Geochimica et Cosmochimica Acta, Vol. 69, No. 7, pp. 1861–1871, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2004.09.018

H2S fluxes from Mt. Etna, Stromboli, and Vulcano (Italy) and implications for the sulfurbudget at volcanoes

A. AIUPPA,1,* S. INGUAGGIATO,2 A. J. S. MCGONIGLE,3 M. O’DWYER,4 C. OPPENHEIMER,3 M. J. PADGETT,4 D. ROUWET,2 andM. VALENZA1,2

1Dipartimento Chimica e Fisica della Terra ed applicazioni, Università di Palermo, Palermo, Italy2Instituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy

3Department of Geography, University of Cambridge, Cambridge, UK4Department of Physics and Astronomy, University of Glasgow, Glasgow, UK

(Received April 14, 2004; accepted in revised form September 14, 2004)

Abstract—We present here new measurements of sulfur dioxide and hydrogen sulfide emissions fromVulcano, Etna, and Stromboli (Italy), made by direct sampling at vents and by filter pack and ultravioletspectroscopy in downwind plumes. Measurements at the F0 and FA fumaroles on Vulcano yielded SO2/H2Smolar ratios of �0.38 and �1.4, respectively, from which we estimate an H2S flux of 6 to 9 t · d�1 for thesummit crater. For Mt. Etna and Stromboli, we found SO2/H2S molar ratios of �20 and �15, respectively,which combined with SO2 flux measurements, suggest H2S emission rates of 50 to 113 t · d�1 and 4 to 8t · d�1, respectively. We observe that “source” and plume SO2/H2S ratios at Vulcano are similar, suggestingthat hydrogen sulfide is essentially inert on timescales of seconds to minutes. This finding has importantimplications for estimates of volcanic total sulfur budget at volcanoes since most existing measurements do

0016-7037/05 $30.00 � .00

not account for H2S emission. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

It is widely recognized that volcanic sulfur dioxide (SO2)and hydrogen sulfide (H2S) emissions are significant sources ofsulfur release to the atmosphere (Bates et al., 1992; Berner andBerner, 1996). The relative proportions of these two species inthe volatile phase, exsolved from magmas, are primarily con-trolled by temperature (T), pressure (P) and melt redox condi-tions (Giggenbach, 1987; Carroll and Webster, 1994; Scaillet etal., 1998; Moretti et al., 2003):

H2S � 2H2O � SO2 � 3H2, (1)

and

log (SO2 ⁄ H2S) � log K(1,T) � 3RH � log P · XH2O, (2)

where K(1,T) is the corresponding equilibrium constant of re-action (1) at temperature T, RH � log (H2/H2O), and XH2O

isthe mole fraction of water vapor in the gas.

According to Eqn. 2, SO2 is favored over H2S in oxidizedmelts (e.g., low values of RH), with decreasing pressure andincreasing temperature. SO2 is thus the prevalent sulfur speciesin high-temperature volcanic gases released at atmosphericpressure (Gerlach and Nordlie, 1975; Symonds et al., 1994;Giggenbach, 1996). In contrast, H2S dominates in low-temper-ature fumaroles and solfataras, where discharges arise fromdeep hydrothermal systems (Giggenbach, 1980). According toEqn. 1 and Eqn. 2, low SO2/H2S ratios reflect re-equilibrationof magmatic gases within colder, reducing (high RH) hydro-thermal envelopes (Giggenbach, 1987; Symonds et al., 1996;Symonds et al., 2001). Thus, the SO2/H2S ratio is a valuablegeochemical indicator for volcano monitoring.

* Author to whom correspondence should be addressed(aiuppa@unipa.it).

1861

For the last thirty years, volcanic SO2 fluxes have beenmeasured using the Correlation Spectrometer (COSPEC).This ultraviolet (UV) device has had a profound impact onthe development of modern volcanology, enabling assess-ments of total global volcanogenic volatile fluxes to theatmosphere (Stoiber et al., 1983; Stoiber et al., 1987; Sy-monds et al., 1988; Nriagu, 1989; Williams et al., 1992;Halmer et al., 2002; Robock and Oppenheimer, 2003). Cur-rent global volcanic SO2 flux estimates (Bluth et al., 1993;Andres and Kasgnoc, 1998; Halmer et al., 2002) vary from10 to 20 Tg · yr�1, corresponding to �10% of total SO2

emissions (Berner and Berner, 1996). In contrast, the totalvolcanogenic H2S flux (estimated to be 1–37 Tg · yr�1) isonly poorly constrained (Berresheim and Jaeschke, 1983;Halmer et al., 2002). This is partly because simultaneousmeasurements of volcanic SO2 and H2S are rare (Symonds etal., 1994; Halmer et al., 2002), mostly due to analyticaldifficulties inherent in resolving between the sulfur species(Giggenbach and Goguel, 1988; Montegrossi et al., 2001). Incontrast to SO2, there are very few remotely sensed H2Smeasurements, as the gas is much harder to detect spectro-scopically (Radke et al., 1976; Hobbs et al., 1981; Bandy etal., 1982; Hobbs et al., 1982; Jaeschke et al., 1982; McGeeet al., 2001; Toda et al., 2004).

Due to the rarity of these investigations, reaction kinetics forreduced S-bearing compounds in high-temperature volcanicplumes remain poorly constrained (e.g., Allen et al., 2002;Mather et al., 2003). Reaction pathways of H2S oxidation in theatmosphere are complex and may include both homogeneous(gas phase) and heterogeneous (involving the suspended liquid/solid aerosol phase) processes (Seinfeld and Pandis, 1998).Estimated lifetimes for H2S in the troposphere range between4.4 and 70 days for homogeneous reactions (Cox and Sheppard,1980; Warneck, 1988; Seinfeld and Pandis, 1998). Heteroge-

neous H2S reactions rates are faster, and complex functions of

1862 A. Aiuppa et al.

relative humidity, ash content, and the presence of aqueousphase catalysts (Warneck, 1988). Amongst volcanologists, SO2

flux measurements are often taken to represent total volcanicsulfur emissions. However, if the lifetime of volcanic H2S involcanic plumes is longer than frequently assumed, then thetotal S emissions of many volcanoes could be significantlyunderrepresented by current SO2 flux estimates.

