Original source: Andres RJ, Kasgnoc AD (1998) A time-averaged inventory of subaerial volcanic sulfur emissions. Journal of Geophysical Research 103: 25,251-25,261. Copyright 1998 by the American Geophysical Union. Further electronic distribution is not allowed.

A TIME-AVERAGED INVENTORY OF SUBAERIAL VOLCANIC SULFUR EMISSIONS
RJ Andres and AD Kasgnoc

Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775-5910 USA

ABSTRACT

A time-averaged inventory of subaerial volcanic sulfur (S) emissions was compiled primarily for the use of global S and sulfate modelers. This inventory relies upon the 25-year history of S, primarily sulfur dioxide (SO₂), measurements at volcanoes. Subaerial volcanic SO₂ emissions indicate a 13 Tg/a SO₂ time-averaged flux, based upon an early 1970's to 1997 time frame. When considering other S species present in volcanic emissions, a time-averaged inventory of subaerial volcanic S fluxes is 10.4 Tg/a S. These time-averaged fluxes are conservative minimum fluxes since they rely upon actual measurements. The temporal, spatial and chemical inhomogeneities inherent to this system gave higher S fluxes in specific years. Despite its relatively small proportion in the atmospheric S cycle, the temporal and spatial distribution of volcanic S emissions provide disproportionate effects at local, regional and global scales. This work contributes to the Global Emissions Inventory Activity.

INTRODUCTION

Volcanoes display a wide activity spectrum from continuous to ephemeral emission events. An average of 56 volcanoes erupted each year from 1975 to 1985. While some showed continuous activity, others erupted less frequently or only once, so that 158 volcanoes actually erupted over this time period. This number increases to 380 volcanoes with known eruptions this century, 534 volcanoes with eruptions in historical times, and more than 1500 volcanoes with documented eruptions in the last 10,000 years [McClelland et al., 1989].

Volcanoes emit many compounds containing sulfur (S). The primary S-containing gases emitted are sulfur dioxide (SO₂) and hydrogen sulfide (H₂S). The other S-containing species usually occur in far smaller quantities than SO₂ and H₂S. The distribution between these various species differs not only between volcanoes, but also during a single volcanic episode at one volcano. Likewise, the flux, or S mass emitted per unit time, varies for one volcano and between volcanoes.

Like many anthropogenic S emissions, volcanoes can be represented as point sources for S venting to the atmosphere. However, many volcanoes occur in areas where anthropogenic point sources are not so significant. In addition, volcanoes can loft S high into the troposphere and stratosphere where the S residence times increase significantly.

This paper presents a time-averaged inventory of subaerial (directly venting to the atmosphere) volcanic S emissions. Primarily for the use of global S and sulfate modelers, it incorporates the temporal, chemical and spatial inhomogeneities inherent to the subject which are necessary for more accurate regional and global atmospheric chemistry transport models [e.g., Chuang et al., 1997; Weisenstein et al., 1997; Chin et al., 1996; Feichter et al., 1996; Pham et al., 1995]. The secondary purposes of this inventory are to compile the existing SO₂ data and facilitate the creation of other emission inventories for volcanic species amenable to specie/S ratios [e.g., Andres et al., 1993; Nriagu, 1989, Symonds et al., 1988; Lantzy and Mackenzie, 1979].

This work contributes to the Global Emissions Inventory Activity (GEIA). GEIA is part of the International Global Atmospheric Chemistry (IGAC) Project which, in turn, is part of the International Geosphere-Biosphere Programme (IGBP). The goal of GEIA is to create, maintain and distribute reliable inventories of species important for understanding global atmospheric chemistry at a one degree global scale [Pacyna and Graedel, 1995]. More information about GEIA, its products, and the inventory presented here can be accessed at http://www.geiacenter.org/.

