Validation of a novel Multi-Gas sensor for volcanic HCl alongside H2S and SO2 at Mt. Etna

Abstract

Volcanic gas emission measurements inform predictions of hazard and atmospheric impacts. For these measurements, Multi-Gas sensors provide low-cost in situ monitoring of gas composition but to date have lacked the ability to detect halogens. Here, two Multi-Gas instruments characterized passive outgassing emissions from Mt. Etna's (Italy) three summit craters, Voragine (VOR), North-east Crater (NEC) and Bocca Nuova (BN) on 2 October 2013. Signal processing (Sensor Response Model, SRM) approaches are used to analyse H 2 S/SO 2 and HCl/SO 2 ratios. A new ability to monitor volcanic HCl using miniature electrochemical sensors is here demonstrated. A "direct-exposure" Multi-Gas instrument contained SO 2 , H 2 S and HCl sensors, whose sensitivities, cross-sensitivities and response times were characterized by laboratory calibration. SRM analysis of the field data yields H 2 S/SO 2 and HCl/SO 2 molar ratios, finding H 2 S/ SO 2 = 0.02 (0.01-0.03), with distinct HCl/SO 2 for the VOR, NEC and BN crater emissions of 0.41 (0.38-0.43), 0.58 (0.54-0.60) and 0.20 (0.17-0.33). A second Multi-Gas instrument provided CO 2 /SO 2 and H 2 O/SO 2 and enabled crosscomparison of SO 2 . The Multi-Gas-measured SO 2 -HCl-H 2 S-CO 2 -H 2 O compositions provide insights into volcanic outgassing. H 2 S/SO 2 ratios indicate gas equilibration at slightly below magmatic temperatures, assuming that the magmatic redox state is preserved. Low SO 2 /HCl alongside low CO 2 / SO 2 indicates a partially outgassed magma source. We highlight the potential for low-cost HCl sensing of H 2 S-poor HClrich volcanic emissions elsewhere. Further tests are needed for H 2 S-rich plumes and for long-term monitoring. Our study brings two new advances to volcano hazard monitoring: real-time in situ measurement of HCl and improved MultiGas SRM measurements of gas ratios.

Dynamics of Outgassing and Plume Transport Revealed by Proximal Unmanned Aerial System (UAS) Measurements at Volcán Villarrica, Chile

“…During sampling and measurement, the sensors themselves act like filters and therefore the measured gas concentrations may be different from the true input signals, in particular if these undergo high frequency or rapid variations (Roberts et al, ). The results presented here should therefore be looked at in conjunction with knowledge of the dynamic response of the specific sensors and system integration used.…”

Volcanological applications of unoccupied aircraft systems (UAS): Developments, strategies, and future challenges

“…For gas measurements, speciesspecific differences in sensor response times introduces uncertainties in derived ratios for ground-based measurements [e.g. Roberts et al 2017], and this effect is amplified for UAS-based instruments. With sensor response times of order 10 s or more Roberts et al 2017], and fixed-wing platforms flying at 15-25 m s −1 , UAS-based measurements represent spatially averaged concentrations with potential location uncertainties of up to order 100 m in the direction of travel.…”

“…Roberts et al 2017], and this effect is amplified for UAS-based instruments. With sensor response times of order 10 s or more Roberts et al 2017], and fixed-wing platforms flying at 15-25 m s −1 , UAS-based measurements represent spatially averaged concentrations with potential location uncertainties of up to order 100 m in the direction of travel. Accounting for such effects requires de-Volcanica 3(1): 67 -114. doi: 1 .3 9 9/vol.…”

“…3. 1.67114 convolution of the sensor response (based on quantitative sensor-specific characterisation and platform motion information) to reconstruct the real signal Roberts et al 2017;Rüdiger et al 2018]. Similarly, for multi-rotor platforms, rotor-driven atmospheric mixing results in spatially averaged sampling of poorly constrained plume volumes Villa et al 2016;Yeo et al 2015], with implications for the discrimination of closely spaced emission sources and for representative sampling of particle and aerosol size distributions.…”

Diode laser-based gas analyser for the simultaneous measurement of CO<sub>2</sub> and HF in volcanic plumes

“…Open-path Fourier transform spectroscopy (Burton et al, 2000(Burton et al, , 2007Duffel et al, 2001Duffel et al, , 2003Horrocks et al, 2003) measures both CO 2 and HF directly in the field. In situ sensors such as MultiGas (Aiuppa et al, 2005Shinohara et al, 2005Shinohara et al, , 2008Roberts et al, 2017) have allowed automated measurements of plume gas compositions, primarily CO 2 and S species, but not HF to date, and very fast…”