Exploiting Sentinel-5P TROPOMI and Ground Sensor Data for the Detection of Volcanic SO2 Plumes and Activity in 2018–2021 at Stromboli, Italy

Sulfur dioxide (SO2) degassing at Strombolian volcanoes is directly associated with magmatic activity, thus its monitoring can inform about the style and intensity of eruptions. The Stromboli volcano in southern Italy is used as a test case to demonstrate that the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (Sentinel-5P) satellite has the suitable spatial resolution and sensitivity to carry out local-scale SO2 monitoring of relatively small-size, nearly point-wise volcanic sources, and distinguish periods of different activity intensity. The entire dataset consisting of TROPOMI Level 2 SO2 geophysical products from UV sensor data collected over Stromboli from 6 May 2018 to 31 May 2021 is processed with purposely adapted Python scripts. A methodological workflow is developed to encompass the extraction of total SO2 Vertical Column Density (VCD) at given coordinates (including conditional VCD for three different hypothetical peaks at 0–1, 7 and 15 km), as well as filtering by quality in compliance with the Sentinel-5P Validation Team’s recommendations. The comparison of total SO2 VCD time series for the main crater and across different averaging windows (3 × 3, 5 × 5 and 4 × 2) proves the correctness of the adopted spatial sampling criterion, and practical recommendations are proposed for further implementation in similar volcanic environments. An approach for detecting SO2 VCD peaks at the volcano is trialed, and the detections are compared with the level of SO2 flux measured at ground-based instrumentation. SO2 time series analysis is complemented with information provided by contextual Sentinel-2 multispectral (in the visible, near and short-wave infrared) and Suomi NPP VIIRS observations. The aim is to correctly interpret SO2 total VCD peaks when they either (i) coincide with medium to very high SO2 emissions as measured in situ and known from volcanological observatory bulletins, or (ii) occur outside periods of significant emissions despite signs of activity visible in Sentinel-2 data. Finally, SO2 VCD peaks in the time series are further investigated through daily time lapses during the paroxysms in July–August 2019, major explosions in August 2020 and a more recent period of activity in May 2021. Hourly wind records from ECMWF Reanalysis v5 (ERA5) data are used to identify local wind direction and SO2 plume drift during the time lapses. The proposed analysis approach is successful in showing the SO2 degassing associated with these events, and warning whenever the SO2 VCD at Stromboli may be overestimated due to clustering with the plume of the Mount Etna volcano.

Tropospheric Volcanic SO2 Mass and Flux Retrievals from Satellite. The Etna December 2018 Eruption

The presence of volcanic clouds in the atmosphere affects air quality, the environment, climate, human health and aviation safety. The importance of the detection and retrieval of volcanic SO2 lies with risk mitigation as well as with the possibility of providing insights into the mechanisms that cause eruptions. Due to their intrinsic characteristics, satellite measurements have become an essential tool for volcanic monitoring. In recent years, several sensors, with different spectral, spatial and temporal resolutions, have been launched into orbit, significantly increasing the effectiveness of the estimation of the various parameters related to the state of volcanic activity. In this work, the SO2 total masses and fluxes were obtained from several satellite sounders—the geostationary (GEO) MSG-SEVIRI and the polar (LEO) Aqua/Terra-MODIS, NPP/NOAA20-VIIRS, Sentinel5p-TROPOMI, MetopA/MetopB-IASI and Aqua-AIRS—and compared to one another. As a test case, the Christmas 2018 Etna eruption was considered. The characteristics of the eruption (tropospheric with low ash content), the large amount of (simultaneously) available data and the different instrument types and SO2 columnar abundance retrieval strategies make this cross-comparison particularly relevant. Results show the higher sensitivity of TROPOMI and IASI and a general good agreement between the SO2 total masses and fluxes obtained from all the satellite instruments. The differences found are either related to inherent instrumental sensitivity or the assumed and/or calculated SO2 cloud height considered as input for the satellite retrievals. Results indicate also that, despite their low revisit time, the LEO sensors are able to provide information on SO2 flux over large time intervals. Finally, a complete error assessment on SO2 flux retrievals using SEVIRI data was realized by considering uncertainties in wind speed and SO2 abundance.