METHODS

Volcanic Locations and Elevations

Figure 1 shows the locations of 218 subaerial volcanoes that erupted at least once since 1 January 1964. Non-emergent, suboceanic volcanoes are not displayed in white cells as the S emitted from these volcanoes is assumed to be dissolved into the overlying water column and thus not emitted directly into the atmosphere. Subglacial and sublacustrine volcanoes are included here because prolonged or explosive activity could quickly allow these volcanoes to emit S directly into the atmosphere through removal of the overlying water column. Mild activity at these subglacial and sublacustrine volcanoes does not usually release S directly into the atmosphere. For example, Agustsdottir and Brantley [1994] estimate Grķmsvötn Volcano, Iceland, released 5300 Mg/a S into the overlying glacial-covered lake. Jökulhlaups (large outburst events) occasionally release the water in the lake. Such fluxes are excluded from this inventory. Likewise, S fluxes from volcanoes which developed extensive hydrothermal systems are omitted. This is due to the efficient scrubbing of magmatic S from these systems by the hydrothermal system, the predominance of S existing as H₂S in the systems, and a lack of reliable means to measure the H₂S flux to the atmosphere. When volcanic activity begins to dominate these hydrothermal systems again, many have their S fluxes measured, and those data are included here.

The volcanoes plotted in Figure 1 were taken from Smithsonian [1997]. This source contains volcano name, latitude, longitude, elevation and some eruptive parameters. This source was augmented four times to account for volcanic features not originally incorporated but made necessary by the following S flux data compilation. All four augmentations relate to the various vents and craters of Kilauea Volcano, Hawaii.

Volcanic S Fluxes - Measured

Volcanic S fluxes can be measured many ways and the correlation spectrometer (COSPEC) method is the most commonly used [Andres and Rose, 1995; Stoiber et al., 1983]. COSPEC was first used for such measurements in the early 1970's at Mount Mihara (Izu-Oshima), Japan [Okita, 1971]. However, COSPEC measures SO₂ emissions only. SO₂ is also the most abundant S species emitted by most volcanoes. COSPEC is useful under many field and volcanic conditions, but is used most routinely under quiet to mildly explosive conditions. COSPEC is not routinely used in the largest volcanic eruptions due to logistical and instrumental limitations. However, large eruptions can sometimes be monitored by satellite methods, especially with the Total Ozone Mapping Spectrometer (TOMS) [Krueger et al., 1995]. The first TOMS measurements of volcanic emissions occurred in the late 1970's. Like COSPEC, TOMS measures SO₂emissions only.

Since fluxes incorporate time, it is important to note the time interval over which measurements are made. An individual COSPEC measurement can last seconds to hours, depending upon field logistics. With COSPEC, plume speed is used to calculate the SO₂ flux and the results are usually reported after extrapolating to one day. TOMS measurements usually take only seconds (assuming the entire plume lies in one satellite pass). Like COSPEC, TOMS essentially measures the SO₂ burden (column abundance in each field of view). Unlike COSPEC however, TOMS data do not routinely incorporate plume speed. Therefore, TOMS data are reported commonly in mass, not mass per time units. Fluxes can be calculated from TOMS data if the time interval elapsed to vent the volcanic plume is measured. However, this time interval is not always well constrained.

The accuracy of COSPEC measurements is generally 10 to 40% [Stoiber et al., 1983]. Field and instrumental conditions, as well as operator ability, can lower or raise the error reported. The accuracy of TOMS measurements is generally 10 to 30% [Krueger et al., 1995]. The presence of volcanic ash and sulfate aerosols, as well as background corrections, are principal error components. Unfortunately, directly comparing the output of these two instruments is difficult. Since COSPEC better suits smaller eruptions and TOMS larger eruptions, few volcanic plumes occur which provide an appropriate opportunity. The differing fields of view and deployment conditions further complicate direct comparisons. A dedicated campaign at Popocatepetl, Mexico gave good agreement, within respective error limits, between COSPEC and TOMS measurements [S. Schaefer, personal communication].

To date, there is no widely used method to directly measure the flux of other volcanic S species. Fluxes for other species can be calculated by directly sampling the volcanic plume, measuring the ratio of the S species of interest to SO₂, and multiplying that ratio by an SO₂ flux determined independently [e.g., Andres et al., 1993; Symonds et al., 1992]. Alternatively, some researchers collect samples by flying aircraft equipped with online instruments through the plume. These point measurements are then plotted, plume cross-sectional concentrations integrated, and flux calculated using a measured wind speed [e.g., Radke, 1982].

An extensive compilation of the available, measured volcanic S fluxes has been conducted. These data were collected from 214 published references, personal communications, and three volcanological conference presentations. Two to three times more references were searched. The conference presentations, as well as two electronic mail messages to the VOLCANO listserv, allowed many opportunities for inventory data discussion with volcanologists and atmospheric scientists.

