Geochemistry Geophysics Geosystems 3 Article G Volume 6, Number 8 25 August 2005 Q08008, doi:10.1029/2005GC000965 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society ISSN: 1525-2027 Emission of bromine and iodine from Mount Etna volcano A. Aiuppa Dipartimento CFTA, Università di Palermo, Via Archirafi 36, Palermo 90123, Italy (aiuppa@unipa.it) C. Federico Istituto Nazionale di Geofisica e Vulcanologia – Seizione di Palermo, Via Ugo La Malfa 153, Palermo 90146, Italy A. Franco Dipartimento CFTA, Università di Palermo, Via Archirafi 36, Palermo 90123, Italy G. Giudice, S. Gurrieri, S. Inguaggiato, and M. Liuzzo Istituto Nazionale di Geofisica e Vulcanologia – Seizione di Palermo, Via Ugo La Malfa 153, Palermo 90146, Italy A. J. S. McGonigle Department of Geography, University of Cambridge, Downing Place, CB2 3EN Cambridge, UK Also at Department of Geography, University of Sheffield, Sheffield, UK M. Valenza Dipartimento CFTA, Università di Palermo, Via Archirafi 36, Palermo 90123, Italy [1] Constraining fluxes of volcanic bromine and iodine to the atmosphere is important given the significant role these species play in ozone depletion. However, very few such measurements have been made hitherto, such that global volcanic fluxes are poorly constrained. Here we extend the data set of volcanic Br and I degassing by reporting the first measurements of bromine and iodine emissions from Mount Etna. These data were obtained using filter packs and contemporaneous ultraviolet spectroscopic SO2 flux measurements, resulting in time-averaged emission rates of 0.7 kt yr 1 and 0.01 kt yr 1 for Br and I, respectively, from April to October 2004, from which we estimate global Br and I fluxes of order 13 (range, 3–40) and 0.11 (range, 0.04–6.6) kt yr 1. Observed changes in plume composition highlight the coherent geochemical behavior of HCl, HF, HBr, and HI during magmatic degassing, and strong fractionation of these species with respect to SO2. Components: 4685 words, 4 figures, 2 tables. Keywords: bromine and iodine in volcanic gases; halogen atmospheric chemistry; volcanic degassing; volcanic plumes. Index Terms: 0370 Atmospheric Composition and Structure: Volcanic effects (8409); 8499 Volcanology: General or miscellaneous. Received 9 March 2005; Revised 18 May 2005; Accepted 21 June 2005; Published 25 August 2005. Aiuppa, A., C. Federico, A. Franco, G. Giudice, S. Gurrieri, S. Inguaggiato, M. Liuzzo, A. J. S. McGonigle, and M. Valenza (2005), Emission of bromine and iodine from Mount Etna volcano, Geochem. Geophys. Geosyst., 6, Q08008, doi:10.1029/ 2005GC000965. 1. Introduction [2] Volcanoes are major point source emitters of reactive volatiles to the atmosphere. During the last Copyright 2005 by the American Geophysical Union decades considerable effort has been invested in determining global volcanogenic fluxes of species such as CO2 [Gerlach, 1991; Williams et al., 1992; Hilton et al., 2002], SO2 [Stoiber et al., 1987; 1 of 8 Geochemistry Geophysics Geosystems 3 G aiuppa et al.: mount etna volcano emissions Bluth et al., 1993; Andres and Kasgnoc, 1998], H2S [Berresheim and Jaeschke, 1983; Halmer et al., 2002], HCl and HF [Cadle, 1975; Symonds et al., 1988; Halmer et al., 2002], Hg vapor [Pyle and Mather, 2003], and many trace metals [Nriagu, 1989; Hinkley et al., 1999, Mather et al., 2003]. Relative to these burdens, estimates of total Br and I degassing from volcanoes are considerably less well constrained [Bureau et al., 2000]. Better determining these latter two fluxes is of great importance; while they are very minor constituents of volcanic fluids, Br and I play a significant role in atmospheric chemistry, depleting ozone in both the stratosphere [McElroy et al., 1986; Daniel et al., 1999; Coffey, 1996], and troposphere [von Glasow and Crutzen, 2003]. [3] Estimates of total volcanic Br emission to the atmosphere are typically based upon measurements of bromine concentrations in volcanic gas condensate samples [e.g., Yung et al., 1980; Sugiura et al., 1963]. However, such concentrations are scarce [Gerlach, 2004], they are almost completely exclusive to arc volcanoes, they vary widely through time for individual volcanoes, and from volcano to volcano, producing large uncertainty in computed global Br fluxes (estimates vary between 2.6–78 kt yr 1) [Cadle, 1980; Halmer et al., 2002; Bobrowski et al., 2003]. The importance of volcanic bromine emission has been further emphasized by recent budgets for