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RESEARCH ARTICLE
Imaging DOAS for volcanological applications
Ilia Louban & Nicole Bobrowski & Dmitri Rouwet &Salvatore Inguaggiato & Ulrich Platt
Received: 26 March 2008 /Accepted: 15 December 2008 /Published online: 16 January 2009# Springer-Verlag 2009
Abstract The simultaneous quantitative determination oftwo-dimensional bromine monoxide (BrO) and sulphurdioxide (SO2) distributions in volcanic gas plumes isdescribed. Measurements at the fumarolic field on the islandVulcano (autumn 2004) and in the plume of Mt. Etnavolcano (spring 2005) were carried out with an ImagingDOAS instrument. The SO2 fluxes of several fumaroles wereestimated from two-dimensional distributions of SO2. Addi-tionally, the first two-dimensional distributions of BrO withina volcanic plume were successfully retrieved. Slant column
densities of up to 2.6×1014 molecules per square centimetrewere detected in the plume of Mt. Etna. The investigation ofthe BrO/SO2 ratio, calculated from the two-dimensionaldistributions of SO2 and BrO, shows an increase from thecentre to the edge of the volcanic plume. These results havesignificance for the involvement of ozone during BrOformation processes in volcanic emissions.
Keywords DOAS . Imaging DOAS . Volcanic plume .
BrO . SO2
Introduction
The position and shape of plumes from many sources, suchas volcanoes, are often visible to the eye due to aerosols(dust or condensing water vapour). However, not all plumescontain aerosols. Moreover, the distribution of gases in theplume may differ from the aerosol distribution. For thesereasons and due to limited investigation of the variousemitted volcanic gases, the spatial distribution of the gasesin volcanic plumes is still not well known. This is aparticular problem when chemical transformations occur inthe plume. In this paper, we describe a novel application ofimaging spectroscopy (Lohberger et al. 2004) usingsunlight scattered in the atmosphere as a light source. Thedata evaluation follows the principle of differential opticalabsorption spectroscopy (DOAS; Platt et al. 1979), which isbased on the characteristic absorption of electromagneticradiation of trace gas molecules along the light path in theopen atmosphere. The DOAS technique relies on highresolution (≈0.5 nm) spectral information within a wave-length interval of ≈100 nm. The technique therefore permitsthe retrieval of unambiguous information on several tracegas species simultaneously in addition to SO2. In particular,
Bull Volcanol (2009) 71:753–765DOI 10.1007/s00445-008-0262-6
Editorial responsibility: P. Delmelle
I. Louban :N. Bobrowski :U. PlattInstitute for Environmental Physics, University of Heidelberg,Im Neuenheimer Feld 229,69120 Heidelberg, Germany
N. Bobrowskie-mail: [email protected]
U. Platte-mail: [email protected]
N. Bobrowski :D. Rouwet : S. InguaggiatoIstituto Nazionale di Geofisica e Vulcanologia,Via Ugo la Malfa 153,90 146 Palermo, Italy
D. Rouwete-mail: [email protected]
S. Inguaggiatoe-mail: [email protected]
Present address:I. Louban (*)Institute for Physical Chemistry, Biophysical Chemistry Group,University of Heidelberg,Neuenheimer Feld 253,69120 Heidelberg, Germanye-mail: [email protected]
it allows us to map weak absorbers (e.g. BrO) whilstproviding good linearity. Recent measurements of two-dimensional SO2 distributions by “SO2 cameras” werereported (Mori and Burton 2006; Bluth et al. 2007). Thesedevices rely on the ratio of two-dimensional images takenin two narrow UV-spectral ranges: one encompassingstrong SO2 absorption bands and one in a spectral regionwith only weak SO2 absorption. Whilst SO2 cameras givegood spatial information, they suffer from relatively poorsensitivity which allows use for SO2 only.
