Abstract
AbstractRadiative transfer models used in remote sensing and hazard assessment of volcanic ash require knowledge of ash optical parameters. Here we characterize the bulk and glass compositions of a representative suite of volcanic ash samples with known complex refractive indices (n + ik, where n is the real part and k is the imaginary part). Using a linear regression model, we develop a new parameterization allowing the complex refractive index of volcanic ash to be estimated from ash SiO2 content or ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/T). At visible wavelengths, n correlates better with bulk than with glass composition (both SiO2 and NBO/T), and k correlates better with SiO2 content than with NBO/T. Over a broader spectral range (0.4–19 μm), bulk correlates better than glass composition, and NBO/T generally correlates better than SiO2 content for both parts of the refractive index. In order to understand the impacts of our new parameterization on satellite retrievals, we compared Infrared Atmospheric Sounding Interferometer satellite (wavelengths 3.62–15.5 μm) mass loading retrievals using our new approach with retrievals that assumed a generic (Eyjafjallajökull) ash refractive index. There are significant differences in mass loading using our calculated indices specific to ash type rather than a generic index. Where mass loadings increase, there is often improvement in retrieval quality (corresponding to cost function decrease). This new parameterization of refractive index variation with ash composition will help to improve remote‐sensing retrievals for the rapid identification of ash and quantitative analysis of mass loadings from satellite data on operational timescales.
Highlights
Satellite retrievals of volcanic ash clouds are important for hazard assessment and the understanding of volcanic processes
Using a linear regression model, we develop a new parameterization allowing the complex refractive index of volcanic ash to be estimated from ash SiO2 content or ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/T)
In this study we characterize the compositions of a suite of volcanic ash samples and explore the relationship between composition (SiO2 and NBO/T) and the complex refractive index measurements of the same samples reported in Ball et al (2015) and Reed et al (2018)
Summary
Satellite retrievals of volcanic ash clouds are important for hazard assessment and the understanding of volcanic processes. Vogel et al (2017) provided a new comprehensive data set of volcanic ash optical and physicochemical properties in the range of 0.3–1.5 μm 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). In this study we characterize the compositions of a suite of volcanic ash samples and explore the relationship between composition (SiO2 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 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
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call