Volcanic gases are central to eruption dynamics, and their flux and composition provide insights into subsurface processes. Once released into the atmosphere, these gases also influence atmospheric composition and climate, although the global volcanic fluxes remain poorly constrained. Only remote sensing methods allow measurements of these gases from a safe distance, including during periods of high volcanic activity.

As early as the 19th century, suggested that spectroscopy could provide an important tool in volcanology. performed the first successful spectroscopic gas measurements, using light in the visible range. and performed the first measurements in the near-infrared (NIR) at Kilauea and Nyiragongo, respectively, both detecting CuCl in the volcanic gas plumes. In the late 1960s, were the first to determine the main components of a volcanic plume by remote sensing, namely H2O, CO2, and SO2. They used infrared absorption between 2.5 and 14.5 µm, with the light emitted by a lava fountain of Kilauea volcano, Hawaii, United States, as the source.

The commercial availability of rugged, compact Fourier transform infrared (FTIR) spectrometers in the 1990s allowed for more frequent volcanological applications of infrared (IR) spectroscopy e.g.,. Operating in the infrared region (500–6000 cm-1) at spectral resolutions of up to 0.5 cm-1, these devices opened up the possibility of measuring many volcanic species of interest (including HCl, H2O, SO2, HF, CO2, SiF4, OCS, and CO) using their rotation-vibration line structures. Today, several research groups use IR spectroscopy to investigate volcanic degassing and have contributed to advancing our understanding of volcanic processes e.g.,.

However, IR remote sensing measurements are still seldom employed for volcanic monitoring, i.e., for ideally continuous and automated observations. The instruments are typically deployed in stationary positions on the ground and pointed at a hot source, either an artificial one or volcanically heated rock or lava. The availability of such a light source is the most serious limitation encountered in deployment: the number of active lava domes and lava fountains is small, and relying on them restricts the measurements to periods of eruptive activity. It is possible to exploit the thermal emission of the volcanic gases themselves when measuring in the thermal IR , but this requires an estimate of the gas temperature, and strong water absorption quickly limits the viewing distance in more humid conditions . Additionally, careful and regular radiometric calibration is necessary to allow for accurate retrieval of gas amounts and to correct for the thermal emission of the instrument itself. Such measurements can even enable the retrieval of CO2 for high emissions, but the high atmospheric background concentration remains a major challenge . demonstrated sky-looking remote sensing in the mid-infrared (MIR), where thermal emission is less relevant, using sunlight scattered by bright high-altitude cloud layers. It is also possible to make use of solar radiation directly , but the sun is rarely in the required geometric relationship to plume and instrument.

Today, successful monitoring relies mainly on scattered sunlight in the ultraviolet (UV) region . Using scattered sunlight has the advantage of not restricting the measurements to clear-sky conditions, and it resolves many geometric limitations. However, these monitoring efforts are mostly limited to SO2, since other gases of interest lack suitable absorption features in the UV and visible (UV-VIS) region, where plenty of scattered light is available. The question is therefore whether the comparatively low intensity of scattered sunlight in the NIR region permits detection limits small enough to resolve the expected plume enhancements of those gases of interest to volcanology that have characteristic absorption lines in this spectral range (e.g., H2O, CO2, HCl, and HF).

Section  introduces the concepts of measuring volcanic emissions by spectroscopy of sky-scattered sunlight. Section  presents the instrument used in this study, as well as proposed changes to optimize it for volcano measurements. Section  presents the predicted measurement performance of an optimized FTIR setup in the NIR region. To this end, we develop an instrument model for the spectral signal-to-noise ratio (SNR), validate it against laboratory measurements, and combine it with an information content analysis that links the SNR to the precision of retrieved trace gas columns for different target gases. Finally, Sect.  summarizes our findings and discusses their implications, including a multi-instrument strategy for volcanic monitoring, the radiative transfer challenges of scattered-sunlight observations, and the transferability of the approach to other emission sources.