Imaging DOAS (IDOAS) basically combines two princi-ples: imaging spectroscopy and the DOAS technique.Imaging of an object produces a two-dimensional dataset ofspatial information, i.e. each picture element of an image(image pixel) corresponds to a defined solid angle of space. Inthe case of imaging spectroscopy a third dimension of data isadded: the high resolved spectral information of each pixel.Therefore, imaging spectroscopic instruments require bothimaging and dispersive optical components. Detailed charac-terisation of the IDOAS principle and IDOAS instrument isgiven elsewhere (Lohberger et al. 2004; Louban 2005);therefore, only a brief description is given here.
Instrumental
The IDOAS principle is schematically shown in Fig. 1, whilstFig. 2 shows the corresponding instrumental setup. Incidentscattered sunlight, originating from the direction of theinvestigated object (Fig. 1a), is deflected by the scanningmirror to a quartz lens, which focuses the light onto thevertical entrance slit of an imaging spectrograph (Fig. 2a).The spectrograph images spatial and spectral resolved lighton the CCD matrix, assigning the horizontal dimension ofthe detector (1024 detector pixel) to spectral information andvertical dimension of the detector (255 detector pixel) to thespatial information (Fig. 1b). Upon the readout of the CCD,255 spectra (Fig. 2b) originating from one spatial column areacquired. Using a horizontal scanning system (push broomscanning principle), many columns up to a viewing angle of70° can be scanned (Fig. 1a).
The number of horizontally resolved image pixels isgiven by the number of acquired columns. The solid anglecorresponding to one image column is defined by the widthof the entrance slit and the displacement angle of thescanning mirror between two acquisitions. The number ofvertically resolved image pixels is limited by the number ofpixel rows of the detector (255 in our case) and can bescaled down by averaging over several adjacent detectorpixel rows (Fig. 2b).
After the acquisition of an entire image, the spectracorresponding to each image pixel of the data have to beevaluated with the DOAS method by the fitting software
(Fig. 1c). The derived slant column densities (SCD) arethen colour-coded and assembled into an image (Fig. 1d).
Figure 2 describes the configuration of the instrument andimaging properties. Main components of the instrument,placed in a weatherproof casing, are a stepper motor rotatingthe scanning mirror, a quartz lens (f=30.0 mm; d=17 mm),an optical shutter, an imaging spectrograph (entrance slit:height=6.9 mm; width=46 μm) insulated by polystyrenefoam, a CCD detector, and several controlling devicesoperated by a personal computer.
The central component of the spectrograph used in theIDOAS instrument is the holographic, flat field aberrationcorrected, concave grating (Jobin Yvon UFS 200; f/#=3.2; f=21 cm) with an efficiency of 30–40% in the spectralrange of 273–385 nm.
The back illuminated CCD detector (ANDOR 420-BU)features 1,024 pixel columns and 255 pixel rows; itsquantum efficiency in the measured spectral range (273–385 nm) ranges from 50% to 80% (ANDOR 2004). Duringall measurements, the CCD detector was cooled to −30°Cto minimise the dark current.
The acquisition software controls inter alia the rotationof the scanning mirror. This is realised with a stepper motorgear assembly, with minimum step size of Δδ=0.026°.However, the instantaneous field of view is determined bythe dimensions of the entrance slit and the lens. It results inan instantaneous horizontal resolution of Δα=0.087° perimage column and a vertical field of view of 13.1°(Fig. 2b). At least three steps of the motor are required tochange the viewing direction by Δα. To reduce the photonnoise, spectra of several columns could be co-added by theacquisition software. Adaptation of the exposure time to thelight conditions at the measurement site was performedautomatically during the measurement (Louban 2005).
Data evaluation
High-resolution trace gas absorption cross-sections wereconvoluted with the instrument function of the IDOASinstrument prior the DOAS evaluation procedure. Tocompensate for the effect of the rotational Raman scatteringin the atmosphere, so-called Ring spectra were included inthe evaluation procedure (Grainger and Ring 1962; Fish andJones 1995; Burrows et al. 1996). They were calculated onthe basis of known rotational states of the main constituentsof air, N2 and O2, from the Fraunhofer reference spectrum(FRS) by the MFC software (Gomer et al. 1996).
