An Online Service for Near Real-Time Satellite Monitoring of Volcanic Plumes

icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Open Access Nat. Hazards Earth Syst. Sci. Discuss., 1, 5935–6000, 2013 Natural Hazards www.nat-hazards-earth-syst-sci-discuss.net/1/5935/2013/ and Earth System doi:10.5194/nhessd-1-5935-2013 NHESSD © Author(s) 2013. CC Attribution 3.0 License. Sciences Discussions 1, 5935–6000, 2013

This discussion paper is/has been under review for the journal Natural Hazards and Earth An online service for System Sciences (NHESS). Please refer to the corresponding ﬁnal paper in NHESS if available. near real-time satellite monitoring Support to Aviation Control Service of volcanic plumes (SACS): an online service for near H. Brenot et al. real-time satellite monitoring of volcanic Title Page plumes Abstract Introduction

H. Brenot1, N. Theys1, L. Clarisse2, J. van Geﬀen3, J. van Gent1, M. Van Conclusions References 1 3 2 2 2,4 Roozendael , R. van der A , D. Hurtmans , P.-F. Coheur , C. Clerbaux , Tables Figures P. Valks5, P. Hedelt5, F. Prata6, O. Rasson1, K. Sievers7, and C. Zehner8

1 Belgisch Instituut voor Ruimte-Aeronomie – Institut d’Aéronomie Spatiale de Belgique J I (BIRA-IASB), Brussels, Belgium 2Spectroscopie de l’Atmosphère, Service de Chimie Quantique et Photophysique, J I Université Libre de Bruxelles (ULB), Brussels, Belgium Back Close 3Koninklijk Nederlands Meteorologisch Instituut (KNMI), De Bilt, the Netherlands 4 UPMC Univ. Paris 6; Université de Versailles St.-Quentin; CNRS/INSU, LATMOS-IPSL, Full Screen / Esc Paris, France 5Institut für Methodik der Fernerkundung (IMF), Deutsches Zentrum für Luft und Raumfahrt Printer-friendly Version (DLR), Oberpfaﬀenhofen, Germany 6 Norsk Institutt for Luftforskning (NILU), Kjeller, Norway Interactive Discussion 7German ALPA, Frankfurt, Germany 5935 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 8European Space Agency (ESA-ESRIN), Frascati, Italy NHESSD Received: 23 August 2013 – Accepted: 9 October 2013 – Published: 31 October 2013 1, 5935–6000, 2013 Correspondence to: H. Brenot ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. An online service for near real-time satellite monitoring of volcanic plumes

H. Brenot et al.

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5936 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract NHESSD Volcanic eruptions emit plumes of ash and gases in the atmosphere, potentially at very high altitudes. Ash rich plumes are hazardous for airplanes as ash is very 1, 5935–6000, 2013 abrasive and easily melts inside their engines. With more than 50 active volcanoes 5 per year and the ever increasing number of commercial flights, the safety of airplanes An online service for is a real concern. Satellite measurements are ideal for monitoring global volcanic near real-time activity and, in combination with atmospheric dispersion models, to track and forecast satellite monitoring volcanic plumes. Here we present the Support to Aviation Control Service (SACS, of volcanic plumes http://sacs.aeronomie.be), which is a free online service initiated by ESA for the near H. Brenot et al. 10 real-time (NRT) satellite monitoring of volcanic plumes of SO2 and ash. It combines data from two UV-visible (OMI, GOME-2) and two infrared (AIRS, IASI) spectrometers. This new multi-sensor warning system of volcanic plumes, running since April 2012, is Title Page based on the detection of SO2 and is optimised to avoid false alerts while at the same time limiting the number of notifications in case of large plumes. The system shows Abstract Introduction 15 successful results with 95 % of our notifications corresponding to true volcanic activity. Conclusions References

1 Volcanic eruptions: a threat to aviation safety Tables Figures

Volcanic eruptions are known to emit large amounts of aerosols (silicate ash, sulphates J I and ice particles) and gases (mostly water vapour, CO2, SO2,H2S and halogen species). The composition of each volcanic cloud is unique, as it is the result of J I

20 a complex processes driven by the chemistry and motion of magma inside the mantle of Back Close the Earth and its ejection at the surface (Robock and Oppenheimer, 2003). Emission can sometimes have severe implications for the atmosphere, life on the Earth, and Full Screen / Esc human society. Volcanic clouds can have a significant impact on atmospheric chemistry and climate, both locally and on a global scale (Robock, 2000; Oppenheimer et al., Printer-friendly Version 25 2011), and can strongly affect human health in the vicinity of the volcano, but the effects of a volcanic cloud may even felt far from volcano (Forbes et al., 2003; Baxter et al., Interactive Discussion 5937 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 1999). Volcanic ash and – to a lesser extent – sulphuric gases are also major hazards to aviation. The largest threat to aviation safety is that volcanic ash can damage plane NHESSD engines and cause them to stall as a result of ash melting (Miller and Casadevall, 1999; 1, 5935–6000, 2013 Prata, 2009). Volcanic ash can abrade windscreens, damage avionic equipment and 5 navigation systems, and reduce the visibility of the pilots. Moreover, sulphuric gases, such as H2SO4 and SO2, may also damage the aircraft (paint and windows) and create An online service for sulphate deposits on and inside the engines. The gases might also be dangerous for near real-time the health of the passengers. In the past there have been several incidents as a result of satellite monitoring encounters of aircraft with volcanic plumes (Casadevall, 1994; Casadevall et al., 1996; of volcanic plumes 10 Guffanti et al., 2010). A significant difficulty in mitigating volcanic hazard to aviation is that, because of strong winds at high altitudes, fine ash can rapidly be transported over H. Brenot et al. long distances (> 1000 km from the volcano) and in the process cross major air routes. As only a small number of the active volcanoes on Earth are regularly monitored using Title Page ground equipment, the use of space-based instruments (see Carn et al., 2008 and 15 Thomas and Watson, 2010) is particularly relevant for aviation safety, as it enables the Abstract Introduction continuous and global monitoring of volcanic plumes in an effective, economical and risk-free way. Conclusions References

Nine worldwide Volcanic Ash Advisory Centres (VAACs) have been designated Tables Figures to advise civil aviation authorities in case of volcanic eruptions. These centres are 20 part of the International Airways Volcano Watch (IAVW), established in 2002 by the J I International Civil Aviation Organization (ICAO) with the co-operation of numerous countries and several international organisations like the International Atomic Energy J I Agency (IAEA), the International Air Transport Association (IATA), the International Back Close Federation of Air Line Pilots’ Associations (IFALPA), the International Union of Geodesy 25 and Geophysics (IUGG) and the World Meteorological Organization (WMO). For more Full Screen / Esc details see ICAO (2012). The nine VAACs each have their own area of responsibility, which together cover the whole globe. They make advisory information available en- Printer-friendly Version route on the extent and movement of volcanic ash in the atmosphere. This information is then used by aviation safety control bodies and by pilots via SIGMET (SIGniﬁcant Interactive Discussion

5938 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion METeorological Information) advisories. The VAACs provide volcanic ash forecasts using atmospheric dispersion models, by making use of all available information NHESSD on volcanic clouds from observatories, pilot reports and measurements from the 1, 5935–6000, 2013 ground, aircrafts and above all from space instruments (mostly satellite imagery from 5 geostationary instruments). Until recently, a policy of zero tolerance regarding volcanic ash was applied by ICAO An online service for on airlines (Cantor, 1998). This regulation has changed with the introduction of ash near real-time concentration thresholds over Europe after the eruptions of the Icelandic volcanoes satellite monitoring Eyjafjallajökull in April/May 2010 and Grímsvötn in May 2011, as both caused partial or of volcanic plumes 10 total closure of airspace over many European countries and led to social and economic upheaval across Europe (IATA, 2010). The introduction of ash concentration thresholds H. Brenot et al. translates to important needs and requirements for improved volcanic ash monitoring and forecasting services (see Zehner, 2010). These needs are: Title Page – Early detection of volcanic emissions. Abstract Introduction 15 – NRT global monitoring of volcanic plumes, with open access and delivery of data. Conclusions References – Quantitative retrievals of volcanic ash (concentration, altitude and particle size Tables Figures distribution) and SO2 (concentration, altitude) from (but not limited to) satellite instruments. Ash data at high-temporal resolution from geostationary payloads (Prata and Prata, 2012) has priority. J I

20 – Accurate source term parameters: time-, particle size- and height-resolved ash J I source emissions for the entire eruptive period. Back Close – Improved inversion techniques involving atmospheric models for forecasting ash Full Screen / Esc dispersion and removal (Stohl et al., 2011).

– Validation of satellite observations. Printer-friendly Version

25 In this paper, we give an overview of the Support to Aviation Control Service (SACS) Interactive Discussion which deals with the first two points listed above as it is an automated global system 5939 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion for the monitoring and warning of volcanic plumes. It makes use of NRT ash and SO2 data products from satellite instruments operating in the UV-visible (OMI and GOME-2) NHESSD and Thermal Infrared (IASI and AIRS) wavelength ranges. A system has been set-up 1, 5935–6000, 2013 to notify the users by e-mail in case of exceptional volcanic emissions. Here we present 5 the strategy adopted to detect and monitor volcanic clouds. With examples of recent eruptions, we demonstrate the service and show the information available to the SACS An online service for user and detail the different aspects of our work. near real-time satellite monitoring of volcanic plumes 2 Overview of SACS H. Brenot et al. The Support to Aviation Control Service is an ESA-funded project developed within 10 the Data User Element (DUE-TEMIS, http://www.temis.nl) and the GMES Service Element (GSE-PROMOTE, http://www.gse-promote.org) programmes (Van Geffen Title Page et al., 2007). SACS is a collaborative project that currently involves the following partners: the Belgian Institute for Space Aeronomy (BIRA-IASB; leader), the Royal Abstract Introduction

Netherlands Meteorological Institute (KNMI), the University of Brussels (ULB) and the Conclusions References 15 German Aerospace Center (DLR). The SACS project is also indirectly supported by the following agencies/institutions: EUMETSAT (via the O3M SAF program), Centre Tables Figures National d’Etudes Spatiales (CNES), Finnish Meteorological Institute (FMI, Ilmatieteen Laitos), Norsk Institutt for Luftforskning (NILU), Jet Propulsion Laboratory (JPL), and J I the National Aeronautics and Space Administration (NASA). The primary objective J I 20 of SACS is to support the VAACs (key users of the service) in their oﬃcial tasks of informing aviation control organisations about the risks associated with volcanic Back Close activity. SACS is a free service available through a single user-friendly web portal (http://sacs.aeronomie.be) that centralises the data (near real-time and archive) in the Full Screen / Esc form of maps (images) globally and for a set of pre-deﬁned regions. This service is also Printer-friendly Version 25 linked to other European initiatives such as ESA SAVAA (http://savaa.nilu.no), VAST (http://vast.nilu.no) and EU FP7 EVOSS (http://www.evoss.eu), whose general aims Interactive Discussion

5940 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion are to define and demonstrate an optimal system for volcanic ash plume monitoring and prediction. NHESSD SACS delivers global data sets for sulphur dioxide (SO ) and aerosol (ash) related 2 1, 5935–6000, 2013 to volcanic eruptions from measurements by space-based instruments. At the time of 5 writing, SACS is based on polar sun-synchronous satellite instruments widely used to sound atmospheric composition and for meteorological and scientific applications: An online service for AIRS/Aqua (Aumann et al., 2000, 2003; Hofstadter et al., 2000), OMI/Aura (Levelt et al., near real-time 2006), GOME-2/MetOp-A (Munro et al., 2006) and IASI/MetOp-A (Clerbaux et al., satellite monitoring 2009; Hilton et al., 2012). All SACS data retrievals are available in NRT (which means of volcanic plumes 10 as soon as the data process allows to retrieve SO2 and ash observations). Figure 1 shows the different sounders that SACS currently uses and their local overpass time. H. Brenot et al. It shows the advantage of combining multiple sensors which have different overpass times in a single system as it allows the reliable and timely monitoring of volcanic Title Page plumes on the global scale. Note that until April 2012, the NRT SACS system also used 15 data from SCIAMACHY/Envisat (Bovensmann et al., 1999) for which archive data can Abstract Introduction still be consulted. Table 1 presents a summary of the SACS products and their main characteristics: satellite platform, data type and availability, equatorial local overpass Conclusions References

time, spatial resolution, retrieved quantity, data provider, units, delays of retrievals, Tables Figures swath widths, and global coverage. Note that the size of the footprint of each instrument 2 20 (resolution in km ) is an approximate value at nadir. A description of the different data J I products used in the SACS system is given in Sect. 3. A notification system has been set-up to warn people (by email) in case of J I exceptional concentrations of SO2, pointing them to a web page with relevant Back Close information on the location of the plume. Users also have the possibility to consult the 25 notification service which compiles notifications of on-going or past eruptive events. Full Screen / Esc Data archives contain the observations starting from September 2002 for AIRS, from January 2004 until April 2012 for SCIAMACHY, from September 2004 for OMI, from Printer-friendly Version January 2007 for GOME-2, and from October 2007 for IASI. Interactive Discussion

5941 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3 Description of satellite data products used by SACS NHESSD The detection of volcanic ash is far from trivial, especially for dispersed plumes. Most of the current satellite algorithms use differential absorption by ash between two 1, 5935–6000, 2013 channels, which can yield false detection in the presence of absorbing aerosols other 5 than ash (e.g., desert dust). Conversely, satellite detection of SO2 is more reliable An online service for and when it is present in the upper troposphere/lower stratosphere it is a more robust near real-time indicator of volcanic activity. Therefore, the SACS system at present only uses satellite satellite monitoring SO2 data products to trigger and issue plume notifications. SO2 is often a good proxy of volcanic plumes for volcanic plumes and therefore possibly of volcanic ash (Thomas and Prata, 2011). H. Brenot et al. 10 This being said, not all eruptions are accompanied by detectable amount of SO2 and even then it is not uncommon for ash and SO2 to follow different trajectories because of differences in injection altitudes. For these reasons, SACS is providing both SO2 and aerosol/ash information in NRT. In this section, we briefly present the different products Title Page

from the ﬁve instruments listed in Table 1 and their main features. Abstract Introduction

