The CO2 laser-based lidar ATLAS has been used to study the Stromboli volcano plume. ATLAS measured water
vapor concentration in cross-sections of the plume and wind speed at the crater. Water vapor concentration
and wind speed were retrieved by differential absorption lidar and correlation technique, respectively. Lidar
returns were obtained up to a range of 3 km. The spatial resolution was 15 mand the temporal resolution was
20 s. By combining these measurements, the water vapor flux in the Stromboli volcano plume was found. To
our knowledge, it is the first time that lidar retrieves water vapor concentrations in a volcanic plume.Published1295–12981.10. TTC - TelerilevamentoJCR Journalreserve

Aiuppa, A.

Colao, F.

Fiorani, L.

Giudice, G.

Palucci, A.

Poreh, D.

AIUPPA, Alessandro

Archivio istituzionale della ricerca - Università di Palermo

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Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyrightAuthor's personal copyFirst-time lidar measurement of water vapor ﬂux in a volcanic plumeLuca Fiorani a,⁎, Francesco Colao a, Antonio Palucci a, Davod Poreh b, Alessandro Aiuppa c,d, Gaetano Giudice da UTAPRAD-DIM, ENEA, Via Enrico Fermi 45, 00044 Frascati, Italyb ENEA guest with ICTP fellowship, ENEA, Via Enrico Fermi 45, 00044 Frascati, Italyc CFTA, Università di Palermo, Via Archiraﬁ 36, 90123 Palermo, Italyd Istituto Nazionale di Geoﬁsica e Vulcanologia, Sezione di Palermo, Via La Malfa 143, 90146 Palermo, Italya b s t r a c ta r t i c l e i n f oArticle history:Received 17 June 2010Received in revised form 15 September 2010Accepted 24 October 2010Keywords:LidarDIALCorrelation techniqueWater vaporWind speedVolcanic plumeThe CO2 laser-based lidar ATLAS has been used to study the Stromboli volcano plume. ATLAS measured watervapor concentration in cross-sections of the plume and wind speed at the crater. Water vapor concentrationand wind speed were retrieved by differential absorption lidar and correlation technique, respectively. Lidarreturns were obtained up to a range of 3 km. The spatial resolution was 15 m and the temporal resolution was20 s. By combining these measurements, the water vapor ﬂux in the Stromboli volcano plume was found. Toour knowledge, it is the ﬁrst time that lidar retrieves water vapor concentrations in a volcanic plume.© 2010 Elsevier B.V. All rights reserved.1. IntroductionStromboli is a small island in the Tyrrhenian Sea, off the northcoast of Sicily, hosting one of the most active volcanoes in Italy. Themost recent volcano eruptions (on December 2002–July 2003 andFebruary–April 2007) generated a tsunami and two paroxysmalexplosions, which caused serious damages to the Stromboli villagesand created strong alarm. In order to assess the potential volcanichazard and the risks associated with the population, the Italian CivilProtection National Service (DPC) supports continuous monitoring ofthe volcano, and eventually restricts the access to dangerous zones.Researchers of the Italian National Institute of Geophysics andVolcanology (INGV) maintain this activity investigating, amongothers, the volcanic emissions and relating variations in gaseousconcentrations to eruptive events [1]. In this framework, theDiagnostics and Metrology Laboratory (UTAPRAD-DIM) of the ItalianNational Agency for New Technologies, Energy and SustainableEconomic Development (ENEA) carried out remote sensing of watervapor in the volcano plume with the Agile Tuner Lidar [2] forAtmospheric Sensing (ATLAS) mounted on the EnvironmentalLaboratory (ENVILAB) hosted in a small truck.2. Instruments and methodsATLAS has already been described in a paper reporting on themeasurement of Mount Etna plume [3]. Let us simply recall here thatit is based on a tunable CO2 laser, a gold coated Newton telescope anda liquid-nitrogen-cooled mercury–cadmium–telluride photodiode. Itsmain speciﬁcations are listed in Table 1. During the Etna campaign,ATLAS operated only vertically and the plume was too far, thuspreventing the detection of water vapor inside it. During summer2009, ATLAS underwent some major improvements: the laser highvoltage circuit was upgraded; the transmitter optics was repolished;the telescope was recoated; a two mirror system, similar to acoelostat, was implemented, in order to scan the whole hemisphereabove the horizon. A gain of a factor 3–4 in terms of signal-to-noiseratio can be estimated as a result of these improvements. Eventually,the water vapor