Trace gas absorption cross-sections, FRS and the Ringspectrum were fitted to the atmospheric spectra by WinD-OAS software by a nonlinear least-squares method (vanRoozendael and Fayt 2001). In order to account for the non-uniformity in the spectral resolution of the spectrograph,each spectrum was evaluated with an individual FRS
754 Bull Volcanol (2009) 71:753–765
(recorded during the measurements) at the correspondingdetector row (or set of co-added detector rows).
The slant column density of the BrO absorptionwas derivedin the spectral interval from 332.4 to 353 nm, encompassingfour BrO absorption bands (Wilmouth et al. 1999). In addition,molecular absorption cross-sections of NO2 (Voigt et al.2002), O3 (Voigt et al. 2001), O4 (Hermans et al. 1999), theFRS and the ring spectrum were simultaneously fitted to the
atmospheric spectra. For NO2 and O3, cross-sections at twodifferent temperatures were included, NO2 at 246 K and293 K; O3 at 223 K and 246 K, to account for thetemperature pattern in different atmospheric layers. A third-order polynomial was used by the fitting procedure (Fig. 3a)to account for continuous absorption. Due to the low BrOconcentration, the I0 effect (Johnston 1996; Frieß 1997)needed to be considered as well.
Fig. 1 Principle of ImagingDOAS. Using the horizontalscanning system, a wide view-ing angle can be scanned (a).The grating inside the spectro-graph images spatial and spec-tral resolved light on the CCDmatrix. It assigns the verticaldimension of the detector tospatial information and horizon-tal dimension of the detector tothe spectral information (b).After the acquisition, data ofeach pixel has to be evaluatedwith the DOAS method byfitting software (c). The derivedSCD’s are colour-coded andassembled into an image (d)
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For the SO2 evaluation, we chose the spectral rangebetween 307 and 317.5 nm encompassing four absorptionbands of SO2. Measured spectra were evaluated using thehigh-resolution molecular cross-sections of SO2 (Vandaeleet al. 1994) and O3 at 223 K and at 246 K, the FRS and theRing spectra. A polynomial of third degree was used by thefitting procedure (Fig. 3a).
Measurements
Here, we present two applications of the Imaging DOASinstrument: first, determination of the two-dimensional SO2
distribution performed on October 6th, 2004 at the islandVulcano, Italy and second, measurements of BrO and SO2
distributions at the plume of Mt. Etna during May 2005.
Fig. 2 a Setup of the IDOAS instrument (top view). The opticsfocuses incident light on the vertical entrance slit of the spectrograph.The concave grating images the entrance slit on the vertical (spatial)dimension of the detector; at the same time, the light is spatiallydispersed in the horizontal (spectral) dimension. A two-dimensionalCCD detector records the spectra, the PC (not shown), inside the
instrument controls the scanning mirror, optical shutter, CCD readoutand other processes of the measurement routine. b Instantaneousinstrumental field of view (13.1°×0.087°) is subdivided by 255 CCDdetector pixels. The resulting field of view of an image pixel isdetermined by the scanning mirror displacement and number of binneddetector pixels
Fig. 3 Two examples of theDOAS evaluation (using dataof the same detector row). Themeasured spectra are plotted inblack, the FRS in blue and theliterature trace gas cross-sectionin red. Topmost spectra: mea-sured spectra and FRS. Below:evaluated spectra; all features(trace gas spectra, FRS and Ringspectra) are removed excepttrace gas indicated. a BrO eval-uation (only O3 of 223 K isshown). b SO2 evaluation
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Measurements of SO2 distribution at the island Vulcano
A small fumarolic field at the edge of the crater (Fig. 4a)was scanned by the IDOAS instrument on October 6th,2004. The measurement covered a solid angle of (vertical ×horizontal) 13.1°×27.3° corresponding to an area of about46×97 m2 at the emission sources at a distance of 200±30 m from the instrument. A total of 105 columns were
acquired. The size of an image pixel at the position of thefumarolic emissions was (0.7±0.1)×(0.91±0.14) m2.
