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co-auteur étranger; International audience; Tephra fallout hazard assessment is undertaken with probabilistic maps that rely on numerical models. Regarding maps production, the input parameters of the model (including atmospheric conditions), the physical approximations of the numerical simulations, and the probabilities of occurrence of different eruption types in specific time frames are among the most critical sources of uncertainty. We therefore present a tephra fallout hazard assessment study for two active volcanoes (Cotopaxi and Guagua Pichincha) in Ecuador. We utilize PLUME-MoM/HYSPLIT models, and a procedure for uncertainty quantification where: i) the uncertainty on eruptive source parameters and eruption type occurrence is quantified through expert elicitation; ii) we implement a new procedure for correlations between the different parameters, and iii) we use correction coeffients to take into account the uncertainty of the numerical model. Maps of exceedance probability given a deposit thickness threshold, and thickness maps given a robability of exceedence, are produced 1) for two eruptive scenarios (sub-plinian and Plinian) and 2) as a combination of these scenarios in case the next eruption will be sub-Plinian or Plinian. These maps are described according to the uncertainty distribution of eruption type occurrence probabilities, considering their 5th percentile, mean, and 95th percentile values. We finally present hazard curves describing exceeding probabilities in 10 sensitive sites within the city of Quito. Additional information includes the areal extent and the population potentially affected by different isolines of tephra accumulation. This work indicates that full uncertainty quantification helps in providing more robust scientific information, improving the hazard assessment reliability.

Co-auteur étranger; International audience; Volcanic emissions can significantly affect the Earth's radiation budget by emitting aerosol particles and gas‐phase species that can result in the new particle formation (NPF). These particles can scatter solar radiation or modify cloud properties, with consequences on health, weather, and climate. To our knowledge, this is the first dedicated study detailing how gas‐phase precursors emitted from volcanic plumes can influence the NPF. A series of airborne measurements were performed around the Etna and Stromboli volcanoes within the framework of the CLerVolc and STRAP projects. The ATR‐42 aircraft was equipped with a range of instrumentation allowing the measurement of particle number concentration in diameter range above 2.5 nm, and gaseous species to investigate the aerosol dynamics and the processes governing the NPF and their growth within the volcanic plumes. We demonstrate that NPF occurs within the volcanic plumes in the Free Troposphere (FT) and Boundary Layer (BL). Typically, the NPF events were more pronounced in the FT, where the condensational sink was up to two orders of magnitude smaller and the temperature was ~20°C lower than in the BL. Within the passive volcanic plume, the concentration of sulfur dioxide, sulfuric acid, and N2.5 were as high as 92 ppbV, 5.65×108 and 2.4×105 cm–3, respectively. Using these measurements, we propose a new parameterization for NPF rate (J2.5) within the passive volcanic plume in the FT. These results can be incorporated into mesoscale models to better assess the impact of the particle formed by natural processes, i.e. volcanic plumes, on climate.

International audience; Volcanic risk at Vesuvius is one of the highest in the world due to the ~670,000 inhabitants living in the Red Zone, the area exposed to both pyroclastic flows and tephra fallout, to be evacuated before renewal of any eruptive activity. The national emergency plan for Vesuvius builds its risk zonation on a scenario similar to the last sub-Plinian eruption, which occurred in 1631. This study aims at providing new insights on the geometry of the caldera associated with this historical eruption. The impact of past Vesuvius eruptions on present-day soil CO2 concentration has been investigated by means of an extended geochemical survey carried out for identifying the circulation pathways of hydrothermal fluids inside the volcano. We performed 4018 soil CO2 concentration measurements over the whole Somma-Vesuvius volcanic complex, covering an area of 50 km2. Besides relatively low values, the results show a significant spatial CO2 concentration heterogeneity over Somma-Vesuvius ranging from the atmospheric value (~400 ppm) up to ~24,140 ppm. The summit of Vesuvius shows an area with anomalous CO2 concentrations well matching the crater rim of the 1906 eruption. Along the cone flanks, secondary CO2 anomalies highlight a roughly circular preferential pathway detected along 8 radial profiles at distances between ~840 m and ~1150 m from the bottom of the present-day crater resulting from the last eruption in 1944. In depth review of the available literature highlights an agreement between this circle-like shaped anomaly and the 1631 sub-Plinian eruption caldera boundary. Indeed, based on the historical chronicles the depression produced by the 1631 eruption had a diameter of 1686 m, whereas the CO2 circular anomaly indicates a diameter of 1956 m. Finally, the results were compared with a 3-D density model obtained from a recent gravity survey that corroborates both the literature and the CO2 data in terms of potential buried structure at the base of the Vesuvius cone.

