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Volcanic emissions of mercury to the atmosphere: global and regional inventories. Comment
Science of the Total Environment 327 (2004) 323–329
Letter to the editor Volcanic emissions of mercury to the atmosphere: global and regional inventories. Comment 1. Introduction In a recent paper, Nriagu and Becker (2003) highlighted the importance of understanding the contribution of volcanoes to the natural atmospheric mercury budget, in view of the wide disparity in estimates of global volcanic Hg emission rates (e.g. Varekamp and Buseck, 1986; Nriagu, 1989; Ferrara et al., 2000). Nriagu and Becker correctly pointed out that, since previous inventories were based on analyses from just a few volcanoes, these prior inventories overlooked the fact that volcanoes display a wide spectrum of activity, are of many different types and widely dispersed geographically. Nriagu and Becker set out to improve on previous emission rate estimates by developing a time-averaged inventory of SO2 and Hg emissions for the volcanoes active between 1980 and 2000, in order to capture the spatial, temporal and chemical variability that arises from the sporadic behaviour of volcanoes. They concluded that the total measured global flux of Hg from volcanoes is ;94 tyyear (112 tyyear after accounting for unmeasured emissions). While their efforts in attempting to develop such a database represent a worthy advance, nonetheless a significant number of errors and ambiguities crept into their analysis, both through errors and omissions in the database, and as a result of neglecting a number of methodological issues that arose from their use of the published data. In addition, Nriagu and Becker used data from the literature collected over a span of 30 years, with vastly differing sampling and analytical protocols, but
they offered no critical assessment of the quality or comparability of these data; nor did they attempt to quantify the uncertainties inherent in their analysis. Together, these factors call into question the reliability and the quoted accuracy (3 significant figures) of both their global and regional inventories. 2. Errors and omissions in the volcanic emissions database Nriagu and Becker (2003) augmented the database of volcanic SO2 emissions compiled by Andres and Kasgnoc (1998) to derive a global Hg emission rate estimate. Several errors and omissions crept in during this process: a For various reasons emissions from several volcanoes were counted twice: – Because of confusion over volcano synonyms (Lascar and Volcan Lascar, Chile; Lonquimay and Volcan Lonquimay, Chile, for example). – Because of the existence of different names for active cones and their surrounding craters (Santiaguito, Guatemala, is the active volcanic dome of Santa Maria Volcano; Satsuma-Iwojima is the active vent of the Kikai caldera, Japan; Kudryavy is the active cone of Medvezhia volcano, Russia). – Emissions from Indonesia were also counted twice, as Nriagu and Becker included in their inventory both the whole volcanic chain (‘Indonesian Arc’, based on work by Nho et al., 1996) and its constituent volcanoes (e.g. Galunggung, Merapi, Slamet, Tengger Caldera, Tangkubanparahu). Nho et al. (1996) calculated the total volcanic SO2 flux from
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Letter to the editor
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Table 1 Compilation of published HgySO2 for volcanic plumes, aerosol and particulates Location
Comment
Bulk plume HgySO2 mass ratio
‘Quiescent’ plumes associated with explosive activity St Helens Plume gas phase St Helens Total plume 9.3=10y5 St Helens Aerosol Popocatepetl Aerosol Popocatepetl Bulk plume 2.5=10y7 –1.5=10y5 Augustine Plume particulates El Chichon Plume particulates High temperature volcanic plumes Etna Plume particulates Etna ‘Hot Vent’ particulates Etna Total plume Etna Mean plume aerosol, central crater Etna Mean plume aerosol, lateral lava vent Stromboli Plume Hgo Kilauea Vent, Halemaumau Erebus Minimum estimates Erebus Filters did not sample gaseous Hg Arenal Unfiltered plume
6.3=10y6
Gas phase HgySO2 mass ratio
Aerosol HgySO2 mass ratio*
Sampling method (Gold trap used, unless otherwise noted)
Ref.
1.4=10y3 )5.2=10y5 5.2=10y5
1 2 2 3 4 5 6
)1.9=10y3 )8=10y5
F F VT F F
8.9=10y5 8=10y5
F F
1=10y5
F
7 7 8 9
4.7=10y5
F
9
2.4=10y6
-1.3=10y7
LIDAR
10 11 12 12
噛
5=10y4 8.8=10y6 4=10y5
F F
6=10y4
A
13 q
References: 1 – Varekamp and Buseck, 1981; 2 – Phelan et al., 1982; 3 – Galindo et al., 1998; 4 – Goff et al., 1998 ; 5 – Lepel et al., 1978q; 6 – Phelan Kotra et al., 1983; 7 – Buat-Menard and Arnold 1978; 8 – Dedeurwaerder et al., 1982; 9 – Aiuppa et al., 2003q; 10 – Edner et al., 1994; 11 – Siegel and Siegel, 1984; 12 – Kyle et al., 1990; 13 – Ballantine et al., 1982. F – particulates (andyor acid gases) collected on filters; VT – passive sampling with a KOH volatile trap. Neither of these techniques quantitatively sample Hgo vapour. LIDAR – Lidar remote sensing of atomic Hgo in the plume. 噛Total Hg measured in nitric acid impingers, compared to gold traps and Millipore filters (see Siegel and Siegel 1987); A – abstract only, so methodology hard to assess. *Particle ‘HgySO2’ ratios are often calculated as equivalents from Hg and SO2y data due to lack of SO2 data. SO2y in the 4 4 particle phase bears no clear and simple relationship to SO2 in the gas phase further undermining the validity of this data. q Not cited in Nriagu and Becker (2003).