15 3.1 SO2 column retrievals from UV-visible sensors (SCIAMACHY, OMI, GOME-2) Conclusions References

The SCIAMACHY, GOME-2 and OMI instruments are UV-visible spectrometers Tables Figures measuring solar light backscattered by the atmosphere or reflected by the Earth (and hence can only measure during daytime). The observations are done in nadir viewing J I geometry, except for SCIAMACHY, which had alternating viewing modes in nadir J I 20 and limb (not used here). Once a day, a direct measurement of the solar irradiance spectrum is also acquired. All three instruments have spectral channels covering the Back Close UV wavelength range (240–400 nm), where SO2 has strong and distinctive absorption bands. The spectral resolution in the UV range is 0.2–0.6 nm. Full Screen / Esc The retrieved quantity from the three algorithms is the so-called SO2 Vertical Column Printer-friendly Version 25 Density (VCD). It represents the SO2 concentration integrated along the vertical axis 16 −2 and is generally expressed in Dobson Units (1DU = 2.69 × 10 moleculescm ). The Interactive Discussion SO2 retrievals of SCIAMACHY (Van Geffen et al., 2007, 2008) and GOME-2 (Rix et al., 5942 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 2009, 2012) use a Differential Optical Absorption Spectroscopy technique (DOAS, Platt and Stutz, 2008) where all measurements in a wavelength range from 315 to 326 nm NHESSD are fitted to laboratory absorption data of SO (and other atmospheric gases) to yield 2 1, 5935–6000, 2013 the SO2 slant column, i.e. the SO2 concentration integrated along the mean optical light 5 path in the atmosphere. The OMI SO2 retrieval is slightly different from SCIAMACHY and GOME-2 in that it uses only a small number of wavelengths (between 310 and An online service for 345 nm) in and outside the SO2 band, following the Linear Fit algorithm (Yang et al., near real-time 2007) and it does not retrieve an SO2 slant column as an intermediate step. Note that satellite monitoring for all instruments, several background and offset corrections are generally applied, to of volcanic plumes 10 ensure geophysical consistency of the data. H. Brenot et al. The SO2 vertical column is obtained using radiative transfer calculations that account for important parameters influencing the UV light path in the atmosphere: solar zenith angle, viewing angles, clouds, atmospheric absorption and scattering, surface albedo. Title Page As the measurement sensitivity strongly depends on the altitude where SO2 resides 15 and because we have no knowledge what this altitude is prior to the measurement, the Abstract Introduction SO2 vertical column estimation is provided on three hypothetical atmospheric layers representative of different scenarios of emissions: planetary boundary layer (typically Conclusions References

the first km above the surface), upper troposphere (∼ 6 km) and lower stratosphere Tables Figures (∼ 15 km). The SO2 images displayed in SACS all use the data assuming lower 20 stratospheric plumes. J I The SO2 algorithms from OMI, SCIAMACHY and GOME-2 are able to detect similar SO2 patterns for modest and strong eruptions. Figure 2 shows that the measurements J I (SO2 VCD images from SCIAMACHY, GOME-2 and OMI for an eruption of Kilauea Back Close volcano in Hawaii, 17 May 2008) are able to consistently observe small (and low) 25 SO2 plumes. Despite the different overpass times, we can see a very good spatial Full Screen / Esc correspondence of the location of the plume between the 3 instruments. To compare ◦ the SO2 VCDs, a fixed latitude of 19.4 N has been chosen for the 3 instruments, and Printer-friendly Version the measurements have been plotted as a function of longitude (Fig. 2d). A fairly good agreement between the VCD values is found, especially taken into account the fact Interactive Discussion

5943 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the SO2 plume is moving westward. This example shows and conﬁrms that SO2 VCD measurements from UV-visible instruments allow a good estimation and monitoring of NHESSD volcanic emissions even for such a small eruption (the highest SO VCD is 5.2 DU as 2 1, 5935–6000, 2013 recorded by GOME-2 during this day). Note that the operational cloud cover fraction 5 products for all UV-visible instruments are also available on the SACS webpage as additional information. Note that the measurement times are indicated in UTC on all An online service for SACS images. near real-time satellite monitoring 3.2 SO2 index retrievals from thermal IR sensors (IASI and AIRS) of volcanic plumes

The IASI instrument is a Fourier transform spectrometer. The spectral coverage H. Brenot et al. −1 −1 10 is from 645 to 2760 cm (with no gaps) with a spectral resolution of 0.5 cm (apodised) and a spectral sampling of 0.25 cm−1. The AIRS instrument is an echelle −1 grating spectrometer covering the range between 650 and 2665 cm (with gaps) and Title Page a spectral resolution of 0.5–2 cm−1. The instruments are operating in a nadir view with Abstract Introduction typical footprints (circular to elliptical depending on the position on the swath) of 12 and 15 15 km diameters (at nadir), for IASI and AIRS respectively. Both sensors measure the Conclusions References spectrum of the outgoing thermal radiation emitted by the Earth-atmosphere system (and hence can operate during both day and night). Tables Figures Both IR instruments cover the strong ν3 (asymmetric stretch) SO2 absorption band around 1362 cm−1. For AIRS, three wavenumbers are used (1395 cm−1, 1327 cm−1 J I −1 20 and 1328 cm ), while the IASI algorithm is based on measurements around J I 1408 cm−1, 1372 cm−1 and 1385 cm−1. The AIRS L1 data are provided in near real- time by NASA’s Earth Observing System Data and Information System (EOSDIS, Back Close https://urs.eosdis.nasa.gov), and the SO retrievals are processed by BIRA-IASB 2 Full Screen / Esc using the algorithm from NILU. The retrieval scheme of AIRS is a two-step process 25 with a first step to identify pixels that contain SO , and a second step to adjust 2 Printer-friendly Version the amount of SO2 based on off-line radiative transfer calculations (see details in Prata and Bernardo, 2007). The retrievals from IASI SO2 are provided in NRT by the Interactive Discussion ULB (http://cpm-ws4.ulb.ac.be/Alerts). The IASI SO2 algorithm (see details in Clarisse 5944 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion et al., 2012) also involves the conversion of the measured signal into an SO2 vertical column by making use of a large look-up-table and EUMETSAT operational pressure, NHESSD temperature and humidity profiles. These are only available for data with L2 information. 1, 5935–6000, 2013 To ensure a maximum amount of IASI data, only the brightness temperature difference 5 index is used in the NRT treatment of SACS. Figure 3 shows IASI and AIRS SO2 images from SACS after the beginning of the An online service for eruption of Nabro’s volcano (Eritrea). IASI was the first instrument to observe the SO2 near real-time plume at 06:25 a.m. and 08:05 a.m. UTC (for two consecutive orbits) on 13 June 2011. satellite monitoring Nevertheless these observations did not cover the full plume. At 10:50 a.m., a full of volcanic plumes 10 coverage of the plume was possible by combining IASI with AIRS measurements. This illustrates the benefit of our multi-sensor approach. As a side note, the delay time for H. Brenot et al. receiving the data was about 1 h50 and 1 h30 for IASI and AIRS, respectively.

3.3 Absorbing aerosol index retrievals from UV-visible sensors (SCIAMACHY, Title Page

OMI, GOME-2) Abstract Introduction

15 The Absorbing Aerosol Index (AAI) indicates the presence of elevated absorbing Conclusions References aerosols in the atmosphere. It is often named the residue, the spectral contrast anomaly, or simply aerosol index. Because the presence of ash can be the dominant Tables Figures part of the AAI detection, SACS provides these products in near real-time. The AAI separates the spectral contrast at two ultraviolet wavelengths caused by absorbing J I 20 aerosols from that of other effects, including molecular Rayleigh scattering, surface J I reflection, gaseous absorption, and aerosol/cloud scattering (Herman et al., 1997; Torres et al., 1998). Back Close The UV wavelengths used are 340 nm and 380 nm for SCIAMACHY and GOME- Full Screen / Esc 2, and 354 nm and 388 nm for OMI. Note that AAI retrieval is possible over cloud 25 covered areas and is performed equally well over ocean and land (except over ice). However, the final AAI products are sensitive to calibration issues and sunglint over Printer-friendly Version the ocean and therefore ad-hoc flags are applied. Figure 4c and d show an example Interactive Discussion

3.4 Ash index retrieval from the thermal IR sensors (IASI, AIRS) 1, 5935–6000, 2013

Clarisse et al. (2010, 2013) have demonstrated the potential of hyperspectral thermal An online service for 5 infrared sounders to detect volcanic ash with a high sensitivity and differentiate it near real-time from other airborne aerosol (including windblown sand). The IASI and AIRS ash index satellite monitoring products used in SACS are based on a three-step process (see Clarisse et al., 2013 of volcanic plumes for details). The first step calculates the relative distance (weighted projection) between the measured spectra and fixed ash spectral signatures. Based on the magnitude of H. Brenot et al. 10 these distances, only those observations are kept which are likely to be ash (called ash candidates). The second step finds the subset of observations which are almost certainly ash by imposing very strict criteria based on absolute distance criteria (called Title Page high confidence detections). The third steps considers spatial context to promote Abstract Introduction ash candidates to low and medium confidence detections in the neighbourhood of 15 high confidence detections. The procedure ensures a very low false detection rate Conclusions References (because of step 2), while at the same time identifying as much as possible ash in the neighbourhood of high confidence detections (because of steps 1 and 3). Figure 4a Tables Figures presents the AIRS detection of ash at 03:40 UTC on 22 May 2011. Note that IASI was the first instrument on a polar orbiting satellite able to clearly distinguish ash emitted J I 20 at the start of the Grímsvötn eruption at 20:45 UTC on 21 May 2011. Figure 4b shows J I the IASI ash detection at 10:30 UTC on 22 May. We can see in Fig. 4c and d the displacement of the ash cloud during the next 2 days (ash detected by AAI retrievals Back Close from GOME-2 and OMI). Full Screen / Esc

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5946 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3.5 Limitations of satellite products in detecting volcanic plumes NHESSD 3.5.1 SO2 products 1, 5935–6000, 2013 Because of competing water vapour absorption in the same IR wavelength window as SO2, the vertical sensitivity of IASI and AIRS to SO2 is limited to the atmospheric An online service for 5 layers above 3–5 km depending on the humidity vertical profile. This limitation might near real-time seem serious, but in the context of aviation it is actually an advantage as the detection satellite monitoring of SO2 by these two sensors means that SO2 is present at altitudes where airplanes of volcanic plumes spend most of their travel time. This fact combined with four overpasses per day make the IR sensors a central component of the SACS system. As a complement, the UV H. Brenot et al. 10 sensors are characterised by a relatively good measurement sensitivity for SO2 down to the surface. While this allows monitoring of low altitude volcanic degassing, it also renders the measurements sensitive to anthropogenic emissions. As most of the SO2 Title Page pollution hotspots are confined to a limited number of regions (China, South Africa, Abstract Introduction Siberia), the SACS system can easily be tuned to avoid false notifications there. As 15 a general comment, SO2 retrievals are also affected by clouds and instrumental noise Conclusions References (especially at high solar zenith angles for the UV sensors). The latter generally shows Tables Figures up in the SO2 images and evolves as the instrument is degrading over time.

3.5.2 Aerosol products J I

Besides the fact that the aerosol products are qualitative products (indexes), J I

20 measurements from UV-visible sensors are sensitive to all types of absorbing aerosols Back Close (desert dust, biomass burning aerosols, volcanic ash) and are therefore not purely Full Screen / Esc selective for ash. However, the combined detection of both elevated aerosol and SO2 is highly selective for the volcanic plume (in fact even more than SO2 detection alone). Note that the ash product of IASI and AIRS sensors yields very few false detections. Printer-friendly Version 25 But it should be noted that because of the strict conditions of step 2, low concentration ash plumes are often not detected in the current implementation. Interactive Discussion 5947 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 3.5.3 Known anomalies impacting the data NHESSD The Earth’s dipolar magnetic field is offset by about 500 km from the rotational axis. As a result of this, the inner Van Allen belt (doughnut-shaped regions of high-energy 1, 5935–6000, 2013 charged particles) is on one side closer to the Earth’s surface than the other side. 5 This region is named the South Atlantic Anomaly (SAA) and it covers a part of South An online service for ◦ America and the Southern Atlantic Ocean: it lies roughly between latitudes 5 and 40 S, near real-time ◦ and between longitudes 0 and 80 W (the precise strength, shape and size of the SAA satellite monitoring varies with the seasons). This dip in the Earth’s magnetic field allows charged particles of volcanic plumes and cosmic rays to penetrate lower into the ionosphere (at ∼ 500 km of altitude). Low- 10 orbiting satellites, such as Envisat, Aura and MetOp-A, pass daily through the inner H. Brenot et al. radiation belt in the SAA-region. Upon passing the inner belt, charged particles may impact on the detector, causing higher-than-normal radiance values, which in turn decreases the quality of the measurements, notably in the UV. The SAA affects the Title Page

SO2 retrievals and results in noise artefacts, as is visible in Fig. 5a and b. Thanks to Abstract Introduction 15 its design which gives it better protection to radiation than other sensors, OMI is less affected by the SAA than SCIAMACHY or GOME-2 (as can be seen Fig. 5c). Conclusions References The OMI instrument has started to suffer from a so-called “row anomaly” since 25 Tables Figures June 2007. This anomaly affects particular viewing direction angles, corresponding to rows on the CCD detector. The row anomaly has increased over time (see www.knmi. J I 20 nl/omi/research/product). Currently, the affected rows are not treated in SACS, leading

to a reduction in the data coverage. Several oﬀ-line schemes to correct for the row J I anomaly exist (e.g. Yan et al., 2012) but are not implemented yet in the NRT data- processing. Back Close

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4 Global monitoring of volcanic SO2 and ash emissions Printer-friendly Version

25 In this section we present results from the SACS global monitoring system of SO2 Interactive Discussion emissions and the strategy adopted to warn users of exceptional SO2 concentrations. 5948 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4.1 Temporal and spatial sampling NHESSD It is important to know precisely the time and space sampling of the SACS monitoring system. Because of the use of polar-orbiting satellites with different overpass times, 1, 5935–6000, 2013 some geographical regions are observed more frequently than others. To evaluate the 5 revisiting frequency of SACS, we have defined seven geographical latitude zones (see An online service for Fig. 6a) and we have estimated, based on data of October 2012, the percentage of the near real-time areas that have been observed at least once by one of the satellite instruments (OMI, satellite monitoring GOME-2, IASI, AIRS). The calculation was done for several time intervals (see e.g. the of volcanic plumes coverage of the Earth by SACS after 8 h of monitoring in Fig. 6b). Table 2 presents the 10 results for the different zones and time durations. A set of two parameters has been H. Brenot et al. evaluated: pxl is the percentage of the region of interest sampled by the SACS system; and max is the maximum number of overpasses. From Table 2, it can be seen that with four instruments, SACS monitors in near real-time any location in the world within Title Page

a 24 h interval. The percentage value drops to 99.5 %, 95 %, 90 %, 75 %, 60 %, 50 % Abstract Introduction 15 and 30 % for periods respectively of 12, 8, 6, 4, 3, 2 and 1 h. Also apparent is the higher sampling rates for high-latitude regions due to Conclusions References overlapping orbits. This is particularly interesting for Icelandic volcanoes (zone E) that Tables Figures will be sampled e.g. every 2 h 50 % of the time (see Table 2). Note that the geographical coverage of D-E-F zones depends on the day of year. For example in December, J I 20 the coverage of Iceland is less good (due to the SZA limit for UV-vis instruments).