Due to the large number of potential sources for volcanic S emissions and the limited number of measurements at these sources over time, volcanoes were grouped into two major activity categories: continuously and sporadically erupting. Continuously erupting volcanoes exhibit relatively constant activity since the early 1970's, when measurements began. Many of these volcanoes exhibit persistent hawaiian, strombolian or vulcanian activity. The flux given in the database tries to incorporate the naturally variable fluxes of these volcanoes by averaging all measurements taken. This assumes that measurements are in proportion to the flux levels which accompany their states of quiescent and eruptive activity. For three sites, however, personal communications supplanted the average. These personal communications relied upon published and unpublished data for Etna, Kilauea and Kilauea East Rift Zone.

Sporadically erupting volcanoes exhibit distinct increases in activity over this time frame. The flux given in the database is the maximum flux measured. Unmeasured higher fluxes may have occurred, and obviously lower fluxes were possible.

S flux data for 72 volcanoes have been accumulated. More than 21,000 individual measurements were tallied. Five volcanoes account for more than 17,000 of these measurements, and some volcanoes have only one reported measurement. This tally excludes TOMS measurements, which will be discussed later. Fifty-one of these 72 volcanoes erupted continuously over the past 25 years. Twenty-five erupted sporadically. Five volcanoes appear on both lists because their relatively constant degassing natures were punctuated by more intense activity of short duration. These volcanoes include Aso, Augustine, Kilauea East Rift Zone, Mayon and San Cristobal (Nicaragua). This leaves 1442 volcanoes in the Smithsonian database for which S flux data do not exist (excluding TOMS data). Of course, most of these volcanoes were inactive over the period during which S flux measurements were made. A list of data sources, by volcano, is available on the GEIA web site.

GEIA Inventory

Due to the nature of the available data and the volcanic source inhomogeneities, the GEIA inventory presented here is time-averaged. If the inventory was constructed for the benchmark GEIA year of 1990, then there would be too few data for a reliable extrapolation. Additionally, if the focus is long-term volcanic source modeling, then a multi-year source average will produce more accurate results than relying on one index year.

DATA

Table 1 lists 49 volcanoes which erupted SO₂ continuously over the last 25 years. Not included in Table 1 are Mounts Mageik and Baker, USA, which had measured continuous H₂S fluxes, but not SO₂. The sum of the SO₂fluxes is approximately 26,200 Mg/d. Figure 2 plots these fluxes on a map similar to Figure 1.

Table 2 lists 25 volcanoes which erupted SO₂ sporadically over the last 25 years. The sum of the SO₂ fluxes is approximately 278,000 Mg/d. Figure 3 plots these fluxes on a map similar to Figure 1. TOMS data are discussed later.

Flux measurements of other S species are reported much less frequently. Table 3 summarizes these measurements. Fluxes are given as a ratio to simultaneously determined SO₂ fluxes so that a total volcanic S flux can be determined later. See the original references for actual flux values.

DISCUSSION

Global SO₂ Fluxes

The average flux for the 25 sporadically emitting volcanoes is 11,000 Mg/d SO₂. Using the McClelland et al. [1989] average of 56 erupting volcanoes per year and the 49 continuously emitting volcanoes tabulated in Table 1, then there are seven sporadic eruptions per year represented by the volcanoes in Table 2. Since the average sporadic flux is almost three times greater than the largest continuous flux, and since by their nature sporadic fluxes are short term events (especially the maximum flux values reported here), it is assumed that these sporadic fluxes last only one day. Thus the sum of the measured SO₂ fluxes from continuously and sporadically emitting volcanoes is 26,400 Mg/d or 9.66 Tg/a. On an annual basis, sporadically emitting volcanoes account for less than 1% of this total.

The 9.66 Tg/a SO₂ flux from sporadic and continuously erupting volcanoes is a minimum flux because it depends upon the actual measurement data applied to the average eruptive record. No provision is made for volcanoes with unmeasured SO₂ fluxes, especially continuously emitting volcanoes. Some of these volcanoes were not measured due to logistical, political or economic reasons. However, this inventory assumes that these limitations were overcome for all major SO₂emitters and that the absent few will not significantly affect the inventory presented here.