all erupting and non-erupting Indonesian volcanoes by extrapolating from measurements made at Galunggung, Merapi and Slamet. b Other volcanoes were inferred to have been active for considerably more of the 20-year period under consideration than was actually the case (Augustine, USA; Mt St Helens, USA; El Chichon, Mexico; Galeras, Colombia), or the length of time for which ‘peak’ emissions were sustained was grossly overestimated (e.g. Rabaul, Papua New Guinea; Ruapehu, New
Zealand), leading to significant overestimates of total SO2 (and Hg) emission. For example, the total amount of SO2 we infer to have been released from Mt St Helens from the information in Nriagu and Becker’s Tables 1 and 2 is 13.2 Mt. The published estimate, and generally accepted value, for the total amount of SO2 emitted from Mt St Helens from 1980–1988 is just 2 Mt (Gerlach and McGee, 1994). c. The separation by Nriagu and Becker of volcanoes into those with ‘explosive’ emissions (their Table 1) and those with continuous or
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Table 2 Compilation of published HgySO2 for fumarolic emissions and condensates Location Volcanic fumaroles Colima Shasta Hood St Helens Etna Kudryavy Vulcano Vulcano (LIDAR) Masaya Momotombo Cerro Negro Poas Kilauea Kilauea: Halemaumau Kilauea: Sulfur bank Kilauea: 1971 fissure, in 1977 Kilauea: 1974 fissure, in 1978
Comment
HgySO2 (equivalent) mass ratio*
Ref.
Fumaroles (150–410 8C) Fumarole, 85 8C Fumarole, 85 8C Fumaroles, 100–750 8C Fumarole, 420 8C Fumarolic condensates, 585–940 8C Fumaroles, 210–341 8C Plume Hgo Fumarolic condensates, 85–100 8C Fumarolic condensates 471–666 8C Fumarolic condensates 170–315 8C Fumarolic condensates, 344–852 8C Fumarolic ‘gas’, 100 8C
2.5–5.2=10y6 3.9=10y5 7.1=10y6 3.2=10y5-5=10y7 2=10y6 0.8–3.6=10y6 1.5=10y7 -1=10y7 2=10y4 4.2=10y5 3.1=10y5 4.6=10y5 1=10y6 4.96=10y4 1.02=10y3 1.31=10y2 3.26=10y2
14 14 14 14 14 15 16 10 17 17 17 17 18 19 19 19 19
References: 14 – Varekamp and Buseck, 1986; 15 – Taran et al., 1995; 16 – Ferrara et al., 2000; 17 – Gemmell, 1987; 18 – Unni et al., 1978; 19 – Siegel and Siegel, 1987q. *‘HgySO2’ ratios calculated from published Hg and S concentrations in the aerosol or particulate phase, expressed as equivalent HgySO2 to facilitate direct comparison with plume and vapour measurements. q Not cited in Nriagu and Becker (2003).
passive degassing (their Table 2) is also the source of potential confusion. Some volcanoes were misclassified as having ‘explosive emissions’ and therefore included in their Table 1, rather than their Table 2 (for example, the degassing lava lake of Erta Ale, Ethiopia; the non-explosive emissions from Ol Doinyo Lengai, Tanzania, and so on). In other cases the SO2 (and Hg) fluxes ascribed to ‘explosive’ and ‘passive’ activity appear to be presented the wrong way round (e.g. Augustine, USA; Colima, Mexico); and other volcanoes were included whose eruptive activity ceased long before 1980 (Heimay, 1973; Mauna Ulu, 1969–1974). d SO2 (and, by implication, Hg) emissions associated with the climactic phases of large explosive eruptions that have been measured by the Total Ozone Mapping Spectrometer (TOMS, Bluth et al., 1993; Carn et al. 2003) were neglected entirely. Key omissions from their SO2 database include eruptions of Alaid, Russia (1 Mt in 1981); El Chichon, Mexico (7 Mt,
1982), Hudson, Chile (4 Mt, 1991); and Pinatubo, Philippines (20 Mt, 1991). Correcting for factors (a) and (b) alone reduces Nriagu and Becker’s estimate of the total annual measured Hg flux (approx. 94 tyyear) by 20% to ;73 tyyear. If the several large eruptions in (d) are included in the analysis, this would contribute an additional time-averaged annual Hg flux of 10– 20 tyyear, if we adopt Nriagu and Becker’s assumed HgySO2 ratios (but see later discussion). Correcting for factors (a) and (b) alone also has significant implications for the regional emissions inventories derived by Nriagu and Becker: we estimate that these corrections would reduce the ‘North American’ flux by 50% from 24 t Hgyyear to ;12 tyyear; and the ‘South American’ flux by 30%, from 29 tyyear to ;19 tyyear. The errors and omissions noted here cast doubt on the validity of Nriagu and Becker’s assessment of the proportionate amount of Hg released by passive volcanic degassing, as opposed to explosive
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Letter to the editor
activity; as well as casting doubt on their breakdown of emissions by geographical region. Given the difficulties associated with the development of volcano by volcano inventories, we suggest that it would be better to concentrate on the development of global or regional inventories of emissions by volcano type (e.g. sporadic emissions compared to continuous emissions), following the lead of Stoiber et al., 1987; Andres and Kasgnoc, 1998; Halmer et al., 2002. We have developed a preliminary analysis elsewhere (Pyle and Mather, 2003), and suggest that the current ‘best estimate’ of the total time-averaged volcanic mercury flux is of the order of 700 tyyear, with uncertainty limits of 80–4000 tyyear. Approximately 10% of this total flux (75 tyyear) is accounted for by continuous emissions from passively degassing volcanoes. There is much uncertainty in the estimates of emissions from large explosive eruptions. 3. Issues surrounding the HgySO2 of volcanic emissions and the global volcanic flux of mercury In addition to errors in their database, there are some methodological issues and practical limitations in Nriagu and Becker’s analysis that should be highlighted. As Nriagu and Becker rightly state, it is the general practice to estimate the flux of many volatile substances from volcanoes by normalising their concentration with respect to SO2. For most metals this involves simultaneous measurement of particulate phase metal and gaseous SO2. However, estimation of volcanic Hg emission rates is more complicated. Hg released from volcanoes may exist as gas phase Hgo, and Hg(II); or as Hg in the particle phase (e.g. Varekamp and Buseck, 1986; Siegel and Siegel, 1987). Since even ash-poor volcanic plumes comprise a mixture of gas, small aerosol particles, and solid particulates, to determine the ‘bulk plume’ HgySO2 ratio requires both gaseous and condensed phases to be adequately characterised. Nriagu and Becker (2003) have assumed that most Hg is in the gaseous phase. By their own admission (p. 4, Nriagu and Becker, 2003), this is an uncertain assumption: at Etna, for example, Dedeurwaerder et al. (1982) estimated that ;62%