This paper presents results from an extensive samplingsurvey carried out in September to October 2003 on thesouthern Italian volcanoes: Vulcano, Etna, and Stromboli(see Fig. 1), aimed at characterizing their emitted SO2/H2Sratios. This was accomplished by integrating contemporane-ous H2S and SO2 measurements by three independent tech-niques: “conventional” direct fumarole sampling by alkalineflasks (Sicardi et al., 1955; Giggenbach, 1975; Chiodini etal., 1993); UV spectroscopy (O’Dwyer et al., 2003); andactive plume sampling by specific sorbent-impregnated fil-ters (Natusch, 1972; Finnegan et al., 1989). The most com-plete dataset was acquired at La Fossa Crater, VulcanoIsland (Fig. 2), where hydrothermal activity is manifested indegassing from a readily accessible 0.045-km2 fumarolicfield (T � 450°C). Vulcano offers an ideal natural laboratoryfor measurement of both fumarolic and downwind (in-plume) H2S and SO2, enabling investigation of the extent towhich in-plume measured SO2/H2S ratios are representativeof vent source signatures. Measurements on Etna and Strom-boli concentrated on the plumes released from the summitcraters. These stronger sources represent a significant frac-tion of the global volcanogenic volatile budget (Allard et al.,1991; Allard et al., 1994; Francis et al., 1998; Gauthier andLe Cloarec, 1998; Allard et al., 2000). Based on our mea-surements, we estimate H2S fluxes for these volcanoes andinvestigate rates of H2S oxidation in volcanic plumes, with

Fig. 1. Quaternary volcanoes of Southern Italy including Stromboli,Etna, and Vulcano, showing the Campanian magmatic province (CMP)and the roman magmatic province (RMP).

implications for total sulfur budgets at volcanoes.

2. METHODS

Fumarole effluents were collected at La Fossa Crater, Vulcano,which has been the subject of several investigations since the start ofthe latest heating and degassing crisis in the late 1970s (Chiodini et al.,1993; Chiodini et al., 1995; Capasso et al., 1997; Nuccio and Paonita,2001). The approach used for direct fumarole sampling was based ona modified version of the Giggenbach (1975) method, involving col-lection of fumarolic gases into evacuated glass bottles containingAgNO3-NH4OH solutions (Sicardi, 1955; Chiodini et al., 1993). TheAg2S precipitate, formed when fumarolic gas was bubbled into theflask, was used for gravimetric determination of H2S concentrations.SO2 concentrations were measured by ion chromatography, after oxi-dation of the AgNO3-NH4OH solution. The data quality was validatedby comparing total sulfur from the AgNO3-NH4OH condensate (St �H2S� SO2) with total sulfur measured from conventional NaOH-filledGiggenbach (1975) flask measurements. The differences between thetwo determinations were generally within 5 to 10%.

Downwind plume measurements were carried out by filter packs atall three volcanoes and by UV spectroscopy at Vulcano and Etna. Filterpack measurements involved pumping plume air at a constant flow rate(4 L · min�1 for 30 min) through a sampler consisting of two filterholders arranged in parallel. The first filter holder contained one par-ticulate and three 1M NaOH-impregnated filters in series (Aiuppa et al.,2002; Aiuppa et al., 2004a), allowing determination of SO2 accordingto a standard procedure (Pate et al., 1963; Johnson et al., 1975;Finnegan et al., 1989). (Note that NaOH-treated filters do not appre-ciably retain H2S, due to its weak Lewis acid nature; Finnegan et al.,1989.) The H2S was efficiently collected with two 1M AgNO3-impreg-nated filters (Natusch et al. 1972) mounted in series in the second filterholder (this sampling procedure is commonly used for ship-bornemeasurements of low-level H2S concentrations in the ocean atmo-sphere; Saltzman and Cooper, 1988; Andreae et al., 1993). Aftercollection, the dark-brown precipitate on the filters was hot-digested inTeflon beakers by a HNO3 (2 vol.% in bidistilled water) -H2O2 mixture,then evaporated until dry. The solid residue was finally eluted inbidistilled water (Sicardi, 1955), and the resulting solution was ana-lyzed for SO4

2� by ion chromatography.Ultraviolet measurements were made with a miniature OceanOptics

Inc. USB-powered spectrometer that has previously been used to mea-sure volcanic SO2 fluxes (Galle et al., 2003; McGonigle et al., 2003).As the UV absorption features of H2S are located around 200 nm, it isnot possible to use UV skylight as a radiation source for H2S detection.We therefore used a deuterium bulb source, as described in O’Dwyeret al. (2003), to access the deeper UV absorption features of H2S andSO2 at 200 to 240 nm. A 50-cm-long optical rail defined the measure-ment path. The lamp and associated collimating lens were mounted atone end of the rail. At the other end, a lens was used to focus the UVlight into an optical fiber that coupled the light into the spectrometer.Each of these components was contained within a tube, with holes in itssides, to allow free circulation of volcanic gases in the optical path. ALabview program was used to acquire, save, and analyze spectra witha time-step of �1 s (individual spectral integration time of 4 ms and100 co-adds per saved spectrum), providing real time H2S and SO2

concentrations. The detection limit of this device for both two-sulfurspecies was �2 ppmv based on fluctuations in retrieved concentrationswhen the instrument monitored clean air samples.