Nevertheless, because the IR sensors are not aﬀected by the SZA limit criteria (day and J I night measurements), the coverage of the high-latitude regions always takes advantage of overlapping orbits over North and South Poles. Back Close

Full Screen / Esc 4.2 SACS strategy for volcanic SO2 notiﬁcations

Printer-friendly Version 25 Here, we detail the implementation of the global multi-sensor notification system for SO . It requires two steps: (1) proper establishment of warnings criteria for the 2 Interactive Discussion different sensors, (2) combining the information from the sensors in one system. 5949 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Particular attention is given to the avoidance of false notifications (due to noise or retrieval failures) or overly frequent/redundant notifications (caused by highly dispersed NHESSD plumes). 1, 5935–6000, 2013

4.2.1 Criteria for SO2 notification An online service for 5 The criteria for detection of exceptional SO2 concentrations from the five instruments near real-time has been derived from the analysis of historical results (see Table 1 for the data periods satellite monitoring considered). Each instrument and retrieval method has different characteristics, of volcanic plumes therefore for each data source different notification threshold values have been established. Note that, even though it no more provides data, SCIAMACHY is included H. Brenot et al. 10 here because it has been used in the past and notifications based on its data are in the SACS archive. Title Page SO2 notification from UV-visible sensors Abstract Introduction The instrumental noise on the UV-visible data is a limitation and can lead to false Conclusions References notifications. For this reason, a first selection criterion is based on the maximum 15 observed value for the SO2 column which must exceed the relevant threshold. An Tables Figures additional criterion is based on the neighbouring pixels (inside a threshold radius, see

Table 3 and Fig. 7), for which the observed SO2 column for more than half of the J I observations must also exceed the threshold value. A notification is generated only J I when both criteria are fulfilled. An illustration is given in Fig. 7 for GOME-2 for an SO2 20 plume from the Copahue volcano (23 December 2012). Back Close Another limitation of UV-visible sensors comes from the South Atlantic Anomaly (see Sect. 3.5.3). To avoid false notifications in this region, a more strict threshold value is Full Screen / Esc considered (see Table 3). Note also that special settings are used for SO2 plumes from small eruptions and degassing, in that the system considers lower threshold values Printer-friendly Version 25 for measurements close to a given volcano (distances less than 300 km). Finally, one Interactive Discussion more limitation of UV-visible sensors is that they are sensitive to anthropogenic SO2 5950 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion emissions. To avoid false notifications, more strict criteria are used for some specific areas (West China, Central North Russia and South Africa). NHESSD 1, 5935–6000, 2013 SO2 notification from thermal IR sensors

IASI and AIRS are much less or not at all affected by instrumental noise, the SAA, or An online service for 5 pollution. For IASI, rather than considering threshold values based on VCD, the criteria near real-time apply directly on the measured Difference of Brightness Temperatures (called DBT and satellite monitoring expressed in Kelvin [K]), and is therefore straightforward. The threshold values (DBT of volcanic plumes and VCD) used are given in Table 3 for respectively IASI and AIRS, and they produce almost no false notifications. H. Brenot et al.

10 4.2.2 SACS multi-sensor warning system Title Page The SO2 warning service relies on pre-defined geographical regions. SACS sends a notification to users as soon as a new volcanic plume is detected. If within a period Abstract Introduction of 12 h, a plume is detected again in the same region no new notification is generated Conclusions References (to avoid sending redundant information). An illustration of the SACS multi-sensors 15 warning system is presented in Fig. 8. The different parts are described in more detail Tables Figures below.

J I SO2 notification for a dedicated region J I As soon as SACS harvests new SO2 observations (from one of the instruments), the first step is the analysis of the data using the criteria defined in Sect. 4.2.1. After Back Close

20 a volcanic eruption, there are potentially multiple warnings generated by the system. Full Screen / Esc For this reason, it has been decided to consider a set of world regions of 30◦ by 30◦ plus two polar regions polewards of 75◦ in latitude. Figure 9 shows the locations (and Printer-friendly Version associated name/number) of the 62 regions of the SACS system. Note that the same regions are used in the near real-time monitoring and archive services to display the Interactive Discussion

The second step is to check the “warning status” of this region. If there is no on-going An online service for 5 notification for this region (meaning no notification since 12 h), the warning status near real-time can possibly become ON. The system compares the time of observations with the satellite monitoring processing time. If the delay is less than 8 h, the notification is issued. This set-up of volcanic plumes enables to provide timely information to the users and also to avoid issuing too many notifications (maximum one notification per region and per 12 h). H. Brenot et al.

10 E-mail notification Title Page Important information related to the warning is gathered into a notification sent by email to users who subscribed to SACS (http://sacs.aeronomie.be/alert/subscribe.php). The Abstract Introduction email notification (see also the example shown in Fig. 10) contains: the time of the notification and the instrument, the region number, some details about the processing, Conclusions References 15 a link to a dedicated web page generated by the system, coordinates of the maximum Tables Figures SO2 signal along with the date and time of observations (UTC), a flag indicating whether a cloud correction has been applied or not, and finally the solar zenith angle. J I Note that a solar zenith angle cut-off is used for each UV-visible sensor depending on the day of the year. The values have been determined empirically to have the best J I 20 coverage while limiting the increase in the noise (instrument-specific). Back Close

Dedicated SO2 notiﬁcation’s web page Full Screen / Esc

An example of a web page generated by SACS using GOME-2 data for a specific Printer-friendly Version notification is shown in Fig. 11. This notification was sent to the users 1 h01 after MetOp-A passed over this area. The page shows a box with the information of the email Interactive Discussion 25 notification (time of measurement, location of the warning, SZA, maximum SO2 value, 5952 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion and the use of cloud data in calculations), and the images of SO2 and aerosols/ash side-by-side. Below the SO2 image, one can find two links to the same image but using NHESSD different mapping tools (interpolated image using the Generic Mapping Tools from the 1, 5935–6000, 2013 University from Hawaii, see http://gmt.soest.hawaii.edu, and a Google-Earth file, see 5 http://www.google.com/earth). For the UV-visible instruments, there is also a link to the corresponding cloud cover fraction (CCF) image. In case of a IASI notification, in An online service for addition to the SO2 brightness temperature index image there is also a link to the SO2 near real-time VCD image. As a complement, a map showing the volcanoes in the region (with a link satellite monitoring to a listing including more information about each of them) is also provided on this web of volcanic plumes 10 page. Furthermore, a direct link to the near real-time monitoring web page is given with all corresponding SACS images (for all instruments), and a link to the data (files or link H. Brenot et al. to data providers), for the region considered.

Title Page Confirmation of SO2 notification Abstract Introduction For the example shown in Fig. 11, IASI also detected SO2 signal and ash in the region 15 112 (see Fig. 12) and acted as a confirmation. In fact the processing of IASI was Conclusions References just 3 min after the one of GOME-2, which took place at 23:42 UTC, as show Fig. 10. No notification has been sent to the users, as there already was a notification based Tables Figures on GOME-2, but the dedicated web page has been automatically updated with the notification box for the IASI warning using the same displaying template. Note that two J I 20 and ten hours after this notification from GOME-2, two others confirmations from AIRS J I and IASI took place. Back Close Update of SO2 warning archive Full Screen / Esc As soon as a warning is detected by the system, the warning archives are automatically updated on the SACS web site. The warning subpage allows the users to select the Printer-friendly Version 25 instruments of interest, for a given month/year and to visualise a map with the location of all the corresponding warnings (see next section). For a given region, a minimum Interactive Discussion 5953 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion delay of 12 h separates the time between two warnings, as explained above. The map of the warnings is an interactive map. When moving the cursor on a warning point, NHESSD the user can visualise the images corresponding to the warning. In addition to this 1, 5935–6000, 2013 interactive tool, a common list with all the warnings (chronologically sorted) is also 5 available with the relevant links to the different webpages. An online service for 4.3 Nine years of data and notifications archive near real-time satellite monitoring Although the multi-sensor notification system is only operational since April 2012, nine of volcanic plumes years of historical data have been reanalysed for the development of the warning service. This section presents an overview of all the notifications identified by SACS H. Brenot et al. 10 using the five sensors. For each year (from 2004 to 2012), an image of all the notifications is shown with a highlight of the main events. This tool, available on the SACS portal, allows any interested person to investigate historical eruptive emissions Title Page in a quick and user-friendly way. Note that our system also allows the monitoring of Abstract Introduction small volcanic eruptions which might not always lead to a notification. Conclusions References 15 4.3.1 Notifications from 2004 to 2006 (SCIAMACHY, OMI, and AIRS) Tables Figures Data from the SCIAMACHY, OMI, and AIRS instruments are available for this

period. The main volcanic activity detected by SACS in 2004 is in the Independent J I State of Papua New Guinea, the archipelago of the Republic of Vanuatu, and the Democratic Republic of Congo (DRC), as shown in Fig. 13a. The borderland between J I

20 Rwanda, the DRC and Uganda is one of the most active regions containing eight Back Close volcanoes, including Mount Nyamuragira and Mount Nyiragongo which were both Full Screen / Esc erupting on 25 May 2004. Details about the satellite monitoring of SO2 emissions from Nyiamuragira from 1979 to 2005 can be found in Bluth and Carn (2008). Figure 13a presents also several SO2 notifications over Europe in 2004. During the first week of Printer-friendly Version 25 November, an eruption occurred in Iceland (Grìmsvötn volcano). Volcanic ash from the eruption fell as far away as mainland Europe and caused short-term disruption of Interactive Discussion 5954 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion airline traffic into Iceland. Assuming an ash density of 2200 kgm−3 and using dispersion models from the VAACs, Witham et al. (2007) gives an emission up to 9.50×1012 gh−1 NHESSD 11 for the larger plume, that corresponds to an erupted mass of 3.3×10 kg and a volume 1, 5935–6000, 2013 of 0.15 km3 over the 35 h release period. Note that at this time of the year, the thermal 5 IR sensor (AIRS) was the best instrument (best coverage and time laps between observations) to monitor the volcanic plume. An online service for The volcanic activity in DRC (Galle et al., 2005) and in Galapagos and Vanuatu near real-time has continued in 2005 as can be seen in Fig. 13b. The signatures of two additional satellite monitoring eruptions are also visible: (1) the submarine eruption of the Japanese Fukutoku- of volcanic plumes 10 Okanoba volcano, located 6 km northeast of the pyramidal island of Minami-Iwojima H. Brenot et al. (about 1300 km south-west of Tokyo); (2) the eruption of the Sierra Negra volcano, Galápagos, Ecuador. After 26 yr being inactive and six months after an earthquake on

an active fault, the volcanic activity of the Sierra Negra raised again on 22 October 2005 Title Page with a ﬁssure opening inside the rim of the caldera. This eruption emitted a volcanic 3 15 cloud with an estimated volume of 150 millions m of gas with a 2 km long curtain of Abstract Introduction lava fountains (Geist et al., 2008). During this eﬀusive eruption, a huge amount of gas Conclusions References and particles was ejected in the low troposphere. The plume of SO2 and ash reached an altitude estimated below 5 km (Yang et al., 2009). At the end of October 2005, Tables Figures atmospheric emissions of this volcano ceased.

20 In 2006, volcanic activity has been detected by SACS (Fig. 13c) in the Andes J I Cordillera (Colombia and Peru), but the three strongest eruptions occurred in other parts of the world: (1) on 6 October, a large explosive eruption took place at Rabaul J I

volcano (Papua New Guinea), sending a plume of gas and ash up to the lower Back Close stratosphere. Several notifications have been issued by SACS showing the volcanic 25 cloud traveling westward; (2) the Nyamuragira volcano was also active in 2006. Full Screen / Esc A strong eruption began in the night of the 27 November (see e.g. Prata and Bernardo, 2007). The volcanic cloud initially moved north-eastward to finally travel to the west and Printer-friendly Version pass over Pakistan and India; (3) the 20 May eruption of Soufrière Hills (Montserrat, British Overseas Territory) released a volcanic cloud which travelled over a very long Interactive Discussion 5955 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion distance (more than 18 000 km westward). A large dome collapse was accompanied by a large explosion, producing an ash column of about 18 km height (Prata et al., 2007). NHESSD The movement of this volcanic cloud has been monitored during 23 days (see Fig. 14), 1, 5935–6000, 2013 drifting westward and causing diversion of many planes. The 20 May, at 17:02 UTC, the 5 OMI sensor measured a VCD up to 122.2 DU. Several days after the eruption, the wind rapidly dispersed the volcanic cloud on the north-east and the south-west directions An online service for (22 and 24 May, Fig. 14). Note that for this date, the OMI sensor was not yet affected near real-time by the row anomaly and was still able to provide global coverage. satellite monitoring of volcanic plumes 4.3.2 Notifications from 2007 to 2012 (SCIAMACHY, OMI, AIRS, GOME-2 and 10 IASI) H. Brenot et al.