ﬂux in the Stromboli volcano plumewasmeasured on17 September 2009. To our knowledge, it is the ﬁrst time that thewater vapor ﬂux in a volcanic plume is retrieved by lidar, althoughlaser remote sensing has been already applied to sound volcanicparticulate in the lower stratosphere [4] and ﬂuxes of aerosol [5] andSO2 [6] in volcanic plumes.The water vapor concentration in the volcanic plume wasmeasured by differential absorption lidar (DIAL) [2] using the 10R18(λOFF) and 10R20 (λON) lines of the CO2 laser [7].The wind speed was measured observing by successive lidarechoes the transport of the volcanic aerosol by the surrounding airmass, a method called “correlation technique”, pioneered in theseventies [8] and applied in the nineties [9] to retrieve trace gas ﬂuxes.Optics Communications 284 (2011) 1295–1298⁎ Corresponding author.E-mail address: luca.ﬁorani@enea.it (L. Fiorani).0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.optcom.2010.10.082Contents lists available at ScienceDirectOptics Communicationsj ourna l homepage: www.e lsev ie r.com/ locate /optcomAuthor's personal copy3. Results and discussionATLAS was located at latitude 38.80074 N and longitude 15.24064E (Fig. 1), i.e. near the harbor and about 2 km north east from themaincrater. The repetition rate was 10 Hz and 200 lidar returns wereaveraged, corresponding to a temporal resolution of 20 s. The spatialresolution depends on the sampling rate and is 15 m.The wind came from south west, blowing the plume south ofATLAS. Consequently, in order to obtain cross-sections of water vaporconcentration in the volcanic plume, the lidar was aimed south westwith a ﬁxed azimuth angle (Fig. 1) while varying the elevation angle.During the ﬁrst scan the azimuth angle (starting from north androtating clockwise) was 237°, i.e. the laser beam was over themountain ﬂank between the peak crater and the east coast ofStromboli (Fig. 1). The elevation angles (starting from the horizon andgoing to the vertical) were from 23° to 37° at steps of 2°. The volcanicplume was detected at elevation angles 31° and 33°. The plume scanwas carried out between 12:20 and 12:27 pm (local civil time).The azimuth angle of the second scan was 192°, i.e. the laser beamwas over the sea surface, nearly parallel to the east coast of Stromboli(Fig. 1). The elevation angles were from 5° to 85° at steps of 10°. Forthe elevation angle 75° (16:24 pm) the plume signal was particularlyevident.The two scans obtained in this way are shown in Fig. 2. Fixing aCartesian coordinate system with origin in ATLAS, x-axis from west toeast, y-axis from south to north and z-axis vertical, the water vaporconcentration maxima of the two scans occurred at the followingpoints:P237 = −1274m;−827m; 863mð Þ;P192 = −52m;−243m; 927mð Þ:The unit vector going from P237 to P192 is:uw = 0:901253; 0:430714; 0:047201ð Þand gives the wind direction. The azimuth angle of the wind directionis 64°. This value is in good agreement withmeasurements carried outin the time interval of lidar operations with a meteo station mountedon the top of the truck (61°±72°).The cross-sectional areas of the volcanic plume corresponding tothe two scans were identiﬁed by the points with concentrationgreater than 10 Torr, the approximate level of water vapor back-ground, i.e. not linked to the Stromboli emission, as it can be inferredfrom Fig. 2. The concentrations averaged in these cross-sectional areaswere:C237 = 20:3 4:1torr;C192 = 17:3 2:3torr:The concentration error was inferred from themeasured signal-to-noise ratio of the lidar signal and is about 10% at the range where thelaser beam intersects the volcanic plume. The measurements ofconcentration are in good agreement for the two scans. Their averageis:C= 18:8 2:4Torr = 1:81 × 10−2  0:23 × 10−2 kgm−3:This value is consistent with simultaneous in-plume measure-ments carried out in situ (i.e. on the volcano summit, ~150 mdownwind the active open vents) using a near-dispersive infrared(NDIR) spectrometer [13], from which a time-averaged plume H2Oconcentration of ~19 Torr was derived. H2O concentrations in thebackground atmosphere – measured with the same method –averaged at ~14 Torr on 17 September 2009. The discrepancybetween the water vapor natural background measured by lidar andNDIR spectrometer is explained by the different sampling zones:while the lidar scans the atmosphere also far from the volcanic plume,the NDIR spectrometer operates only in proximity of the active