We describe here an instrument concept that exploits spectra of sky-scattered sunlight in the NIR spectral region. The instrument itself, or a separate telescope, collects light from a chosen viewing direction (Fig. a) and forwards it to the spectrometer for analysis (Fig. b). A fitting routine retrieves the total-column abundance of the target trace gases, e.g., CO2, HF, and HCl, and provides ancillary information such as water-vapor content or total air mass, the latter derived from the O2 column.

By acquiring a series of measurements while scanning across the plume of an emission source, e.g., a volcano, we can infer the plume's target-gas enhancement and its spatial profile. These data, in turn, constrain the source's total emissions e.g.,. Alternatively, when the plume's location is known a priori, measurements taken on-plume and off-plume provide direct information on plume composition.

The NIR spectral range offers a distinct advantage for these measurements: although its absorption features are generally weaker and the set of detectable species is smaller than in the MIR, Rayleigh and Mie scattering are stronger at shorter wavelengths, yielding a considerably brighter background signal.

For laboratory verification and for reference measurements of sky-scattered sunlight, we employ an existing FTIR instrument that was originally developed for ground-scattered sunlight and is the successor of the instrument presented in . In this work we refer to it as the “EM27/SCAv2”. The instrument is based on the commercially available Bruker EM27/SUN spectrometer. We equipped it with a Hamamatsu G12183-210KA-03 InGaAs photodiode, cooled by a two-stage thermoelectric cooler. In addition, we adapted its focal length and replaced the Jacquinot stop with a custom aperture optimized for maximum light throughput. A schematic of the instrument's internal layout is shown in Fig. .

To assess how such a system could perform for volcanic plume observations, we devised a second instrument version that is specifically optimized for the anticipated measurement conditions. For the purpose of this study we designate it “EM27/Volcano”. The principal modification is the substitution of the detector with a Hamamatsu G12181-210K photodiode. This device exhibits a higher responsivity around the target wavelength of 1.6 µm and possesses a substantially larger shunt resistance owing to its narrower spectral bandwidth.

The narrower bandwidth means that the strong CO2 absorption bands near 2 µm are no longer accessible. However, synthetic performance studies, similar in spirit to the analysis presented here but beyond the scope of this article, indicate that the resulting increase in spectral SNR more than compensates for the loss of the 2 µm lines. Consequently, the EM27/Volcano detector choice is expected to deliver the best overall CO2 performance for volcanic plume measurements.

Spectral fitting is performed with a variant of the RemoTeC algorithm that assumes a ground-based upward-looking observer in a non-scattering atmosphere. To mimic the viewing geometry depicted in Fig. , we divide the atmosphere into six equidistant-pressure levels and assume that in the upper five layers the lightpath follows the direction defined by the solar zenith angle. The lowest layer is assumed to contain the path component along the instrument's viewing direction, and the gas concentrations are free to be fitted, essentially delivering the slant column densities (SCDs), i.e., the gas concentrations integrated along the lightpath. This procedure neglects that the actual scattering-modulated lightpath in the atmosphere might have experienced slightly different ambient pressure and temperature conditions. Note that our assessment only uses the algorithm to propagate the spectral noise into SCD errors; it does not make use of the SCDs per se.

The prediction of the overall measurement performance consists of two largely independent parts: (i) an estimation of the quality of the optical measurement itself, and (ii) an information content analysis, which links the spectral performance to the measurement target, i.e., the retrievable trace gas columns.

In the first component, we estimate the spectral SNR using an instrument model based on fundamental principles of optics and detector electronics, complemented by FTIR-specific characteristics.

The second component translates the achievable SNR into trace gas retrieval performance using an information content analysis. For this purpose, we select Mount Etna as a test case. It is one of the largest halogen point sources on Earth , among the strongest volcanic emitters of CO2 , and one of the most significant continuous emitters of volcanic gases worldwide . In addition, the results of  provide a benchmark for expected gas column enhancements at Mount Etna, and radiometrically calibrated measurements of sky radiances were available to us to constrain our simulations. This use of real observations also allows us to consider systematic effects that typically occur in total column measurements of trace gases.