of plume Hg was present in the particle phase. While there have been no recent attempts to quantify the partitioning of Hg between the different phases in high-temperature volcanic plumes, it is certainly the case that volcanic aerosol is rich in Hg (e.g. Galindo et al., 1998; Aiuppa et al., 2003). Since a significant proportion of volcanic aerosol comprises species that are in the accumulation mode (e.g. Mather et al., 2003a), the trace metal species associated with volcanic aerosol (which, we suggest, includes particulate Hg) will experience extended atmospheric lifetimes, and may contribute substantially to the global atmospheric particulate mercury burden. There is an urgent need for improved characterisation of volcanic emissions. The choice of HgySO2 mass ratio to use when calculating emission rates is one of the most important determinants of the final value arrived at for the total flux. Nriagu and Becker indicated that they had compiled estimates of the HgySO2 mass ratio in a range of volcanic gases, and condensates, from ‘well over 100 volcanoes’ that were active between 1980 and 2000. (We can only find simultaneous Hg and SO2 measurements at approximately 20 volcanoes in the published literature). They report that they then derived estimates of HgySO2 for each type of volcanic activity. Unfortunately, they neither publish the details of their compilation, nor explain the basis of the values they deduced (or the uncertainties in these values); nor did they show graphically the correlations on which they reportedly based their analysis. In addition, Nriagu and Becker (2003) made no comment on the reliability or comparability of the published data that they used in their compilation. From Nriagu and Becker’s summary tables we infer that they adopted the following HgySO2 mass ratios for different sorts of volcanic emission, by taking the ratio of their reported Hg fluxes to SO2 fluxes: 1.18=10y5 (erupting volcanoes); 1.16=10y5 (continuously degassing volcanoes) and 5.88=10y6 for ash rich plumes. These ratios lie within Nriagu’s earlier (1989) estimated range (10y6 to 2=10y5), but differ substantially from Varekamp and Buseck’s (1986) estimates of 3.7=10y6 for passively degassing volcanoes and
Letter to the editor
10y4 as representative of the ‘higher but varied’ ratios in eruptive plumes. Inspection of published data on Hg and SO2 in volcanic plumes, which we have compiled in Tables 1 and 2 (the majority of which Nriagu and Becker also cite as their sources of data), suggests two problems with Nriagu and Becker’s analysis that the authors did not address: 3.1. Paucity of truly relevant data There are remarkably few estimates of ‘bulk plume’ HgySO2 mass ratios; and those few that do exist (for Etna, Kilauea, Mt St Helens, El Chichon and Erebus) vary by several orders of magnitude (Table 1). For example, measurements of the Mt St Helen’s plume just a few days apart in September 1980 led to estimates of HgySO2 ranging from 1.4=10y3 (Varekamp and Buseck, 1981) to 9=10y5 (Phelan et al., 1982). We note that very few measurements of high-temperature ‘bulk plume’ HgySO2 ratios have been published within the past decade, and without further work it is not possible to establish whether the variability in published measurements reflects methodological problems, or the natural variability between or within individual volcanic systems. The majority of published data on Hg and S in volcanic emanations cited by Nriagu and Becker refer either to Hg in the condensed particle phase, or in fumarolic (geothermal) emissions (Table 2). Nriagu and Becker rightly note that the HgySO2 model is not readily applicable to geothermal sources (p. 9, Nriagu and Becker, 2003). Hence, until it is demonstrated that there is equivalence between the HgySO2 in the high temperature gas that escapes from magma, and the lower temperature fluids sampled in fumaroles, it may be unwise to place too much weight on data from the latter when trying to evaluate the former (see discussion in Mather et al., 2003b): after all, most volcanic SO2 is released from high-temperature magmas (Stoiber et al., 1987; Halmer et al., 2002). Nonetheless, Nriagu and Becker (2003) utilise data from fumarolic emissions in their inventory without critical evaluation (papers summarised in our Table 2). Further, while they state that their analysis assumes all Hg to be in the gaseous phase, their
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cited sources include data on volcanic aerosol and particulates (see our Table 1). More significantly, there are no published measurements of the total HgySO2 ratio in ash-rich eruptive plumes. Measurements of total Hg were published for Mt St Helens, but in the quiescent plume rather than during the main eruptive phase (Varekamp and Buseck, 1981; Phelan et al., 1982). Other measurements from plumes are solely of particulate Hg from dilute eruptive plumes (Lepel et al. 1978; Phelan Kotra et al., 1983; Galindo et al., 1998). We remain puzzled by Nriagu and Becker’s choice of HgySO2 for ash-rich eruptive plumes of 5.88=10y6 (the value we infer from their tables: this is the ratio between their quoted Hg fluxes and their quoted SO2 fluxes at a number of volcanoes), and we suggest that this value is highly uncertain. 3.2. Orders of magnitude variation in all data types In Tables 1 and 2 we compile measurements of HgySO2 from the literature, broken down by type of volcanic emission. All except three of the data sources that we list in our Tables 1 and 2 were cited as data sources by Nriagu and Becker (2003). The main feature to note is that the ratio HgySO2 varies by orders of magnitude within each emission type. It seems unlikely at this stage that there is a simple relationship between eruptive style and Hgy SO2 ratio, but this is certainly an area that needs further investigation. Again, we note the lack of recent data, and suggest that the time is ripe for a concerted effort to characterise volcanic mercury emissions, and their fate in the environment, using state-of-the-art instruments and a consistent sampling protocol. We suggest that the ideal volcanic sampling system for simultaneous measurement of total gaseous mercury, reactive gaseous mercury and particulate mercury might comprise a serial KCl denuder, particle filter and gold trap array coupled to an automated Tekran mercury analyser (cf. ¨ Munthe et al., 2001; Landis et al., 2002; Wangberg et al., 2003). Given the orders of magnitude of variation in the HgySO2 ratios measured in volcanic emanations, we suggest that it is not at this stage possible to derive an estimate of the volcanic mercury flux
