A Novel and Inexpensive Method for Measuring Volcanic Plume Water Fluxes at High Temporal Resolution

Abstract

Abstract:Water vapour (H 2 O) is the dominant species in volcanic gas plumes. Therefore, measurements of H 2 O fluxes could provide valuable constraints on subsurface degassing and magmatic processes. However, due to the large and variable concentration of this species in the background atmosphere, little attention has been devoted to monitoring the emission rates of this species from volcanoes. Instead, the focus has been placed on remote measurements of SO 2 , which is present in far lower abundances in plumes, and therefore provides poorer single flux proxies for overall degassing conditions. Here, we present a new technique for the measurement of H 2 O emissions at degassing volcanoes at high temporal resolution (≈1 Hz), via remote sensing with low cost digital cameras. This approach is analogous to the use of dual band ultraviolet (UV) cameras for measurements of volcanic SO 2 release, but is focused on near infrared absorption by H 2 O. We report on the field deployment of these devices on La Fossa crater, Vulcano Island, and the North East Crater of Mt. Etna, during which in-plume calibration was performed using a humidity sensor, resulting in estimated mean H 2 O fluxes of ≈15 kg·s −1 and ≈34 kg·s −1 , respectively, in accordance with previously reported literature values. By combining the Etna data with parallel UV camera and Multi-GAS observations, we also derived, for the first time, a combined record of 1 Hz gas fluxes for the three most abundant volcanic gas species: H 2 O, CO 2 , and SO 2 . Spectral analysis of the Etna data revealed oscillations in the passive emissions of all three species, with periods spanning ≈40-175 s, and a strong degree of correlation between the periodicity manifested in the SO 2 and H 2 O data, potentially related to the similar exsolution depths of these two gases. In contrast, there was a poorer linkage between oscillations in these species and those of CO 2 , possibly due to the deeper exsolution of carbon dioxide, giving rise to distinct periodic degassing behaviour.

The Difficulty of Measuring the Absorption of Scattered Sunlight by H2O and CO2 in Volcanic Plumes: A Comment on Pering et al. “A Novel and Inexpensive Method for Measuring Volcanic Plume Water Fluxes at High Temporal Resolution,” Remote Sens. 2017, 9, 146

“…This value Remote Sens. 2017, 9, 534 3 of 11 differs from the 5.7 × 10 −22 cm 2 /molecule (=19 cm 2 /g) cited by Pering et al [8], but this could simply be a result of using a greater line width for the calculation of the cross-section. In the end, it is the area under each line that will determine the absorbed energy.…”

“…Relative humidity can then be obtained by comparing the actual water vapor partial pressure (or concentration) to the saturation pressure. This equation, or a variant thereof, is probably what Pering et al [8] meant to refer to in their Equation (2), though their equation appears to be reproduced incorrectly.…”

“…The cross-sections of both CO 2 and H 2 O are relatively well-known and available for download from the HITRAN database [11]. For the sake of brevity, and because this was the topic of the original study by Pering et al [8], the following sections will focus on H 2 O, but the discussed concepts apply to CO 2 as well.…”

“…Though barely different from the initial Gaussian, the spectrum of transmitted light is plotted in Figure 2b. Integration over the spectral radiance gives the total incident radiance, which is the parameter that would be measured by a camera or other photodetector (denoted by IAs and IAb by Pering et al [8]). The optical depth a' is then obtained by taking the negative logarithm, analogous to the authors' Equation (1).…”

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Abstract:In their recent study, Pering et al (2017) presented a novel method for measuring volcanic water vapor fluxes. Their method is based on imaging volcanic gas and aerosol plumes using a camera sensitive to the near-infrared (NIR) absorption of water vapor.

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