Section  describes the instrument model and SNR estimation. Section  presents a laboratory validation of the model. Section  reports on the information content analysis and links spectral SNR to the precisions of trace gas column densities. Finally, Sect.  brings all the steps together and delivers the final assessment of the trace gas performance in the volcano setting.

In this study, we assessed the feasibility of constraining volcanic emissions by passive FTIR spectroscopy of sky-scattered sunlight in the NIR. We developed an instrument SNR model, validated it with laboratory measurements, and combined it with an information-content analysis to estimate detection limits for trace gas columns under realistic conditions.

Our results indicate that reliably measuring CO2 columns in volcanic plumes remains challenging. Even under bright sky conditions, the SNR is insufficient to resolve typical plume enhancements with the precision required for robust flux estimates. As a result, constraining CO2 emissions via plume transects would likely require alternative observational strategies or measurement concepts. In contrast, halogen species such as HCl and HF are detectable at volcanoes with high emissions. Their low atmospheric background concentrations result in substantially higher signal-to-background ratios. The expected performance for a sky of average brightness is comparable to that of the measurements of Fig. 2c, performed in the MIR with bright cloud layers as the light source: they measured signals of 100 ppmm (250 ppmm) for HF (HCl) with a relative precision of approximately 10 % in 8 min. Converting these values using an air density of 1.5×1025 molec.m-3 yields 1.5×1017 molec.cm-2 for HF and 3.8×1017 molec.cm-2 for HCl. A precision of about 10 % of these values for measurement times of 8 min is very similar to the respective averages of our estimated performance ranges in Fig.  for both gases. Such averaging times are still likely too long for plume transect measurements, since a full scan of the plume would require hours, even under bright sky conditions.

As an alternative to plume transect measurements, a multi-instrument approach, in which the FTIR instrument measures only the plume enhancement, would be more promising: a co-aligned differential optical absorption spectroscopy (DOAS) measurement as in, e.g., could extend the measurement to SO2, allowing for the retrieval of gas ratios, which are in themselves already a useful tool in volcanology. In combination with an SO2 observatory such as the NOVAC stations, this would then give access to the emissions of HCl and HF. An additional SO2 camera could provide real-time information on the position of the plume, ensuring that the FTIR and its co-aligned DOAS instrument sample the center of the plume. While constraining halogen emissions with such a setup seems feasible, it is less likely for CO2: the lower sensitivity of the measurement would probably restrict the approach to a single retrieved value per day even under favorable conditions, but more importantly, the high atmospheric background concentration of CO2 and its variability make the retrieval itself significantly more challenging. We therefore cannot rule out CO2 by this route, but any attempt would have to rest on the multi-instrument approach and likely also on a measurement concept of substantially better precision.

If an instrument such as the one described here, with potentially better noise performance, were to be deployed for remote sensing at a volcano, the follow-up challenge for obtaining meaningful information about plume gas concentrations would be to develop accurate radiative transfer simulations of the lightpath. The lightpath conceptually divides into a component behind the plume, where sunlight is scattered into the viewing direction, a component inside the plume, where the targeted gas enhancements are accumulated, and a component between the plume and the observer . The solution to this problem largely depends on the scattering properties of the background atmosphere and of the plume in terms of aerosol load, aerosol types, and height distributions, which are all highly variable and poorly constrained by external information for the instantaneous and local measurements envisioned here. The complexity also depends on whether the target gas occurs in large background concentrations such as for CO2 or whether it only occurs within the plume such as for HF and HCl. For the latter, observations of the plume geometry might deliver sufficient information to obtain a meaningful estimate of the respective plume concentrations, provided the plume is not too opaque, so that the (infrared) sunlight can be assumed to travel through the entire plume cross section. For the former, one would need either accurate radiative transfer simulations fed with accurate aerosol properties or a proxy gas that shares the same lightpath but is not emitted by the volcano. In this case, the radiative transfer problem is similar to the one encountered in UV SO2 measurements e.g.,. For CO2, such a proxy is typically methane (CH4), which has a similar distribution of background concentrations but is not emitted in relevant amounts by the volcano. CH4 also has absorption features in spectral proximity to CO2 (around 1650 nm), such that the spectral variation of the optical scattering properties is mild enough to assume very similar lightpaths. Overall, the complexity of radiative transfer is a further major challenge to the type of volcano remote sensing we investigated here.