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Letter to the editor
that is constrained better than to an order of magnitude. We explore this, along with other evidence that considerable amounts of mercury may be released during large explosive eruptions, elsewhere (Pyle and Mather, 2003). 4. In summary Nriagu and Becker’s intentions, when setting out to inventory volcanic mercury emissions in detail, by separating volcanoes by emission style and geographic location, were laudable and present a conceptual advance in this area. However, a number of errors occurred during the compilation of this database, and some key issues regarding published measurements of volcanic HgySO2 ratios were overlooked or inadequately explained. Both factors serve to compromise the clarity of their final results and conclusions and call into question the accuracy to which they quote their final results (3 significant figures). Nriagu and Becker suggested that previous inventories of volcanic Hg emission ‘be regarded as order-of-magnitude estimates’. We suggest that the sparse and inconsistent volcanic Hg dataset, and uncertainties in our understanding of the nature and fate of Hg in volcanic emissions, means that this statement still holds true. Acknowledgments We thank Mike Goodsite and Uwe Grunewald for comment and discussion. References Aiuppa A, Dongarra` G, Valenza M, Federico C, Pecoraino G. Degassing of trace volatile metals during the 2001 eruption of Etna. In: Robock A, Oppenheimer C, editors. Volcanism and the earth’s atmosphere. Geophys Monogr 2003;139. Andres RJ, Kasgnoc AD. A time averaged inventory of subaerial volcanic sulfur emissions. J Geophys Res 1998;103:25 251 –25 261. Ballantine DS, Finnegan DL, Phelan JM, Zoller WH. Measurement of HgyS ratios from five volcanoes. EOS Trans Am Geophys Union 1982;63:1152. Bluth GJS, Schnetzler CC, Krueger AJ, Walter LS. The contribution of explosive volcanism to global atmospheric sulfur dioxide concentrations. Nature 1993;366:327 –329. Buat-Menard P, Arnold M. The heavy metal chemistry of atmospheric particulate matter emitted by Mount Etna volcano. Geophys Res Lett 1978;5:245 –248.
Carn SA, Krueger AJ, Bluth GJS, Schaefer SJ, Krotkov NA, Watson IM, Datta S. Volcanic eruption detection by the Total Ozone Mapping Spectrometer (TOMS) instruments: a 22year record of sulfur dioxide and ash emissions. In: Oppenheimer C, Pyle DM, Barclay J, editors. Volcanic degassing, vol. 213Geol Soc London Spec Pub, 2003. p. 177 –202. Dedeurwaerder H, Decadt G, Bayens W. Estimations of Hg fluxes emitted by Mount Etna volcano. Bull Volcanol 1982;45:191 –196. Edner H, Ragnarson P, Svanberg S, Wallinder E, Ferrara R, Cioni R, Raco B, Taddeucci G. Total fluxes of sulfur dioxide from the Italian volcanoes Etna, Stromboli and Vulcano measured by differential absorption lidar and passive differential optical absorption spectroscopy. J Geophys Res 1994;99:18 827 –18 838. Ferrara R, Mazzolai B, Lanzillotta E, Nucaro E, Pirrone N. Volcanoes as emission sources of atmospheric mercury in the Mediterranean basin. Sci Total Environ 2000;259:115 – 121. Galindo I, Ivlev LS, Gonzalez A, Ayala R. Airborne measurements of particle and gas emissions from the December 1994–January 1995 eruption of Popocatepetl volcano (Mexico). J Volcanol Geotherm Res 1998;83:197 –217. Gemmell JB. Geochemistry of metallic trace elements in fumarolic condensates from Nicaraguan and Costa Rican volcanoes. J Volcanol Geotherm Res 1987;33:161 –181. Gerlach TM, McGee KA. Total sulfur dioxide emissions and pre-eruption vapor-saturated magma at Mount St Helens, 1980–1988. Geophys Res Lett 1994;2833 –2836. Goff F, Janik CJ, Delgado H, Werner C, Counce D, Stimac JA, Siebe C, Love SP, Williams SN, Fischer T, Johnson L. Geo´ chemical surveillance of magmatic volatiles at Popocatepetl volcano, Mexico. Geol Soc Am Bull 1998;110(6):695 –710. Halmer MM, Schmincke H-U, Graf HF. The annual volcanic gas input into the atmosphere, in particular into the stratosphere: a global data set for the past 100 years. J Volcanol Geotherm Res 2002;115:511 –528. Kyle PR, Meeker M, Finnegan D. Emission rates of sulfur dioxide, trace gases and metals from Mount Erebus, Antarctica. Geophys Res Lett 1990;17:2125 –2128. Landis MS, Stevens RK, Schaedlich F, Prestbo EM. Development and characterisation of an annular denuder methodology for the measurement of divalent inorganic reactive gaseous mercury in ambient air. Environ Sci Technol 2002;36:3000 –3009. Lepel EA, Stefansson KM, Zoller WH. The enrichment of volatile elements in the atmosphere by volcanic activity: Augustine volcano 1976. J Geophys Res 1978;83:6213 –6220. Mather TA, Allen AG, Oppenheimer C, Pyle DM, McGonigle AJS. Size-resolved particle compositions of soluble ions in the tropospheric plume of Masaya volcano, Nicaragua: origins and plume processing. J Atmos Chem 2003a;139. Mather TA, Pyle DM, Oppenheimer C. Tropospheric volcanic aerosol. In: Robock A, Oppenheimer C, editors. Volcanism and the earth’s atmosphere. Geophys Monogr 2003b;139. Munthe J, Wangberg I, Pirrone N, Iverfeldt A, Ferrara R, Ebinghaus R, Feng X, Gardfeldt K, Keeler G, Lanzillotta E,
Letter to the editor Lindberg SE, Lu J, Mamane Y, Prestbo E, Schmolke S, Schroeder WH, Sommar J, Sprovieri F, Stevens RK, Stratton W, Tuncel G, Urba A. Intercomparison of methods for sampling and analysis of atmospheric mercury series. Atmos Environ 2001;35:3007 –3017. Nho E-Y, Le Cloarec MF, Ardouin B, Tjetjep WS. Source strength assessment of volcanic trace elements emitted from the Indonesian arc. J Volcanol Geotherm Res 1996;74:121 – 129. Nriagu J, Becker C. Volcanic emissions of mercury to the atmosphere: global and regional inventories. Sci Total Environ 2003;304:3 –12. Nriagu JO. A global assessment of natural sources of atmospheric trace metals. Nature 1989;338:47 –49. Phelan JM, Finnegan DL, Ballantine DS, Zoller WH. Airborne aerosol measurements in the quiescent plume of Mount St Helens: September, 1980. Geophys Res Lett 1982;9:1093 – 1096. Phelan Kotra J, Finnegan DL, Zoller WH, Hart MA, Moyers JL. El Chichon: composition of plume gases and particles. Science 1983;222:1018 –1021. Pyle DM, Mather TA. The importance of volcanic emissions for the global atmospheric mercury cycle. Atmos Environ 2003;37:5115–5124. Siegel SM, Siegel BZ. First estimate of annual mercury flux at the Kilauea main vent. Nature 1984;309:146 –147.