Since 2007, SO2 observations from the GOME-2 and IASI instruments became available and have been included in SACS. Volcanic emissions in 2007 have been Title Page observed by our system for some volcanoes of the Russian Federation (located in the Abstract Introduction Northern Kurile Islands and in Kamchatka), as shown in Fig. 15a. In January 2007, the 15 Kliuchevskoi volcano started another eruption cycle. On 28 June, this volcano began Conclusions References to experience the largest explosions so far recorded in that eruption cycle, disrupting air traffic from the United States to Asia and causing ash falls on Alaska’s Unimak Tables Figures Island. Volcanic activity has also be monitored by SACS in many other regions on Earth (although not all events are relevant for aviation), notably in Papua New Guinea (Rabaul J I 20 volcano), the archipelago of Vanuatu (Ambrym and Lopevi volcanoes), in Ecuador J I (Tungurahua volcano), the Bonin Islands of Japan (Fukutoku-Okanoba), Italy (Etna), and in Ethiopia (Manda Hararo). The two largest eruptions of 2007 were observed at Back Close Jebel al-Tair Island (Yemen) and at Réunion Island (France). Full Screen / Esc The Jebel al-Tair eruption started unexpectedly on 30 September 2007 after 124 yr 25 of dormancy. SO2 emitted during the initial eruption travelled rapidly northward over Egypt and eastward over Pakistan and India (Clarisse et al., 2008). The volcanic Printer-friendly Version plume reached the low stratosphere at an altitude of about 16 km (Eckhardt et al., Interactive Discussion 2008; Clarisse et al., 2008; Yang et al., 2009). The eruption of Piton de la Fournaise 5956 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion (Réunion Island) started during the night of 30 March 2007. This was the largest eruption in 100 yr in this region (Deroussi et al., 2009). On 2 April, the opening of NHESSD a fissure produced a lava flow that reached the sea with a flux estimated at 100 m3 s−1. 1, 5935–6000, 2013 A degassing up to 1800 kgs−1 was estimated for this day by Tulet and Villeneuve 5 (2011). Their study of the temporal evolution of the SO2 emission shows a budget of 230 kt. This eruption stopped on 1 May. An online service for The SACS notifications of 2008 are presented in Fig. 15b. Besides a few eruptions near real-time on the Galapagos islands (Cerro Azul volcano) and the eruption of Dalafilla volcano satellite monitoring (Ethiopia), the two most significant explosive eruptions happened at the Okmok and of volcanic plumes 10 Kasatochi volcanoes (Aleutian Islands, Alaska, USA), respectively, in July and August. These two eruptions have been studied extensively and more information can be found H. Brenot et al. in the literature (e.g. Prata et al., 2010; Karagulian et al., 2010; Krotkov et al., 2010; Clarisse et al., 2011). These strong eruptions caused a real threat to air traffic over the Title Page North American continent (see affected regions Fig. 15b). 15 Figure 16 presents the SO2 cloud from Kasatochi as observed by IASI on 10 and 11 Abstract Introduction August (interpolation with a grid of 0.25◦ by 0.25◦). The ash images from IASI (showing Conclusions References the level of confidence of the detection) have been superimposed over the SO2 images. The ash and SO2 clouds show a similar pattern on 10 August (see Fig. 16a). Note that Tables Figures ash observed with a low level of confidence over the south of Alaska and the west of 20 Canada is an artefact in the analysis of the spectrum and the process of ash detection. J I The volcanic plume emitted by Kasatoshi was a SO2-rich cloud (up to 676.4 DU on 8 August). We can see on 11 August that the detected ash cloud is smaller than the SO2 J I cloud (see Fig. 16b). Back Close Two months after Kasatochi eruption, Dalaffilla volcano erupted on 3 November. This 25 volcano, located in the Afar region of Ethiopia, is one of the six volcanoes in the Erta Full Screen / Esc Ale Range. We can see in Fig. 15b that a few days after this eruption, which was the largest observed in Ethiopia in historical times, the volcanic cloud generated six SO2 Printer-friendly Version notifications over India and the south of China. Interactive Discussion

5957 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Three major eruptions that occurred in 2009 were identified by the SACS warning service (Fig. 15c): redoubt volcano in Alaska, Fernandina volcano in Galápagos NHESSD islands, and Sarychev volcano in the Kuril islands. The explosive eruption of Sarychev 1, 5935–6000, 2013 (11–19 June 2009) emitted a huge amount of ash and SO2 into the atmosphere for 5 a range of altitudes of 10–16 km (Carn and Lopez, 2011; Clarisse et al., 2012). The mass of the SO2 plume was estimated to be 1.2 ± 0.2 Tg by Haywood et al. (2010). An online service for This eruption, which was one of the 10 largest stratospheric injections in the last 50 yr, near real-time significantly affected the radiative budget of the atmosphere (Haywood et al., 2010). satellite monitoring Images of the SO2 and ash clouds observed by AIRS are shown in Fig. 17 (ash image of volcanic plumes 10 superimposed over the SO2 image). We can see that the ash detection (with a low level of confidence) over the Russian land on the north of the Sea of Okhotsk is an artefact. H. Brenot et al. On the morning (UTC time) of 16 June, SO2 and ash cloud are well co-located (see Fig. 17a). Note that the first SO2 plume was emitted two days before and was observed Title Page on the West coast of USA. Figure 17b shows the SO2 cloud observed on the afternoon 15 of 17 June. The ash cloud detected by AIRS is located over the Sakhalin Russian Abstract Introduction Island. No ash was detected by AIRS over the Pacific ocean. Conclusions References Figure 18a shows all the SO2 notifications identified by SACS for the year 2010. Many regions were affected by volcanic activity. The 2010 eruptions of Eyjafjallajökull Tables Figures volcano in Iceland were relatively small, but unusual meteorological conditions brought 20 volcanic ash clouds over Europe, causing enormous disruption to air traffic across J I western and northern Europe over an initial period of six days in April 2010 (Zehner, 2010). Additional localised disruption continued into May 2010. The eruption was J I declared officially over in October 2010, when snow falling on the glacier did not melt Back Close anymore. From 14 to 20 April, ash covered large areas of northern Europe when the 25 volcano erupted (see Fig. 19 with images of the level of confidence of ash detection Full Screen / Esc from AIRS and IASI from 14 to 17 April). Most of the airports of Northern Europe were closed by the authorities. During these days, the quantity of SO2 observed by UV-visible Printer-friendly Version and IR satellites was low (less than 5 DU), possibly due to local chemical reactions Interactive Discussion between SO2 and water vapour creating sulfides or sulphuric acid. In May 2010, during 5958 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the second phase of emissions, a more significant amount of SO2 was measured (up to 28.6 DU for AIRS, on 6 May). NHESSD The strongest eruption of 2010 was the one of Merapi on 3 November on the Island 1, 5935–6000, 2013 of Java, Indonesia. Rapid ascent of magma from the depths of the volcano and the 5 collapse of the lava dome put the large population living around the volcano in danger. People had to move away from their homes around the flanks of Merapi. For the first An online service for time, remote sensing (ground deformation by aperture radar, and monitoring of gas near real-time emissions by spectroscopic technique), together with the monitoring of the seismicity satellite monitoring and the ground deformation by geodetic technique, provided precursory signals of this of volcanic plumes 10 volcanic crisis, avoiding the death of several thousand people living around (Surono H. Brenot et al. et al., 2012). Explosive and effusive phases took place and released a mass of SO2 of about 0.44 Tg. The altitude of the SO2- and ash-rich plume was estimated to be about 12 km. Figure 20 presents SO2 and ash measured by IASI on 5 November. Title Page Ash and SO2 were monitored respectively by IASI during 10 and 15 days after the 15 start of the eruption. The five instruments used by SACS (with their space- and time- Abstract Introduction complementarities) identified many SO2 notifications (see Fig. 18a) and allowed a good monitoring of the Merapi’s eruption and its volcanic cloud (eastward movement and Conclusions References

then southward). Tables Figures The largest number of notifications of the SACS system to date occurred during 20 the year 2011 (Fig. 18b). The three major events of 2011 were the eruptions of the J I Icelandic Grìmsvötn volcano (21–28 May), the eruption of the Chilean Puyehue-Cordón Caulle volcano (4–7 June), and the eruption of the Eritrean Nabro volcano (12 June– J I 7 July). A characteristic feature of the Grìmsvötn eruption is that a large amount of Back Close SO2 was ejected northwards while the ash cloud went to the southeast (see GOME- 25 2 image Fig. 21). In this figure, the image of the absorbing aerosols index has been Full Screen / Esc superimposed onto the SO2 image. A null, low, medium and high level of aerosols/ash detected by GOME-2 corresponds respectively to AAI under 2, AAI of 2.5, AAI of 3, Printer-friendly Version and AAI over 3.5. Aerosol detection on the north coast of Norway is not related to the Interactive Discussion Grìmsvötn eruption. The SO2 cloud, which travelled over Canada and came back over 5959 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Europe, was monitored during 3 weeks. The ash cloud, which travelled over Europe, was under the limit of detection 3 days after the start of the eruption. NHESSD On 4 June 2011, the eruption of the Puyehue-Cordón Caulle volcano started. Out 1, 5935–6000, 2013 of the nine years the SO2 notification archives span, this eruption is the most ash-rich 5 recorded by our system. The volcanic cloud emitted was an ash-laden plume. During the first two days of this eruption, the SO2 and ash clouds were collocated. Afterwards, An online service for the volcanic cloud dispersed and circled around the South pole (see Fig. 22). Within near real-time a few day, the cloud of ash covered the main part of Southern high latitudes. Note that satellite monitoring for this particular eruption, that occurred during the Austral winter, the UV sensors had of volcanic plumes 10 limited coverage and IASI and AIRS were the only sensors able to track globally the H. Brenot et al. volcanic plume. The SO2 and ash clouds were observed by IR sensors respectively during 3 and 5 weeks. The volcanic ash emitted by Puyehue consequently disturbed the air traffic of the Southern Hemisphere during the month of June 2011 in Chili, Title Page Argentina, Uruguay, Brazil, South Africa, Australia and New Zealand. 15 During the night of 12 June 2011, the Eritrean Nabro volcano started to erupt (see Abstract Introduction Fig. 3). This explosive eruption ejected a huge amount of SO2 in the atmosphere, threating international routes from the Far East to Europe. On 14 June, the Nabro Conclusions References

volcano spewed a volcanic plume across the route of many flights over East Africa Tables Figures and the Middle East. Satellite information was critical in monitoring the journey of the 20 volcanic cloud. We can see in Fig. 3 the cloud of SO2 detected by IASI and AIRS on J I 13 and 14 June. Figure 23 shows the SO2 plume observed by IASI during the night of 15–16 June. The amount of ash emitted by Nabro for this eruption was very low (and J I largely undetected by IASI and AIRS), while the ash emitted by the Puyehue volcano Back Close was still detected by IASI in Fig. 23. 25 Figure 18c shows the SACS notifications for 2012. Continuous activity was recorded Full Screen / Esc at the Nyamuragira volcano (DRC). Summit eruptions of Etna (Italy) were also detected by SACS in January–April 2012, and again in July–October 2012, but no strong Printer-friendly Version disruption of air traffic occurred. On 1 September 2012, an explosive eruption took Interactive Discussion place at Bezymianni volcano (Kamchatka), producing an SO2 and ash cloud (see 5960 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Figs. 11 and 12). With a cloud rising to an altitude of about 10 km, the Tokyo VAAC sent out an ash-cloud aviation warning and the major intercontinental routes that pass NHESSD in that area were cancelled. Less than two months after this eruption, a nearby volcano, 1, 5935–6000, 2013 the Tolbachik, erupted on 27 November. The main part of the volcanic ejection drifted 5 north and northwest (NNW), as observed by IASI and AIRS sensors. At this time of the year and for this location, the thermal IR instruments were the only instruments used An online service for by SACS that were able to monitor the volcanic cloud. The ash advisory board from the near real-time Tokyo VAAC reported of an ash cloud at an altitude of 10 km, spreading to the NNW. satellite monitoring Despite this, no ash was detected by the instruments used by SACS. of volcanic plumes 10 The last 2012 eruption which caused a disruption to air traffic, was the eruption of H. Brenot et al. the Chilean Copahue volcano (23–25 December). An SO2-rich plume associated with ash emissions was spewed into the sky (Fig. 24). The volcanic plume was transported over Chile and aviation authorities in South America warned airlines to avoid the area. Title Page Contrarily to the previous eruption of Puyehue in June 2011, the Copahue’s volcanic 15 cloud was not particularly ash-rich. We can see in Fig. 24 that for the first day of Abstract Introduction this eruption, SO2 and ash clouds took the same trajectory, nevertheless the high Conclusions References concentrations of SO2 and ash are not exactly at the same locations. In this section, we have shown the major volcanic events which have disturbed Tables Figures air traffic during the last 10 yr. We can see the uniqueness of each eruption. This 20 diversity is characterised by different volcanic cloud compositions (sometimes SO2-rich J I and/or sometimes ash-rich) driven by to specific meteorological conditions occasionally dispersing the plume over several continents. To synthetize the full performance of our J I notification system from 2010 to 2012, the diagnosis of SACS monitoring of the volcanic Back Close activity is presented in the Appendix A. Full Screen / Esc

