| 24 Mar 2026
Feasibility of Measuring Volcanic Gas Composition Using Sky-scattered Sunlight and FTIR Spectroscopy
Abstract. Monitoring volcanic emissions is essential for understanding volcanic processes and predicting eruption dynamics. Remote sensing is the only method that allows safe measurements right before, during, and after eruptions. Current monitoring is mostly limited to the ultraviolet and visible (UV-VIS) spectral ranges due to the high sky brightness in that region, restricting observations largely to SO2.
Here, we assess the feasibility of constraining volcanic emissions using passive Fourier transform infrared (FTIR) spectroscopy of sky-scattered sunlight in the near-infrared (NIR), where absorption features of a broader range of gases of interest are available. Using an instrument model for spectral signal-to-noise ratio (SNR) combined with an information-content analysis, we estimate detection limits for individual trace gas columns under realistic conditions. To address systematic uncertainties always present in atmospheric total column measurements, we developed an innovative approach of incorporating actual measurements into our estimations. The instrument model accurately reproduced laboratory validation experiments. To assess applicability of the method, this study focuses on Mount Etna as a representative high-emission volcano. Our results indicate that CO2 column measurements remain challenging. Even under bright sky conditions, measurement times of 30 minutes are necessary to reach detection limits on the scale of the expected total column enhancement. This renders flux estimations via plume transects not feasible, while plume-composition measurements might be possible. In contrast, strongly emitted halogen species such as HCl and HF are detectable within tens of seconds under bright skies and up to approximately 10 minutes for dark conditions, owing to their low atmospheric background concentrations. The limitations identified for CO2 are largely independent of the specific spectroscopic implementation, since they arise fundamentally from the low amount of available light. Finally, the SNR and detection-limit analysis using actual measurements is broadly applicable to other instruments, spectral regions, and target species.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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General Comments:
In this manuscript, the authors describe a data-informed numerical evaluation and instrument model for volcanic gas analysis using scattered near-infrared light. They provide an approach to estimating signal-to-noise ratio under varied conditions and thereby measurement precision for target gas species. While the text is clear in its derivation of SNR and concise in the outcome for three gas species (CO2, HCl, and HF), the overall presentation would benefit from a stronger emphasis on the novelty of the work and impact on future directions for atmospheric measurements.
Specific Comments:
For example, the authors state they “developed an innovative approach of incorporating actual measurements into our estimations” to address systematic uncertainties. However, the text presents a data-informed model and analysis which did not read as new – while the pre-existing dataset itself may be unique or rare. The model was presented, and data (or previously determined correlation factors) was supplemented. Additional detail describing the unique aspects of the “information-content analysis” would strengthen the narrative.
Further, the manuscript would benefit from additional discussion around the implications or impact of the findings. It’s clear that the work effort informed the authors’ decision to not proceed with building a new instrument. There is a missed opportunity to emphasize why the reader should be motivated to use the results going forward. For example, a quick comparison of findings with the current measurement approaches – going beyond feasibility/utility and motivating where this technique may best be pursued (particularly for HF and HCl, where responsivity was deemed reasonable) – would help orient and motivate the reader.