New insights into magmatic processes from integrated satellite observation, trajectory analysis and magma ascent modelling

<p>Analysis of TROPOMI data with plume trajectory tools opens the possibility of new insights into volcanic / magmatic processes from two data sources: SO2 flux time series and plume height time series. In this paper we investigate results from explosive eruptions and attempt to explain the results with a magma ascent conduit model. The combination of plume height and gas flux data with a model of the magma ascent process provides a toolkit which allows us to constrain magma reservoir processes from satellite monitoring data. The combination of modelling and observations opens a new volcanological research frontier, because the TROPOMI sensor has daily global coverage, a high spatial resolution and is sensitive enough to detect many small-medium explosions globally, so that a large inventory of explosive activity can be characterised. </p>

Inferring a shallow degassing model for Villarrica Volcano from seismic explosion signals and SO2 flux

<p>Villarrica is a basaltic volcano with an active lava lake in South Central Chile. The lava lake displays a variety of degassing styles from gentle seething to more violent Strombolian explosions. This activity is accompanied by sequences of transient seismic waveforms suggesting the presence of discrete gas bubbles in the upper magma column. Gas bubbles flow through liquid-filled pipes according to distinct patterns depending on viscosity of the liquid and volumetric gas flow rate. Laboratory experiments indicate that these regimes are characterized by distinct frequency distributions of bubble sizes and spacings. By assuming that these parameters are reflected by the magnitude of the transients and the time between them, we compared their statistical distributions to infer a flow regime for the shallow conduit of Villarrica. The approximately log-normal distributions indicate a sustained slug flow regime in which the gas ascends in trains of conduit-wide gas slugs. The event catalog for our analysis contained about 20,000 events and was generated from 12 days of seismic data from March 2012 acquired by a dense local network. A well-known problem in earthquake statistics is the incompleteness of event catalogs towards low magnitudes due to decreasing detectability in the ambient noise. We estimated the actual distribution of magnitudes by using a Monte Carlo simulation of the event detection based on the statistical properties of the observed seismic noise. The unknown source depth and mechanism introduce further ambiguity regarding the distributions. Nevertheless, we hope to refine the degassing model by taking into account degassing rates, magma properties and more detailed analysis of the nature of the seismic events.</p>

Quantification of SO2 emission rates from the Kilauea volcano in Hawaii by the divergence of the SO2 flux using S5P-TROPOMI satellite measurements

<div> <div> <div> <div> <p>With a nearly continuously effusive eruption since 1983, the Kilauea volcano (Hawaii, USA) is one of the most active volcanoes in the world. At the beginning of May 2018, a sequence of eruptions on the Lower East Rift Zone (LERZ) caused an enhanced outbreak of volcanic gases and aerosols, releasing them into the troposphere. Since these gases and particles affect climate, environment, traffic, and health on regional to global scales, a continuos monitoring of the emission rates is essential.</p> <p>As satellites provide the opportunity to observe and quantify the emissions remotely from space, their contribution to the monitoring of volcanoes is significant. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite was successfully launched by the end of 2017 and provides measurements with unprecedented level of details with a resolution of 3.5 x 7 km2. This also allows for an accurate retrieval of trace gas species such as volcanic SO2.</p> <p>Here, it will be shown that the location and strength of SO2 emissions from Kilauea can be determined by the divergence of the temporal mean SO2 flux. This approach, which is based on the continuity equation, has been demonstrated to work for NOX emissions of individual power plants (Beirle et al., Sci. Adv., 2019).</p> <p>The present state of our work indicates that emission maps of SO2 can be derived by the combination of satellite measurements and wind fields on high spatial resolution. As the divergence is highly sensitive on point sources like the erupting fissures in the 2018 Kilauea eruption, they can be localized very precisely. The obtained emission rates are slightly lower than the ones reported from ground-based measurements in other studies like the one from Kern et al. (Bull. Volcanol., 2020). The effects of suboptimal conditions like high cloud fractions on the method probably affect the derived emission rates and have to be further analyzed.</p> </div> </div> </div> </div>

Elevated CO2 Emissions during Magmatic-Hydrothermal Degassing at Awu Volcano, Sangihe Arc, Indonesia

Awu is a remote and little known active volcano of Indonesia located in the northern part of Molucca Sea. It is the northernmost active volcano of the Sangihe arc with 18 eruptions in less than 4 centuries, causing a cumulative death toll of 11,048. Two of these eruptions were classified with a Volcanic Explosivity Index (VEI) of 4. Since 2004, a lava dome has occupied the centre of Awu crater, channelling the fumarolic gas output along the crater wall. A combined Differential Optical Absorption Spectroscopy (DOAS) and Multi-component Gas Analyzer System (Multi-GAS) study highlight a relatively small SO2 flux (13 t/d) sustained by mixed magmatic–hydrothermal emissions made-up of 82 mol.% H2O, 15 mol.% CO2, 2.55 mol.% total S (ST) and 0.02 mol.% H2. The CO2 emission budget, as observed during a short observation period in 2015, corresponds to a daily contribution to the atmosphere of 2600 t/d, representing 1% of the global CO2 emission budget from volcanoes. The gas CO2/ST ratio of 3.7 to 7.9 is at the upper limit of the Indonesian gas range, which is ascribed to (i) some extent of S loss during hydrothermal processing, and perhaps (ii) a C-rich signature of the feeding magmatic gas phase. The source of this high CO2 signature and flux is yet to be fully understood; however, given the peculiar geodynamic context of the region, dominated by the arc-to-arc collision, this may result from either the prolonged heating of the slab and consequent production of carbon-rich fluids, or the recycling of crustal carbon.