3. RESULTS

3.1. Vulcano

Measurements were made at Vulcano on September 23 and24, 2003, by direct fumarole sampling, in-plume filter packs,and UV spectroscopy. During the sampling, the wind wasblowing toward the SW at an average speed of �2 m · s�1.Direct fumarole sampling focused on two main vents, hence-forth referred to as the F0 and FA fumaroles, within the F0 andFA fumarolic areas, located on La Fossa’s rim and inner craterslope, respectively (see Fig. 2a and b). For each fumarole, three

replicates were collected, to verify data reproducibility. The

n for th

1863H2S fluxes from Mt. Etna, Stromboli and Vulcano Island

results of these direct sampling measurements are reported inTable 1.

La Fossa’s fumarolic emissions arise from mixing of H2O-rich hydrothermal and CO2-He-rich magmatic end members(Chiodini et al., 1993; Chiodini et al., 1995; Nuccio and Pa-onita, 2001). The relative contributions from these two fluidsdefine the chemistry of individual fumaroles. The sampled FA

Fig. 2. Measurements at La Fossa Crater, Vulcano IslaFossa Crater, showing the locations of the FA and F0 fum(FP) measurements downwind from F0; (d) instrumentatiotraverses.

Table 1. Composition of FA and FO fumaroles (in mole %): in eachcase derived by averaging the results from three direct sampling mea-surements.

FA 09/23/03 FO 09/23/03

mol% mol%H2O 91.5 93.3CO2 7.8 6.1Stot 0.32 0.31(H2S) 0.13 0.23(SO2) 0.19 0.09HCl 0.28 0.13N2 0.14 0.098H2 0.017 0.026CO 1.0·10�4 4.1·10�5

He 4.3·10�5 5.9·10�5

HF 3.1·10�5 0.017

CH4 1.0·10�5 4.0·10�6

fumarole has high-temperature (T � 380°C in September 2003)gas discharges with a composition (H2O � 91–94 vol.%; CO2

� 5–8 vol.%) indicative of the magmatic component (Nuccioand Paonita, 2001), whereas F0 has lower temperatures and acomposition reflecting a greater contribution from the hydro-thermal system (270°C; H2O � 93–94 vol.%; CO2 � 5–6.5vol.%). We found the directly sampled total sulfur contents tobe similar for both fumaroles (St from 0.31 to 0.33 vol. %),with SO2/H2S molar ratios of 1.3 to 1.5 and 0.33 to 0.42 for FAand F0, respectively. These SO2/H2S ratios correspond well topreviously reported ranges for fumaroles at La Fossa (e.g., 0.25to 1.3; Chiodini et al., 1993) (all ratios quoted or reported hereare molar). They also serve to highlight the contrasting geo-chemical signatures of the inner crater and rim fumaroles(SO2/H2S �1 and �1, respectively, reflecting the lower tem-peratures of the crater rim vents).

SO2/H2S ratios and concentration data for the in-plumemeasurements are shown in Table 2. The filter pack measure-ments were made in proximity of these vents (Fig. 2c). Eightand three replicates, lasting 30 min each, were taken for the F0and FA fumaroles, respectively. Ultraviolet spectroscopic datawere acquired during walking traverses (McGonigle et al.,2002; Oppenheimer et al., 2004) through the emissions fromthe F0 and FA fumaroles, a few meters downwind (Fig. 2d),

panoramic view of Vulcano Island (small insert) and La; (b) direct sampling (DS) at F0 fumarole; (c) filter packe UV spectroscopic measurements configured for walking

nd: (a)aroles

providing a total of �1400 spectra.

erestim

1864 A. Aiuppa et al.

In Figure 3a, H2S vs. SO2 concentrations are plotted for theUV and filter pack measurements for the F0 vent. The scatter inthe UV spectroscopy measurements about the mean trend-line(characteristic of emissions from the F0 fumarole) arises fromvariable mixing of emissions from F0 with gases originatingfrom other vents, captured by this high time-resolution tech-nique but averaged out by the filter pack measurements. Thein-plume measurements for F0 fumarole indicated SO2/H2Sratios of 0.35 � 0.04 (filter packs) and 0.45 � 0.15 (UVspectroscopy—defined by the gradient of the lines of best fit tothe data in Fig. 3a). Despite the 300-fold dilution of the gas in

Table 2. Summary of results from filter pack (FP) and UV spectroscoSO2/H2S molar ratios, with errors in parentheses.

Method Replicates

La Fossa-Vulcano FP 8La Fossa-Vulcano UVS 865La Fossa-Vulcano FP 3La Fossa-Vulcano UVS 551

The average and standard deviation (1�: in parentheses) of the filtemeasurements, the FA SO2 filters saturated and the FA UVS data werethe resulting SO2/H2S ratios are an underestimate (�30%), and an ov

Fig. 3. (a) Plot of H2S vs. SO2 for the in-plume measurements ofemissions from the F0 fumarole, La Fossa crater, made by filter packsand UV spectroscopy. The black line denotes the (directly sampled)source SO2/H2S ratio of 0.38. Note the close correspondence betweenall three datasets. This points to good intercalibration of the techniques,and suggests that rapid atmospheric oxidation of H2S on timescales ofseconds does not occur; (b) plot of H2S vs. SO2 for the in-plumemeasurements of emissions from the FA fumarole. Discrepancy be-tween direct sampling and plume measurements are ascribed to either

contamination from other fumaroles (UV spectroscopy) or to nonquan-titative sulfur dioxide retention by filter packs.

air at the downwind measurement sites, these in-plume SO2/H2S ratios closely match the directly sampled source signatures(0.38 � 0.07). This finding suggests chemical stability of H2Sand SO2 in the atmosphere, at least on a timescale of seconds,and good intercalibration of the three techniques. Indeed UVspectroscopic measurements even further from the source (upto 20 m, plume age �10 s) show no deviation from the �0.4source SO2/H2S ratio. Further away, the effect of mixing withgases from other fumaroles is too pronounced to permit accu-rate measurements of the emissions of F0.

