The Complex Refractive Index of Volcanic Ash Aerosol Retrieved From Spectral Mass Extinction

Abstract

The complex refractive indices of eight volcanic ash samples, chosen to have a representative range of SiO2 contents, were retrieved from simultaneous measurements of their spectral mass extinction coefficient and size distribution. The mass extinction coefficients, at 0.33–19 μm, were measured using two optical systems: a Fourier transform spectrometer in the infrared and two diffraction grating spectrometers covering visible and ultraviolet wavelengths. The particle size distribution was measured using a scanning mobility particle sizer and an optical particle counter; values for the effective radius of ash particles measured in this study varied from 0.574 to 1.16 μm. Verification retrievals on high‐purity silica aerosol demonstrated that the Rayleigh continuous distribution of ellipsoids (CDEs) scattering model significantly outperformed Mie theory in retrieving the complex refractive index, when compared to literature values. Assuming the silica particles provided a good analogue of volcanic ash, the CDE scattering model was applied to retrieve the complex refractive index of the eight ash samples. The Lorentz formulation of the complex refractive index was used within the retrievals as a convenient way to ensure consistency with the Kramers‐Kronig relation. The short‐wavelength limit of the electric susceptibility was constrained by using independently measured reference values of the complex refractive index of the ash samples at a visible wavelength. The retrieved values of the complex refractive indices of the ash samples showed considerable variation, highlighting the importance of using accurate refractive index data in ash cloud radiative transfer models.

A New Parameterization of Volcanic Ash Complex Refractive Index Based on NBO/T and SiO2 Content

“… Note . Samples used in Ball et al () are labeled as Data Set S1, and those used in Reed et al () are labeled as Data Set S2. References where samples have previously been described are included.…”

“…In this study we characterize the compositions of a suite of volcanic ash samples and explore the relationship between composition (SiO 2 and NBO/T) and the complex refractive index measurements of the same samples reported in Ball et al () and Reed et al (). We have used the Reed et al () data set to develop an empirically based parameterization of complex refractive index.…”

“… R 2 for the linear regressions between the real part of the refractive index ( n ) and ash sample nonbridging oxygens to tetrahedrally coordinated cations (NBO/T; upper panel) or SiO 2 (middle panel) versus wavelength of measurement. The n measurements are from Reed et al () and are represented by colored dashed lines (lower panel). R 2 values for the correlation between n and bulk ash composition are represented by the solid lines and for the correlation between n and glass composition by the dashed lines.…”

“…This data set includes a broad range of compositions and provides a useful reference for complex refractive indices in the ultraviolet to near-infrared range but does not include longer wavelengths (midinfrared range), which are necessary for deriving mass loadings from the silicate absorption feature between 8 and 12 μm (Clarisse & Prata, 2016). Complex refractive indices for the samples used in the present study have been presented in Grainger et al (2013), Ball et al (2015), and Reed et al (2018) over a wide spectral range (0.33-20 μm). One of the main challenges in selecting an appropriate refractive index in the event of an eruption is that the volcanic ash composition is typically not known until afterward and composition of the cloud becomes more silicic downwind, as denser crystals settle more rapidly than the glassy components .…”

“…In this study we characterize the compositions of a suite of volcanic ash samples and explore the relationship between composition (SiO 2 and NBO/T) and the complex refractive index measurements of the same samples reported in Ball et al (2015) and Reed et al (2018). We have used the Reed et al (2018) data set to develop an empirically based parameterization of complex refractive index. Finally, we investigate the recent eruptions of Grímsvötn, Iceland (May 2011); Chaitén, Chile (May 2008); and Okmok, Aleutian Peninsula (July 2008), in order to assess the usefulness of our parameterization in infrared satellite ash retrieval schemes.…”