Feasibility of volcanic CO2, HF, and HCl remote sensing measurements using Fourier-transform infrared spectrometry in scattered sunlight geometry

Carbon dioxide (CO₂) is usually the second most abundant gas in volcanic plumes. Its early dissolution from rising magmas can allow insights into magmatic source regions and subsurface volcanic structures. Due to its chemical inertness, CO₂ measurements are particularly valuable for studying plume chemistry using CO₂ as a mixing tracer. The also relatively abundant volcanic gases, hydrogen fluoride (HF) and hydrogen chloride (HCl), dissolve at shallower depths. Their measurements, along with those of other halogen and sulfur compounds, complement the insights into the volcanic system and plume composition. Continuous measurements of volcanic CO₂, HF, and HCl emissions could therefore enhance our understanding of volcanic activity and improve hazard assessment.

Remote sensing measurements of volcanic gases in the shortwave infrared (SWIR) spectral range offer the potential for automated, continuous data acquisition of the above-mentioned gases. However, current studies are limited to direct sun geometry which restricts measurement opportunities to cases where the volcanic plume is positioned between the instrument and the sun. Therefore, we investigated the feasibility of operating a portable Fourier-transform infrared (FT-IR) spectrometer in scattered sunlight geometry for CO₂, HF, and HCl measurements. This approach would enable greater measurement flexibility but introduces the challenge of a significantly weaker light source in the SWIR spectral range.

We present a framework for calculating the required averaging time to achieve specific detection limits for CO₂, HF, and HCl based on instrument parameters (e.g. detector specifications, field of view, beam diameter). For Bruker’s EM27/SUN instrument, modified and optimized for scattered sunlight measurements, we estimate an averaging time of several hours to detect CO₂ slant column densities at levels typical for Mt. Etna (Italy). This makes the setup studied highly impractical for volcanic CO₂ measurements. In contrast, the required averaging times for HF and HCl detection are of the order of 10 minutes.

These results underscore the utility of performance calculations in guiding instrument design while highlighting the challenges associated with scattered sunlight being a weak light source in the SWIR spectral range.