Co-auteur étranger; International audience; Future occurrence of explosive eruptive activity at Cotopaxi and Guagua Pichincha volcanoes, Ecuador, is assessed probabilistically, utilizing expert elicitation. Eight eruption types were considered for each volcano. Type event probabilities were evaluated for the next eruption at each volcano and for at least one of each type within the next 100 years. For each type, we elicited relevant eruption source parameters (duration, average plume height, and total tephra mass). We investigated the robustness of these elicited evaluations by deriving probability uncertainties using three expert scoring methods. For Cotopaxi, we considered both rhyolitic and andesitic magmas. Elicitation findings indicate that the most probable next eruption type is an andesitic hydrovolcanic/ash-emission (~ 26–44% median probability), which has also the highest median probability of recurring over the next 100 years. However, for the next eruption at Cotopaxi, the average joint probabilities for sub-Plinian or Plinian type eruption is of order 30–40%—a significant chance of a violent explosive event. It is inferred that any Cotopaxi rhyolitic eruption could involve a longer duration and greater erupted mass than an andesitic event, likely producing a prolonged emergency. For Guagua Pichincha, future eruption types are expected to be andesitic/dacitic, and a vulcanian event is judged most probable for the next eruption (median probability ~40–55%); this type is expected to be most frequent over the next 100 years, too. However, there is a substantial probability (possibly >40% in average) that the next eruption could be sub-Plinian or Plinian, with all that implies for hazard levels.

Monitoring active volcanoes activity passes through the detection of fluctuations in degassing levels which may reflect changes in the magma supply rate and help inform a short-term forecast of on-going eruptions. Infrared hyperspectral imagers, which is an imaging technology still little used for volcanoes monitoring, have been deployed for various field campaigns on active volcanoes recently. For example, the Hyper-Cam LWIR (LongWave InfraRed) ranging between 850-1300 cm-1 (7.7 - 11.8 µm) with a spectral resolution up to 0.25 cm-1, provided high spectral resolution images from ground-based measurements of the Mount Etna (Sicily, Italy) plume during IMAGETNA campaign in June 2015. Processing the raw data and retrieving the infrared spectra with the LATMOS (Laboratoire Atmosphères Milieux Observations Spatiales) Atmospheric Retrieval Algorithm (LARA), a robust and a complete radiative transfer model, require a calculation time of ~7 days per image.One of the main ways of risk mitigation effects of explosive eruptions is to get a fast and accurate quantification of SO2 fluxes emitted by volcanoes. In this context, using the dataset acquired during IMAGETNA campaign at Mount Etna, a spectra classification methodology has been developed to drastically decrease the calculation time and reach near real-time retrievals of SO2 slant column densities. The methodology is based on a network built on two layers of information from the extraction of spectral features in the O3 and SO2 emission bands. A training dataset of five SO2 slant column densities images retrieved with the time-consuming pixel-by-pixel retrieval method allowed the creation of a library. The spectra classification makes it possible to process each hyperspectral image in less than 40 seconds. It opens the possibility to infer near real-time estimation of SO2 emission fluxes from IR hyperspectral imager measurements.

International audience; Volcanoes are complex systems that evolve in space and time as a result of their eruptive activity. Volcanic eruptions represent the ultimate expression of a complex interplay between internal and external processes that span across different time scales. Deciphering how internal and external processes interact at the time scale of eruptions may provide key insights on the temporal evolution of eruptions and also help to better evaluate associated volcanic hazards. Studies of the tidal influence on volcanic activity have fallen within this context, although the cause-effect relationship between tides and eruptions is still unclear. In this study, we used Singular Spectrum Analysis to analyze three time-series, namely the seismic tremor, SO2 emission and lava volume flow rate, which cover the first month of effusive activity at Fogo volcano, Cape Verde, in 2014-2015. We detect 9 tidal periodicities and up to 5 in each time-series ranging from semi-diurnal to fortnightly periods. We show that the movement of magma at crustal depths and at surface as well as gas emission during the effusive eruption are all modulated by lunisolar gravitational forces. We highlight the relevance of the volcano location on Earth, which together with the timing of the eruption, associated with a specific astronomical configuration, result in a specific combination of tides that directly influence the volcano eruptive activity. With this data set, we further investigate the response of Fogo volcano to this external forcing. We show that during the 2014-2015 eruption, Fogo volcano acted as a bandpass filter to quasi-permanent tidal oscillations.

