MTU GE 4250 - Measuring volcanic degassing of SO2 in the lower troposphere with ASTER band ratios

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Measuring volcanic degassing of SO2 in the lower troposphere with ASTER band’ratiosIntroductionMethodologyRadiative transfer simulationsRetrieval schemeSensitivity analysisSulfur dioxideThermal contrastRadiometric noiseAtmospheric humidity and surface elevationSurface emissivitySulfate aerosolsCase studiesMiyakejimaEtna, 3 August 2006ConclusionAcknowledgementsAppendix AReferencesMeasuring volcanic degassing of SO2in the lower troposphere with ASTERband ratiosRobin Campiona,⁎, Giuseppe Giovanni Salernob,d, Pierre-François Coheure, Daniel Hurtmanse,Lieven Clarissee, Kohei Kazahayaf, Michael Burtonc, Tommaso Caltabianob,Cathy Clerbauxe, Alain BernardaaUniversité Libre de Bruxelles, Département des Sciences de la Terre et de l'Environnement. 50 Av. Roosevelt, CP160/02, 1050 Bruxelles, BelgiumbIstituto Nazionale di Geofisica e Vulcanologia—Sezione di Catania. Piazza Roma 2, 95125, Catania, ItalycIstituto Nazionale dei Geofisica e Vulcanologia—Sezione di Pisa. Via della Faggiola 32, 56126, Pisa, ItalydUniversity of Cambridge, Department of Geography. Downing Place, Cambridge, CB2 3EN, United KingdomeUniversité Libre de Bruxelles, Chimie Quantique et Photophysique, 50 Av. Roosevelt, CP160/09, 1050 Bruxelles, BelgiumfGeological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japanabstractarticle infoArticle history:Received 23 November 2009Accepted 21 April 2010Keywords:remote sensingSO2ASTERDOASEtnaMiyakejimaWe present a new method for measuring SO2with the data from the ASTER (Advanced Spaceborne ThermalEmission and Reflectance radiometer) orbital sensor. The method consists of adjusting the SO2columnamount until the ratios of radiance simulated on several ASTER bands match the observations. We present asensitivity analysis for this method, and two case studies. The sensitivity analysis shows that the selectedband ratios depend much less on atmospheric humidity, sulfate aerosols, surface altitude and emissivity thanthe raw radiances. Measurements with b 25% relative precision are achieved, but only when the thermalcontrast between the plume and the underlying surface is higher than 10 K. For the case studies we focusedon Miyakejima and Etna, two volcanoes where SO2is measured regularly by COSPEC or scanning DOAS. TheSO2fluxes computed from a series of ten images of Miyakejima over the period 2000–2002 is in agreementwith the long term trend of measurement for this volcano. On Etna, we compared SO2column amountsmeasured by ASTER with those acquired simultaneously by ground-based automated scanning DOAS. Thecolumn amounts compare quite well, providing a more rigorous validation of the method. The SO2mapsretrieved with ASTER can provide quantitative insights into the 2D structure of non-eruptive volcanicplumes, their dispersion and their progressive depletion in SO2.© 2010 Elsevier B.V. All rights reserved.1. IntroductionVolcanism is the leading natural source of sulfur dioxide (SO2)totheatmosphere, with average annual emissions estimated at 10 to40.109Gg/year, according to different authors (Le Guern, 1982; Stoiberet al., 1987; Andres and Kasgnoc, 1997; Graf et al., 1997). Theseemissions occur through eruptions and passive degassing. The lowatmospheric background of SO2, together with its strong absorptionfeatures in the ultra-violet (UV) and infrared (IR), makes this gas a goodtarget for spectrometric measurement from field and space. SO2fluxmeasurements have been performed since the 1970s by UV correlationspectrometry (e.g.: Newcomb and Millán, 1970; Stoiber et al., 1983).They have proven to be valuable for our understanding of eruptiondynamics and eruption forecasting (see Williams-Jones et al., 2008,foran extensive review of theory, practice and case studies of SO2fieldmeasurements on volcanoes). The most commonly used instrumentsfor SO2field measurements are now compact UV spectrometers with orwithout calibration cells (Galle et al., 2002, Elias et al., 2006). However,these sensors can only measure scans or transects of SO2columnamounts, while volcanic plumes are actually dynamical 3D objects. Toexpand the investigations on volcanic plumes, field UV imaging deviceshave been developed recently (Bobrowski et al., 2006; Mori and Burton,2006; Bluth et al., 2007, Dalton et al., 2009) and these technologies arecurrently being rapidly developed as volcano monitoring tools. Inaddition, techniques using several spectrometers are starting to beapplied to evaluate vertical distribution of SO2in the plume throughtomography algorithms (e.g. Kazahaya et al., 2008).Space based sensorsare alsoableto detect and measure volcanic SO2.They provide a more complete view of large scale volcanic plumes andare the only tools allowing a quantification of the material (gas, ash andaerosols) injected into the atmosphere by large eruptions. Currentspace-borne sensors sensitive to SO2(see Annex table of Appendix A)are MODIS, ASTER, AIRS, SEVIRI and IASI operating in the thermalInfrared (TIR), and GOME-2, SCIAMCHY, and OMI in the UV. Thanks toJournal of Volcanology and Geothermal Research 194 (2010) 42–54⁎ Corresponding author. Tel.: +32 26502239.E-mail address: [email protected] (R. Campion).0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2010.04.010Contents lists available at ScienceDirectJournal of Volcanology and Geothermal Researchjournal homepage: www.elsevier.com/locate/jvolgeorestheir high sensitivity and daily (or nearly daily) global coverage, OMI(Carn et al., 2008), AIRS (Prata and Bernardo, 2007) and IASI (Clarisseet al., 2008) have already proved very useful for quantifying eruptiveemissions of SO2as well as tracking SO2transport up to several weeksafter the eruption. Thanks to its high acquisition frequency (1 imageevery 15 min), the geostationary sensor SEVIRI can also be very usefulfor near real-time monitoring of volcanoes located in its observationzone. However the low spatial resolution of these three sensors, and/ortheir weak sensitivity to tropospheric SO2complicate the quantificationof SO2flux in passive degassing plumes.ASTER was launched in December 1999 on board the EOS Terrasatellite on a sun-synchronous orbit (Pieri and Abrams, 2004). Dependingon latitude, the minimal time delay between two images varies between 1and 7 days, somewhat limiting its use for routine volcano monitoring. Thepresence of a tight cloud cover at the time of overflight can further


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