The limitations of the spectral measurement itself arise fundamentally from the limited amount of scattered NIR light and are largely independent of the specific spectroscopic implementation (FTIR in this study). Any method aiming to overcome this limitation would likely need to either collect substantially more light, restrict observations to a narrow spectral region around the CO2 lines to increase the ratio of signal photons to irrelevant background photons, or combine both approaches. Restricting the spectral range introduces additional challenges: it can complicate the baseline estimation and it reduces the information available to characterize scattering in the retrieval. If the spectral range were narrowed enough to improve the performance for a specific trace gas appreciably, the measurement would likely be restricted to that single species. A meaningful increase in the light throughput also appears unlikely: the proposed instrument already uses 2-inch optics, no suitable FTIR platforms with larger optics are commercially available, and the added size and mass would make the instrument too bulky and heavy for field deployment. An increase in the FOV, achieved either by enlarging the field stop (which is the detector size) or by further shortening the focal length of the focusing optics, is also unlikely to help: a larger detector quickly results in worse noise performance, while a shorter focal length cannot be realized with off-the-shelf mirrors and would lead to significant alignment challenges and to detrimental effects on the instrument line shape, and thus on the quality of the spectral retrieval. Additionally, increasing the FOV beyond that of the proposed instrument would quickly result in self-apodization, reducing the effective resolution and with it the spectral signatures of the trace gases. It is possible that a setup with a reduced resolution (meaning a reduced maximal OPD) and a larger FOV could still provide an overall performance gain, but the information content does not scale linear as Eq. () implies. A smaller maximal OPD also causes a linear decrease in spectral sampling. This should result in an effective scaling of the information content closer to the inverse square root of the maximal OPD. So even if this parameter is tuned (and this would require a different development platform) there is likely just a factor, not an order of magnitude to gain and the detrimental impact of a decreased spectral resolution on the trace gas performance is complex and challenging to predict .

Despite these constraints, the concept retains a decisive advantage in its viewing geometry: it depends neither on a hot emission source nor on a favorable solar position, so the instrument can be pointed freely and operated continuously and automatically, which is the prerequisite for monitoring rather than campaign-style measurements. For the strongly emitted halogens HCl and HF, the required measurement times are short enough that such monitoring appears within reach, particularly when the FTIR measurement is combined with co-aligned UV observations of SO2. For CO2, the limiting factor is the small plume enhancement relative to the large and variable atmospheric background, which in addition leaves the retrieval far more sensitive to errors in the radiative transfer. Additionally, the detection limits reported here are a property of the method and instrument concept and can therefore be applied to any source whose expected column enhancement is known. Anthropogenic super-emitters such as power plants or industrial facilities, for instance, produce CO2 enhancements far larger than those of a volcanic plume. For such targets, CO2 observations in sky-scattered sunlight may well prove feasible, and a flexible viewing geometry would be equally valuable there.

This study provides a systematic assessment of NIR FTIR spectroscopy of sky-scattered sunlight, in general and for volcanic applications in particular, and it identifies the limitations of the concept. Turning the concept into an operational measurement requires two further steps: a retrieval that solves the lightpath problem outlined above and, particularly for the multi-instrument approach proposed here, the assembly and co-deployment of the required instrumentation. Alternatively, one could pursue a measurement concept that overcomes the light-throughput and resolution constraints identified in this study. The framework we used, i.e., an instrument SNR model combined with an information content analysis anchored in actual measurements, applies equally to other instruments, spectral regions, and target species, and can guide such assessments before hardware is built.