The Multi-GAS technique is the current standard for measuring the composition of volcanic gases. However, they are prohibitively expensive, immediately restricting the measurement and understanding of gas release to financially able organisations. These instruments use non-dispersive (NDIR) spectroscopy to measure CO2 concentrations and electrochemical sensors for other gases, such as SO2 and H2S. Whilst NDIR spectroscopy for CO2 allows for precise measurements with a high temporal resolution, they typically come with a relatively high cost (£1k – £6k). In comparison, electrochemical sensors are more affordable (~£100), but due to their exposure to harsh volcanic conditions, they require frequent replacement and regular calibration. Comparisons between electrochemical and NDIR measurements are additionally challenging, due to differing response times. Therefore, new low-cost techniques which can allow for high accuracy measurements possible of capturing multiple sources need to be developed.

Here, we present the development and initial testing of a new low-cost Multi-GAS instrument, called the SpectroGas, based solely on spectroscopic approaches. The SpectroGas employs broadband cavity-enhanced absorption spectroscopy (BBCEAS) which utilises optical cavities to enhance the interaction pathlength of light with the sample, the first such attempt in volcanology, for the measurement of SO2 and H2S. The overall instrument is lower in cost and allows for high-accuracy spatially distributed measurements at volcanoes.

The first SpectroGas prototype was deployed on the Reykjanes Peninsula, Iceland – a region experiencing frequent volcanic and geothermal activity since 2021. The aim was to assess the instrument’s precision and stability in real volcanic environments by comparing its measurements to those of an Icelandic Meteorological Office (IMO)-owned Multi-GAS and a lower-cost system called the PiGas. Initial results are encouraging. SpectroGas detected peaks in H₂S that matched those observed by PiGas, though the values were consistently higher – typically around 25 ppm compared to PiGas readings of 12 ppm. One likely explanation is the presence of water vapour inside the optical cavity, which could interfere with UV light through absorption or scattering, artificially elevating the apparent H₂S concentration. Addressing this is now a key focus of ongoing refinement.

Planned future deployments will target sites with higher SO₂ concentrations to evaluate performance across a broader range of conditions. The long-term goal is to develop a reliable, low-cost gas sensor that can be built and used more widely, helping to fill monitoring gaps around the world.