25 5 Conclusions and perspectives Printer-friendly Version Ash emitted from volcanoes poses a significant threat to aircraft because once it is Interactive Discussion sucked into the aircraft’s engines, it readily melts and can potentially cause engine 5961 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion failure. Europe, North America, the north Atlantic and Asia regions have the highest air traffic densities with an estimated rate of growth of about 2–5 % per year during NHESSD the period 1990–2050 (ESCAP, 2005). With the increase of commercial and freight air 1, 5935–6000, 2013 traffic, the increase of volcano surveillance is a critical need for future aircraft safety 5 (Prata, 2009). The near real-time monitoring of ash and SO2 from satellite sensors is essential for An online service for the international Volcanic Ash Advisory Centres (VAACs). Focussing on instruments near real-time onboard polar orbiting platforms, this study presents the Support to Aviation Control satellite monitoring Service (SACS). This free service provides images of SO2 and ash in near real-time on of volcanic plumes 10 a global scale via a web interface (http://sacs.aeronomie.be). We created a notification system and demonstrated that it is able to detect all volcanic events that occurred H. Brenot et al. during the last decade that were a threat to aviation. The current system uses in synergy data products from UV-visible sensors (OMI and GOME-2) and thermal IR Title Page sensors (AIRS and IASI) with a revisiting time of (at least) 6 h anywhere on Earth. We 15 have assessed the limitations of our system and estimated the success rate of the Abstract Introduction notifications. At the time of writing, the SACS multi-sensor warning system generated 444 notifications (emails sent with information and links to images) since its activation Conclusions References

in April 2012. With a number of 17 false notifications provided by UV-visible sensors Tables Figures (mainly due to observations at high solar zenith angle or during an eclipse of the 20 sun), more than 95 % of the notifications have been successful. The warning system J I actually considers only the SO2 detection and not the more difficult detection of ash, however for each SO2 notification and dedicated webpage creation, an associated J I image of ash/aerosols is provided. In particular, a level of confidence is included for Back Close the observations of ash from IASI and AIRS (Clarisse et al., 2013). 25 Finally, the next objectives for our service are: Full Screen / Esc – to incorporate more sensors in the system (in particular IASI and GOME-2 on MetOp-B), and possibly also improved satellite data products for ash (as an Printer-friendly Version outcome of the on-going SACS2 project); Interactive Discussion

– to provide information in NRT on volcanic SO2 plume height from satellite 1, 5935–6000, 2013 observations (e.g. Van Gent et al., 2013);

5 – to implement new visualisation tools and different levels of notification, in line with An online service for the users needs. In particular, customised notifications will be set-up to trigger near real-time volcanic plume dispersion modelling and forecasts as part of the ESA VAST satellite monitoring project (http://vast.nilu.no). of volcanic plumes H. Brenot et al. Appendix A

Title Page 10 Diagnosis of SACS from 2010 to 2012

Abstract Introduction For the period 2010–2012, we have investigated the success rate of SACS in detecting volcanic eruptions. We have listed all volcanic eruptions detected by the SACS warning Conclusions References system and compared them to the volcanoes known to be active for this period (80 in total), according to the Global Volcanism Programme (www.volcano.si.edu). The Tables Figures 15 results can be found in Table 4, which gives the location of the 80 volcanoes and the corresponding SACS regions. The volcanoes in red correspond to the volcanoes J I for which SACS has detected volcanic activity. The detection scores by SACS are the J I following: 37 % in 2010, 62 % in 2011, and 71 % detected in 2012. Note that the criteria of volcanic activity considered by GVP is not only based on the emission of gas and Back Close 20 particles in the atmosphere, as seismic activity plays a key role in their survey. From Full Screen / Esc the SACS point of view and air traffic safety, all the strong eruptions with a threat to aviation have been detected. These results prove the quality of our system. For the year 2012, the SACS multi sensors system has issued 316 notifications with only 11 Printer-friendly Version false notifications (success rate of 96 %). Interactive Discussion

5963 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Acknowledgements. Many thanks to ESA for the funding (SACS, SACS+ and SACS2 projects) and the support. We are grateful to NASA and FMI for the very fast delivery of OMI, NHESSD CNES and EUMETSAT for data of IASI and GOME-2. We thank also JPL/NASA for the free access to AIRS data on EOSDIS. We would like also to acknowledge the team from 1, 5935–6000, 2013 5 the Toulouse VAAC (Météo-France), the London VAAC (UK-MetOffice), the Darwin VAAC (Bureau of Meteorology), the Anchorage VAAC (NOAA), the Wellington VAAC (MetService), the Montréal VAAC (Government of Canada), and all the users for their very useful feedback, An online service for which brings us energy and motivation and helps us to continue and to improve our service. near real-time This work was also undertaken under the auspices of the O3M SAF project of the EUMETSAT. satellite monitoring 10 P. F. Coheur and L. Clarisse are respectively Research Associate and Postdoctoral Researcher of volcanic plumes with F.R.S.-FNRS. The research in Belgium was funded by the F.R.S.-FNRS, the Belgian State Federal Office for Scientific, Technical and Cultural Affairs and the European Space Agency H. Brenot et al. (ESA-Prodex arrangements C-4000103226). Financial support by the “Actions de Recherche Concertées” (Communauté Française de Belgique) is also acknowledged. Title Page

15 References Abstract Introduction

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5965 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Clarisse, L., Hurtmans, D., Clerbaux, C., Hadji-Lazaro, J., Ngadi, Y., and Coheur, P.-F.: Retrieval of sulphur dioxide from the infrared atmospheric sounding interferometer (IASI), Atmos. NHESSD Meas. Tech., 5, 581–594, doi:10.5194/amt-5-581-2012, 2012. Clarisse, L., Coheur, P.-F., Prata, F., Hadji-Lazaro, J., Hurtmans, D., and Clerbaux, C.: A unified 1, 5935–6000, 2013 5 approach to infrared aerosol remote sensing and type specification, Atmos. Chem. Phys., 13, 2195–2221, doi:10.5194/acp-13-2195-2013, 2013. Clerbaux, C., Boynard, A., Clarisse, L., George, M., Hadji-Lazaro, J., Herbin, H., Hurtmans, D., An online service for Pommier, M., Razavi, A., Turquety, S., Wespes, C., and Coheur, P.-F.: Monitoring of near real-time atmospheric composition using the thermal infrared IASI/MetOp sounder, Atmos. 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5967 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Levelt, P. F., van den Oord, G. H. J., Dobber, M. R., Mälkki, A., Visser, H., de Vries, J., Stammes, P., Lundell, J. O. V., and Saari, H.: The Ozone Monitoring Instrument, IEEE Trans. NHESSD Geosci. Remote, 44, 1093–1101, doi:10.1109/TGRS.2006.872333, 2006. Miller, T. P. and Casadevall, T. J.: Volcanic ash hazards to aviation, in: Encyclopedia of 1, 5935–6000, 2013 5 Volcanoes, edited by: Sigurdsson, H., Houghton, B., McNutt, S. R., Ryman, H., and Stix, J., Academic Press, San Diego, 915–930, 1999. Munro, R., Eisinger, M., Anderson, C., Callies, J., Corpaccioli, E., Lang, R., Lefebvre, A., An online service for Livschitz, Y., and Albinana, A. 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Table 1. List of data products available from SACS. H. Brenot et al.

Instruments Data Overpass Data Resolution Swath Global Data Participants Units Delay type time availability (km2) (km) coverage products (h) Title Page

SCIAMACHY UV/visible 10:00 a.m. Apr 2004–2012 30 × 60 960 96 SO2 vertical columns BIRA DU 2–3 h (ENVISAT) Absorbing aerosol index KNMI – 2–3 h Abstract Introduction OMI UV/visible 01:30 p.m. Sep 2004–present 13 × 24 2600 24 SO2 vertical columns NASA/KNMI/FMI DU 2–3 h (EU: 45’) (Aura) Absorbing aerosol index KNMI – 2–3 h GOME-2 UV/visible 09:30 a.m. Jan 2007–present 40 × 80 1920 24 SO2 vertical columns DLR DU 1–2 h Conclusions References (MetOp-A) SO2 plume height BIRA km oﬀ-line Absorbing aerosol index KNMI – 2–3 h

IASI Infrared 09:30 a.m. Oct 2007–present 12 × 12 2200 12 SO2 index (and columns) ULB K (DU) 1–2 h Tables Figures (MetOp-A) 09:30 p.m. Ash indicator ULB – 1–2 h

AIRS Infrared 01:30 a.m. Sep 2002–present 15 × 15 1650 12 SO2 vertical columns NILU DU 1–2 h (Aqua) 01:30 p.m. Ash indicator ULB – 1–2 h J I

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H. Brenot et al.

Table 2. Geographical coverage of SACS monitoring for October 2012. Title Page Duration of monitoring Zone 1 h 2 h 3 h 4 h 6 h 8 h 12 h 24 h Pixel (max) Pixel (max) Pixel (max) Pixel (max) Pixel (max) Pixel (max) Pixel (max) Pixel (max) Abstract Introduction A 14.5 % (5) 28.8 % (5) 42.5 % (5) 55.0 % (7) 75.0 % (8) 88.5 % (8) 98.0 % (8) 100 % (13) B 15.9 % (5) 31.2 % (5) 46.0 % (6) 59.0 % (7) 78.5 % (8) 90.9 % (8) 98.5 % (9) 100 % (15) C 18.8 % (5) 36.1 % (7) 51.3 % (8) 64.8 % (9) 84.9 % (10) 97.0 % (10) 99.7 % (11) 100 % (17) Conclusions References D 24.7 % (7) 44.8 % (9) 57.9 % (11) 71.0 % (12) 90.3 % (13) 99.8 % (14) 100 % (15) 100 % (23) E 31.3 % (8) 50.1 % (10) 61.9 % (14) 72.5 % (19) 87.9 % (22) 96.0 % (30) 99.4 % (38) 100 % (71) F 72.5 % (8) 94.8 % (10) 97.3 % (14) 98.8 % (19) 100.0 % (22) 100 % (30) 100 % (38) 100 % (71) World 31.6 % (8) 50.6 % (10) 62.3 % (14) 72.8 % (19) 88.1 % (22) 96.1 % (30) 99.4 % (38) 100 % (71) Tables Figures

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An online service for near real-time satellite monitoring of volcanic plumes Table 3. Key criteria used by the SACS SO2 notiﬁcation system. H. Brenot et al. Instruments SCIAMACHY GOME-2 OMI IASI AIRS Threshold radius 80 km 85 km 50 km No need No need Type of observation VCD VCD VCD DBT VCD Title Page Threshold values 1.8 DU 1.8 DU 1.45 DU 2.9 K 3 DU Abstract Introduction Threshold value 1.25 DU 1.45 DU 1.25 DU No need No need proximity volcano Conclusions References (< 300 km) Avoid SAA Yes 3.25 DU Yes 2 DU No need No need No need Tables Figures threshold value Avoid pollution Yes 3.25 DU Yes 2 DU Yes 1.6 DU No need No need threshold value J I

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5973 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Table 4. List of the volcanoes having their last eruptions in years 2010, 2011 or 2012. The NHESSD corresponding SACS region of these volcanoes, their positions (latitude, longitude and the altitude of the summit) are presented. The red band corresponds to the volcanoes for which 1, 5935–6000, 2013 SACS system has identiﬁed an SO2 notiﬁcations.

SACS Longitude Latitude Summit Year Volcano name An online service for region (◦)(◦) (m) near real-time satellite monitoring 106 −19.62 63.63 1666 2010 Eyjafjallalökull 111 156.02 50.68 1156 2010 Ebeko of volcanic plumes 111 158.03 52.56 1829 2010 Gorely H. Brenot et al. 111 153.93 48.96 1170 2010 Ekarma 203 −84.70 10.46 1670 2010 Arenal 203 −90.60 14.38 2552 2010 Pacaya 210 139.53 34.08 815 2010 Miyake-jima Title Page 210 123.68 13.26 2462 2010 Mayon Abstract Introduction 211 145.80 18.13 570 2010 Pagan 211 144.77 14.60 −517 2010 NW Rota-1 Conclusions References 211 141.49 24.28 −14 2010 Fukutoku-Okanoba 303 −77.37 1.22 4276 2010 Galeras Tables Figures 307 35.91 −2.76 2962 2010 Ol Doinyo Lengai 310 123.58 −7.79 748 2010 Batu Tara 310 116.57 −8.42 3726 2010 Rinjani J I 311 148.42 −5.53 1330 2010 Langila J I 311 167.50 −14.27 797 2010 Gaua 404 −72.65 −42.83 1122 2010 Chaitén Back Close 408 55.71 −21.23 2632 2010 Piton de la Fournaise 101 −169.94 52.83 1730 2011 Cleveland Full Screen / Esc 106 −17.33 64.42 1725 2011 Grímsvötn 106 −19.05 63.63 1512 2011 Katla Printer-friendly Version 111 160.64 56.06 4835 2011 Kliuchevskoi 111 160.32 55.13 2376 2011 Kizimen Interactive Discussion

5974 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Table 4. Continued. NHESSD SACS Longitude Latitude Summit Year Volcano name ◦ ◦ 1, 5935–6000, 2013 region ( )( ) (m) 203 −87.00 12.70 1745 2011 San Cristóbal 203 −85.62 11.54 1610 2011 Concepción An online service for 203 −86.85 12.60 1061 2011 Telica near real-time 209 93.86 12.28 354 2011 Barren Island satellite monitoring 210 130.86 31.93 1700 2011 Kirishima of volcanic plumes 210 124.05 12.77 1565 2011 Bulusan 210 131.11 32.88 1592 2011 Aso H. Brenot et al. 303 −77.66 −0.08 3562 2011 Reventador 307 41.70 13.37 2218 2011 Nabro 310 125.40 2.78 1827 2011 Karangetang [Api Siau] Title Page 310 124.73 1.11 1784 2011 Soputan 310 110.44 −7.54 2968 2011 Merapi Abstract Introduction 310 112.95 −7.94 2329 2011 Tengger Caldera 311 152.20 −4.27 688 2011 Rabaul Conclusions References 311 145.04 −4.08 1807 2011 Manam 311 151.33 −5.05 2334 2011 Ulawun Tables Figures 311 167.83 −15.40 1496 2011 Aoba 312 −175.07 −19.75 515 2011 Tofua J I 403 −70.57 −35.24 4107 2011 Planchón-Peteroa 403 −72.12 −40.59 2236 2011 Puyehue-Cordon Caulle J I 512 167.17 −77.53 3794 2011 Erebus 111 161.36 56.65 3283 2012 Shiveluch Back Close 111 159.45 54.05 1536 2012 Karymsky Full Screen / Esc 111 160.59 55.98 2882 2012 Bezymianny 111 160.33 55.83 3682 2012 Tolbachik 201 −155.29 19.42 1222 2012 Kilauea Printer-friendly Version 203 −103.62 19.51 3850 2012 Colima 203 −84.23 10.20 2708 2012 Poás Interactive Discussion

5975 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Table 4. Continued. NHESSD SACS Longitude Latitude Summit Year Volcano name ◦ ◦ 1, 5935–6000, 2013 region ( )( ) (m) 203 −90.88 14.47 3763 2012 Fuego 203 −91.55 14.76 3772 2012 Santa Maria An online service for 203 −98.62 19.02 5426 2012 Popocatépetl near real-time 203 −83.77 10.02 3340 2012 Turrialba satellite monitoring 204 −62.18 16.72 915 2012 Soufrière Hills of volcanic plumes 206 15.21 38.79 924 2012 Stromboli 206 15.00 37.73 3330 2012 Etna H. Brenot et al. 207 40.67 13.60 613 2012 Erta Ale 210 130.66 31.58 1117 2012 Sakura-jima 210 129.72 29.64 799 2012 Suwanose-jima Title Page 303 −75.32 4.89 5321 2012 Nevado del Ruiz 303 −78.34 −2.00 5230 2012 Sangay Abstract Introduction 303 −78.44 −1.47 5023 2012 Tungurahua 303 −76.03 2.93 5364 2012 Nevado del Huila Conclusions References 307 29.25 −1.52 3470 2012 Nyiragongo 307 29.20 −1.41 3058 2012 Nyamuragira Tables Figures 309 100.47 −0.38 2891 2012 Marapi 310 127.63 1.49 1325 2012 Ibu J I 310 127.88 1.68 1335 2012 Dukono 310 112.92 −8.11 3676 2012 Semeru J I 310 105.42 −6.10 813 2012 Krakatau 310 124.79 1.36 1580 2012 Lokon-Empung Back Close 311 155.20 −6.14 1750 2012 Bagana Full Screen / Esc 404 −71.93 −39.42 2847 2012 Villarrica 404 −71.17 −37.85 2997 2012 Copahue 412 169.44 −19.53 361 2012 Yasur Printer-friendly Version 412 168.12 −16.25 1334 2012 Ambrym 412 165.80 −10.38 851 2012 Tinakula Interactive Discussion

5976 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion NHESSD 1 (http://savaa.nilu.no), VAST (http://vast.nilu.no) and EU FP7 EVOSS (http://www.evoss.eu), whose general 1, 5935–6000, 2013 2 aims are to define and demonstrate an optimal system for volcanic ash plume monitoring and prediction.

