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Retrieval of refractive indices of ten volcanic ash samples in the infrared, visible and ultraviolet spectral region

par Deguine, Alexandre;Petitprez, Denis;Clarisse, Lieven ;Deschutter, Lise;Fontijn, Karen ;Herve, Hervé
Référence Journal of aerosol science, 167, 106100
Publication Publié, 2023-01-01

Article révisé par les pairs

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Résumé :

Volcanic eruptions can emit large amounts of ash into the atmosphere, which can have significant impacts on infrastructure, human health, agriculture and air traffic. Remote sensing instruments can efficiently detect airborne ash plumes, and the measured spectra can be exploited to obtain information on the physical characteristics of ash (grain size distribution, concentration, optical depth). The key parameter on which all such satellite retrievals depend is the complex refractive index (CRI) which remains one of the largest sources of uncertainty in the retrieval process. Here we present a complementary dataset of refractive indices of volcanic ash to that published by Deguine et al. (2020), to cover a part of the major explosive eruptions occurred during the past 50 years. These CRIs were obtained using an innovative experimental methodology which consists in measuring simultaneously the extinction spectra in the IR and UV/visible domain and the size distribution of ash in suspension in a nitrogen flow. These experimental data are the main input to the retrieval process of CRI. The numerical routine uses Mie theory coupled with Kramers–Kronig relationship to retrieve the imaginary and the real part of the complex refractive index. This methodology has been successfully applied on samples collected from various eruptions and deposits in Indonesia (Kelud), Chile (Chaitén), Italy (Stromboli), Russia (Karymsky), Tanzania (Rungwe, Mount Meru), Ethiopia (Corbetti), Philippines (Taal, Pinatubo) and USA (Mount St. Helens). Significant variations of the real and imaginary part of the CRI are observed according to the chemical composition of the samples. Moreover, the sensitivity of the CRI to chemical composition and mineralogical structure (amorphous/crystalline fraction) has been investigated and shows a strong dependence of the CRI on these parameters.