Feasibility of measuring volcanic gas composition using sky-scattered sunlight and FTIR spectroscopy

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 relies on scattered sunlight, whose essentially unconstrained viewing geometry permits continuous and automated observation. It is, however, mostly limited to the ultraviolet and visible (UV-VIS) spectral ranges by the available sky brightness, restricting observations largely to SO 2 . Here, we assess the feasibility of constraining volcanic emissions by passive Fourier transform infrared (FTIR) spectroscopy of sky-scattered sunlight in the near-infrared (NIR), where more gases of interest have absorption features. Combining an instrument model for the spectral signal-to-noise ratio (SNR) with an information-content analysis, and incorporating actual measurements to capture the systematic uncertainties inherent to atmospheric total column retrievals, we estimate detection limits for individual trace gas columns. The instrument model accurately reproduces the results of laboratory validation experiments. We use Mount Etna as a representative high-emission volcano. We find that CO 2 column measurements remain challenging: the plume enhancement is small compared to the high and variable atmospheric background, and little scattered light is available in the NIR. Even under bright skies, reaching a detection limit comparable to the expected column enhancement takes about 5 min, and up to 2.5 h under dark conditions. Plume transects, which require many such measurements at substantially better precision, are therefore out of reach, whereas individual plume-composition measurements remain conceivable. In contrast, the strongly emitted halogen species HCl and HF, whose atmospheric background is low, are detectable within seconds under bright skies and within a few minutes under dark conditions. For these species, a multi-instrument approach makes plume-composition measurements practical: pairing the FTIR with co-aligned UV observations of SO 2 yields gas ratios that, combined with established SO 2 flux networks, give access to the halogen emissions. For CO 2 this route is not excluded, but limited precision and the impact of radiative transfer errors on a background-dominated retrieval make the outcome hard to predict. Finally, this SNR and detection-limit analysis transfers to other instruments, spectral regions, target species, and emission sources.