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J I 3 Back Close Fig. 1. Illustration of the satellite equatorial overpasses (solar local time). Figure 1: Illustration of the satellite equatorial overpasses (solar local time). Full Screen / Esc

4 SACS delivers global data sets for sulphur dioxide (SO2) and aerosol (ash) related to volcanic eruptions fromPrinter-friendly Version

5 measurements by space-based instruments. At the time of writing, SACS is based on polar sun-synchronousInteractive Discussion 6 satellite instruments widely used to sound atmospheric composition and for meteorological and scientific 5977 7 applications: AIRS/Aqua (Aumann et al., 2000, 2003; Hofstadter et al. 2000), OMI/Aura (Levelt et al., 2006), 8 GOME-2/MetOp-A (Munro et al., 2006) and IASI/MetOp-A (Clerbaux et al., 2009; Hilton et al., 2012). All

9 SACS data retrievals are available in NRT (which means as soon as the data process allows to retrieve SO2 and 10 ash observations). Figure 1 shows the different sounders that SACS currently uses and their local overpass time. 11 It shows the advantage of combining multiple sensors which have different overpass times in a single system as 12 it allows the reliable and timely monitoring of volcanic plumes on the global scale. Note that until April 2012, 13 the NRT SACS system also used data from SCIAMACHY/Envisat (Bovensmann et al., 1999) for which archive 14 data can still be consulted. Table 1 presents a summary of the SACS products and their main characteristics: 15 satellite platform, data type and availability, equatorial local overpass time, spatial resolution, retrieved quantity, 16 data provider, units, delays of retrievals, swath widths, and global coverage. Note that the size of the footprint of 17 each instrument (resolution in km²) is an approximate value at nadir. A description of the different data products 18 used in the SACS system is given in Section 3.

Table 1: List of data products available from SACS.

19 4 1 The SO2 vertical column is obtained using radiative transfer calculations that account for important parameters 2 influencing the UV light path in the atmosphere: solar zenith angle, viewing angles, clouds, atmospheric 3 absorption and scattering, surface albedo. As the measurement sensitivity strongly depends on the altitude where

4 SO2 resides and because we have no knowledge what this altitude is prior to the measurement, the SO2 vertical 5 column estimation is provided on three hypothetical atmospheric layers representative of different scenarios of icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 6 emissions: planetary boundary layer (typically the first km above the surface), upper troposphere (~6km) and

7 lower stratosphere (~15km). The SO2 images displayed in SACS all use the data assuming lower stratospheric NHESSD 8 plumes. 1, 5935–6000, 2013

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Figure 2: SO2 VCD images by respectively a) OMI, b) GOME-2 and c) SCIAMACHY. The lower right plot (d) shows Fig. 2. SO2VCD images by respectively (a) OMI, (b) GOME-2 and (c) SCIAMACHY. The lower Back Close the SO VCDs for a fixed latitude of 19.4°N (blue line in the SO2 images) as a◦ function of longitude during an eruption right plot2 (d) shows the SO2 VCDs for a ﬁxed latitude of 19.4 N (blue line in the SO2 images) ◦ ◦ asof a Kilauea function volcano of (155.29°W; longitude 19.42° duringN), 17 an-18 eruptionMay 2008. of Kilauea volcano (155.29 W; 19.42 N), 17– Full Screen / Esc 18 May 2008. 9 The SO algorithms from OMI, SCIAMACHY and GOME-2 are able to detect similar SO patterns for modest 2 2 Printer-friendly Version 10 and strong eruptions. Figure 2 shows that the measurements (SO2 VCD images from SCIAMACHY, GOME-2 11 and OMI for an eruption of Kilauea volcano in Hawaii, 17 May 2008) are able to consistently observe small (and Interactive Discussion

12 low) SO2 plumes. Despite the different overpass times, we can see a very good spatial correspondence of the 5978 13 location of the plume between the 3 instruments. To compare the SO2 VCDs, a fixed latitude of 19.4°N has been 14 chosen for the 3 instruments, and the measurements have been plotted as a function of longitude (Fig. 2d). A

15 fairly good agreement between the VCD values is found, especially taken into account the fact the SO2 plume is

16 moving westward. This example shows and confirms that SO2 VCD measurements from UV-visible instruments

17 allow a good estimation and monitoring of volcanic emissions even for such a small eruption (the highest SO2 18 VCD is 5.2 DU as recorded by GOME-2 during this day). Note that the operational cloud cover fraction products 19 for all UV-visible instruments are also available on the SACS webpage as additional information. Note that the 20 measurement times are indicated in UTC on all SACS images. 6 1 3.2 SO2 index retrievals from thermal IR sensors (IASI and AIRS)

2 The IASI instrument is a Fourier transform spectrometer. The spectral coverage is from 645 cm-1 to 2760 cm-1 3 (with no gaps) with a spectral resolution of 0.5 cm-1 (apodised) and a spectral sampling of 0.25 cm-1. The AIRS -1 4 instrument is an echelle grating spectrometer covering the range between 650 and 2665 cm (with gaps) and a 5 spectral resolution of 0.5-2 cm-1. The instruments are operating in a nadir view with typical footprints (circular to icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 6 elliptical depending on the position on the swath) of 12 and 15 km diameters (at nadir), for IASI and AIRS

7 respectively. Both sensors measure the spectrum of the outgoing thermal radiation emitted by the Earth- NHESSD 8 atmosphere system (and hence can operate during both day and night). 1, 5935–6000, 2013

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Back Close th th Fig.Figure 3. SACS3: SACS SO SO22 vertical column column density density images images from IASI from and IASIAIRS andfor the AIRS 13 and for 14 the of 13 Ju andne 2011 14 at June the 2011beginning at the of the beginning eruption of of Nabro. the eruption Note that of IASI Nabro. and AIRS Note observations that IASI are and plotted AIRS using observations circles with mean are Full Screen / Esc plotted using circles with mean diameters of respectively 12 km and 15 km. diameters of respectively 12 km and 15 km.

-1 Printer-friendly Version 9 Both IR instruments cover the strong asymmetric stretch SO2 absorption band around 1362 cm . For AIRS, 10 three wavenumbers are used (1395 cm-1, 1327 cm-1 and 1328 cm-1), while the IASI algorithm is based on Interactive Discussion 11 measurements around 1408 cm-1, 1372 cm-1 and 1385 cm-1. The AIRS L1 data are provided in near real-time by 5979 12 NASA's Earth Observing System Data and Information System (EOSDIS, https://urs.eosdis.nasa.gov), and the

13 SO2 retrievals are processed by BIRA-IASB using the algorithm from NILU. The retrieval scheme of AIRS is a

14 two-step process with a first step to identify pixels that contain SO2, and a second step to adjust the amount of

15 SO2 based on off-line radiative transfer calculations (see details in Prata and Bernardo, 2007). The retrievals

16 from IASI SO2 are provided in NRT by the ULB (http://cpm-ws4.ulb.ac.be/Alerts). The IASI SO2 algorithm (see

17 details in Clarisse et al., 2012) also involves the conversion of the measured signal into an SO2 vertical column 18 by making use of a large look-up-table and EUMETSAT operational pressure, temperature and humidity

7 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion NHESSD 1 4b shows the IASI ash detection at 10:30 UTC the 22nd of May. We can see in Fig. 4cd the displacement of the 2 ash cloud during the next 2 days (ash detected by AAI retrievals from GOME-2 and OMI). 1, 5935–6000, 2013

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3 J I Fig. 4. AshFigure index 4: Ash from index from(a) a)IASI IASI and b) (b)AIRSAIRS and AAI and from AAIc) GOME from-2 and(c) d) GOME-2OMI during the and Grímsvötn(d) OMI eruption during the Back Close Grímsvötn eruptionon Iceland. Images on Iceland. show the first Images days of the show eruption the, which ﬁrst start daysed on ofthe theafternoon eruption, of the 21st which of May 2011. started on the Full Screen / Esc afternoon of 21 May 2011. 4 3. 5 Limitations of satellite products in detecting volcanic plumes Printer-friendly Version 5 3.5.1 SO2 products Interactive Discussion

6 Because of competing water vapour absorption in the same IR wavelength window as SO2, the vertical 7 sensitivity of IASI and AIRS to SO2 is limited to5980 the atmospheric layers above 3-5 km depending on the 8 humidity vertical profile. This limitation might seem serious, but in the context of aviation it is actually an

9 advantage as the detection of SO2 by these two sensors means that SO2 is present at altitudes where airplanes 10 spend most of their travel time. This fact combined with four overpasses per day make the IR sensors a central 11 component of the SACS system. As a complement, the UV sensors are characterised by a relatively good

12 measurement sensitivity for SO2 down to the surface. While this allows monitoring of low altitude volcanic

13 degassing, it also renders the measurements sensitive to anthropogenic emissions. As most of the SO2 pollution 14 hotspots are confined to a limited number of regions (China, South Africa, Siberia), the SACS system can easily

15 be tuned to avoid false notifications there. As a general comment, SO2 retrievals are also affected by clouds and 16 instrumental noise (especially at high solar zenith angles for the UV sensors). The latter generally shows up in

17 the SO2 images and evolves as the instrument is degrading over time. 9 1 3.5.2 Aerosol products

2 Besides the fact that the aerosol products are qualitative products (indexes), measurements from UV-visible 3 sensors are sensitive to all types of absorbing aerosols (desert dust, biomass burning aerosols, volcanic ash) and

4 are therefore not purely selective for ash. However, the combined detection of both elevated aerosol and SO2 is

5 highly selective for the volcanic plume (in fact even more than SO2 detection alone). Note that the ash product of 6 IASI and AIRS sensors yields very few false detections. But it should be noted that because of the strict 7 conditions of step 2, low concentration ash plumes are often not detected in the current implementation.

8 3.5.3 Known anomalies impacting the data

9 The Earth's dipolar magnetic field is offset by about 500 km from the rotational axis. As a result of this, the inner 10 Van Allen belt (doughnut-shaped regions of high-energy charged particles) is on one side closer to the Earth's 11 surface than the other side. This region is named the South Atlantic Anomaly (SAA) and it covers a part of icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 12 South America and the Southern Atlantic Ocean: it lies roughly between latitudes 5°S and 40°S, and between 13 longitudes 0°W and 80°W (the precise strength, shape and size of the SAA varies with the seasons). This dip in NHESSD 14 the Earth's magnetic field allows charged particles and cosmic rays to penetrate lower into the ionosphere (at 15 ~500 km of altitude). Low-orbiting satellites, such as Envisat, Aura and MetOp-A, pass daily through the inner 1, 5935–6000, 2013 16 radiation belt in the SAA-region. Upon passing the inner belt, charged particles may impact on the detector, 17 causing higher-than-normal radiance values, which in turn decreases the quality of the measurements, notably in An online service for 18 the UV. The SAA affects the SO2 retrievals and results in noise artefacts, as is visible in Fig. 5ab. Thanks to its near real-time 19 design which gives it better protection to radiation than other sensors, OMI is less affected by the SAA than satellite monitoring 20 SCIAMACHY or GOME-2 (as can be seen Fig 5c). of volcanic plumes

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Abstract Introduction 21 Conclusions References Figure 5: Example of SO2 VCD from a) SCIAMACHY, b) GOME-2, and c) OMI on 2 January 2008 in the region Fig. 5. Example of SO2 VCD from (a) SCIAMACHY, (b) GOME-2, and (c) OMI on 2 January affected by the SAA. The SO2 plume is related to the eruption of the Llaima volcano (Chile), marked by a blue 2008 in the region aﬀected by the SAA. The SO2 plume is related to the eruption of the Tables Figures triangle in the plots. Artefacts in the SO2 VCD from SCIAMACHY and GOME-2 instruments induced by the South Llaima volcano (Chile), marked by a blue triangle in the plots. Artefacts in the SO2 VCD from SCIAMACHYAtlantic andAnomaly GOME-2 are visible on instruments these images; OMI induced is less affected. by the South Atlantic Anomaly are visible on these22 images;The OMI OMI instrument is less has astartedﬀected. to suffer from a so-called "row anomaly" since 25 June 2007. This anomaly J I 23 affects particular viewing direction angles, corresponding to rows on the CCD detector. The row anomaly has J I 24 increased over time (see www.knmi.nl/omi/research/product). Currently, the affected rows are not treated in 25 SACS, leading to a reduction in the data coverage. Several off-line schemes to correct for the row anomaly exist Back Close 26 (e.g. Yan et al., 2012) but are not implemented yet in the NRT data-processing. Full Screen / Esc

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5981 1 4 Global monitoring of volcanic SO2 and ash emissions

2 In this section we present results from the SACS global monitoring system of SO2 emissions and the strategy

3 adopted to warn users of exceptional SO2 concentrations.