In-plume measurements for FA fumarole (Fig. 3b) wereaffected far more by emissions from nearby vents than for thedata of F0, where such perturbations were minor. Thus, the FAmeasurements provide limited insights into H2S reaction rates.Nevertheless, the SO2/H2S ratio derived by UV spectroscopy(2.1 � 1) matches the directly sampled value (1.4 � 0.17)within error. The UV spectroscopic data overestimate F0’sSO2/H2S source ratio of a factor �40%, probably because thecontaminating adjacent fumaroles are at higher temperatures(up to 450°C) and emit gas with higher SO2/H2S than the380°C FA fumarole. Unfortunately, and in contrast to the F0measurements, the SO2 filters saturated whilst capturing FA’sSO2 rich emissions, as evidenced by appreciable quantities of Son the backup filter. Thus, the measured H2S/SO2 ratio (0.97� 0.24) is probably underestimated by �30% (Fig. 3b).

3.2. Etna

Figure 4 shows a plot of H2S vs. SO2 concentrations fromfilter packs and UV spectroscopic measurements made near thesummit craters of Mt. Etna. Filter pack samples were collectedon several occasions (5 samples between September 29 and 30and 3 samples on October 22) from the rims of the Voragine(VOR), Bocca Nuova (BN) and North-East (NEC) craters, themain degassing sites at the time (Fig. 5). CO2 concentrations atthe sampling points, measured by infrared spectroscopy, wereup to 140 ppmv above background due to the presence of theplume released from the open craters. The filter pack measure-ments from these locations varied within a rather narrow range:H2S between 0.3 and 1.4 ppmv and SO2 from 6.7 to 34.7 ppmv.The SO2/H2S ratios from each of these craters were similar,ranging from 14 to 25 (average of 20). The UV spectroscopicmeasurements were made by a walking traverse on the summitarea on September 30. These results indicate a wider range ofconcentrations and ratios, with H2S between 0.2 and 31.2ppmv, SO between 0.1 and 19.4 ppmv, corresponding to

S) measurements at La Fossa crater, Vulcano Island, showing derived

role SO2 H2S SO2/H2S

2.7 (0.8) 7.6 (2.3) 0.35 (0.04)0.45 (0.14)

38 (14) 39 (8) 0.97 (0.24)2.1 (1)

oncentrations (ppmv) are also shown. Note that, in contrast to the F0ly affected by emissions from surrounding hotter fumaroles, such that

ate (�40%), respectively.

py (UV

Fuma

F0F0FAFA

r pack cadverse

2

SO2/H2S ratios of 0.04 to 100. Whilst the filter pack results are

1865H2S fluxes from Mt. Etna, Stromboli and Vulcano Island

representative of magmatic crater degassing alone, the spectro-scopic data were acquired over a wider area, and thereforereflect mixing between gases sourced from three end members:diffuse soil degassing, fumaroles, and the main magmatic ventswithin the craters.

Diffuse degassing from Etna’s summit region releases a cold(T �� 100°C) CO2-rich component containing minor He,CH4, and N2 (Giammanco et al., 1998). As the main sulfurspecies in these discharges is hydrogen sulfide (up to 23 ppmvH2S, SO2 �1 ppmv detection limit, based on soil gas measure-ments with specific electrochemical sensors), diffuse degassing(labeled as (a) in Fig. 4) is responsible for the SO2-poor(subdetection limit) gas compositions we found on the flanks ofthe active craters, far from the main plume and fumarole vents.Numerous high-temperature fumaroles are also found at thesummit. Ultraviolet measurements at the fumarolic field bor-dering the eastern rim of BN crater (Fig. 5) indicate high andcomparable H2S and SO2 concentrations (see (b) in Fig. 4).Theoretical lines representing mixing between the three endmembers are shown in Figure 4, demonstrating that such amixing process can account for the heterogeneity of plumecompositions evident in the spectroscopic measurements. Twodistinctive mixing trends are suggested by these data, corre-sponding to the La Voragine and Bocca Nuova crater magmaticcomponents (c= and c in Fig. 4), with different H2S and SO2

concentrations but consistent SO2/H2S ratios (�20).In summary, we propose that a SO2/H2S ratio of �20 is

representative of “passive” magmatic degassing at Etna inSeptember 2003, but that secondary processes such as coolingand re-equilibration, and scrubbing by hydrothermal fluids (Sy-monds et al., 2001), yield lower SO2/H2S ratios for the fu-marolic and soil degassing components. The ratio for the bulkplume will depend on the relative contributions from these

Fig. 4. Plot of H2S vs. SO2 for measurements of Mt. Etna plume. Thefilter pack data have similar SO2/H2S ratios (�20), representative ofmagmatic degassing from within the crater, in contrast to the largevariation in concentrations and SO2/H2S ratios evident in the UVspectroscopy data. The UV measurements were acquired over a widerarea, demonstrating mixing between emissions from (a) diffuse soildegassing (H2S dominant and most evident far from craters and fuma-roles); and (b) rim and flank fumaroles (see Fig. 5) (high and compa-rable H2S and SO2 concentrations) and the magmatic vents in thecraters. Dark grey and light grey lines characterizing this three com-ponent mixing are also shown for the two distinct magmatic compo-nents (of differing concentrations but similar SO2/H2S ratios) corre-sponding to the Voragine (c=) and Bocca Nuova (c) summit craters,respectively.

three components.

3.3. Stromboli

During our survey on Stromboli summit on September 27,wind direction was variable, and the plume intermittently fu-migated the Pizzo Sopra La Fossa, �500 m from the activecraters, where plume sampling is routinely undertaken. Gasconcentrations were below the detection limit for UV spectros-copy. However, both SO2 and H2S were measured in filterpacks that were exposed for 45 min, yielding SO2/H2S ratios of14 to 17, close to the ratio we report for Mt. Etna.