International audience; Eruptive columnmodels are powerful tools for investigating the transport of volcanic gas and ash, reconstructing past explosive eruptions, and simulating future hazards. However, the evaluation of these models is challenging as it requires independent estimates of themainmodel inputs (e.g.mass eruption rate) and outputs (e.g. column height). There exists no database of independently estimated eruption source parameters (ESPs) that is extensive, standardized, maintained, and consensus-based. This paper introduces the Independent Volcanic Eruption Source Parameter Archive (IVESPA, ivespa.co.uk), a community effort endorsed by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Commission on Tephra HazardModelling.We compiled data for 134 explosive eruptive events, spanning the 1902-2016 period, with independent estimates of: i) total erupted mass of fall deposits; ii) duration; iii) eruption column height; and iv) atmospheric conditions. Crucially, we distinguish plume top versus umbrella spreading height, and the height of ash versus sulphur dioxide injection. All parameter values provided have been vetted independently by at least two experts. Uncertainties are quantified systematically, including flags to describe the degree of interpretation of the literature required for each estimate. IVESPA also includes a range of additional parameters such as total grain size distribution,eruption style, morphology of the plume (weak versus strong), and mass contribution from pyroclastic density currents, where available. We discuss the future developments and potential applications of IVESPA and make recommendations for reporting ESPs to maximize their usability across different applications. IVESPA covers an unprecedented range of ESPs and can therefore be used to evaluate and develop eruptive column models across a wide range of conditions using a standardized dataset.

In 2010, the Eyjafjallaj&ouml, kull volcano erupted, generating an ash cloud causing unprecedented disruption of European airspace. Despite an exceptional situation, both the London and Toulouse Volcanic Ash Advisory Centres (VAAC) provided critical information on the location of the cloud and on the concentration of ash, thus contributing to the crisis management. Since then, substantial efforts have been carried out by the scientific community in order to improve remote sensing techniques and numerical modeling. Satellite instruments have proven to be particularly relevant for the characterization of ash cloud properties and a great help in the operational management of volcanic risk. In this study, we present the satellite-based system HOTVOLC developed at the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC) using Meteosat geostationary satellite and designed for real-time monitoring of active volcanoes. After a brief presentation of the system we provide details on newly developed satellite products dedicated to the ash cloud characterization. This includes, in particular, ash cloud altitude and vertical column densities (VCD). Then, from the Stromboli 2018 paroxysm, we show how HOTVOLC can be used in a timely manner to assist the Toulouse VAAC in the operational management of the eruptive crisis. In the second part of the study, we provide parametric tests of the MOCAGE-Accident model run by the Toulouse VAAC from the April 17 Eyjafjallaj&ouml, kull eruption. For this purpose, we tested a range of eruption source parameters including the Total Grain Size Distribution (TGSD), the eruptive column profile, the top plume height and mass eruption rate (MER), as well as the fine ash partitioning. Finally, we make a comparison on this case study between HOTVOLC and MOCAGE-Accident VCD.

Co-auteur étranger; International audience; Numerical modelling of tephradispersaland depositionis essential forevaluation of volcanichazards. Manymodels consider reasonable physical approximations in order toreducecomputational times, but this may introduce a certain degree of uncertainty in the simulation outputs.The important step of uncertainty quantification is dealt in this paper with respect to a coupled version of a plume model (PLUME-MoM) and a tephra dispersal model (HYSPLIT). The performances of this model are evaluated through simulationsoffourpasteruptions of different magnitudesand styles from three Andean volcanoes, and the uncertainty is quantified by evaluating the differences between modeled and observed data of plume height (at different time steps above the vent)as well asmass loading and grain sizeat givenstratigraphic sections.Different meteorological datasets were alsotestedand hada sensible influence on the model outputs. Other results highlight that the modeltendsto underestimate plume heights whileoverestimatingmass loading values, especially for higher magnitude eruptions.Moreover, the advective part of HYSPLIT seems to work more efficiently than the diffusive part.Finally, though the coupledPLUME-MoM/HYSPLIT model generally is less efficient inreproducing deposit grain sizes, we propose itmay be used for hazard maps production for higher magnitude eruptions (sub-Plinian or Plinian)for what concern mass loading.