Figure 4b shows a greyscale intensity image obtainedby integrating the radiation intensity of each image pixelbetween 273 and 385 nm (intensity profile). By overlay-ing Fig. 4b and the evaluation result (Fig. 5) to thephotograph (Fig. 4a), the position of the trace gasemissions could be determined. The exposure time for eachcolumn was manually set to 8 s. The whole measurementprocedure took about 28 min due to delays caused byoptical alignment and data saving procedures. The windspeed, measured close to the fumaroles, varied between 0and 1 m/s. Figure 5 shows the evaluated SO2 columndensities. Figure 6 shows the corresponding statisticalerror of the DOAS fitting procedure. To avoid distortionof the image, the width-to-height ratio of all IDOASimages presented here were adjusted to the correspondingangle scale in horizontal and vertical direction.
Eight individual fumaroles (see black arrows at the topof Fig. 5) could be spatially resolved and their individualSO2 source strength was measured (see below). Threemajor SO2 sources, “F0”, “F5AT” and “F11”, are clearlydetectable. The large sources in the centre of Fig. 5(“F5AT” and “F11”) were observed completely and closeinspection shows that they are composed of at least fiveminor SO2 sources.
It is remarkable that the SO2 seems to disappear near thetop of the image (approximately above image row no. 55).Unfortunately, there were no vertical wind profiles mea-sured, but the dominant wind direction during the mea-surement is indicated by vapour sources at the crater edge(Fig. 4a). As shown on the left part of Fig. 4, theinvestigated fumarole group is situated at the lee side ofthe crater edge. The height of the discernible SO2 plumes is40 image pixels (Fig. 5) corresponding to 35 m. At thisaltitude, higher wind speeds than the measured ones couldhave occurred, which could have significantly diluted theplumes.
Fig. 4 a Fumaroles and steam emissions blown in the direction of thewind (black arrows) at the edge of the active crater “La Fossa”, onVulcano Island, Italy. The white frame indicates the approximate fieldof view of the instrument during the measurement. b Integratedradiation intensities of each image pixel in the spectral range 273–385 nm (intensity profile of the IDOAS measurement). The crateredge is clearly visible, black areas left and right are caused by theinstrument casing and are removed in Figs. 5 and 6
Fig. 5 Two-dimensional distri-bution of SO2. 64×105 image-pixel, colour-coded SCD.Different emission sources areclearly separable (black arrowsat the top). Image pixels withfailed evaluation process arebrown coloured. The dashedframe indicates the cutout ana-lyzed in Fig. 7
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SO2 flux calculation of the “F5AT”/“F11” fumarole
The flux ΦSO2 of a gas source (molecules per second) isapproximately the product of the trace gas slant columndensities SSO2 (which includes integration along the lightpath Δy) integrated along the width of the source x, timesthe updraft velocity vz. In the case of discrete image pixel(of a width Δx) SCD values, the integral can be replaced bythe sum.
ΦSO2 ¼ vz �Z
SSO2dx ¼ vzXni
SSO2ð Þi�Δx: ð1Þ
As an example, the flux of the fumarole group “F5AT”/“F11”(Fig. 7) was calculated from the SCD profiles of image rows
16, 26, 36 and 46. The updraft could not be measured andmust therefore be estimated here. The water vapour con-densing in clouds is also emitted from the fumaroles withapproximately the same temperature and updraft velocity.The ascent of these clouds up to a height of 1.7–1.8 mtook—according to visual inspection—approximately 1–1.5 s.From this observation, we estimated an updraft velocity of vz=(1.5± 0.5) m/s. The calculated fluxes, ΦSO2, at the height ofthe image rows 16–46 are equal within the error and arepresented in Table 1.
