Validation of a novel Multi-Gas sensor for volcanic HCl alongside H(2)S and SO(2) at Mt. Etna

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 cross-comparison 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 HCl-rich 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 Multi-Gas SRM measurements of gas ratios. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00445-017-1114-z) contains supplementary material, which is available to authorized users.