First In-Situ Measurements of Plume Chemistry at Mount Garet Volcano, Island of Gaua (Vanuatu)

Recent volcanic gas compilations have urged the need to expand in-situ plume measurements to poorly studied, remote volcanic regions. Despite being recognized as one of the main volcanic epicenters on the planet, the Vanuatu arc remains poorly characterized for its subaerial emissions and their chemical imprints. Here, we report on the first plume chemistry data for Mount Garet, on the island of Gaua, one of the few persistent volatile emitters along the Vanuatu arc. Data were collected with a multi-component gas analyzer system (multi-GAS) during a field campaign in December 2018. The average volcanic gas chemistry is characterized by mean molar CO2/SO2, H2O/SO2, H2S/SO2 and H2/SO2 ratios of 0.87, 47.2, 0.13 and 0.01, respectively. Molar proportions in the gas plume are estimated at 95.9 ± 11.6, 1.8 ± 0.5, 2.0 ± 0.01, 0.26 ± 0.02 and 0.06 ± 0.01, for H2O, CO2, SO2, H2S and H2. Using the satellite-based 10-year (2005–2015) averaged SO2 flux of ~434 t d−1 for Mt. Garet, we estimate a total volatile output of about 6482 t d−1 (CO2 ~259 t d−1; H2O ~5758 t d−1; H2S ~30 t d−1; H2 ~0.5 t d−1). This may be representative of a quiescent, yet persistent degassing period at Mt. Garet; whilst, as indicated by SO2 flux reports for the 2009–2010 unrest, emissions can be much higher during eruptive episodes. Our estimated emission rates and gas composition for Mount Garet provide insightful information on volcanic gas signatures in the northernmost part of the Vanuatu Arc Segment. The apparent CO2-poor signature of high-temperature plume degassing at Mount Garet raises questions on the nature of sediments being subducted in this region of the arc and the possible role of the slab as the source of subaerial CO2. In order to better address the dynamics of along-arc volatile recycling, more volcanic gas surveys are needed focusing on northern Vanuatu volcanoes.

Ultraviolet Camera Measurements of Passive and Explosive (Strombolian) Sulphur Dioxide Emissions at Yasur Volcano, Vanuatu

Here, we present the first ultraviolet (UV) camera measurements of sulphur dioxide (SO2) flux from Yasur volcano, Vanuatu, for the period 6–9 July 2018. These data yield the first direct gas-measurement-derived calculations of explosion gas masses at Yasur. Yasur typically exhibits persistent passive gas release interspersed with frequent Strombolian explosions. We used compact forms of the “PiCam” Raspberry Pi UV camera system powered through solar panels to collect images. Our daily median SO2 fluxes ranged from 4 to 5.1 kg s−1, with a measurement uncertainty of −12.2% to +14.7%, including errors from the gas cell calibration drift, uncertainties in plume direction and distance, and errors from the plume velocity. This work highlights the use of particle image velocimetry (PIV) for plume velocity determination, which was preferred over the typically used cross-correlation and optical flow methods because of the ability to function over a variety of plume conditions. We calculated SO2 masses for Strombolian explosions ranging 8–81 kg (mean of 32 kg), which to our knowledge is the first budget of explosive gas masses from this target. Through the use of a simple statistical measure using the moving minimum, we estimated that passive degassing is the dominant mode of gas emission at Yasur, supplying an average of ~69% of the total gas released. Our work further highlights the utility of UV camera measurements in volcanology, and particularly the benefit of the multiple camera approach in error characterisation. This work also adds to our inventory of gas-based data, which can be used to characterise the spectrum of Strombolian activity across the globe.