The calculations presented here were only made for thefumarole group “F5AT”/“F11”; however, the order ofmagnitude of calculated SO2 fluxes is comparable withresults reported elsewhere (Aiuppa et al. 2005). Theseauthors obtained the SO2 flux of the “La Fossa” volcanic
Fig. 6 Colour-coded statisticalerror of the DOAS fitting pro-cedure. Areas of low UV inten-sity are visible, especially theedge of the crater. Image pixelswhere the evaluation processfailed are brown coloured
Fig. 7 Investigated fumarolegroup “F5AT”/“F11” (cutoutfrom Fig. 5). The graph showsSCD along the image rows 16,26, 36 and 46 with statisticalerror of the DOAS fitting pro-cedure. The horizontal extent ofthe emission source is 40 imagepixels, corresponding to an an-gular extend of 10° and a widthof (36.4±5.5) m
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plume traversing with a zenith sky-pointed Ocean OpticsUSB2000 UV spectrograph (Galle et al. 2003). Five vehicle-borne traverses were performed and a flux of 18±3 t/d wascalculated.
Measurements of trace gas distribution in the plumeof Mt. Etna
On May 9th and on May 10th of 2005, SO2 and BrOdistributions were measured in the plume of Mt. Etna.Two different positions of the instrument were chosen tobe able to scan (anti)parallel (May 9th) and perpendicular(May 10th) to the propagation direction of the plume(Fig. 8a). This results in lateral and cross-section views ofthe SO2 and BrO distributions within the volcanic plume(Figs. 9 and 11). Due to the exposure time adaptation(Louban 2005), no usable intensity profile exists for thesemeasurements. Therefore, the allocation of the instru-mental scanning area had to be determined by comparisonwith photographs of the plume taken during the measure-ments (Fig. 8b, c) in order to compare their shape andposition with the evaluated data. Table 2 containstechnical characteristics of the measurements presentedbelow.
Lateral view of Mt. Etna’s plume
Figure 9 presents the results of the SO2 and the BrO dataevaluation. At Mt. Etna, 140 columns were acquired,whereas only the first 68 columns were acquired within thevolcanic plume.
The analysis of the evaluated data permits the assump-tion that the gas emissions of Mt. Etna do not occurcontinuously, but rather that the emission appears to beintermittent. Both evaluations (SO2 and BrO) show localminima of the SCD at the columns 6, 19, 33, 46 and 56(Fig. 9). Furthermore, the BrO distribution shows a closesimilarity to the SO2 distribution; even the aforementionedlocal minima occur at almost the same positions.
Figure 10 shows the DOAS fit errors of the BrO andSO2 DOAS evaluation procedure. Despite the goodinstrumental signal-to-noise ratio (Louban 2005), due tothe low concentration of the trace gas, the relative fit errorof the BrO evaluation can reach up to 40% within theplume. This also explains the noisy appearance in thebackground of the BrO evaluation result (Fig. 9b). Incontrast to that, the relative fit error of the SO2 evaluationis very low due to the high concentration of SO2 in theinvestigated plume.
Table 1 Row-integrated SO2—SCDs and annual SO2 fluxes of the fumarole group ‘F5AT’, ‘F11’
Row no. Row-integrated Σ SCD (molecules/cm) Uncertainty Δ(Σ SCD) (molecules/cm) SO2 flux ΦSO2 (t/d) Uncertainty ΔΦSO2 (t/d)
16 3.4×1018 ±6.8 ×1016 17.2 ±5.8
26 3.5×1018 ±6.2 ×1016 17.6 ±6.3
36 3.9×1018 ±5.8 ×1016 19.6 ±7.1
46 3.1×1018 ±6.3×1016 15.5 ±5.8
Fig. 8 a The IDOAS instrument positions on May 9th and 10th (blackdots), schematically constructed wind directions (orange) and scan-ning angle of the measurement on May 9th (red dashed lines) relativeto the summit of Mt. Etna. Instrument positions were calculated fromGPS coordinates. The angle γ denotes the wind direction for the
respective measurements, α denotes the direction from the crater to theinstrument position (see also Table 2). b Volcanic plume of Mt. Etna atthe time of the measurements: lateral view on May 9th. c Cross-sectionview on May 10th
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Cross-section view of the plume of Mt. Etna
The SO2 and BrO evaluation results are presented inFig. 11. Here, the measurement routine includes acquisi-tion of 100 columns (Table 2). It is obvious that the SO2
and BrO trace gas distributions are very similar. The
“core” of the plume and especially the structure of itscross-section are congruent and clearly visible. Threeparts of the plume are recognisable, corresponding to thenumber of significant active craters of Mt. Etna (Bonaccorso2004). The separation at column no. 57 is hardly presentat the upper edge and only recognisable in the shape ofthe plume core. In columns 70–85, a clearly separatedsecondary plume can be seen. The DOAS fit errorsshown in Fig. 12 are comparable to the data presented inFig. 10.