stratospheric Br inputs from individual explosive eruptions (ranging from 0.1 to >33,000 kt) [Sachs and Harms, 1998; Bureau et al., 2000], which can greatly exceed, on a temporary basis, the total Br loading of the atmosphere from all other natural sources and human activities combined (100 kt/yr [Khalil et al., 1993]). [4] Measurements of I concentrations in volcanic gas samples are even less common than for bromine [Honda et al., 1966; Honda, 1970; Tedesco and Toutain, 1991; Snyder et al., 2002; Snyder and Fehn, 2002]. As for Br, virtually all data correspond to volcanic arc tectonic settings, where volcanic fluids are expected to be enriched in I, due to recycling of marine sedimentary components [Muranatsu and Wedepohl, 1998]. The only estimate of global volcanogenic iodine flux to date (0.2–7.7 kt yr 1) was made by Snyder and Fehn [2002]. [5] It is therefore vital that the meager current data for Br and I degassing are augmented by further bromine and iodine flux measurements, particularly for open-conduit basaltic volcanoes, in order to 10.1029/2005GC000965 better constrain the total global volcanogenic emissions of these species. Here we report the first measurements of Br and I emissions from Mount Etna: one of the most prominent volcanic point source emitters on earth [Allard et al., 1991; Allard, 1997; Francis et al., 1998; Caltabiano et al., 2004]. These data were obtained throughout 2004, using filter packs, in parallel with HCl, HF, and SO2 concentrations; simultaneous ultraviolet spectroscopic SO2 flux determinations allowed quantification of Br and I fluxes from this target. 2. Techniques [6] Our Br and I compositional data were obtained from Mount Etna’s Voragine (VOR) and NorthEast (NEC) craters during periodic surveys throughout 2004. In both cases, data were collected on the rims of the degassing craters  a few hundred meters from the gas sources, in the centre of the dense plumes. The sampled plume ages were 0.5 minute: established by measuring the time lags between individual mild strombolian explosions, and the arrival of plume ‘‘puffs’’ at the measurement sites. In total 52 measurements were made by pumping plume air (at a constant flow rate of 4 l min 1) through ‘‘filter packs’’, each containing four filters, assembled in series. The first filter blocked solid and liquid particles, and the latter three were impregnated with 1M NaHCO3, to trap acidic gaseous volatiles (e.g., SO2, HCl, HF, HBr, and HI) [Aiuppa et al., 2002, 2004a]. [7] Thermodynamical equilibrium calculations indicate that HBr(g) and HI(g) are the primary Br and I species at Etnean magmatic temperatures (T between 1000 and 1100
C; Figure 1). Furthermore, HBr(g) and HI(g) dominate over the other species in high-temperature volcanic gas/air mixtures [Gerlach, 2004]. Thermodynamical computations were not performed for T < 500
C, as kinetic effects are likely to become prevalent below that temperature. However, we assume that Br and I were primarily in the forms of HBr(g) and HI(g), even in the ‘‘cold’’ (a few degrees above ambient air temperature) plume sampled during our measurements, as (1) Br and I concentrations on the particulate filters were negligible and (2) BrO, which should be the secondary Br-bearing gas species (formed through photochemical reactions within the plume), could not be detected spectroscopically at our measurement locations [Bobrowski et al., 2005]. This reflects the short-lived plume sampled in this study, thus mitigating against any significant photochemical oxidation prior to mea2 of 8 Geochemistry Geophysics Geosystems 3 G aiuppa et al.: mount etna volcano emissions 10.1029/2005GC000965 Figure 1. Equilibrium concentrations (c as mole fraction%) of (left) iodine and (right) bromine species versus temperature, for pressure = 1 bar. These data were determined by solving a set of mass balance and mass action relations, using a Gibbs free energy minimization algorithm, with HSC software (v. 2.1). The mass balance equations were parameterized using the H-O-C-S-Cl-I-Br composition of Etna’s magmatic gases, derived from the H2O-CO2SO2 data of Allard et al. [1991], and S/Cl, S/Br, and S/I ratios from this study. HI(g), I(g), and HBr(g) are the prevalent I and Br gas species in Etna’s emissions at magmatic (1050 – 1100
C) temperatures. Simulations were not performed for T < 500