co-auteur étranger; International audience; Thermal infrared (TIR) imaging is a common tool for the monitoring of volcanic activity. Broadband cameras with increasing sampling frequency give great insight into the physical processes taking place during effusive and explosive event, while Fourier transform infrared (FTIR) methods provide high resolution spectral information used to assess the composition of volcanic gases but are often limited to a single point of interest. Continuing developments in detector technology have given rise to a new class of hyperspectral imagers combining the advantages of both approaches. In this work, we present the results of our observations of volcanic activity at Stromboli volcano with a ground-based imager, the Telops Hyper-Cam LW, when used to detect emissions of sulfur dioxide (SO2) produced at the vent, with data acquired at Stromboli volcano (Italy) in early October of 2015. We have developed an innovative technique based on a curve-fitting algorithm to quickly extract spectral information from high-resolution datasets, allowing fast and reliable identification of SO2. We show in particular that weak SO2 emissions, such as inter-eruptive gas puffing, can be easily detected using this technology, even with poor weather conditions during acquisition (e.g., high relative humidity, presence of fog and/or ash). Then, artificially reducing the spectral resolution of the instrument, we recreated a variety of commonly used multispectral configurations to examine the efficiency of four qualitative SO2 indicators based on simple Brightness Temperature Difference (BTD). Our results show that quickly changing conditions at the vent – including but not limited to the presence of summit fog – render the establishment of meaningful thresholds for BTD indicators difficult. Building on those results, we propose recommendations on the use of multispectral imaging for SO2 monitoring and routine measurements from ground-based instruments.

A volcanic eruption initiated on 23/Nov/2014 on Fogo Island in Cape Verde (14.91°N, 24.38°W, Figure 1 ) lasted for 78 days until 08/Feb/2015 [1] – [4] . Volcanic eruptions are phenomena with strong heat fluxes due to the release of lava and gases that can be suitably monitored through thermal remote sensing and its principles [5] . The sensors acquiring such type of data have generally a high temporal resolution but a low spatial one. But at the same time, non-thermal imagery captured by other sensors can also be very useful to better understand the ongoing event and track its temporal evolution [6] . Therefore, we propose a methodology to reconstruct the 2014-2015 Fogo eruption based on multi-platform imagery.

Volcanic ash clouds are common, often unpredictable, phenomena generated during explosive eruptions. Mainly composed of very fine ash particles, they can be transported in the atmosphere at great distances from the source, having detrimental socio-economic impacts. However, proximal settling processes controlling the proportion (ε) of the very fine ash fraction distally transported in the atmosphere are still poorly understood. Yet, for the past two decades, some operational meteorological agencies have used a default value of ε = 5% as input for forecast models of atmospheric ash cloud concentration. Here we show from combined satellite and field data of sustained eruptions that ε actually varies by two orders of magnitude with respect to the mass eruption rate. Unexpectedly, we demonstrate that the most intense eruptions are in fact the least efficient (with ε = 0.1%) in transporting very fine ash through the atmosphere. This implies that the amount of very fine ash distally transported in the atmosphere is up to 50 times lower than previously anticipated. We explain this finding by the efficiency of collective particle settling in ash-rich clouds which enhance early and en masse fallout of very fine ash. This suggests that proximal sedimentation during powerful eruptions is more controlled by the concentration of ash than by the grain size. This has major consequences for decision-makers in charge of air traffic safety regulation, as well as for the understanding of proximal settling processes. Finally, we propose a new statistical model for predicting the source mass eruption rate with an unprecedentedly low level of uncertainty.