Investigation of chemical processes using IDOAS results
The IDOAS technique can provide insights into halogenchemistry of volcanic emissions. It is assumed thatdirect volcanic emissions generally contain halides, butprobably no bromine monoxide (Oppenheimer et al.2006; Bobrowski et al. 2007). Therefore, the detectedBrO has to be produced within the plume after emissioninto the atmosphere. Possible reactions forming BrOin the troposphere are listed elsewhere (Platt andHönninger 2002) and could basically occur due tophotochemical processes and oxidation by troposphericozone (O3). Br2 will rapidly be photolysed (on the orderof minutes) to form bromine atoms (McElroy et al. 1986;Molina and Molina 1987; Bobrowski et al. 2007), whichare most likely to react with ozone (typical time constants1 s).
Br þ O3 ! BrOþ O2 ð2Þ
Table 2 Parameters of two plume measurements of Mt. Etna
Trace gas SO2; BrO SO2; BrO
Date 9th May 2005 10th May 2005
Time 13:17 local time 14:04 local time
Total time (min) 43 28
Co-added spectra 10 10
Exposure time (s)
per column
Auto Auto
Number of columns 140 100
Binned detector rows 2 2
Resolution (pixel; v×h) 127×140 127×100
Covered solid angle (v×h) 13.1°×36.4° 13.1°×26°
Solid angle of one
image pixel (v×h)
0.1°×0.26° 0.1°×0.26°
Distance to the
emission source
10 km ≈10 km
Covered area (v×h) (2.23×6.58) km2 –
Pixel size at the object
distance (v×h)
(36.7×45.4) m2 –
Wind direction at
summit (3,340 m)a275°–282° 274°–280°
Wind speed at
summit (3,340 m)a14 kn=7 m/s 29 kn=14.5 m/s
aWind data: Department of Atmospheric Sciences, University ofWyoming 2005
Fig. 9 Colour-coded SCD ofthe SO2 (a) and BrO (b) evalu-ation of the IDOAS data mea-sured on May 9th 2005. Lateralview of the Mt. Etna plume. Themarks on the upper edge indi-cate its structure characteristics
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Bromine atoms are regenerated by photolysis of BrO
BrOþ hn ! Br þ O ð3Þwith a photolysis frequency of 4×10−2 s−1 (in fullsunshine), thus leading to a photo-stationary state be-
tween Br and BrO with Br/BrO of the order of 10−3 aslong as ozone is available. Furthermore, the BrO could berapidly (k≈3×10−11 cm3 per molecule per second) fedinto other important reservoirs for inorganic halogenspecies,
BrOþ H2O ! HOBr þ O2 ð4Þ
Fig. 12 Colour-coded statistical error (ΔSCD) of the DOAS fittingprocedure: a SO2 evaluation, b BrO evaluation
Fig. 11 Colour-coded SCD of the SO2 (a) and BrO (b) evaluation ofthe IDOAS data measured on May 10th 2005. Mt. Etna plume cross-section; the marks on the upper edge indicate structures characteristicof the measured plume cross-section
Fig. 10 Colour-coded statisticalerror (ΔSCD) of the DOAS-fitting procedure: a SO2 evalua-tion, b BrO evaluation
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and can then be recycled through liberation of gaseousbromine species by the following heterogeneous mecha-nism (Vogt et al. 1996).
HOBr þ Br�ð ÞsurfþHþ ! Br2 þ HO2: ð5Þ
This reaction occurs at appreciable rates only at pH<6.5(Fickert et al. 1999). The required H+ are supplied bystrong acids, such as H2SO4, which are abundant involcanic plumes. The halogen molecules (Br2) are rapidlyphotolysed to release the halogen atoms, which againreact rapidly with O3 if available (see above).