C, because kinetic effects are likely to become important below that temperature. surement. By analogy, it is known that F and Cl are mainly present as HF(g) and HCl(g) in volcanic gases [Symonds et al., 1988]. Therefore, while the base-treated filters only capture the acidic Br and I species (e.g., HBr(g) and HI(g)), we suggest that any underestimations in the bromine measurements, due to the presence of other Br bearing species, are insignificant. Assuming thermodynamic equilibrium, just under 50% of iodine in the plume is I(g) for magmatic temperatures (Figure 1); therefore our HI(g) data could underrepresent total I by a factor of 2, in principle. However, with cooling of the plume the I[g] is converted to stable HI(g) (Figure 1); therefore underestimations are likely to be considerably lower for our data. [8] Br and I concentrations were determined by ICP-MS (Perkin Elmer ELAN-DRC-e), following leaching of the exposed filters using a standard elution procedure [Aiuppa et al., 2002]. An external standard calibration method (+ In as internal standard) was used throughout. In order to ensure similar matrices in blanks, calibration standards and samples, the standards were prepared by spiking blanks, from the filter extraction procedure, with known amounts of Br and I. In plume SO2, HCl and HF concentrations were also determined using these filters, as described by Aiuppa et al. [2002, 2004a]. [9] SO2 flux measurements were made by traversing the plume from below by road on Etna’s eastern flank (distance from source, 6–10 km), and measuring zenith SO2 concentrations with an Ocean Optics Inc. USB2000 spectrometer, of 245– 400 nm spectral range and 0.5 nm spectral resolution, connected to a laptop PC with a USB cable. Data capture, and analysis and instrumental control were achieved using JScripts executed with DOASIS (University of Heidelberg) software. During measurements, spectra were acquired with a time step of 2.5 s. The spectra were evaluated in real time, using a differential optical absorption spectroscopy routine, as described elsewhere [e.g., Galle et al., 2003]. Net SO2 fluxes from the volcano were calculated by combining overhead SO2 concentration measurements with wind speed data (at 3400 m) from twice daily balloon soundings at the Trapani-Birgi meteorological station (http://www.weather.uwyo.edu). 3. Iodine and Bromine Abundance in Etna’s Plume [10] The mean derived bromine and iodine concentrations from our study were 18 ppb and 0.31 ppb for the VOR plume, and 14 ppb and 0.16 ppb, respectively, for the NEC plume (see Table 1). These concentrations greatly exceed (by 3 of 8 Geochemistry Geophysics Geosystems 3 G aiuppa et al.: mount etna volcano emissions Table 1. Gas-Phase Concentrations of Halogens and Sulfur in Etna’s Summit Plume, Determined by in Situ Active Sampling From North-East Crater and Voragine Crater Rimsa Element I, ppb Br, ppb F, ppm Cl, ppm S, ppm NEC 0.16 (0.21) 14 (13) 5.0 (5.1) 11.6 (10.5) 13.6 (11.8) VOR 0.31 (0.3) 18 (17) 1.6 (1) 11.4 (8.5) 35.0 (30.7) a NEC, North-East crater; VOR. Voragine crater. Mean concentrations (±1s) over 52 filter pack measurements between January and October 2004 are reported. up to four orders of magnitude) the concentrations of the major tropospheric Br (CH3Br: 10 ppt [Schauffler et al., 1998]; BrO: 0.5 –2 ppt [von Glasow et al., 2004]) and I (CH3I: 0.2 –5 ppt [Shallcross et al., 2003]; IO: 1 – 10 ppt [von Glasow and Crutzen, 2003]) species. [11] Figure 2 shows temporal trends throughout 2004 in (a) our plume compositional data (S/I, S/Br, S/F and S/Cl ratios) and (b) SO2 fluxes measured by ultraviolet spectroscopy [Galle et al., 2003; McGonigle et al., 2003]. We estimated the random error on the former data points to be ±10%, by finding the standard deviations of results from contemporaneously obtained triplicate filter pack samples; the latter data were obtained either by ourselves, or from http://www.ct.ingv.it. [12] Figures 3a and 3b are plots of chlorine plotted against bromine and iodine concentration, respectively, for our samples. The mean Cl/Br ratios for the VOR and NEC plumes are similar: 1036 and 1017, respectively (Table 2), falling within the range of those previously obtained for volcanic condensate samples (140 – 2128; arc volcano mean = 455; see Figure 3a) [Gerlach, 2004]. The very few Cl/Br data reported to date for non-arc volcanoes (e.g., 1100 and 2000 for Sierra Negra and Kilauea, respectively [Goff and McMurtry, 2000]), along with our results, may indicate higher mean Cl/Br ratios for hot spot and rift volcanism, relative to arc volcanoes. The average Cl/I ratios for VOR and NEC were 5.0
