Validation of a novel Multi-Gas sensor for volcanic HCl alongside HS and SO 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 HS/SO and HCl/SO ratios. A new ability to monitor volcanic HCl using miniature electrochemical sensors is here demonstrated. A "direct-exposure" Multi-Gas instrument contained SO, HS and HCl sensors, whose sensitivities, cross-sensitivities and response times were characterized by laboratory calibration. SRM analysis of the field data yields HS/SO and HCl/SO molar ratios, finding HS/SO = 0.02 (0.01-0.03), with distinct HCl/SO 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/SO and HO/SO and enabled cross-comparison of SO. The Multi-Gas-measured SO-HCl-HS-CO-HO compositions provide insights into volcanic outgassing. HS/SO ratios indicate gas equilibration at slightly below magmatic temperatures, assuming that the magmatic redox state is preserved. Low SO/HCl alongside low CO/SO indicates a partially outgassed magma source. We highlight the potential for low-cost HCl sensing of HS-poor HCl-rich volcanic emissions elsewhere. Further tests are needed for HS-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.