4 4.1 Temporal and spatial sampling

5 It is important to know precisely the time and space sampling of the SACS monitoring system. Because of the 6 use of polar-orbiting satellites with different overpass times, some geographical regions are observed more 7 frequently than others. To evaluate the revisiting frequency of SACS, we have defined seven geographical 8 latitude zones (see Fig. 6a) and we have estimated, based on data of October 2012, the percentage of the areas 9 that have been observed at least once by one of the satellite instruments (OMI, GOME-2, IASI, AIRS). The 10 calculation was done for several time intervals (see e.g. the coverage of the Earth by SACS after 8 hours of 11 monitoring in Fig. 6b). Table 2 presents the results for the different zones and time durations. A set of two 12 parameters has been evaluated: pxl is the percentage of the region of interest sampled by the SACS system; and 13 max is the maximum number of overpasses. From Table 2, it can be seen that with four instruments, SACS 14 monitors in near real-time any location in the world within a 24 hours interval. The percentage value drops to

15 99.5%, 95%, 90%, 75%, 60%, 50% and 30% for periods respectively of 12, 8, 6, 4, 3, 2 and 1 | hour(s). Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Table 2: Geographical coverage of SACS monitoring for October 2012. NHESSD 1, 5935–6000, 2013

An online service for near real-time satellite monitoring of volcanic plumes H. Brenot et al.

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Tables Figures Fig. 6.Figure (a) 6Geographical: a) Geographical zones used used in Table in 2; Table b) 8 hours 2; (b)of monitoring8 h of monitoring from SACS instruments from SACS (example instruments of the (example of 27 October 2012). 27th of October 2012). J I 16 Also apparent is the higher sampling rates for high-latitude regions due to overlapping orbits. This is particularly 17 interesting for Icelandic volcanoes (zone E) that will be sampled e.g. every 2 hours 50% of the time (see Table J I

18 2). Note that the geographical coverage of D-E-F zones depends on the day of year. For example in December, Back Close 19 the coverage of Iceland is less good (due to the SZA limit for UV-vis instruments). Nevertheless, because the IR Full Screen / Esc 11

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rd Figure 7: Eruption of the Copahue volcano the 23 of December 2012. a) SO2 vertical column density from GOME-2 . Fig. 7. Eruption of the Copahue volcano 23 December 2012. (a) SO2 vertical column density Tables Figures from GOME-2.A zoom for A two zoom pixels above for two the “threshold pixels value” above is shown the with “threshold the issues ofvalue” warning. The is shownblack ellipse with represents the issues of the area defined by the “threshold radius” considered in the SACS warning system (see Table 3); b) locations of SO warning. The black ellipse represents the area deﬁned by the “threshold radius” considered2 in warnings pixels. J I the SACS warning system (see Table 3); (b) locations of SO2 warnings pixels. 2 Another limitation of UV-visible sensors comes from the South Atlantic Anomaly (see section 3.5.3). To avoid J I 3 false notifications in this region, a more strict threshold value is considered (see Table 3). Note also that special

4 settings are used for SO2 plumes from small eruptions and degassing, in that the system considers lower Back Close 5 threshold values for measurements close to a given volcano (distances less than 300 km). Finally, one more Full Screen / Esc 6 limitation of UV-visible sensors is that they are sensitive to anthropogenic SO2 emissions. To avoid false 7 notifications, more strict criteria are used for some specific areas (West China, Central North Russia and South 8 Africa). Printer-friendly Version

9 Interactive Discussion

10 SO2 notification from thermal IR sensors 5983 11 IASI and AIRS are much less or not at all affected by instrumental noise, the SAA, or pollution. For IASI, rather 12 than considering threshold values based on VCD, the criteria apply directly on the measured Difference of 13 Brightness Temperatures (called DBT and expressed in Kelvin [K]), and is therefore straightforward. The 14 threshold values (DBT and VCD) used are given in Table 3 for respectively IASI and AIRS, and they produce 15 almost no false notifications.

1 4.2.2 SACS multi-sensor warning system An online service for near real-time satellite monitoring of volcanic plumes

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J I 2 Back Close Fig. 8. Overview of the SACSFigure notiﬁcation 8: Overview system. of the SACS notification system.

3 The SO2 warning service relies on pre-defined geographical regions. SACS sends a notification to users as soon Full Screen / Esc 4 as a new volcanic plume is detected. If within a period of 12 hours, a plume is detected again in the same region 5 no new notification is generated (to avoid sending redundant information). An illustration of the SACS multi- Printer-friendly Version 6 sensors warning system is presented in Fig. 8. The different parts are described in more detail below. Interactive Discussion 7 SO2 notification for a dedicated region 5984

Figure 9: Predefined world regions used by the SACS notification

system and for the SO2 monitoring service.

8 As soon as SACS harvests new SO2 observations (from one of the instruments), the first step is the analysis of 9 the data using the criteria defined in section 4.2.1. After a volcanic eruption, there are potentially multiple

14 1 4.2.2 SACS multi-sensor warning system

Figure 8: Overview of the SACS notification system.

3 The SO2 warning service relies on pre-defined geographical regions. SACS sends a notification to users as soon icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 4 as a new volcanic plume is detected. If within a period of 12 hours, a plume is detected again in the same region NHESSD 5 no new notification is generated (to avoid sending redundant information). An illustration of the SACS multi- 1, 5935–6000, 2013 6 sensors warning system is presented in Fig. 8. The different parts are described in more detail below.

7 SO notification for a dedicated region An online service for 2 near real-time satellite monitoring of volcanic plumes

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J I Fig. 9. PredeﬁnedFigure 9: worldPredefined regions world used regions by the SACSused by notiﬁcation the SACS system notification and for the SO2 monitoring service. J I system and for the SO monitoring service. 2 Back Close

8 As soon as SACS harvests new SO2 observations (from one of the instruments), the first step is the analysis of Full Screen / Esc 9 the data using the criteria defined in section 4.2.1. After a volcanic eruption, there are potentially multiple Printer-friendly Version

14 Interactive Discussion

5985 1 warnings generated by the system. For this reason, it has been decided to consider a set of world regions of 30° 2 by 30° plus two polar regions polewards of 75° in latitude. Figure 9 shows the locations (and associated 3 name/number) of the 62 regions of the SACS system. Note that the same regions are used in the near real-time

4 monitoring and archive services to display the SO2 and ash images. As soon as a notification is issued, the related 5 region is flagged ‘ON’.

6 Check of the warning status

7 The second step is to check the “warning status” of this region. If there is no on-going notification for this region 8 (meaning no notification since 12 hours), the warning status can possibly become ON. The system compares the 9 time of observations with the processing time. If the delay is less than 8 hours, the notification is issued. This 10 set-up enables to provide timely information to the users and also to avoid issuing too many notifications 11 (maximum one notification per region and per 12 hours).

12 E-mail notification | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion

13 Important information related to the warning is gathered into a notification sent by email to users who subscribed NHESSD 14 to SACS (http://sacs.aeronomie.be/alert/subscribe.php). The email notification (see also the example shown in 1, 5935–6000, 2013 15 Fig. 10) contains: the time of the notification and the instrument, the region number, some details about the

16 processing, a link to a dedicated web page generated by the system, coordinates of the maximum SO2 signal An online service for 17 along with the date and time of observations (UTC), a flag indicating whether a cloud correction has been near real-time 18 applied or not, and finally the solar zenith angle. satellite monitoring of volcanic plumes

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Tables Figures 19 J I Figure 10: Example of an email notification sent to users in case of an exceptional SO concentration detected. Fig. 10. Example of an email notiﬁcation sent to users in case2 of an exceptional SO2 concentration detected. 20 Note that a solar zenith angle cut-off is used for each UV-visible sensor depending on the day of the year. The J I

21 values have been determined empirically to have the best coverage while limiting the increase in the noise Back Close 22 (instrument-specific). Full Screen / Esc

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Full Screen / Esc st Figure 11: SO2 and aerosol images from GOME-2 (notification web page), the 1 September 2012 (eruption of the Fig. 11. SO2 and aerosol images from GOME-2 (notiﬁcation web page), on 1 September 2012 Bezymianny volcano in the Kuril islands). (eruption of the Bezymianny volcano in the Kuril islands). Printer-friendly Version 2 An example of a web page generated by SACS using GOME-2 data for a specific notification is shown in Fig.

3 11. This notification was sent to the users 1h01 after MetOp-A passed over this area. The page shows a box with Interactive Discussion

4 the information of the email notification (time of measurement, location of the warning, SZA, maximum SO2 5987 5 value, and the use of cloud data in calculations), and the images of SO2 and aerosols/ash side-by-side. Below the

6 SO2 image, one can find two links to the same image but using different mapping tools (interpolated image using 7 the Generic Mapping Tools from the University from Hawaii, see http://gmt.soest.hawaii.edu, and a Google- 8 Earth file, see http://www.google.com/earth). For the UV-visible instruments, there is also a link to the

9 corresponding cloud cover fraction (CCF) image. In case of a IASI notification, in addition to the SO2 brightness

10 temperature index image there is also a link to the SO2 VCD image. As a complement, a map showing the 11 volcanoes in the region (with a link to a listing including more information about each of them) is also provided 12 on this web page. Furthermore, a direct link to the near real-time monitoring web page is given with all 13 corresponding SACS images (for all instruments), and a link to the data (files or link to data providers), for the

14 region considered.

An online service for 1 Confirmation of SO2 notification near real-time satellite monitoring of volcanic plumes

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Figure 12: SO2 and ash images from IASI, on 1 September 2012. Confirmation of notification by IASI. J I

Fig. 12. SO2 and ash images from IASI, on 1 September 2012. Conﬁrmation of notiﬁcation by J I IASI.2 For the example shown in Fig. 11, IASI also detected SO2 signal and ash in the region 112 (see Fig. 12) and 3 acted as a confirmation. In fact the processing of IASI was just 3 minutes after the one of GOME-2, which took Back Close 4 place at 23:42 UTC, as show Fig. 10. No notification has been sent to the users, as there already was a 5 notification based on GOME-2, but the dedicated web page has been automatically updated with the notification Full Screen / Esc 6 box for the IASI warning using the same displaying template. Note that two and ten hours after this notification 7 from GOME-2, two others confirmations from AIRS and IASI took place. Printer-friendly Version

8 Update of SO warning archive 2 Interactive Discussion 9 As soon as a warning is detected by the system, the warning archives are automatically updated on the SACS 10 web site. The warning subpage allows the users to select5988 the instruments of interest, for a given month/year and 11 to visualise a map with the location of all the corresponding warnings (see next section). For a given region, a 12 minimum delay of 12 hours separates the time between two warnings, as explained above. The map of the 13 warnings is an interactive map. When moving the cursor on a warning point, the user can visualise the images 14 corresponding to the warning. In addition to this interactive tool, a common list with all the warnings 15 (chronologically sorted) is also available with the relevant links to the different webpages.

16 4.3 Nine years of data and notifications archive

17 Although the multi-sensor notification system is only operational since April 2012, nine years of historical data 18 have been reanalysed for the development of the warning service. This section presents an overview of all the 19 notifications identified by SACS using the five sensors. For each year (from 2004 to 2012), an image of all the 20 notifications is shown with a highlight of the main events. This tool, available on the SACS portal, allows any 21 interested person to investigate historical eruptive emissions in a quick and user-friendly way. Note that our 22 system also allows the monitoring of small volcanic eruptions which might not always lead to a notification. 17 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion

1 4.3.1 Notifications from 2004 to 2006 (SCIAMACHY, OMI, and AIRS) NHESSD 1, 5935–6000, 2013

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2 Printer-friendly Version Figure 13: SACS notifications for 2004, 2005, and 2006. Fig. 13. SACS notiﬁcations for 2004, 2005, and 2006. 18 Interactive Discussion

5989 1 Data from the SCIAMACHY, OMI, and AIRS instruments are available for this period. The main volcanic 2 activity detected by SACS in 2004 is in the Independent State of Papua New Guinea, the archipelago of the 3 Republic of Vanuatu, and the Democratic Republic of Congo (DRC), as shown in Fig. 13a. The borderland 4 between Rwanda, the DRC and Uganda is one of the most active regions containing eight volcanoes, including 5 Mount Nyamuragira and Mount Nyiragongo which were both erupting on May 25, 2004. Details about the

6 satellite monitoring of SO2 emissions from Nyiamuragira from 1979 to 2005 can be found in Bluth and Carn

7 (2008). Figure 13a presents also several SO2 notifications over Europe in 2004. During the first week of 8 November, an eruption occurred in Iceland (Grìmsvötn volcano). Volcanic ash from the eruption fell as far away 9 as mainland Europe and caused short-term disruption of airline traffic into Iceland. Assuming an ash density of 10 2200 kg.m-3 and using dispersion models from the VAACs, Witham et al. (2007) gives an emission up to 9.50 × 11 1012 g.hr-1 for the larger plume, that corresponds to an erupted mass of 3.3 × 1011 kg and a volume of 0.15 km3 12 over the 35-h release period. Note that at this time of the year, the thermal IR sensor (AIRS) was the best 13 instrument (best coverage and time laps between observations) to monitor the volcanic plume.