The 2014 Barðarbunga rifting event in Iceland resulted in a 6-month long eruption at Holuhraun. This eruption was characterized by high lava discharge rate and significant gas emission. The SO2 flux for the first 3 months was measured with satellite sensors and the petrologic method. High-resolution time series of the satellite data give 1200 kg/s that concurs with 1050 kg/s obtained from melt inclusion minus degassed lava sulfur contents scaled to the mass of magma produced. A high-purity gas sample, with elevated S/Cl due to limited chlorine degassing, reveals a similar degassing pattern of trace metals as observed at Kīlauea (Hawai'i) and Erta Ale (Ethiopia). This suggests a common degassing mechanism at mantle plume-related volcanoes. The trace metal fluxes, calculated from trace element to sulfur ratios in the gas sample and scaled to the sulfur dioxide flux, are 1–2 orders of magnitude stronger at Holuhraun than Kīlauea and Erta Ale. In contrast, volcanoes at convergent margins (Etna and Stromboli, Italy) have 1–2 orders of magnitude higher trace element fluxes, most likely caused by abundant chlorine degassing. This emphasizes the importance of metal degassing as chlorine species. Short-lived disequilibria between radon daughters, 210Pb-210Bi-210Po measured in the gas, suggest degassing of a continuously replenished magma batch beneath the eruption site. Earlier and deep degassing phase of carbon dioxide and polonium is inferred from low (210Po/210Pb) in the gas, consistent with magma transfer rate of 0.75 m/s.

[1] Following the eruption of the Icelandic volcano Eyjafjallajokull on the 14 April 2010, ground-based N2-Raman lidar (GBL) measurements were used to trace the temporal evolution of the ash plume from 16 to 20 April 2010 above the southwestern suburb of Paris. The nighttime overpass of the Cloud-Aerosol LIdar with Orthogonal Polarization onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite (CALIPSO/CALIOP) on 17 April 2010 was an opportunity to complement GBL observations. The plume shape retrieved from GBL has been used to assess the size range of the particles size. The lidar-derived aerosol mass concentrations (PM) have been compared with model-derived PM concentrations held in the Eulerian model Polair3D transport model, driven by a source term inferred from the SEVIRI sensor onboard Meteosat satellite. The consistency between model and ground-based wind lidar and CALIOP observations has been checked. The spatial and temporal structures of the ash plume as estimated by each instrument and by the Polair3D simulations are in agreement. The ash plume was associated with a mean aerosol optical thickness of 0.1 ± 0.06 and 0.055 ± 0.053 for GBL (355 nm) and CALIOP (532 nm), respectively. Such values correspond to ash mass concentrations of ∼400 ± 160 and ∼720 ± 670 μg m−3, respectively, within the ash plume, which was lower than 0.5 km in width. The relative uncertainty is ∼75% and mainly due to the assessment of the specific cross-section assuming an aerosol density of 2.6 g cm−3. The simulated ash plume is smoother leading to integrated mass of the same order of magnitude (between 50 and 250 mg m−2).

International audience; The violent August 16-17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42-57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16-18 km above the vent and making it the strongest eruptive phase since the volcano's magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type.

[1] The 11–13 January 2011 eruptive episode at Etna volcano occurred after several months of increasing ash emissions from the summit craters, and was heralded by increasing SO2 output, which peaked at ∼5000 megagrams/day several hours before the start of the eruptive activity. The eruptive episode began with a phase of Strombolian activity from a pit crater on the eastern flank of the SE-Crater. Explosions became more intense with time and eventually became transitional between Strombolian and fountaining, before moving into a lava fountaining phase. Fountaining was accompanied by lava output from the lower rim of the pit crater. Emplacement of the resulting lava flow field, as well as associated lava fountain- and Strombolian-phases, was tracked using a remote sensing network comprising both thermal and visible cameras. Thermal surveys completed once the eruptive episode had ended also allowed us to reconstruct the emplacement of the lava flow field. Using a high temporal resolution geostationary satellite data we were also able to construct a detailed record of the heat flux during the fountain-fed flow phase and its subsequent cooling. The dense rock volume of erupted lava obtained from the satellite data was 1.2 × 106 m3; this was emplaced over a period of about 6 h to give a mean output rate of ∼55 m3 s−1. By comparison, geologic data allowed us to estimate dense rock volumes of ∼0.85 × 106 m3 for the pyroclastics erupted during the lava fountain phase, and 0.84–1.7 × 106 m3 for lavas erupted during the effusive phase, resulting in a total erupted dense rock volume of 1.7–2.5 × 106 m3 and a mean output rate of 78–117 m3 s−1. The sequence of events and quantitative results presented here shed light on the shallow feeding system of the volcano.