openvents, where some water vapor can be transported from the volcanicplume.Table 1Main speciﬁcations of ATLAS. Note that the laser linewidth is small if comparedwith theH2O absorption linewidth (0.86 cm−1).Transmitter Pulse energy 850 mJ (at the 10R18 and 10R20emission lines)Pulse duration 60 ns (full width at half maximum)Repetition rate 2–20 HzTransmitted wavelength 9.2–10.8 μmLaser linewidth 0.13 cm−1Beam divergence 0.7 mradReceiver Mirror coating AuDiameter 310 mmFocal length 1.2 mRange of overlap 150 m (partial), 300 m (full)Detector Diameter 1 mmSpeciﬁc detectivity 4×1010 cm Hz1/2 W−1Gain 200Linear dynamic range 0.1–1000 mVBandwidth 0–10 MHzAnalog-to-digitalconverterDynamic range 8 bitSampling rate 10 Ms s−1Fig. 1. Location of ATLAS (circle) and directions of the laser beam during themeasurements.Fig. 2.Water vapor concentration retrieved by ATLAS during two scans at 237° (a) and192° (b) azimuth angles. The white points are the lidar measurements. The blackcrosses are the points where the water vapor concentration maxima of the two scansoccurred, i.e. P237 (a) and P192 (b).1296 L. Fiorani et al. / Optics Communications 284 (2011) 1295–1298Author's personal copyThe wind speed along the laser beamwas measured aiming ATLASto the peak crater (azimuth angle 246°, elevation angle 25°): in thisway the lidar direction was as much parallel as possible to the windwhile still intersecting the volcanic plume (if the beam does notinteract with the plume, the correlation technique cannot be applied).The plume was located at the range of 2100–2700 m from ATLAS, as itcan be inferred by the extinction coefﬁcient proﬁle (Fig. 3). As alreadydescribed elsewhere [3], the extinction coefﬁcient was retrieved fromthe lidar signal with an inversion method [10] derived from thealgorithms published by Klett [11] and Fernald [12]. It was assumedthat the extinction coefﬁcient was 10−5 m−1 at the reference range of3000 m, according to our previous measurements [3]. Consequently,the correlation function was calculated in the range interval 2100–2700 m for ten successive lidar proﬁles (one each 10 s), thusobserving the displacement of aerosol inhomogeneities for 100 s. Anexample of correlation function is given in Fig. 3. Ten measurementsof this kind were carried between 13:33 and 14:11 pm. Averagingthese measurements the wind speed along the laser beam resulted in3.4±1.3 m s−1. In order to obtain the actual wind speed w, this lattermeasurement has to be divided by the absolute value of the dotproduct of uw and the unit vector corresponding to the laser beamdirection:u246 = −0:827953;−0:368629; 0:422618ð Þ;resulting in:w = 3:8 1:5ms−1:This value is in reasonable agreement with the measurementscarried out in the time interval of lidar operations with the meteostation mentioned earlier (8.2±2.9 m s−1). Unfortunately, as alreadyshown in a detailed statistical analysis [9], because of the signal noisecontribution to the correlation function, the speed error of thecorrelation technique is rather large (comparablewith thewind speedstep, i.e. 1.5 m s−1 in our case, as it can be inferred from Fig. 3).Another important source of error is the natural variation of the windspeed and direction during the measurement time that, in our case, israther long (38 min). This natural variation dominates the error of thewind measurements carried out with the meteo station. The fact thatthe meteo station error (2.9 m s−1) is larger than the lidar error(1.5 m s−1) is not surprising because the ﬂuctuation of the windmeasurements carried out with the meteo station is larger due to itsproximity with the ground, where air masses are more affected byeddies and turbulences. Nevertheless, the comparison betweenmeteostation and lidar indicates that the contribution of the naturalvariation to the wind uncertainty is comparable with the error itself.In conclusion, the speed error and its contributions from signal noiseand natural variation are all of the order of 1 m s−1 and it is notpossible to separate the different contributions.Although the correlation technique is not very precise, it allowsone to gain information on the wind ﬁeld from lidar returns exactly inthe same space zone and time interval where the water vaporconcentration is measured. Moreover, it does not require thedeployment of any other instrument.Combining the measurements of average concentrations and windspeed, the following water vapor ﬂux can be calculated:Φ= C × w = 0:069 0:029kgm−2 