4. DISCUSSION

Very few determinations of hydrogen sulfide abundance involcanic plumes have been reported to date, despite the ubiq-uity of H2S in volcanic emissions, particularly in low-temper-ature hydrothermal manifestations. The first reported plumemeasurement of hydrogen sulfide appears to be that for Mt.Baker by Radke et al. (1976): the gaseous sulfur detected byairborne measurements with a Flame sulfur analyser was as-sumed to be hydrogen sulfide, as direct measurements at thevents revealed this to be the dominant species at source. Bandyet al. (1982) later made the first simultaneous measurement of

Fig. 5. The summit crater area of Mount Etna in September 2003. (a)Map of the active craters: the white curve represents the walkingtraverse track for UV spectroscopy measurements; filled squares are thesites of the filter pack measurements; areas of enhanced fumarolicactivity are identified by the shaded area; (b) photo of the summit area(photograph courtesy of N. Bobrowski). Low-temperature diffuse soildegassing (�) arises from the whole summit area (component a in Fig.

4). Degassing from the main craters: Northeast (NEC), Voragine(VOR), and Bocca Nuova craters (BN1 and BN2) are also shown.

1866 A. Aiuppa et al.

SO2 and H2S in a volcanic plume (by gas chromatography atMt. St. Helens), finding the in-plume SO2/H2S ratio to beessentially constant with distance from source (from �5 km to�25 km), providing evidence for negligible atmospheric reac-tions of the emitted sulfur species on timescales of tens ofminutes. During the same eruption, Hobbs et al. (1981) re-ported significant temporal fluctuations in the plume’s SO2/H2Sratio (from �0.01 to �10), corresponding to changes in vol-canic activity. Hydrogen sulfide emissions from Mt. Etna andVulcano were first measured by Jaeschke et al. (1982). At Etna,a wide scatter of SO2/H2S values (77–26,000) were observed,whereas hydrogen sulfide was the only species measured atVulcano Island. These data (obtained by sampling with impreg-nate filters and analysis by the fluorescence sampling method;Saltzman and Cooper, 1988) were used in a later quantificationof the global volcanogenic H2S flux (Berresheim and Jaeschke,1983). More recently, McGee et al. (2001) estimated H2Semission rates from Mt. Baker using an electrochemical sensor,and Toda et al. (2004) reported the use of a field instrument forsimultaneous determination of reduced (H2S, CH3SH) and ox-idized (SO2) sulfur species at Mt. Aso volcano, Japan.

4.1. The SO2/H2S Ratio: Characterizing Source Signature

Direct sampling of Mt. Etna’s magmatic gases is hamperedby the inaccessibility of the source vents. However, in the pastsome directly sampled chemical and isotope data have beencollected, mostly at eruptive vents and/or lava flows. An esti-mate of Etna’s magmatic gas SO2/H2S ratio at “source” (�10)was given by Huntingdon (1973), based on composition of gassamples (T � 1000°C) collected near the summit crater vents.Gerlach (1979), however, showed these gas analyses wereaffected by considerable non-equilibrium secondary processesdue to poor sampling conditions and proposed “restored” gascompositions with a higher SO2/H2S ratio (�100). A represen-tative SO2/H2S ratio of � 20 (range, 14–23) was recentlyproposed by Aiuppa et al. (2004b) based on a review ofliterature data. This poorly constrained Etna source composi-tion is represented by a vertical black bar in Figure 6 (labeledas E). These high SO2/H2S ratios for Etna (10–100) are con-sistent with data from other divergent-plate and hot spot vol-canoes (such data, taken from compilations by Symonds et al.[1994] and Halmer et al. [2002], are shown in Fig. 6), e.g.,within the 4 to 296 range for Kilauea (mean 60; Gerlach, 1980;Gerlach, 1993). Curves representing thermal equilibria, accord-ing to Eqn. 2, for log K(1,T) relations given in Giggenbach(1996), and redox conditions (RH) fixed by QFM, NNO, FeO-FeO1.5, and HM buffers (Giggenbach, 1987; Giggenbach,1996), are also shown. We infer that our determined Etneanmagmatic SO2/H2S ratio (�20) is close to thermodynamicequilibrium for T � 1000°C, P �10 bar, and redox conditionsbuffered by the basaltic melt around the nickel-nickel oxide(NNO) buffer (Métrich and Clocchiatti, 1996). A similar mag-matic signature is inferred for Stromboli volcano.

Figure 6 also shows SO2/H2S plotted against temperature fordirectly sampled measurements at Vulcano. At La Fossa, Vul-cano, high-temperature fumarole sampling is possible, and wefind the source signature characterized by sulfur contents of�0.3 mol.% and SO /H S molar ratios ranging between 0.38

2 2

and 1.4 (Table 1). The SO2/H2S ratios show a temperature

dependence (Fig. 6), reflecting higher magmatic contributionsin high-temperature, high SO2 inner crater fumaroles (FA;Table 1 and Fig. 3). According to Chiodini et al. (1993) and DiLiberto et al. (2002), H2S and SO2 in fumarolic emissions atVulcano arise from mixing of deep magmatic and shallowhydrothermal components. Sulfur gases in the hydrothermalcomponent are thought to originate via pyrite hydrolysis assaline (marine) brines seep into the depressurized central con-duit, where they either completely (Chiodini et al., 1993) orpartially (Di Liberto et al., 2002) evaporate. Subsequently theyreact with host rock (remobilizing sulfur from pyrite) and mixwith magmatic fluids ascending from depth. In this model,SO2/H2S ratios in fumarolic effluents reflect variable mixingbetween magmatic and hydrothermal end members and their T,P, and redox conditions, estimated to be 950 to 1000°C, 20 to50 MPa with a NNO buffer and 380 to 400°C, 20 MPa with ahematite-magnetite buffer, respectively (Nuccio et al., 1999; DiLiberto et al., 2002) (Fig. 6).