In summary, reaction 2, followed by photolysis of Br2, 4and 5, leads to a cycle with the net result:
BrOþ O3 þ Br�ð Þsurfþ Hþð Þsurf!Surface;HOX
2BrO
þ Prod: ð6Þ
Effectively, one BrO molecule is converted into two byoxidizing bromide at particle (aerosol) surfaces. Bromide atthe surface is then replenished from gas phase HBr. Thisprocess leads to exponential growth of the gaseous BrOconcentration, which led to the term “bromine explosion”(Platt and Jannsen 1996; Platt and Lehrer 1997).
BrO/SO2 ratio as indicator for BrO formation processes
The chemical processes discussed above describe a possibleBrO formation mechanism within a volcanic plume. Wewill assume for now that the total amount of SO2, in
contrast to BrO, does not change along the first kilometresof the plume (McGonigle et al. 2004), as there are nosubstantial sinks (or sources). In a recent study (e.g.Rodriguez et al. 2008), possible chemical transformationof SO2, partly into sulfate aerosol, was found. But as theauthors of this article state, this may be a problem of theSO2 measurement at lower altitude volcanoes emitting inthe boundary layer in tropical regions. Mt. Etna is situatedneither in the tropics nor is it a low-altitude volcano. Highrelative humidity, which is a cause of the SO2 loss at thestudied Soufriere Hills volcano in Rodriguez et al. (2008),can only rarely be found at Mt. Etna. Furthermore,Rodriguez et al. (2008) studied only plume ages of 6 to35 min in contrast to our studies, which are based onmeasurements during the first 5 min of the plume.
Therefore, we will use the simultaneously measured SO2
and BrO distributions at Mt. Etna to eliminate dilution andthus to investigate the BrO variation due to chemistry only.
Calculation of BrO/SO2 ratios from different volumeelements of the measured distributions could give informa-tion about the BrO formation processes within theinvestigated plume. In the case of no direct BrO sourcesor sinks, the BrO/SO2 ratios should be constant in allvolume elements of the measured distributions.
Figure 13 shows the BrO SCDs of the image column no.10 (Fig. 9 b) plotted vs. the SO2 SCDs from the samecolumn (Fig. 9a). The BrO/SO2 ratio in these volumeelements of the investigated plume is then defined by theslope of the linear fit.
This procedure, repeated for each image column andimage row of corresponding SO2 and BrO data, gives
Fig. 13 Calculation of the SCDBrO/SCDSO2 ratio of image pixelcolumn 10 from SO2 and BrO distributions presented in Fig. 9; thecalculated slope (red line) corresponds to the BrO/SO2 ratio in this
part of the investigated plume. The correlation coefficient R iscalculated during the fitting procedure for each SCDBrO/SCDSO2 ratiovalue
762 Bull Volcanol (2009) 71:753–765
insight into the variation of the relative BrO concentrationwithin the plume and indicates locations of probable BrOsources or sinks.
Results
In order to investigate possible BrO formation processes,the data acquired on May 9th 2005 (Fig. 9) were analyzedas described above.
Figure 14a shows the BrO/SO2 ratio as a function of theimage column number. Here, the calculated ratios are nearlyconstant between the image columns 0 and 60 (≈4.6×10−4).Unfortunately, this data do not show the increase of theBrO/SO2 ratio in the aging plume observed by Oppen-heimer et al. (2006) and Bobrowski et al. (2007). It couldprobably be masked by the relatively high uncertainty ofthe BrO evaluation and the low plume age, since the part ofthe plume investigated here corresponds to only fiveminutes of propagation (Table 2).
Figure 14b shows the BrO/SO2 ratio as a function of theimage row number. Here, two different areas can bedistinguished:
& Area I: The values at the lower edge of the plume (rows0–31) begin at 4.6×10−4 and decrease to 3.3×10−4. Anincrease up to 5.6×10−4 starts at row 32 and continuesto row 51.