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Siegel SM, Siegel BZ. Hawaiian volcanoes and the biogeology of mercury. US Geol Surv Prop Pap 1987; 35:827–839. Stoiber RE, Williams SN, Huebert B. Annual contribution of sulfur dioxide to the atmosphere by volcanoes. J Volcanol Geotherm Res 1987;33:1 –8. Taran YA, Hedenquist JW, Korzhinsky MA, Tkachenko SI, Shmulovich KI. Geochemistry of magmatic gases from Kudryavy volcano, Iturup, Kuril Islands. Geochim Cosmochim Acta 1995;59:1749 –1761. Unni C, Fitzgerald WF, Settle D, Gill G, Ray B, Patterson CC, Duce R. The impact of volcanic emissions on the global atmospheric cycles of sulfur, mercury and lead. EOS Trans Am Geophys Union 1978;59:1223. Varekamp JC, Buseck PR. Global mercury flux from volcanic and geothermal sources. Appl Geochem 1986;1:65 –73. Varekamp JC, Buseck PR. Mercury emissions from Mount St Helens during September 1980. Nature 1981;293:555 –556. ¨ Wangberg I, Edner H, Ferrara R, Lanzillotta E, Munthe J, Som¨ mar J, Sjoholm M, Sune Svanberg S, Weibring P. Atmospheric mercury near a chlor-alkali plant in Sweden. Sci Total Environ 2003;304:29 –41. Tamsin A. Mather David M. Pyle Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK E-mail address: [email protected] Received 23 May 2003
Letter to the editor Volcanic emissions of mercury to the atmosphere: global and regional inventories. Comment 1. Introduction In a recent paper, Nriagu and Becker (2003) highlighted the importance of understanding the contribution of volcanoes to the natural atmospheric mercury budget, in view of the wide disparity in estimates of global volcanic Hg emission rates (e.g. Varekamp and Buseck, 1986; Nriagu, 1989; Ferrara et al., 2000). Nriagu and Becker correctly pointed out that, since previous inventories were based on analyses from just a few volcanoes, these prior inventories overlooked the fact that volcanoes display a wide spectrum of activity, are of many different types and widely dispersed geographically. Nriagu and Becker set out to improve on previous emission rate estimates by developing a time-averaged inventory of SO2 and Hg emissions for the volcanoes active between 1980 and 2000, in order to capture the spatial, temporal and chemical variability that arises from the sporadic behaviour of volcanoes. They concluded that the total measured global flux of Hg from volcanoes is ;94 tyyear (112 tyyear after accounting for unmeasured emissions). While their efforts in attempting to develop such a database represent a worthy advance, nonetheless a significant number of errors and ambiguities crept into their analysis, both through errors and omissions in the database, and as a result of neglecting a number of methodological issues that arose from their use of the published data. In addition, Nriagu and Becker used data from the literature collected over a span of 30 years, with vastly differing sampling and analytical protocols, but
they offered no critical assessment of the quality or comparability of these data; nor did they attempt to quantify the uncertainties inherent in their analysis. Together, these factors call into question the reliability and the quoted accuracy (3 significant figures) of both their global and regional inventories. 2. Errors and omissions in the volcanic emissions database Nriagu and Becker (2003) augmented the database of volcanic SO2 emissions compiled by Andres and Kasgnoc (1998) to derive a global Hg emission rate estimate. Several errors and omissions crept in during this process: a For various reasons emissions from several volcanoes were counted twice: – Because of confusion over volcano synonyms (Lascar and Volcan Lascar, Chile; Lonquimay and Volcan Lonquimay, Chile, for example). – Because of the existence of different names for active cones and their surrounding craters (Santiaguito, Guatemala, is the active volcanic dome of Santa Maria Volcano; Satsuma-Iwojima is the active vent of the Kikai caldera, Japan; Kudryavy is the active cone of Medvezhia volcano, Russia). – Emissions from Indonesia were also counted twice, as Nriagu and Becker included in their inventory both the whole volcanic chain (‘Indonesian Arc’, based on work by Nho et al., 1996) and its constituent volcanoes (e.g. Galunggung, Merapi, Slamet, Tengger Caldera, Tangkubanparahu). Nho et al. (1996) calculated the total volcanic SO2 flux from
0048-9697/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2003.08.031
Letter to the editor
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Table 1 Compilation of published HgySO2 for volcanic plumes, aerosol and particulates Location
Comment
Bulk plume HgySO2 mass ratio
‘Quiescent’ plumes associated with explosive activity St Helens Plume gas phase St Helens Total plume 9.3=10y5 St Helens Aerosol Popocatepetl Aerosol Popocatepetl Bulk plume 2.5=10y7 –1.5=10y5 Augustine Plume particulates El Chichon Plume particulates High temperature volcanic plumes Etna Plume particulates Etna ‘Hot Vent’ particulates Etna Total plume Etna Mean plume aerosol, central crater Etna Mean plume aerosol, lateral lava vent Stromboli Plume Hgo Kilauea Vent, Halemaumau Erebus Minimum estimates Erebus Filters did not sample gaseous Hg Arenal Unfiltered plume
6.3=10y6
Gas phase HgySO2 mass ratio
Aerosol HgySO2 mass ratio*
Sampling method (Gold trap used, unless otherwise noted)
Ref.