4.2. The SO2/H2S Ratio: Plume vs. Source Signatures

We have shown the consistency of plume and source SO /

Fig. 6. Plot of SO2/H2S vs. temperature for directly sampled mea-surements at Vulcano (large black squares � this study; small blacksquares � Chiodini et al., 1993), and source conditions at Etna(Gerlach, 1979; E � black vertical bar; the white square indicates theaverage SO2/H2S ratio reported in Aiuppa et al., 2004b). For compar-ison, compositions of high-temperature fumaroles at volcanoes in di-vergent-hot spot (grey squares) and convergent (white diamonds) tec-tonic settings are also shown (from Symonds et al., 1994). Solid linesrepresent SO2/H2S ratios computed at thermodynamic equilibrium withrelation (2), using LogK(1) vs. T relations given in Giggenbach (1996),and redox conditions fixed by QFM, NNO, FeO-FeO1.5, and HMbuffers (Giggenbach, 1987; Giggenbach, 1996; pressure is 1 bar exceptwhere specified). According to our proposed SO2/H2S signature of�20, Etna’s magmatic volatiles are close to thermodynamic equilib-rium at T � 1000°C, P � 10 bar and redox conditions at the NNObuffer. La Fossa’s fumarole compositions reflect mixing between deep-magmatic (DMC) and shallow-hydrothermal (SHC) components (DiLiberto et al., 2002). In-plume data (15°C temperature) for Etna (craterdegassing component represented by filter pack results) and for Vul-cano (mean � 1� for filter pack and UV spectroscopy data) are alsoshown: represented by grey triangles and a white circle with a verticalblack bar, respectively.

2

H2S ratios at Vulcano, suggesting either that atmospheric re-

1867H2S fluxes from Mt. Etna, Stromboli and Vulcano Island

actions affecting the sulfur species are negligible on timescalesof seconds, or that any transformations or losses occur atsimilar rates for both species. This is also consistent with recentspectroscopic observations of the H2S-rich Bocca Grande fu-marole (T � 160°C) at Solfatara, Campi Flegrei (O’Dwyer etal., 2003), where SO2 is absent, probably due to scrubbing inthe underlying hydrothermal reservoir(s) (Chiodini et al.,2001). Ultraviolet measurements �20 m downwind of thesource revealed H2S concentration of up to 3.7 ppmv, with SO2

below the 0.2 ppmv detection limit, indicating that no rapid(timescale of seconds) conversion of H2S to SO2 was occur-ring. Despite Etna’s sources composition being poorly con-strained (Gerlach, 1979), our estimated plume SO2/H2S ratio of�20 for Etna’s passive magmatic degassing lies within therange of published high-temperature gas analyses from thevolcano and fits the magmatic gas-phase composition reportedin Aiuppa et al. (2004b) (Fig. 6).

Two main processes may alter source SO2/H2S ratios oncefumarole gases are entrained in the atmosphere. H2S and SO2

can oxidize both in the gas phase (homogeneous reactions) andin the liquid/solid aerosol phase (heterogeneous reactions)(Seinfeld and Pandis, 1998). Pathways of gas phase oxidationreactions are complex and not fully understood; however bothspecies are considered to react mainly with OH radicals (War-neck, 1988). Based on a rate constant (k) of 5 1012

cm3 · molecules-1 · s-1 at 298 K (Cox and Sheppard, 1980) anda typical OH atmospheric concentration of 5 105

molecules · cm-3, a tropospheric lifetime of 4.4 days can becomputed for H2S. As SO2 reactions with OH radicals are evenslower (k � 0.7 1012 cm3 · molecules-1 · s�1; Cox andSheppard, 1980), it is therefore unlikely that gas phase reac-tions can appreciably alter SO2/H2S ratios in volcanic plumeson timescales shorter than hours. It is important to note, how-ever, that kinetics of OH radical reactions in high-temperaturevolcanic gases could be much faster than in ambient air, asobserved in the case of jet engine exhausts (Kärcher et al.,1996).

Oxidation kinetics in the aerosol phase are faster than in thegas phase (Möller, 1980), and reactants include ozone, hydro-gen peroxide, and oxygen (Seinfeld and Pandis, 1998). Oppen-heimer et al. (1998) proposed that SO2-depleting heterogeneousreactions in volcanic plumes can be up to four orders ofmagnitude faster than homogeneous reactions, particularly in-side ash-laden plumes (Eatough et al., 1994). In this case, SO2

oxidation is favored by water droplet condensation on thesurfaces of ash particles (Martin et al., 1986; Oppenheimer etal., 1998) and metal chlorides (Ammann and Burtscher, 1993),with a significant catalyzing action played by dissolved heavymetals. Fast oxidative dissolution of H2S may also take place,notably in air-saturated aqueous solutions (Stumm and Morgan,1996).

The magnitude of liquid/solid phase reactions can be evalu-ated by studying the abundance of particulate sulfur in theplume (as sulfate and sulfuric acid), which is the product ofheterogeneous oxidation reactions. Particulate sulfur accountsfor a minor fraction of total sulfur in emissions from Vulcanoand Etna (respectively, �3%, Aiuppa et al., 2004a; and �1% inSeptember 2003; e.g. Aiuppa et al., 2003), measured with filterpacks at the filter pack monitoring locations described above.

Based on these low-sulfur particulate contents, we infer that

heterogeneous reactions in the plume do not significantly altergas phase SO2/H2S ratios on a timescale of seconds to minutes,even in the unlikely case that all particulate sulfur is formed byoxidation of H2S(g).