104 and 2.2
105, in good agreement with the few previously obtained condensate data (Cl/I  6.0
104 [Honda, 1970; Snyder et al., 2002]). The mean S/I and S/Br ratios were 1.5
105, and 3102, for the VOR plume, and 2.5
105, and 1344 for the NEC emissions (Table 2). [13] Figures 4a and 4b show S-Cl-F and S-Cl-Br triangular plots, respectively, for our data, in both cases indicating a compositional trend between a S-depleted Cl-F-Br-rich end-member, best repre- 10.1029/2005GC000965 sented by the late August 2004 NEC plume, and a S-rich end-member, typified by the VOR plume during October 2004. The relatively narrow range of Cl/Br and Cl/F ratios implies that this evolving composition was primarily driven by changes in sulfur degassing, rather than inter-halogen fractionation. In particular, we suggest that the decrease in S/halogen ratios and SO2 fluxes before early September are indicative of S depletion from the summit crater’s plumbing system, and that the subsequent increase of S/halogen ratios and SO2 fluxes, in tandem with the onset of an effusive eruption on 7 September, signifies a fresh magmatic intrusion (Figure 2). As sulfur is less soluble in silicate melts than the halogens [Carroll and Webster, 1994], it is preferentially lost during magmatic degassing, therefore gases emitted from fresh magmas have larger SO2 fluxes and S/halogen ratios than those from degassed melts [Symonds et al., 1996; Aiuppa et al., 2002, 2004b; Allard et al., 2004]. Indeed, the vapor-melt distribution coefficients (D) for S and Cl in Etna’s hawaiitic melts (f(O2)  10 8; i.e., +0.3 to +0.6 log units above the NNO buffer at 1200
C [Métrich and Clocchiatti, 1996]) differ by more than a factor of 10 (i.e., DSO2  1000 and DCl  50 at 0.1 MPa) [Aiuppa et Figure 2. (a) Variations in sulfur to halogen molar ratios in Etna’s North-East crater plume between January and October 2004 (plotted points are averages for all measurements made on a particular day). The primary y axis corresponds to the S/I and S/Br ratios; the secondary y axis corresponds to the S/Cl and S/F ratios. (b) SO2 fluxes, measured by differential optical absorption spectroscopy [Galle et al., 2003]. Data were measured by the authors (gray circles) or obtained from http://www.ct.ingv.it (black circles). The gray shaded area indicates an eruption, which commenced on 7 September, consisting of persistent lava effusion from a fracture on Etna’s eastern flank. 4 of 8 Geochemistry Geophysics Geosystems 3 G aiuppa et al.: mount etna volcano emissions 10.1029/2005GC000965 and iodine emission rates from Mount Etna volcano. On 11 occasions during 2004, in situ plume Br and I sampling on the summit was performed simultaneously with determination of SO2 output rates. By combining the SO2 flux data with the filter pack S/Br and S/I ratio data (Figure 2) for each day of measurements, and integrating, we estimate time integrated Br and I fluxes of 0.7 kt yr 1 and 0.01 kt yr 1, respectively, from April to October 2004. The bromine flux is comparable with that recently estimated for Soufrière Hills volcano, Montserrat (0.35 kt yr 1 [Bobrowski et al., 2003]). Likewise, the iodine flux is similar to those previously measured at arc volcanoes (i.e., White Island, New Zealand, 0.007 kt yr 1 [Tedesco and Toutain, 1991]; Satsuma-Iwojima, Japan; 0.012 kt yr 1 [Snyder et al., 2002]; Poás, Costa Rica, 0.0076 kt yr 1 [Snyder and Fehn, 2002]). Mount Etna’s annual Br flux is comparable to the estimates made by Bureau et al. [2000], of the short-term bromine releases from some large Figure 3. (a) Chlorine versus bromine and (b) chlorine versus iodine concentrations for the Voragine (white circles) and the North-East crater (gray circles) plumes, for all the filter pack measurements. The black triangles signify the volcanic condensate and high-temperature volcanic gas data previously obtained [Gerlach, 2004; Honda et al., 1970; Snyder and Fehn, 2002], and the white triangle and cross indicate average meteoric and seawater compositions, respectively [Snyder and Fehn, 2002]. al., 2004b], explaining these observed temporal trends. This is reinforced by the good correspondence between our data, of Figure 4a, and a model curve, describing the compositional evolution from ‘‘early’’ to ‘‘late vapor’’ of the gases exsolved from Etnean melts [Aiuppa et al., 2002]. 4. Bromine and Iodine Fluxes [14] Our measured compositions of the summit crater’s plumes enabled assessment of bromine Table 2. Characteristic Volatile Ratios for Etna’s Summit Plumea S/I NEC VOR a S/Br 5 2.5