& Area II: A very similar behaviour can be noticedbetween the rows 51 and 120 (higher row numbers lieoutside of the volcanic plume). From row 51 until row
91, the BrO/SO2 ratio decreases to 2.3×10−4. At thehigher row number, the ratio increases again up to 7.8×10−4 (row 119).
Thus, the investigated plume could also be separated intotwo sections:
& Section I includes image columns 0–35. This section ismostly covered by the volcanic plume. The boundariesto the surrounding air are located along image rows100–120 at the sun-faced side and image rows 1–13 atthe lower edge of the plume.
& Section II includes image columns 36–60 and is nearlyhalf-covered. The boundary to the surrounding air at thesun-faced side lies diagonal between the image coor-dinates (column; row) [no. 36; no. 110] and [no. 60; no.30]. There is no clear boundary discernible at the loweredge of the plume due to the scanning geometry.
Apparently, there are interconnections between theincrease of BrO/SO2 ratio (Fig. 14b) and trace gasinteractions with surrounding air and thus ozone (Fig. 9).It is assumed that the plume is initially ozone-free due tothe high temperature of volcanic emissions. In this case, theozone could be transported into the plume by diffusion andturbulent mixing processes with the surrounding air,starting to enrich the ozone in the volcanic plume at itsboundaries. Furthermore, the photochemistry of the BrOformation processes will favour the BrO production at thesun-facing side of the plume. Consequently, the plumegeometry described in “section I” decisively affects theevolution of SCDBrO/SCDSO2 ratio values described in
Fig. 14 Investigation of chemical processes using IDOAS resultsmeasured on May 9th, 2005 as shown in Fig. 9. a Slope (dSCDBrO/dSCDSO2) and correlation coefficient (R) of the BrO vs. SO2
relationship for columns 0–58. Both quantities are calculated for eachcolumn from a correlation plot of dSCDBrO vs. dSCDSO2 as shown inFig. 13 (just for column 10). b Slope (dSCDBrO/dSCDSO2) and
correlation coefficient (R) of the BrO vs. SO2 relationship for rows 0to 120. Both quantities are calculated for each row from a correlationplot of dSCDBrO vs. dSCDSO2 encompassing columns 0 to 60. Bluearrows highlight the variation of the SCDBrO/SCDSO2 ratio across theplume; Roman numerals represent two areas described in the text
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“area II”, and the plume geometry described in “section II”decisively affects the evolution of SCDBrO/SCDSO2 ratiovalues described in “area I”. This can be recognised by thehigher values of the BrO/SO2 ratio at the upper side of thevolcanic plume and at the increase of those around row 32.
Conclusions
Imaging DOAS allows the identification, visualisation andcharacterisation of atmospheric trace gas distributions, interalia of volcanic emission plumes. The relatively simple andcompact instrumentation allows easy transportation to themeasurement sites and working in the field.
The IDOAS data allow straightforward calculation of thetrace gas fluxes at known wind speeds. The gas fluxes of afumarolic field at the island Vulcano, Italy were calculatedat four different heights for the purpose of comparison.
Different trace gas species (BrO and SO2) and theirdistributions were simultaneously detected in volcanicplumes. We present new possibilities to investigate chemicalprocesses within the plumes. Inspecting the variation of theBrO/SO2 ratio along the column and row directions, areas ofenhanced BrO formation are marked by higher ratios. Wepresent examples showing that the BrO/SO2 ratio (and thusthe BrO formation rate) is enhanced at the boundary of theplume to the surrounding air (and thus to undisturbed ozonelevels). Furthermore, higher BrO/SO2 ratios were seen at thesun exposed, upper side of the plume, due to thephotochemistry of the BrO formation processes.
As demonstrated, the IDOAS technique has many usefulapplications for volcanological issues. The cross-sectionand a lateral view of Mt. Etna’s plume could be mappedand investigated. Information about plume morphology(intermittent gas emission patterns), chemical processes(e.g. BrO formation), as well as transport and turbulentmixing processes could be gained from the two-dimensionalhigh-resolved data.
Acknowledgements We like to thank Bo Galle and Tom Wright forreviewing the manuscript and their helpful comments.
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