In summary, both the similarity of source and plume SO2/H2S ratios and the low-plume particulate sulfur content indicatethat oxidation kinetics of both species are slow on timescales ofseconds, or at least of comparable rates to one another. This isconsistent with H2S and SO2 depletion rates of 2.1 10�4 s�1

and (7.8–10) 10�4 s�1, respectively, measured by Jaeschkeet al. (1982) in Etna’s plume at distances up to 30 km fromsource (plume ages of up to 58 min). However, it is likely thatthese k values are overestimates, given that significantly lowerSO2 loss rates (k of 2.2 10�6 - 4.0 10�5 s�1) were laterfound for Mt. Etna by Martin et al. (1986). The more recentinvestigation used Si and Po210 as tracers for plume dispersion(Martin et al., 1986), as opposed to CO2 in the former case. Dueto high variability of background CO2 concentrations and Et-na’s degassing of CO2 through its flanks (Allard et al., 1991),it is likely that the Jaeschke et al. (1982) study overestimates kfor H2S and SO2.

An alternative reaction path that might modify source SO2/H2S ratios in the vent area is the precipitation of native sulfur,according to the disproportionation reaction (Giggenbach,1987; Oppenheimer, 1992):

2H2S(g) � SO2(g) � 2H2O(g) � 3 ⁄ 2S2(s) (3)

From Eqn. 3, deposition of native sulfur consumes twice asmuch H2S(g) as SO2(g). The deposition of native sulfur, whilebeing unlikely for high-temperature magmatic degassing atopen-conduit volcanoes such as Etna and Stromboli, is com-mon for low-temperature fumaroles and has been documentedat La Fossa Crater since the 1950s (Sicardi, 1955). Accordingto Giggenbach (1987), the degree of saturation with respect todeposition of elemental sulfur can be characterized by plottingXSO2

· X2H2S

/X2H2O

against temperature. Such a plot is shownin Figure 7 for our Vulcano data, along with the theoreticalgas/elemental sulfur equilibrium line, computed assuming idealgas behavior (i.e., ƒi � Xi · PTOT), and thermodynamic datareported in Giggenbach (1987) for the equilibrium constant ofEqn. 3. Figure 7 shows a general undersaturation of La Fossa’sgases, indicating that sulfur deposition would only be expectedfor vent temperatures below �150°C. This is consistent withthe observation of sulfur sublimates only at low-temperaturevents (T � 200°C; Garavelli et al., 1997), and indicates thatEqn. 3 plays only a minor role in perturbing volcanic gaschemistry upon entrainment to the atmosphere.

4.3. H2S Emission Rates from Etna, Stromboli,and Vulcano

Our data suggest that volcanic hydrogen sulfide is relativelyinert on short (seconds to minutes) timescales. This resultaugments the earlier finding by Aiuppa et al. (2004a) thatin-plume and fumarolic S/Cl signatures are similar at La Fossa.As atmospheric reactions are likely to slow as the plume istransported further downwind (with decreasing temperatures,

ash load, and liquid water content), this result calls into ques-

1868 A. Aiuppa et al.

tion the common premise that remotely measured SO2 fluxesare a good proxy for total volcanic S emission.

We use our measured SO2/H2S ratios, coupled with SO2 fluxmeasurements (made by UV spectroscopy) to calculate H2Sfluxes for Etna, Stromboli, and Vulcano, as shown in Table 3(in the table, errors in our determined SO2/H2S ratios, given as� 1�, and in the corresponding H2S fluxes are reported). ForEtna and Stromboli, the magmatic degassing SO2/H2S ratios of20 and 15 were used, yielding H2S fluxes of 50 t · d�1 and 8t · d�1, respectively, based on SO2 emission rates measuredduring September 2003; and 113 and 3.9 t.d�1, respectively,using previously published SO2 flux measurements (Allard etal., 1997; Allard et al., 2000).

This is the first estimate of the H2S flux from Stromboli, andthe Etnean emission rate is considerably higher than that pre-viously reported by Jaeschke et al., (1982) (�5 t · d·1, SO2/H2Sof 77–26,000) due to the lower crater degassing SO2/H2S ratios(�20) we observed using two independent techniques. Notethat due caution must be applied in extrapolating data fromonly a few days of measurements to make longer-term time-averaged assessments. The discrepancy between our SO2/H2S

Fig. 7. Saturation with respect to solid native sulfur (S2) for Vulca-no’s fumaroles. Reaction quotients of Eqn. 3, computed from ourobserved fumarole compositions and those of Chiodini et al. (1993), areplotted against observed temperatures and compared with relativetemperature-dependent equilibrium compositions (grey solid line, com-puted from thermodynamic data in Giggenbach, 1987). La Fossa’semissions are undersaturated at these vent temperatures, and depositionof native sulfur would only be expected for vent temperatures below150°C, indicating that this process plays only a minor role in perturbingvolcanic S emissions.

Table 3. Average SO2/H2S ratios (�1�) and estimated H2S emissionvolcanoes SO2 fluxes and those obtained during September 2003.

SO2/H2Smolar

�SO2, t·d�1

September 2003�H2

Septem

Etna 20 � 4 1980a 50Stromboli 15 � 2 230b 8Vulcano Island 1.3 � 0.3 15c 6

a Caltabiano (2004)b This study, average of 8 helicopter-based traverses with Ocean Opc with Ocean Optics spectrometer.d Allard (1997), time average flux for the last twenty years.e

Allard et al. (2000), time average flux for the last twenty years.f Francis et al. (1995); Edner et al. (1994).

ratio and that reported by Jaeschke et al. (1982) is indicative ofthe higher proportionate contributions to the total volcanicdegassing from the fumarolic and diffuse degassing compo-nents during September 2003, due to the very low level ofcrater degassing at that time (e.g., 1000–2000 t · d�1 total SO2

flux; McGonigle et al., 2003; Catalbiano (2004); comparedwith a time-averaged average emission of 4500 t · d�1 for thelast thirty years; Allard, 1997; Caltabiano et al., 1999; Calta-biano et al., 2001) Bruno et al., 1999; Bruno et al., 2001.