10 (2.5
105) 1.5
105 (5.5
104) 1344 (1088) 3102 (486) Cl/I Cl/Br 5 2.2
10 (2.8
105) 5.0
104 (1.9
104) Mean (±1s) molar ratios over 2004 are reported. 1017 (569) 1036 (486) Figure 4. (a) S-Cl-F and (b) S-Cl-Br
300 triangular plots. The model curve [Aiuppa et al., 2002] in Figure 4a describes the evolution of exsolved magmatic gases with time, from ‘‘early’’ to ‘‘late vapors’’ with the transition from fresh magmas to degassed melts. 5 of 8 Geochemistry Geophysics Geosystems 3 G aiuppa et al.: mount etna volcano emissions explosive eruptions (i.e., El Chichón; 0.3 kt), reinforcing the importance of sustained emissions from passively degassing basaltic volcanoes in the total volcanic Br budget. [15] Notwithstanding the significance of our data in augmenting the presently very sparse catalogue of volcanic Br and I emissions, too few targets have been covered to date to enable accurate assessment of the total volcanogenic bromine and iodine release to the atmosphere. Nevertheless, scaling our observed mean Br/S and I/S ratios for Mount Etna by an estimate for global volcanic SO2 flux (14,000 ± 6,000 kt yr 1 [Bluth et al., 1993; Andres and Kasgnoc, 1998; Halmer et al., 2002; Hilton et al., 2002]) provides order of magnitude assessments for global Br and I emissions: 13 (range: 3 – 40), and 0.11 (range: 0.04 – 6.6) kt yr 1, respectively. This scaled mean bromine flux falls near the centre of the range of the few previous estimates (2.6–78 kt yr 1 [Cadle, 1980; Halmer et al., 2002; Bobrowski et al., 2003]) for the global total volcanic burden. In contrast, our mean estimated iodine flux is below the one previous estimate for the overall volcanic output (0.2–7.7 kt yr 1 [Snyder and Fehn, 2002]). However, given that the latter report was based upon measurements of thermal waters and reservoir brines (T < 310
C; Cl/I  5000–7000; compare to 5.0
104 –2.2
105 for our data), we propose our data to be more representative of high-temperature magmatic gases, and therefore of total global volcanic I fluxes. 5. Conclusions [16] One of the primary impediments in accurately constraining global degassing rates for volcanic bromine and iodine, is the virtually uncharacterized contribution from open-conduit persistently degassing basaltic volcanoes. Here we report time-averaged Br and I fluxes (0.7 kt yr 1 and 0.01 kt yr 1) and plume concentrations (SO2, HCl, HF, HBr, and HI), obtained throughout 2004 for one such volcano: Mount Etna, one of the largest point source halogen emitters on Earth. These Br and I data are the first of their kind for Etna; they significantly extend the presently very limited global volcanic bromine degassing data set [Gerlach, 2004], and constitute the first measurement of iodine in the plume of a non-arc volcano. Observed changes in plume chemistry over the measurement period highlight the coherent geochemical behavior of HCl, HF, HBr and HI during the magmatic degassing 10.1029/2005GC000965 process, and substantial fractionation of these species with respect to the more volatile SO2. [17] On the basis of our results, we suggest the total global volcanic degassing budgets of bromine and iodine are of order 13 and 0.11 kt yr 1, respectively. The predominantly inorganic nature of volcanic Br emission is revealed by comparison with a recent estimate for the global methyl bromide source strength from quiescent volcanoes (1
10 3 kt yr 1 [Schwandner et al., 2004]). Our data indicate volcanoes to be significant Br sources regionally, and perhaps globally as well. In comparison, the primary source of bromine to the atmosphere is methyl bromide from the oceans (100 kt yr 1 [Khalil et al., 1993]). In contrast, volcanic I release appears to be a very minor constituent of the global iodine atmospheric budget, dominated by oceanic methyl iodide production (200 kt/yr [Muranatsu and Wedepohl, 1998]). Acknowledgments [18] This paper benefited from constructive reviews from S. Self, D. Pyle, and T. Fischer. R. Favara (INGV-PA) is acknowledged for continuous support. References Aiuppa, A., C. Federico, A. Paonita, G. Pecoraino, and M. Valenza (2002), S, Cl and F degassing as an indicator of volcanic dynamics: The 2001 eruption of Mount Etna, Geophys. Res. Lett., 29(11), 1559, doi:10.1029/2002GL015032. Aiuppa, A., M. Burton, F. Murè, and S. 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