dust particles were revealed through focused ion beam milling and analyzed. Here, we perform a sensitivity study to evaluate the optical impacts of some of the typical internal structures revealed. To obtain suitable model particles, the first step is to generate inhomogeneous particles with varying internal structures by using an algorithm that is based on three-dimensional Voronoi tessellation. The parameters for the particle generation are obtained from studies of real-world Asian dust particles. The second step is to generate homogeneous versions of the generated particles by using an effective-medium approximation, for comparison. Third, light scattering by both versions of these particles is simulated with discrete dipole approximation code. This allows us to see how different internal structures affect light scattering, and how important it is to account for these structures explicitly. Further, this allows us to estimate the potential inaccuracies caused by using only homogeneous model particles for atmospheric studies and remote-sensing measurements. The results show that the effects vary greatly between different kinds of internal structures and single-scattering quantity considered, but for most structure types the effects are overall notable. Most significantly, hematite inclusions in particles impact light scattering heavily. Furthermore, internal pores and hematite-rich coating both affect some form of light scattering noticeably. Based on this work, it seems that it is exceedingly important that the effects of dust particle internal structures on light scattering are accounted for in a wide variety of applications.

Effects of Particle Nonsphericity on Dust Optical Properties in a Forecast System: Implications for Model‐Observation Comparison

“…Several experimental and remote sensing studies have shown that any reasonable distribution of nonspherical particles is superior compared to the spherical particles used by Mie theory when it comes to optical properties of mineral dust (see Nousiainen, 2009, and references therein). Especially, the dust nonsphericity is extensively studied and applied in lidar investigations (e.g., Ansmann et al, 2017;Gaststeiger et al, 2011;Jeong et al, 2016;Kemppinen et al, 2015). However, dealing with nonsphericity is not a completely resolved issue in dust forecast models as most models are still based on the Mie theory.…”

Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event

“…Apparently, clays and iron oxyhydroxides are 10 intimately associated (cemented) with the coarser quartz and feldspar grains (see SEM and TEM results below). This is a very important finding due to the effect that internal mixing (i.e., aggregates) of clays plus iron oxyhydroxides surrounding larger silicate mineral grains has on the radiative properties of desert dust (Sokolik and Toon, 1999;Formenti et al, 2014b;Kemppinen et al, 2015;Zhang et al, 2015;Jeong et al, 2016). Abundant quartz along with small amounts of feldspars, and abundant clay minerals were detected in the silt fraction ( Fig.…”

“…While some researchers consider that the net radiative forcing of mineral dust on the climate system is negative (Gieré and Querol 2010;Allen et al, 2016), others report that under specific scenarios the direct forcing can be positive (Carlson and Benjamin, 1980), leading to regional (Overpeck et al, 1996) or even global warming (Kok et al, 2017). The uncertainties regarding the sign and magnitude of the direct radiative forcing of desert dust are rooted in the fact that they depend on many poorly 20 constrained factors such as: (i) the characteristics of mineral dust (concentration, vertical distribution, PSD, shape, internal/external mixing, and composition/mineralogy) and (ii) external variables such as surface albedo below dust plumes, temperature at ground level, and presence/absence of clouds (Balkanski et al, 2007;Kemppinen et al, 2015). Composition/mineralogy and particle size, as well as mixing state, appear to be the most critical factors controlling dust direct radiative forcing (Zhang et al, 2015).…”

Refractive Index for Asian Dust in the Ultraviolet‐Visible Region Determined From Compositional Analysis and Validated With OMI Observations

“…The mixing rule approximation used to calculate effective optical constants for composite particle could be another source of uncertainty. The validity of the effective optical constants has been discussed in several studies including Liu et al (2014), Mishchenko et al (2014), Kahnert (2015), Kemppinen et al (2015), and Mishchenko et al (2016). Common finding of the studies was that the effective-medium approximation (EMA) could be reasonable when a large number of small inclusions were evenly distributed in a host mineral particle with small refractive-index contrast between the host and inclusion minerals.…”