1.4=10y3 )5.2=10y5 5.2=10y5
1 2 2 3 4 5 6
)1.9=10y3 )8=10y5
F F VT F F
8.9=10y5 8=10y5
F F
1=10y5
F
7 7 8 9
4.7=10y5
F
9
2.4=10y6
-1.3=10y7
LIDAR
10 11 12 12
噛
5=10y4 8.8=10y6 4=10y5
F F
6=10y4
A
13 q
References: 1 – Varekamp and Buseck, 1981; 2 – Phelan et al., 1982; 3 – Galindo et al., 1998; 4 – Goff et al., 1998 ; 5 – Lepel et al., 1978q; 6 – Phelan Kotra et al., 1983; 7 – Buat-Menard and Arnold 1978; 8 – Dedeurwaerder et al., 1982; 9 – Aiuppa et al., 2003q; 10 – Edner et al., 1994; 11 – Siegel and Siegel, 1984; 12 – Kyle et al., 1990; 13 – Ballantine et al., 1982. F – particulates (andyor acid gases) collected on filters; VT – passive sampling with a KOH volatile trap. Neither of these techniques quantitatively sample Hgo vapour. LIDAR – Lidar remote sensing of atomic Hgo in the plume. 噛Total Hg measured in nitric acid impingers, compared to gold traps and Millipore filters (see Siegel and Siegel 1987); A – abstract only, so methodology hard to assess. *Particle ‘HgySO2’ ratios are often calculated as equivalents from Hg and SO2y data due to lack of SO2 data. SO2y in the 4 4 particle phase bears no clear and simple relationship to SO2 in the gas phase further undermining the validity of this data. q Not cited in Nriagu and Becker (2003).
all erupting and non-erupting Indonesian volcanoes by extrapolating from measurements made at Galunggung, Merapi and Slamet. b Other volcanoes were inferred to have been active for considerably more of the 20-year period under consideration than was actually the case (Augustine, USA; Mt St Helens, USA; El Chichon, Mexico; Galeras, Colombia), or the length of time for which ‘peak’ emissions were sustained was grossly overestimated (e.g. Rabaul, Papua New Guinea; Ruapehu, New
Zealand), leading to significant overestimates of total SO2 (and Hg) emission. For example, the total amount of SO2 we infer to have been released from Mt St Helens from the information in Nriagu and Becker’s Tables 1 and 2 is 13.2 Mt. The published estimate, and generally accepted value, for the total amount of SO2 emitted from Mt St Helens from 1980–1988 is just 2 Mt (Gerlach and McGee, 1994). c. The separation by Nriagu and Becker of volcanoes into those with ‘explosive’ emissions (their Table 1) and those with continuous or
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Table 2 Compilation of published HgySO2 for fumarolic emissions and condensates Location Volcanic fumaroles Colima Shasta Hood St Helens Etna Kudryavy Vulcano Vulcano (LIDAR) Masaya Momotombo Cerro Negro Poas Kilauea Kilauea: Halemaumau Kilauea: Sulfur bank Kilauea: 1971 fissure, in 1977 Kilauea: 1974 fissure, in 1978
Comment
HgySO2 (equivalent) mass ratio*
Ref.
Fumaroles (150–410 8C) Fumarole, 85 8C Fumarole, 85 8C Fumaroles, 100–750 8C Fumarole, 420 8C Fumarolic condensates, 585–940 8C Fumaroles, 210–341 8C Plume Hgo Fumarolic condensates, 85–100 8C Fumarolic condensates 471–666 8C Fumarolic condensates 170–315 8C Fumarolic condensates, 344–852 8C Fumarolic ‘gas’, 100 8C
2.5–5.2=10y6 3.9=10y5 7.1=10y6 3.2=10y5-5=10y7 2=10y6 0.8–3.6=10y6 1.5=10y7 -1=10y7 2=10y4 4.2=10y5 3.1=10y5 4.6=10y5 1=10y6 4.96=10y4 1.02=10y3 1.31=10y2 3.26=10y2
14 14 14 14 14 15 16 10 17 17 17 17 18 19 19 19 19
References: 14 – Varekamp and Buseck, 1986; 15 – Taran et al., 1995; 16 – Ferrara et al., 2000; 17 – Gemmell, 1987; 18 – Unni et al., 1978; 19 – Siegel and Siegel, 1987q. *‘HgySO2’ ratios calculated from published Hg and S concentrations in the aerosol or particulate phase, expressed as equivalent HgySO2 to facilitate direct comparison with plume and vapour measurements. q Not cited in Nriagu and Becker (2003).
