Influence of shape on the optical properties of hematite aerosol

Abstract

Mineral dust particles are the second highest emitted aerosol type by mass. Due to changes in particle size, composition, and shape that are caused by physical processes and reactive chemistry, optical properties vary during transport, contributing uncertainty in the calculation of radiative forcing. Hematite is the major absorbing species of mineral dust. In this study, we analyzed the extinction cross sections of nigrosin and hematite particles using cavity ring-down aerosol extinction spectroscopy (CRD-AES) and have measured particle shape and size distributions using transmission electron microscopy. Nigrosin was also used in this study as a spherical standard for absorbing particles. The size-selected nigrosin particles have a narrow size distribution, with extinction cross sections that are described by Mie theory. In contrast, the size distribution of size-selected hematite particles is more polydisperse. The extinction cross sections were modeled using Mie theory and the discrete dipole approximation (DDA). The DDA was used to model more complex shapes that account for the surface roughness and particle geometry. Of the four models used, Mie theory was the simplest to implement, but had significant error with a 26.1% difference from the CRD-AES results. By increasing the complexity of the models using the DDA, we determined that spheroids had a 14.7% difference, roughened spheres a 12.8% difference, and roughened spheroids a 11.2% difference from the experimental results. Using additional parameters that account for particle shape is necessary to model the optical properties of hematite particles and leads to improved extinction cross sections for modeling aerosol optical properties.

Optical properties of non-absorbing mineral dust components and mixtures

“…Particles were aerosolized using the dry generation method that we have previously described (Veghte and Freedman 2012;Veghte et al 2015). Mineral dust analogs studied included: silicon dioxide (quartz; >99%, Sigma Aldrich), kaolinite, Warren County, Georgia, USA (KGa-2; Clay Mineral Society, West Lafayette, IN, USA); montmorillonite, Gonzales County, TX, USA (STx-1b; Clay Mineral Society, West Lafayette, IN, USA), NX-illite (Arginotec, NX Nanopowder, BCM Notenk€ amper, Munich, Germany), and Arizona test dust (0-3 mm fraction, Powder Technology Inc., Burnsville, MN, USA).…”

“…A cavity ring-down aerosol extinction spectrometer was used to determine the extinction cross sections of the size-selected particles at 643 nm as described previously (Veghte and Freedman 2012;Veghte et al 2015). The characteristic ring-down time for our instrument is 165 ms, corresponding to an effective path length longer than 48 km.…”

“…DDA calculations were performed using DDSCAT 7.2 developed for FORTRAN-90 Flatau 1994, 2010). For calculations of the scattering properties of roughened particles, an average of 196 scattering angles is required to obtain convergence of each extinction cross section (Hajihashemi and Jian 2013;Veghte et al 2015). Because spheroidal particles have multiple symmetry axes compared to roughened particles, fewer particle orientations are needed.…”

“…As a result, only 24 scattering angles were used for the spheroidal particles due to the symmetry of the particles. This number corresponds to the same number of particle orientations as used for rough, asymmetric particles in our previous study (Veghte et al 2015).…”

“…Cavity ring-down aerosol extinction spectroscopy (CRD-AES) has been previously used to study mixed organic/salt particles (Riziq et al 2007;Freedman et al 2009;Lang-Yona et al 2009;Mellon et al 2011) and mineral dust aerosol (Attwood and Greenslade 2011;Veghte and Freedman 2012;Veghte et al 2015). CRD-AES has a high sensitivity due to having a long effective path length and independence from laser output intensity (Pettersson et al 2004;Riziq et al 2007;Li et al 2011).…”