The assessment of H2S emission rates from La Fossa craterrequires an estimate of the respective contributions from rim andinner crater fumaroles, which have different SO2/H2S signatures.Steam flux measurements, performed throughout 2002 using themethod of Italiano et al. (1998), showed that 87% of the totalsteam mass output from the volcano comes from inner craterfumaroles, with the remainder arising from the rim fumaroles.Assuming that our directly sampled F0 and FA data are represen-tative of SO2/H2S ratios for the rim and inner crater emissions,respectively, then the bulk plume has an SO2/H2S molar ratio of1.3. Combined with a recent determination of Vulcano’s SO2

emission rate of 15 t · d�1, this indicates a present H2S flux of �6t · d�1. Slightly higher H2S emission rates (7.3–9.2 t · d�1) areobtained using previous SO2 flux estimations (Edner et al., 1994;Francis et al., 1995). These fluxes are in good agreement with theonly other estimate of Vulcano’s H2S emission rate (�10 t · d�1;Jaeschke et al., 1982). Thus �40% of total sulfur emitted by LaFossa is in a chemical form (H2S) not observable by zenith skyUV spectroscopy.

It is important to note that our studies focus on plumes that areseconds, or in the case of Etna, perhaps minutes old, whilst UVspectroscopic evaluations of SO2 flux are typically made forplumes several tens of minutes old. However, the few otherinvestigations into H2S depletion in volcanic plumes do suggestthat SO2/H2S ratios are stable for plume ages of up to an hour(Bandy et al., 1982; Jaeschke et al., 1982), supporting our conclu-sion that sulfur fluxes may be substantially underestimated atmany passively degassing volcanoes. To test this hypothesis fur-ther, and better understand H2S and SO2 reaction kinetics involcanic plumes, will require new measurements of distal plumes.

5. CONCLUDING REMARKS AND IMPLICATIONS FORTHE SULFUR BUDGET AT VOLCANOES

Hitherto, analytical difficulties and the limited understandingof the chemistry of tropospheric volcanic plumes have hindered

r Etna, Stromboli, and Vulcano, based upon past measurements of the

1

03 �SO2, t·d�1 time average �H2S, t·d�1 time average

4500d 113 � 23110e 4 � 0.5

19–24f 7.3 � 1.6 � 9.2 � 5.5

ectrometer.

rates fo

S, t·d�

ber 20

� 10� 1� 1.4

tics sp

1869H2S fluxes from Mt. Etna, Stromboli and Vulcano Island

evaluation of the significance of hydrogen sulfide in volcanicemissions. The few earlier investigations into rates of H2Sdepletion in volcanic plumes (Bandy et al., 1982; Jaeschke etal., 1982) focused on plumes of up to one hour old. In thesestudies reaction rates were found to be similar to those in thebackground atmosphere, with H2S tropospheric lifetime of theorder of days. To our knowledge, no comparable data havebeen reported up to now on close-to-source dense, warmer,water-rich, and aerosol-rich volcanic plumes, where both ho-mogeneous and heterogeneous reaction rates of S-bearing com-pounds can be expected to be appreciably faster (e.g., Allen etal., 2002; Mather et al., 2003). The work reported in this paperrepresents the first systematic attempt to measure both fu-marolic and downwind (in-plume) H2S and SO2, enablingassessment of the extent to which S-bearing compounds par-ticipate in atmospheric oxidative reactions on timescales ofseconds to minutes. We demonstrate close agreement betweenSO2/H2S ratios representative of source composition and thosefor advected atmospheric plumes, arguing against any rapidchemical transformation of H2S, even in the extreme conditionsof a dense volcanic plume. The finding that such transforma-tions do not occur on timescales of seconds to minutes suggeststhat volcanic H2S may indeed be considerably longer lived.

Results presented here could call into question the commonassumption that SO2 measurements characterize total volcanicS emissions, suggesting that total sulfur emissions of manyvolcanoes have been significantly underestimated. By scalingto representative SO2 fluxes our measured H2S to SO2 ratios inemissions from Mt. Etna, Stromboli, and Vulcano, we revealH2S fluxes of 50 to 113, 3 to 6, and 6 to 9 t · d�1, respectively.This indicates that H2S contribution to the total sulfur budget isrelatively insignificant (�5%) at both Mt. Etna and Stromboli,and probably at the scale of global volcanic S emissions (whereopen-conduit persistently degassing from basaltic volcanoesrepresents a main contribution; Andres and Kasgnoc, 1998). Onthe other hand, we demonstrate that H2S contributes to �40%of total sulfur emitted by La Fossa volcano, implying thatzenith sky UV spectroscopy may underestimate total S emis-sions at many quiescent volcanoes in solfatara activity state,particularly if the proposed H2S stability in aged volcanicplumes (Bandy et al., 1982; Jaeschke et al., 1982) is confirmed.Future field measurements, in particular to examine a widerspan of plume ages than covered here, would be of great valueto advance our understanding of volcanic plume sulfur chem-istry.

Time-averaged SO2 output rates measured by UV spectros-copy have been traditionally used in combination with petro-logic constrains on volatile abundances in the melt, before andafter degassing upon surface emplacement, to estimate magmabudgets at volcanoes (e.g., Allard et al., 1994). In the light ofthe evidences raised in the present paper, we recommend thatfuture investigation take into account the potential H2S contri-bution to total S emissions.

Acknowledgments—The authors wish to thank M. Kusakabe, P. Allard,N. Varley, and an anonymous reviewer for constructive suggestions.

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