Near Real-Time Monitoring of the Christmas 2018 Etna Eruption Using SEVIRI and Products Validation

On the morning of 24 December 2018, an eruptive event occurred at Etna, which was followed the next day by a strong sequence of shallow earthquakes. The eruptive episode lasted until 30 December, ranging from moderate strombolian to lava fountain activity coupled with vigorous ash/gas emissions and a lava flow effusion toward the eastern volcano flank of Valle del Bove. In this work, the data collected from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments on board the Meteosat Second Generation (MSG) geostationary satellite are used to characterize the Etna activity by estimating the proximal and distal eruption parameters in near real time. The inversion of data indicates the onset of eruption on 24 December at 11:15 UTC, a maximum Time Average Discharge Rate (TADR) of 8.3 m3/s, a cumulative lava volume emitted of 0.5 Mm3, and a Volcanic Plume Top Height (VPTH) that reached a maximum altitude of 8 km above sea level (asl). The volcanic cloud ash and SO2 result totally collocated, with an ash amount generally lower than SO2 except on 24 December during the climax phase. A total amount of about 100 and 35 kt of SO2 and ash respectively was emitted during the entire eruptive period, while the SO2 fluxes reached peaks of more than 600 kg/s, with a mean value of about 185 kg/s. The SEVIRI VPTH, ash/SO2 masses, and flux time series have been compared with the results obtained from the ground-based visible (VIS) cameras and FLux Automatic MEasurements (FLAME) networks, and the satellite images collected by the MODerate resolution Imaging Spectroradiometer (MODIS) instruments on board the Terra and Aqua- polar satellites. The analysis indicates good agreement between SEVIRI, VIS camera, and MODIS retrievals with VPTH, ash, and SO2 estimations all within measurement errors. The SEVIRI and FLAME SO2 flux retrievals show significant discrepancies due to the presence of volcanic ash and a gap of data on the FLAME network. The results obtained in this study show the ability of geostationary satellite systems to characterize eruptive events from the source to the atmosphere in near real time during the day and night, thus offering a powerful tool to mitigate volcanic risk on both local population and airspace and to give insight on volcanic processes.

The quantification of NO_<i>x</i> and SO₂ point source emission flux errors of mobile DOAS on the basis of the Gaussian dispersion model: A simulation study

Mobile differential optical absorption spectroscopy (mobile DOAS) has become an important tool for the quantification of emission sources, including point sources (e.g., individual power plants) and area emitters (e.g., entire cities). In this study, we focused on the error budget of mobile DOAS measurements from point sources, and we also offered recommendations for the optimum settings of such measurements. First we established a Gaussian plume model from which the NOx and SO2 distribution from the point source was determined. In a second step the simulated distributions are converted into vertical column densities of NOx and SO2 according to the mobile DOAS measurement technique. With assumed parameters, we then drove the forward model in order to simulate the emissions, after which we performed the analysis. Following this analysis, we conclude that: (1) Larger sampling resolution clearly results in larger flux error. The proper resolution we suggest is between 5 m and 50 m. Even larger resolutions may also be viable, but > 100 m is not recommended. (2) Error effects vary with measurement distance from the source. We found that undetectable flux (measured VCDs are under the detection limit) is the main error source when measuring far from the source, for both NOx and SO2. When measuring close to the source, low sampling frequency results in large flux error. (3) The wind field primarily affects 2 aspects of the flux measurement error. When measuring far from the source, dispersion results in more undetectable flux, which is the main error source. When measuring close to the source, wind field uncertainty becomes the main error source of SO2 flux, but not of NOx. We suggested that the proper wind speed for mobile DOAS measurements is between 1 m/s and 4 m/s. (4) The study of NOx atmospheric chemistry reactions indicated that a [NOx]/[NO2] ratio correction has to be applied when measuring very close to the emission source. But even when such a correction is applied, the remaining errors can be significant. To minimize the [NOx]/[NO2] ratio correction error, we recommended 0.05 NO2 maximum reaction rate as the accepted NOx steady-state thus to determine the proper starting measurement distance. (5) The error of the spectral retrieval is not a main emission flux error source and its error budget varies with the measuring distance. (6) Increasing the number of measurements can lower the flux error that results from wind field uncertainty and retrieval error. This directly indicates that SO2 flux error could be lowered if the measurements are repeated when not too far from the emission source. With regard to NOx, more measurement times can only work effectively when not very close or too far from the source. (7) Also the effects of the temporal and spatial sampling are investigated. When the sampling resolution is prescribed, the integration depends on the driving speed and the corresponding flux error is mainly determined by the undetectable flux. When the car speed is prescribed, the integration time is determined by the sampling resolution for measuring near the source, while undetectable flux predominates when far away. (8) As a general recommendation, our study suggests that emission rates < 30 g/s for NOx and < 50 g/s for SO2 are not recommended for mobile DOAS measurements. The source height affects the undetectable flux, but has little influences on the total error. Based on the model simulations our study indicates that mobile DOAS measurements are very well suited tool to quantify point source emissions. The results of our sensitivity studies are important to make optimum use of such measurements.