14 The volcanic activity in DRC (Galle et al., 2005) and in Galapagos and Vanuatu has continued in 2005 as can be icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 15 seen in Fig. 13b. The signatures of two additional eruptions are also visible: 1) The submarine eruption of the 16 Japanese Fukutoku-Okanoba volcano, located 6 km northeast of the pyramidal island of Minami-Iwojima (about NHESSD 17 1300 km south-west of Tokyo); 2) the eruption of the Sierra Negra volcano, Galápagos, Ecuador. After 26 years 18 being inactive and six months after an earthquake on an active fault, the volcanic activity of the Sierra Negra 1, 5935–6000, 2013 19 raised again on 22 October 2005 with a fissure opening inside the rim of the caldera. This eruption emitted a 20 volcanic cloud with an estimated volume of 150 millions m³ of gas with a 2-km long curtain of lava fountains An online service for 21 (Geist et al., 2008). During this effusive eruption, a huge amount of gas and particles was ejected in the low near real-time 22 troposphere. The plume of SO and ash reached an altitude estimated below 5 km (Yang et al., 2009). At the end 2 satellite monitoring 23 of October 2005, atmospheric emissions of this volcano ceased. of volcanic plumes

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Figure 14: OMI images of SO2 emitted by Soufrière Hills volcano from 20 May to 8 June 2006 (from right to left). Fig. 14. OMI images of SO2 emitted by Soufrière Hills volcano from 20 May to 8 June 2006 (from25 rightIn 2006, to left). volcanic activity has been detected by SACS (Fig. 13c) in the Andes Cordillera (Colombia and Peru), J I 26 but the three strongest eruptions occurred in other parts of the world: 1) On 6 October, a large explosive eruption J I 27 took place at Rabaul volcano (Papua New Guinea), sending a plume of gas and ash up to the lower stratosphere. 28 Several notifications have been issued by SACS showing the volcanic cloud traveling westward; 2) The Back Close 29 Nyamuragira volcano was also active in 2006. A strong eruption began in the night of the 27th of November (see Full Screen / Esc 19

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1 Printer-friendly Version Figure 15: SACS notifications for 2007, 2008, and 2009. Fig. 15. SACS notiﬁcations for 2007, 2008, and 2009. 21 Interactive Discussion

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1 Printer-friendly Version Figure 16: IASI SO column density and ash index emitted by the Kasatochi volcano on a) 10 August 2008; b) 11 Fig. 16. IASI SO column2 density and ash index emitted by the Kasatochi volcano on (a) 10 2 August 2008. August 2008; (b) 11 August 2008. Interactive Discussion

2 Figure 16 presents the SO2 cloud from Kasatochi as observed by IASI on 10 and 11 August (interpolation with a 3 grid of 0.25° by 0.25°). The ash images from IASI (showing5992 the level of confidence of the detection) have been

4 superimposed over the SO2 images. The ash and SO2 clouds show a similar pattern on 10 August (see Fig. 16a). 5 Note that ash observed with a low level of confidence over the south of Alaska and the west of Canada is an 6 artefact in the analysis of the spectrum and the process of ash detection. The volcanic plume emitted by

7 Kasatoshi was a SO2-rich cloud (up to 676.4 DU on 8 August). We can see on 11 August that the detected ash

8 cloud is smaller than the SO2 cloud (see Fig. 16b).

9 Two months after Kasatochi eruption, Dalaffilla volcano erupted on 3 November. This volcano, located in the 10 Afar region of Ethiopia, is one of the six volcanoes in the Erta Ale Range. We can see in Fig. 15b that a few days

1 after this eruption, which was the largest observed in Ethiopia in historical times, the volcanic cloud generated NHESSD

2 six SO2 notifications over India and the south of China. 1, 5935–6000, 2013

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Figure 17: AIRS SO2 column density and ash index emitted by the Sarychev volcano on a) 16 June 2009; b) 17 Fig. 17. AIRS SO2 column density and ash index emitted by the Sarychev volcano on (a) 16 Full Screen / Esc June2009. June 2009; (b) 17 June 2009. 4 Three major eruptions that occurred in 2009 were identified by the SACS warning service (Fig. 15c): Redoubt Printer-friendly Version 5 volcano in Alaska, Fernandina volcano in Galápagos islands, and Sarychev volcano in the Kuril islands. The

6 explosive eruption of Sarychev (11-19 June 2009) emitted a huge amount of ash and SO2 into the atmosphere for Interactive Discussion

7 a range of altitudes of 10-16 km (Carn and Lopez, 2011; Clarisse et al., 2012). The mass of the SO2 plume was 8 estimated to be 1.2 ± 0.2 Tg by Haywood et al. 5993 (2010). This eruption, which was one of the 10 largest 9 stratospheric injections in the last 50 years, significantly affected the radiative budget of the atmosphere

10 (Haywood et al., 2010). Images of the SO2 and ash clouds observed by AIRS are shown in Fig. 17 (ash image

11 superimposed over the SO2 image). We can see that the ash detection (with a low level of confidence) over the th 12 Russian land on the north of the Sea of Okhotsk is an artefact. On the morning (UTC time) of the 16 June, SO2

13 and ash cloud are well co-located (see Fig. 17a). Note that the first SO2 plume was emitted two days before and th 14 was observed on the West coast of USA. Figure 17b shows the SO2 cloud observed on the afternoon of the 17 15 of June. The ash cloud detected by AIRS is located over the Sakhalin Russian Island. No ash was detected by 16 AIRS over the Pacific ocean.

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Printer-friendly Version Figure 18: SACS notifications for 2010, 2011, and 2012 Fig. 18. SACS notiﬁcations for 2010, 2011, and 2012 24 Interactive Discussion

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1 Figure 19: Ash index images from thermal IR sensors (AIRS and IASI) during the eruption of Eyjafjallajökull Printer-friendly Version Fig. 19. Ash index images from thermalvolcano IR (Iceland) sensors in spring 2010 (AIRS. and IASI) during the eruption of Eyjafjallajökull volcano (Iceland) in spring 2010. 25 Interactive Discussion

5995 1 Figure 18a shows all the SO2 notifications identified by SACS for the year 2010. Many regions were affected by 2 volcanic activity. The 2010 eruptions of Eyjafjallajökull volcano in Iceland were relatively small, but unusual 3 meteorological conditions brought volcanic ash clouds over Europe, causing enormous disruption to air traffic 4 across western and northern Europe over an initial period of six days in April 2010 (Zehner, 2010). Additional 5 localised disruption continued into May 2010. The eruption was declared officially over in October 2010, when 6 snow falling on the glacier did not melt anymore. From 14 to 20 April, ash covered large areas of northern 7 Europe when the volcano erupted (see Fig. 19 with images of the level of confidence of ash detection from AIRS 8 and IASI from 14 April to 17 April). Most of the airports of Northern Europe were closed by the authorities.

9 During these days, the quantity of SO2 observed by UV-visible and IR satellites was low (less than 5 DU),

10 possibly due to local chemical reactions between SO2 and water vapour creating sulfides or sulphuric acid. In

11 May 2010, during the second phase of emissions, a more significant amount of SO2 was measured (up to 28.6 12 DU for AIRS, on 6 May).

13 The strongest eruption of 2010 was the one of Merapi on 3 November on the Island of Java, Indonesia. Rapid 14 ascent of magma from the depths of the volcano and the collapse of the lava dome put the large population living 15 around the volcano in danger. People had to move away from their homes around the flanks of Merapi. For the 16 first time, remote sensing (ground deformation by aperture radar, and monitoring of gas emissions by 17 spectroscopic technique), together with the monitoring of the seismicity and the ground deformation by geodetic 18 technique, provided precursory signals of this volcanic crisis, avoiding the death of several thousand people

19 living around (Surono et al., 2012). Explosive and effusive phases took place and released a mass of SO2 of

20 about 0.44 Tg. The altitude of the SO2- and ash-rich plume was estimated to be about 12 km. Figure 20 presents

21 SO2 and ash measured by IASI on 5 November. Ash and SO2 were monitored respectively by IASI during 10

22 and 15 days after the start of the eruption. The five instruments used by SACS | (with Paper Discussion | their Paper Discussion | space Paper Discussion | - Paper Discussion and time-

23 complementarities) identified many SO2 notifications (see Fig. 18a) and allowed a good monitoring of theNHESSD 24 Merapi’s eruption and its volcanic cloud (eastward movement and then southward). 1, 5935–6000, 2013

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J I 25 J I Back Close Fig. 20. Ash image superimposed over the SO image from IASI during the Merapi’s eruption Figure 20: Ash image superimposed over the SO2 image from2 IASI during the Merapi’s eruption (5 November 2010). (5 November 2010). Full Screen / Esc

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Interactive Discussion 26 5996 1 The largest number of notifications of the SACS system to date occurred during the year 2011 (Fig. 18b). The 2 three major events of 2011 were the eruptions of the Icelandic Grìmsvötn volcano (21-28 May), the eruption of 3 the Chilean Puyehue-Cordón Caulle volcano (4-7 June), and the eruption of the Eritrean Nabro volcano (12 June

4 - 7 July). A characteristic feature of the Grìmsvötn eruption is that a large amount of SO2 was ejected northwards 5 while the ash cloud went to the southeast (see GOME-2 image Fig. 21). In this figure, the image of the absorbing

6 aerosols index has been superimposed onto the SO2 image. A null, low, medium and high level of aerosols/ash 7 detected by GOME-2 corresponds respectively to AAI under 2, AAI of 2.5, AAI of 3, and AAI over 3.5. Aerosol

8 detection on the north coast of Norway is not related to the Grìmsvötn eruption. The SO2 cloud, which travelled 9 over Canada and came back over Europe, was monitored during 3 weeks. The ash cloud, which travelled over 10 Europe, was under the limit of detection 3 days after the start of the eruption. icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion NHESSD 1, 5935–6000, 2013

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11 Printer-friendly Version Interactive Discussion Fig.Figure 21. SO 212: SOand2 and aerosols aerosols (ash) (ash) detected detected by by GOME-2 GOME- (Grìmsvötn2 (Grìmsvötn eruption, eruption, 23 23 May Ma 2011).y 2011). 5997

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Interactive Discussion Figure 22: SO2 and ash detected by IASI (Puyehue-Cordón Caulle eruption, 6 June 2011). Fig. 22. SO2 and ash detected by IASI (Puyehue-Cordón Caulle eruption, 6 June 2011). 1 On 4 June 2011, the eruption of the Puyehue-Cordón Caulle volcano started. Out of the nine years the SO 5998 2 2 notification archives span, this eruption is the most ash-rich recorded by our system. The volcanic cloud emitted

3 was an ash-laden plume. During the first two days of this eruption, the SO2 and ash clouds were collocated. 4 Afterwards, the volcanic cloud dispersed and circled around the South pole (see Fig. 22). Within a few day, the 5 cloud of ash covered the main part of Southern high latitudes. Note that for this particular eruption, that occurred 6 during the Austral winter, the UV sensors had limited coverage and IASI and AIRS were the only sensors able to

7 track globally the volcanic plume. The SO2 and ash clouds were observed by IR sensors respectively during 3 8 and 5 weeks. The volcanic ash emitted by Puyehue consequently disturbed the air traffic of the southern

28 1 hemisphere during the month of June 2011 in Chili, Argentina, Uruguay, Brazil, South Africa, Australia and 2 New Zealand.

3 During the night of the 12th of June 2011, the Eritrean Nabro volcano started to erupt (see Fig. 3). This explosive

4 eruption ejected a huge amount of SO2 in the atmosphere, threating international routes from the Far East to 5 Europe. The 14th of June, the Nabro volcano spewed a volcanic plume across the route of many flights over East

6 Africa and the Middle East. Satellite information was critical in monitoring the journey of the volcan | Paper Discussion ic cloud | Paper . Discussion | Paper Discussion | Paper Discussion th th 7 We can see in Fig. 3 the cloud of SO2 detected by IASI and AIRS the 13 and 14 of June. Figure 23 shows the

8 SO2 plume observed by IASI during the night of 15-16 June. The amount of ash emitted by Nabro for this NHESSD 9 eruption was very low (and largely undetected by IASI and AIRS), while the ash emitted by the Puyehue 1, 5935–6000, 2013 10 volcano was still detected by IASI in Fig. 23.

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Figure 23: SO2 and ash detected by IASI (Nabro and Puyehue eruptions, 16 June 2011). Fig. 23. SO2 and ash detected by IASI (Nabro and Puyehue eruptions, 16 June 2011). Full Screen / Esc 11 Figure 18c shows the SACS notifications for 2012. Continuous activity was recorded at the Nyamuragira 12 volcano (DRC). Summit eruptions of Etna (Italy) were also detected by SACS in January–April 2012, and again Printer-friendly Version 13 in July–October 2012, but no strong disruption of air traffic occurred. The 1st of September 2012, an explosive Interactive Discussion 14 eruption took place at Bezymianni volcano (Kamchatka), producing an SO2 and ash cloud (see Fig. 11 and 12). 15 With a cloud rising to an altitude of about 10 km, the Tokyo VAAC sent out an ash-cloud aviation warning and 16 the major intercontinental routes that pass in that area5999 were cancelled. Less than two months after this eruption, a 17 nearby volcano, the Tolbachik, erupted on 27th November. The main part of the volcanic ejection drifted north

29 1 and northwest (NNW), as observed by IASI and AIRS sensors. At this time of the year and for this location, the 2 thermal IR instruments were the only instruments used by SACS that were able to monitor the volcanic cloud. 3 The ash advisory board from the Tokyo VAAC reported of an ash cloud at an altitude of 10 km, spreading to the 4 NNW. Despite this, no ash was detected by the instruments used by SACS.

5 The last 2012 eruption which caused a disruption to air traffic, was the eruption of the Chilean Copahue volcano

6 (23-25 December). An SO2-rich plume associated with ash emissions was spewed into the sky (Fig. 24). The 7 volcanic plume was transported over Chile and aviation authorities in South America warned airlines to avoid 8 the area. Contrarily to the previous eruption of Puyehue in June 2011, the Copahue’s volcanic cloud was not

9 particularly ash-rich. We can see in Fig. 24 that for the first day of this eruption, SO2 and ash clouds took the

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11 Printer-friendly Version Interactive Discussion Fig. 24. SO and ash detected by IASI (Copahue eruption, 23 December 2012). Figure2 24: SO2 and ash detected by IASI (Copahue eruption, 23 December 2012). 6000 12 In this section, we have shown the major volcanic events which have disturbed air traffic during the last 10 13 years. We can see the uniqueness of each eruption. This diversity is characterised by different volcanic cloud

14 compositions (sometimes SO2-rich and/or sometimes ash-rich) driven by to specific meteorological conditions 15 occasionally dispersing the plume over several continents. To synthetize the full performance of our notification 16 system from 2010 to 2012, the diagnosis of SACS monitoring of the volcanic activity is presented in the 17 appendix.