The April-May 2010 eruption of Eyjafjallajokull volcano (Iceland) was characterized by a nearly continuous injection of tephra in the atmosphere that affected various economic sectors in Iceland and caused a global-wide interruption of air traffic. Eruptive activity during 4-8 May 2010 was characterized based on short-duration physical parameters in order to capture transient eruptive behavior of a long-lasting eruption (i.e., total grainsize distribution, erupted mass and mass eruption rate averaged over 30-minute activity). Resulting 30-minute total grainsize distribution based on both ground and MSG-SEVIRI satellite measurements is characterized by Mdphi of about 2 phi and a fine-ash content of about 30wt%. Accumulation rate varied by two orders of magnitude with an exponential decay away from the vent, whereas Mdphi shows a linear increase until about 18 km from vent reaching a plateau of about 4.5 phi between 20-56 km. Associated mass eruption rate is in between 0.6-1.2 x 10^5 kg s^-1. In-situ sampling showed how fine ash mainly fell as aggregates of various typologies. About 5 to 9 wt% of the erupted mass remained in the cloud up to 1000 km from the vent, suggesting that nearly half of the ash >7 phi settled as aggregates within the first 60 km. Particle sphericity and shape factor varied between 0.4 and 1 with no clear correlation with size and distance from vent. Our experiments also demonstrate how satellite retrievals and Doppler radar grainsize detection can provide real-time description of the source term but for a limited particle-size range.

[1] Hydrothermal systems are active structures present at many volcanoes. Their characterization is essential as they reveal the evolution of the magmatic source and may play a role in the eruptive style. Many studies have already suggested the existence of a shallow hydrothermal system beneath the summit craters at Piton de la Fournaise volcano (La Reunion), although there is still no clear evidence. Here we present new arguments on the basis of satellite-based data acquired during April 2007 eruption suggesting the existence of a large hydrothermal system beneath the Dolomieu crater at Piton de la Fournaise. SO2 released during the collapse phase of the Dolomieu crater (∼April 6–13) has been estimated at 935 ± 244 kilotons whereas erupted SO2 calculated from lava effusion rates was found to be clearly insufficient (179 ± 89 kilotons). We suggest that the excess of SO2 originated from a large hydrothermal system suddenly opened by the collapse.

In the paper by Gouhier, M., Harris, A., Calvari, S., Labazuy, P., Guehenneux, Y., Donnadieu, F., Valade, S, entitled “Lava discharge during Etna’s January 2011 fire fountain tracked using MSG-SEVIRI” (Bull Volcanol (2012) 74:787–793, DOI 10.1007/s00445-011-0572-y), we present data from a Doppler radar (VOLDORAD 2B). This ground-based Lband radar has been monitoring the eruptive activity of the summit craters of Mt. Etna in real-time since July 2009 from a site about 3.5 km SSE of the craters. Examples of applications of this type of radar are reviewed by Donnadieu (2012) and shown on the VOLDORAD website (http://wwwobs. univbpclermont.fr/SO/televolc/voldorad/). Although designed and owned by the Observatoire de Physique du Globe in Clermont-Ferrand (OPGC), France, VOLDORAD 2B is operated jointly with the INGV-Catania (Italy) in the framework of a technical and scientific collaboration agreement between the INGV of Catania, the French CNRS and the OPGC-Universite Blaise Pascal in ClermontFerrand. The system also utilizes a dedicated micropatch antenna designed at the University of Calabria (Boccia et al. 2010) and owned by INGV. The objective of the joint acquisition of the radar data by INGV-Catania and the OPGC is twofold: (1) to mitigate volcanic risks at Etna by better assessing the hazards arising from ash plumes and (2) to allow detailed study of volcanic activity and its environmental impact. In the paper by Gouhier et al. (2012), we failed to highlight this important collaboration between the INGV Catania and the OPGC; a cooperation essential for the past, current and future generation of such valuable data sets. Specifically we wish to acknowledge the roles of Mauro Coltelli, Michele Prestifilippo and Simona Scollo for their important input into this project, and pivotal role in setting up, and maintaining, this collaborative deployment.

International audience; During the 2010 eruption of Eyjafjallajökull volcano (Iceland) we set up a system designed to ingest satellite data and output volcanic ash cloud products. The system (HVOS = HotVolc Observing System) ingested on-reception data provided every 15 minutes by the SEVIRI sensor flown aboard the Meteosat Second Generation (MSG) satellite. Data were automatically processed and posted on the web to provide plume location maps, as well as to extract plume metrics (cloud top height and mass flux), in near-real time. Given the closing speeds for aircraft approaching such hazardous ash clouds, reporting delays for such products have to be minimised.

