Electrochemical sensing of volcanic gases

We report here the development and application of a compact "geochemical nose" incorporating electrochemical sensors for gas measurements in volcanic plumes. A novel element of the instrument design is the arrangement of the sensors in a parallel array that enables near-simultaneous exposure and fast response. Data analysis methods were developed that utilise the multi-sensor output currents to extract gas mixing ratio abundances and eliminate cross-sensitivities. Use of filter methods is demonstrated to remove baseline drift or instrument noise. We introduce a new approach for analysis of measurements from sensors that have a slower response time (e.g. HCl), and apply this model to estimate HCl/SO ₂ ratios. We deployed the sensor system at Aso volcano, Japan, detecting emissions from its fumarole field hot crater lake, and a mixed plume. We measured SO ₂, H ₂S, CO and HCl, ranging in abundance from ~10 ²-10 ⁴ppbv. Neither NO ₂ nor Cl ₂ were detected. For the fumarolic gases, molar ratios were measured as follows: H ₂S/SO ₂ is ~0.15, H ₂/SO ₂ ~0.25, CO/SO ₂ ~0.02, HCl/SO ₂ ~0.1. The crater lake plume's H ₂S/SO ₂ is ~0.03. The compositions are discussed in terms of degassing equilibria and plume chemistry. Our instrument design represents a cost-effective, low-power and highly portable system that can be readily adapted for operational surveillance of volcanic gases.