s−1:The contribution to this ﬂux from the volcanic plume can beobtained subtracting the natural background from C, thus giving:ΦP = 0:032 0:015kgm−2 s−1;corresponding to the daily emission rate:R = ΦP × A = 118 55kgs−1 = 10200 4800 tday−1;where A, cross-sectional area of the volcanic plume, has been inferredfrom Fig. 2a, considering the points where the water vaporconcentration is larger than 20 Torr.This value has been compared with the conventional indirectmeasurement based on the knowledge of both the plume H2O/SO2mass ratio and the SO2 daily emission rate [14]. On 17 September2009, we estimated:– an average H2O/SO2 mass ratio of ~56 from MultiGAS, thepermanent network of Stromboli [1,13],– an average SO2 daily emission rate of ~100 t day−1 from FLAME,the scanner network based on differential optical absorption in theultraviolet [15],yielding to a water vapor daily emission rate of 5600±1100 t day−1,in reasonable agreement with the lidar measurement.4. ConclusionsThe CO2 laser-based lidar ATLAS was used to scan the plume ofStromboli volcano. Water vapor concentration and wind speed vectorwere measured. Eventually, the water vapor ﬂux was retrieved fromthese two values. The lidar measurement was in agreement with thevalue obtained with conventional techniques, yet based on complete-ly independent and signiﬁcantly different approaches. Although theerror is rather high, it is the ﬁrst time that the water vapor ﬂux in avolcanic plume is retrieved by lidar, representing the ﬁrst directmeasurement of this kind ever performed on an active volcano andFig. 3. Extinction coefﬁcient (a) and correlation versus wind speed (b) retrieved at13:57 pm by ATLAS.1297L. Fiorani et al. / Optics Communications 284 (2011) 1295–1298Author's personal copyshowing the high potential of laser remote sensing in geophysicalresearch.AcknowledgementsThe authors are deeply grateful to R. Fantoni (ENEA) and S.Gurrieri (INGV-Palermo), for constant encouragement. They extend aspecial thank-you to N. du Preez and G. Louwrens (SDI Lasers) foruseful discussions and laser servicing, respectively, R. Giovagnoli(ENEA) for mechanical parts, D. Del Bugaro (ENEA) and M. Liuzzo(INGV-Palermo) for signiﬁcant support to ﬁeld work, and T.Caltabiano (INGV-Catania) for kindly providing SO2 ﬂux data fromthe FLAME network. D. Poreh gratefully acknowledges the hospitalityof ENEA and the support of the Abdus Salam International Centre forTheoretical Physics (ICTP) in the framework of the program Trainingand Research in Italian Laboratories (TRIL). This research was partiallyfunded by Research Contract V2_01 — Convenzione DPC-INGV 2007–2009, Project V2: Paroxysm (responsible, A. Aiuppa).References[1] A. Aiuppa, C. Federico, G. Giudice, G. Giuffrida, R. Guida, S. Gurrieri, M. Liuzzo, R.Moretti, P. Papale, J. Volcanol. Geoth. Res. 182 (2009) 221.[2] R.T. Menzies, M.J. Kavaya, P.H. Flamant, D.A. Haner, Appl. Opt. 23 (1984) 2510.[3] L. Fiorani, F. Colao, A. Palucci, Opt. Lett. 34 (2009) 800.[4] H. Jaeger, Geophys. Res. Lett. 19 (1992) 191.[5] J.N. Porter, K.A. Horton, P.J. Mouginis-Mark, B. Lienert, S.K. Sharma, E. Lau, A.J.Sutton, T. Elias, Geophys. Res. Lett. 29 (2002) 1783, doi:10.1029/2002GL014744.[6] P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, T. Caltabiano,Appl. Phys. B 67 (1998) 419.[7] W.J. Witteman, The CO2 Laser, Springer, Berlin, 1987.[8] E.W. Eloranta, J.M. King, J.A. Weinman, J. Appl. Meteorol. 14 (1975) 1485.[9] L. Fiorani, B. Calpini, L. Jaquet, H. Van den Bergh, E. Durieux, Atmos. Environ. 32(1998) 2151.[10] L. Fiorani, Lasers and Electro-Optics at the Cutting Edge, Nova, New York, 2007.[11] J.D. Klett, Appl. Opt. 20 (1981) 211.[12] F.G. Fernald, Appl. Opt. 23 (1984) 652.[13] A. Aiuppa, A. Bertagnini, N. Métrich, R. Moretti, M. Liuzzo, G. Tamburello, Earth.Planet. Sc. Lett., 295 (2010) 195, doi:10.1016/j.epsl.2010.03.040.[14] A. Aiuppa, G. Giudice, S. Gurrieri, M. Liuzzo, M. Burton, T. Caltabiano, A.J.S.McGonigle, G. Salerno, H. Shinohara, M. Valenza, Geophys. Res. Lett. 35 (2008)L24302, doi:10.1029/2008GL035871.[15] M. Burton, T. Caltabiano, F. Murè, G. Salerno, D. Randazzo, J. Volcanol. Geoth. Res.182 (2009) 214.1298 L. Fiorani et al. / Optics Communications 284 (2011) 1295–1298

First-time lidar measurement of water vapor flux in a volcanic plume

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First-time lidar measurement of water vapor flux in a volcanic plume

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