International audience; Volcanic plumes from Etna volcano (Italy) are governed by easterly winds driving ash over the Ionian Sea. The limited land tephra deposit makes total grain-size distribution (TGSD) assessment and its fine ash fraction highly uncertain. On 23 February 2013, a lava fountain produced a 9-km-high column above sea level (a.s.l.). The atypical north-easterly wind direction dispersed the tephra from Etna to the Puglia region (southern Italy) allowing sampling up to very distal areas. This study uses field measurements to estimate the field-based TGSD. Very fine ash distribution (particle matter below 10 μm—PM10) is explored parameterizing the field-TGSD through a bi-lognormal and bi-Weibull distribution. However, none of the two latter TGSDs allow simulating any far-traveling airborne ash up to distal areas. Accounting for the airborne ash retrieved from satellite (Spinning Enhanced Visible and Infrared Imager), we proposed an empirical modification of the field-based TGSD including very fine ash through a power law decay of the distribution. The input source parameters are inverted by comparing simulations against measurements. Results suggest a column height of 8.7 km a.s.l., a total erupted mass of 4.9 × 109 kg, a PM10 content between 0.4 and 1.3 wt%, and an aggregate fraction of 2 wt% of the fine ash. Aerosol optical depth measurements from the AErosol RObotic NETwork are also used to corroborate the results at 1,700 km from the source. Integrating numerical models with field, ground-based, and satellite-based data aims at providing a better TGSD estimation including very fine ash, crucial for air traffic safety.

In March 2020, the coronavirus disease 2019 outbreak was declared a pandemic by the World Health Organization and became a global health crisis. Authorities worldwide implemented lockdowns to restrict travel and social exchanges in a global effort to counter the pandemic. In France, and in French overseas departments, the lockdown was effective from 17 March to 11 May 2020. It was in this context that the 2–6 April 2020 eruption of Piton de la Fournaise (La Réunion Island, Indian Ocean) took place. Upon the announcement of the lockdown in France, a reduced activity plan was set up by the Institut de Physique du Globe de Paris, which manages the Observatoire Volcanologique du Piton de la Fournaise (OVPF). The aim was to (1) maintain remote monitoring operations by teleworking and (2) authorize fieldwork only for critical reasons, such as serious breakdowns of stations or transmission relays. This eruption provided an opportunity for the observatory to validate its capacity to manage a volcanic crisis with 100% remotely operated monitoring networks. We thus present the long- and short-term precursors to the eruption, and the evolution of the eruption recorded using the real-time monitoring data as communicated to the stakeholders. The data were from both continuously recording and transmitting field instruments as well as satellites. The volcano observatory staff remotely managed the volcano crisis with the various stakeholders based only on these remotely functioning networks. Monitoring duties were also assured in the absence of ad hoc field investigation of the eruption by observatory staff or face-to-face communications. The density and reliability of the OVPF networks, combined with satellite observations, allowed for trustworthy instrument-based monitoring of the eruption and continuity of the OVPF duties in issuing regular updates of volcanic activity in the context of a double crisis: volcanic and health.

Volcanic ash clouds are common, often unpredictable, phenomena generated during explosive eruptions. Mainly composed of very fine ash particles, they can be transported in the atmosphere at great distances from the source, having detrimental socio-economic impacts. However, proximal settling processes controlling the proportion (ε) of the very fine ash fraction distally transported in the atmosphere are still poorly understood. Yet, for the past two decades, some operational meteorological agencies have used a default value of ε = 5% as input for forecast models of atmospheric ash cloud concentration. Here we show from combined satellite and field data of sustained eruptions that ε actually varies by two orders of magnitude with respect to the mass eruption rate. Unexpectedly, we demonstrate that the most intense eruptions are in fact the least efficient (with ε = 0.1%) in transporting very fine ash through the atmosphere. This implies that the amount of very fine ash distally transported in the atmosphere is up to 50 times lower than previously anticipated. We explain this finding by the efficiency of collective particle settling in ash-rich clouds which enhance early and en masse fallout of very fine ash. This suggests that proximal sedimentation during powerful eruptions is more controlled by the concentration of ash than by the grain size. This has major consequences for decision-makers in charge of air traffic safety regulation, as well as for the understanding of proximal settling processes. Finally, we propose a new statistical model for predicting the source mass eruption rate with an unprecedentedly low level of uncertainty.