passive degassing (their Table 2) is also the source of potential confusion. Some volcanoes were misclassified as having ‘explosive emissions’ and therefore included in their Table 1, rather than their Table 2 (for example, the degassing lava lake of Erta Ale, Ethiopia; the non-explosive emissions from Ol Doinyo Lengai, Tanzania, and so on). In other cases the SO2 (and Hg) fluxes ascribed to ‘explosive’ and ‘passive’ activity appear to be presented the wrong way round (e.g. Augustine, USA; Colima, Mexico); and other volcanoes were included whose eruptive activity ceased long before 1980 (Heimay, 1973; Mauna Ulu, 1969–1974). d SO2 (and, by implication, Hg) emissions associated with the climactic phases of large explosive eruptions that have been measured by the Total Ozone Mapping Spectrometer (TOMS, Bluth et al., 1993; Carn et al. 2003) were neglected entirely. Key omissions from their SO2 database include eruptions of Alaid, Russia (1 Mt in 1981); El Chichon, Mexico (7 Mt,
1982), Hudson, Chile (4 Mt, 1991); and Pinatubo, Philippines (20 Mt, 1991). Correcting for factors (a) and (b) alone reduces Nriagu and Becker’s estimate of the total annual measured Hg flux (approx. 94 tyyear) by 20% to ;73 tyyear. If the several large eruptions in (d) are included in the analysis, this would contribute an additional time-averaged annual Hg flux of 10– 20 tyyear, if we adopt Nriagu and Becker’s assumed HgySO2 ratios (but see later discussion). Correcting for factors (a) and (b) alone also has significant implications for the regional emissions inventories derived by Nriagu and Becker: we estimate that these corrections would reduce the ‘North American’ flux by 50% from 24 t Hgyyear to ;12 tyyear; and the ‘South American’ flux by 30%, from 29 tyyear to ;19 tyyear. The errors and omissions noted here cast doubt on the validity of Nriagu and Becker’s assessment of the proportionate amount of Hg released by passive volcanic degassing, as opposed to explosive
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activity; as well as casting doubt on their breakdown of emissions by geographical region. Given the difficulties associated with the development of volcano by volcano inventories, we suggest that it would be better to concentrate on the development of global or regional inventories of emissions by volcano type (e.g. sporadic emissions compared to continuous emissions), following the lead of Stoiber et al., 1987; Andres and Kasgnoc, 1998; Halmer et al., 2002. We have developed a preliminary analysis elsewhere (Pyle and Mather, 2003), and suggest that the current ‘best estimate’ of the total time-averaged volcanic mercury flux is of the order of 700 tyyear, with uncertainty limits of 80–4000 tyyear. Approximately 10% of this total flux (75 tyyear) is accounted for by continuous emissions from passively degassing volcanoes. There is much uncertainty in the estimates of emissions from large explosive eruptions. 3. Issues surrounding the HgySO2 of volcanic emissions and the global volcanic flux of mercury In addition to errors in their database, there are some methodological issues and practical limitations in Nriagu and Becker’s analysis that should be highlighted. As Nriagu and Becker rightly state, it is the general practice to estimate the flux of many volatile substances from volcanoes by normalising their concentration with respect to SO2. For most metals this involves simultaneous measurement of particulate phase metal and gaseous SO2. However, estimation of volcanic Hg emission rates is more complicated. Hg released from volcanoes may exist as gas phase Hgo, and Hg(II); or as Hg in the particle phase (e.g. Varekamp and Buseck, 1986; Siegel and Siegel, 1987). Since even ash-poor volcanic plumes comprise a mixture of gas, small aerosol particles, and solid particulates, to determine the ‘bulk plume’ HgySO2 ratio requires both gaseous and condensed phases to be adequately characterised. Nriagu and Becker (2003) have assumed that most Hg is in the gaseous phase. By their own admission (p. 4, Nriagu and Becker, 2003), this is an uncertain assumption: at Etna, for example, Dedeurwaerder et al. (1982) estimated that ;62%
of plume Hg was present in the particle phase. While there have been no recent attempts to quantify the partitioning of Hg between the different phases in high-temperature volcanic plumes, it is certainly the case that volcanic aerosol is rich in Hg (e.g. Galindo et al., 1998; Aiuppa et al., 2003). Since a significant proportion of volcanic aerosol comprises species that are in the accumulation mode (e.g. Mather et al., 2003a), the trace metal species associated with volcanic aerosol (which, we suggest, includes particulate Hg) will experience extended atmospheric lifetimes, and may contribute substantially to the global atmospheric particulate mercury burden. There is an urgent need for improved characterisation of volcanic emissions. The choice of HgySO2 mass ratio to use when calculating emission rates is one of the most important determinants of the final value arrived at for the total flux. Nriagu and Becker indicated that they had compiled estimates of the HgySO2 mass ratio in a range of volcanic gases, and condensates, from ‘well over 100 volcanoes’ that were active between 1980 and 2000. (We can only find simultaneous Hg and SO2 measurements at approximately 20 volcanoes in the published literature). They report that they then derived estimates of HgySO2 for each type of volcanic activity. Unfortunately, they neither publish the details of their compilation, nor explain the basis of the values they deduced (or the uncertainties in these values); nor did they show graphically the correlations on which they reportedly based their analysis. In addition, Nriagu and Becker (2003) made no comment on the reliability or comparability of the published data that they used in their compilation. From Nriagu and Becker’s summary tables we infer that they adopted the following HgySO2 mass ratios for different sorts of volcanic emission, by taking the ratio of their reported Hg fluxes to SO2 fluxes: 1.18=10y5 (erupting volcanoes); 1.16=10y5 (continuously degassing volcanoes) and 5.88=10y6 for ash rich plumes. These ratios lie within Nriagu’s earlier (1989) estimated range (10y6 to 2=10y5), but differ substantially from Varekamp and Buseck’s (1986) estimates of 3.7=10y6 for passively degassing volcanoes and
Letter to the editor
