Measuring volcanic ash optical properties with high-spectral resolution infrared sounders: role of refractive indices

Hyperspectral infrared sounders like IASI are used to track and quantify volcanic ash in the atmosphere. The retrieval process of physical quantities like particle radius and mass depends critically on the assumed spectrally dependent complex refractive indices that are used. Traditionally, the Pollack et al. (1973) dataset were used almost exclusively. These indices are however based on measurements of rock slabs and in recent years two datasets have become available from laboratory measurements of ash in suspension, the Reed et al. (2018) and Deguine et al. (2020) dataset. In this work, we compare for the first time the three most important datasets of CRI with respect to the three most common ash types (basaltic, andesitic and rhyolitic). The results show significant influence of the dataset used on the retrieved physical quantities. When it comes to basaltic and andesitic ash, both the Deguine and Reed samples outperform Pollack in terms of able to reconstruct the satellite observed spectra. However, all datasets overestimate the extinction near 1250 cm⁻¹, which could possibly be related to the lack of sensitivity of spectrometers (water vapour continuum) leading to a poor signal over noise ratio in this spectral region. While this is not a guarantee that the retrieved quantities are closer to the physical reality, being able to reconstruct the observed spectra is a prerequisite of constructing a consistent physical model. Finally, a case study on the 7 May 2010 plume of the Eyjafjallajokull eruption is presented. For this case study, the differences are found to be mostly related in retrieved altitudes. It is clear that while the availability of CRI based on ash suspended in air is an important milestone, a lot of further research is needed to strengthen the theoretical basis of infrared retrievals of volcanic ash. A comprehensive database of reliable in-situ measurements of volcanic clouds would in this perspective be most welcome.

A. Deguine, D. Petitprez, L. Clarisse, S. Gudmundsson, V. Outes, G. Villarosa, and H. Herbin, “Complex refractive index of volcanic ash aerosol in the infrared, visible, and ultraviolet,” Applied Optics, vol. 59, no. 4, p. 884, jan 2020.

J. B. Pollack, O. B. Toon, and B. N. Khare, “Optical properties of some terrestrial rocks and glasses,” Icarus, vol. 19, no. 3, pp. 372–389, jul 1973.

B. E. Reed, D. M. Peters, R. McPheat, and R. G. Grainger, “The complex refractive index of volcanic ash aerosol retrieved from spectral mass extinction,” Journal of Geophysical Research, vol. 123, no. 2, pp. 1339–1350, jan 2018.