10y4 as representative of the ‘higher but varied’ ratios in eruptive plumes. Inspection of published data on Hg and SO2 in volcanic plumes, which we have compiled in Tables 1 and 2 (the majority of which Nriagu and Becker also cite as their sources of data), suggests two problems with Nriagu and Becker’s analysis that the authors did not address: 3.1. Paucity of truly relevant data There are remarkably few estimates of ‘bulk plume’ HgySO2 mass ratios; and those few that do exist (for Etna, Kilauea, Mt St Helens, El Chichon and Erebus) vary by several orders of magnitude (Table 1). For example, measurements of the Mt St Helen’s plume just a few days apart in September 1980 led to estimates of HgySO2 ranging from 1.4=10y3 (Varekamp and Buseck, 1981) to 9=10y5 (Phelan et al., 1982). We note that very few measurements of high-temperature ‘bulk plume’ HgySO2 ratios have been published within the past decade, and without further work it is not possible to establish whether the variability in published measurements reflects methodological problems, or the natural variability between or within individual volcanic systems. The majority of published data on Hg and S in volcanic emanations cited by Nriagu and Becker refer either to Hg in the condensed particle phase, or in fumarolic (geothermal) emissions (Table 2). Nriagu and Becker rightly note that the HgySO2 model is not readily applicable to geothermal sources (p. 9, Nriagu and Becker, 2003). Hence, until it is demonstrated that there is equivalence between the HgySO2 in the high temperature gas that escapes from magma, and the lower temperature fluids sampled in fumaroles, it may be unwise to place too much weight on data from the latter when trying to evaluate the former (see discussion in Mather et al., 2003b): after all, most volcanic SO2 is released from high-temperature magmas (Stoiber et al., 1987; Halmer et al., 2002). Nonetheless, Nriagu and Becker (2003) utilise data from fumarolic emissions in their inventory without critical evaluation (papers summarised in our Table 2). Further, while they state that their analysis assumes all Hg to be in the gaseous phase, their
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cited sources include data on volcanic aerosol and particulates (see our Table 1). More significantly, there are no published measurements of the total HgySO2 ratio in ash-rich eruptive plumes. Measurements of total Hg were published for Mt St Helens, but in the quiescent plume rather than during the main eruptive phase (Varekamp and Buseck, 1981; Phelan et al., 1982). Other measurements from plumes are solely of particulate Hg from dilute eruptive plumes (Lepel et al. 1978; Phelan Kotra et al., 1983; Galindo et al., 1998). We remain puzzled by Nriagu and Becker’s choice of HgySO2 for ash-rich eruptive plumes of 5.88=10y6 (the value we infer from their tables: this is the ratio between their quoted Hg fluxes and their quoted SO2 fluxes at a number of volcanoes), and we suggest that this value is highly uncertain. 3.2. Orders of magnitude variation in all data types In Tables 1 and 2 we compile measurements of HgySO2 from the literature, broken down by type of volcanic emission. All except three of the data sources that we list in our Tables 1 and 2 were cited as data sources by Nriagu and Becker (2003). The main feature to note is that the ratio HgySO2 varies by orders of magnitude within each emission type. It seems unlikely at this stage that there is a simple relationship between eruptive style and Hgy SO2 ratio, but this is certainly an area that needs further investigation. Again, we note the lack of recent data, and suggest that the time is ripe for a concerted effort to characterise volcanic mercury emissions, and their fate in the environment, using state-of-the-art instruments and a consistent sampling protocol. We suggest that the ideal volcanic sampling system for simultaneous measurement of total gaseous mercury, reactive gaseous mercury and particulate mercury might comprise a serial KCl denuder, particle filter and gold trap array coupled to an automated Tekran mercury analyser (cf. ¨ Munthe et al., 2001; Landis et al., 2002; Wangberg et al., 2003). Given the orders of magnitude of variation in the HgySO2 ratios measured in volcanic emanations, we suggest that it is not at this stage possible to derive an estimate of the volcanic mercury flux
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that is constrained better than to an order of magnitude. We explore this, along with other evidence that considerable amounts of mercury may be released during large explosive eruptions, elsewhere (Pyle and Mather, 2003). 4. In summary Nriagu and Becker’s intentions, when setting out to inventory volcanic mercury emissions in detail, by separating volcanoes by emission style and geographic location, were laudable and present a conceptual advance in this area. However, a number of errors occurred during the compilation of this database, and some key issues regarding published measurements of volcanic HgySO2 ratios were overlooked or inadequately explained. Both factors serve to compromise the clarity of their final results and conclusions and call into question the accuracy to which they quote their final results (3 significant figures). Nriagu and Becker suggested that previous inventories of volcanic Hg emission ‘be regarded as order-of-magnitude estimates’. We suggest that the sparse and inconsistent volcanic Hg dataset, and uncertainties in our understanding of the nature and fate of Hg in volcanic emissions, means that this statement still holds true. Acknowledgments We thank Mike Goodsite and Uwe Grunewald for comment and discussion. References Aiuppa A, Dongarra` G, Valenza M, Federico C, Pecoraino G. Degassing of trace volatile metals during the 2001 eruption of Etna. In: Robock A, Oppenheimer C, editors. Volcanism and the earth’s atmosphere. Geophys Monogr 2003;139. Andres RJ, Kasgnoc AD. A time averaged inventory of subaerial volcanic sulfur emissions. J Geophys Res 1998;103:25 251 –25 261. Ballantine DS, Finnegan DL, Phelan JM, Zoller WH. Measurement of HgyS ratios from five volcanoes. EOS Trans Am Geophys Union 1982;63:1152. Bluth GJS, Schnetzler CC, Krueger AJ, Walter LS. The contribution of explosive volcanism to global atmospheric sulfur dioxide concentrations. Nature 1993;366:327 –329. Buat-Menard P, Arnold M. The heavy metal chemistry of atmospheric particulate matter emitted by Mount Etna volcano. Geophys Res Lett 1978;5:245 –248.
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Siegel SM, Siegel BZ. Hawaiian volcanoes and the biogeology of mercury. US Geol Surv Prop Pap 1987; 35:827–839. Stoiber RE, Williams SN, Huebert B. Annual contribution of sulfur dioxide to the atmosphere by volcanoes. J Volcanol Geotherm Res 1987;33:1 –8. Taran YA, Hedenquist JW, Korzhinsky MA, Tkachenko SI, Shmulovich KI. Geochemistry of magmatic gases from Kudryavy volcano, Iturup, Kuril Islands. Geochim Cosmochim Acta 1995;59:1749 –1761. Unni C, Fitzgerald WF, Settle D, Gill G, Ray B, Patterson CC, Duce R. The impact of volcanic emissions on the global atmospheric cycles of sulfur, mercury and lead. EOS Trans Am Geophys Union 1978;59:1223. Varekamp JC, Buseck PR. Global mercury flux from volcanic and geothermal sources. Appl Geochem 1986;1:65 –73. Varekamp JC, Buseck PR. Mercury emissions from Mount St Helens during September 1980. Nature 1981;293:555 –556. ¨ Wangberg I, Edner H, Ferrara R, Lanzillotta E, Munthe J, Som¨ mar J, Sjoholm M, Sune Svanberg S, Weibring P. Atmospheric mercury near a chlor-alkali plant in Sweden. Sci Total Environ 2003;304:29 –41. Tamsin A. Mather David M. Pyle Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK E-mail address: [email protected] Received 23 May 2003