Brantley and Koepenick [1995] correctly recognized that the S flux of all volcanoes will not be measured. Following their assumption that the distribution of volcanic SO₂ fluxes can be approximated by a power law function of the form

N = af^c, (1)

where N is the number of volcanoes with SO₂ fluxes, f, f and a and c are constants, one can calculate a global total flux, f_gt, if c < 1. Using the 49 volcanoes with measured continuous SO₂fluxes, a power law best-fit does result in c < 1. Following the approximate solution given by Marrett and Allmendinger [1991; 1990] to calculate f_gt, one obtains an additional unmeasured 1.85 Tg/a SO₂ being emitted from volcanoes. This is approximately 19% of the measured SO₂fluxes. The f_gt obtained by this procedure equates to 1.8 x 10¹¹ moles/a SO₂ and agrees well with the lower estimate calculated by Brantley and Koepenick [1995]. One should keep in mind that a power law fit will overestimate the frequency of small emitters. Thus, the 1.85 Tg/a SO₂ may be taken as an upper bound.

Bluth et al. [1997] summarized the available TOMS data on volcanic eruptions. Over 14 years, 55 detected eruptions produced 52 Tg SO₂ or 3.7 Tg/a SO₂. A partial summary of these TOMS measurements appears in Table 4.

Thus, the time-averaged, annual SO₂ flux from subaerial volcanoes to the atmosphere is 13.4 Tg/a. This flux is based upon actual measurements by various techniques. It assumes the aggregation of these measurements is an accurate summary of all the inhomogeneities in volcanic emissions.

The above volcanic SO₂ flux, derived from the COSPEC and TOMS archives, does not include examples of larger eruptions that occurred prior to this modern instrumental record. For example, the 1963 Agung eruption released 7 Tg SO₂ over two days [Self and King, 1996], the 1783-1784 Laki eruption released 122 Tg SO₂ over eight months [Thordarson et al., 1996], and the 14.7 Ma Roza eruption released 12,420 Tg SO₂ over 10 years [Thordarson and Self, 1996].

Comparison with Previous Volcanic SO₂ Flux Estimates

Table 5 summarizes previous estimates of the global, volcanic SO₂ flux. The 13.4 Tg/a flux calculated here falls in the middle of these estimates. Estimates less than 10 Tg/a appear too low, given the COSPEC and TOMS-obtained data presented above. Likewise, estimates more than 20 Tg/a appear too high, given the same data and the possibilities that existing measurements are too sparse in terms of fluxes and possible vent sites. However, the estimate by Lambert et al. [1988] is intriguing because of the unique method by which it was calculated. Using SO₂/²¹⁰Po ratios for a limited number of sampling sites and extrapolating to global totals, the authors estimate a flux of two to four times that based on more direct field measurements. This suggests that these estimates are at least of the right order of magnitude, even though the system inhomogeneities may not allow a more accurate time-averaged flux to be assessed.

Global Fluxes of Volcanic S Species Other than SO₂

Table 6 combines Table 3 data with the 13.4 Tg/a global SO₂ flux derived above to obtain S specie fluxes. The equivalent S fluxes of particulate S, SO₄^2-, OCS and CS₂ are less than 1 Tg/a S each and combined sum to 0.64 Tg/a S. It is important to note that these fluxes ultimately rely on only a few samples and therefore are prone to many error sources. Cadle [1980] used a similarly small database and a different approach for estimating global OCS and CS₂ fluxes; he obtained estimates of 0.011 and 0.017 Tg/a S, respectively. These estimates are an order of magnitude smaller than that determined in this study. Such differences highlight the lack of data that would allow researchers to better constrain these atmospheric inputs.

The volcanic H₂S flux was derived from the most data and shows the most variability. It combines ratio data from eight different volcanoes, which is more volcanoes than that used for any other species listed, except SO₂.

The 12 species are H₂S, S₂, S₂O, H₂S₂, SO, OCS, S₃, HS, AsS, PbS, SbS and BiS. The four samples, all from Augustine, Alaska, and the species listed are the most thermodynamically stable, after SO₂, emitted from this volcano [Symonds et al., 1992]. On average, the S in H₂S comprises 71% of the S reported for the entire 12 species.

Now a time-averaged inventory of subaerial volcanic S emissions can be calculated. Summing the individual species fluxes for SO₂, 12 species, CS₂, SO₄^2- and particulate S from Table 6, one obtains a 10.4 Tg/a S global flux. H₂S and OCS are excluded from this total because they are already incorporated into the 12 species flux and their magnitudes (displayed independently in Table 6) require no additional adjustment. SO₂contributes 64% to this global S flux.

Comparison with Other Natural and Anthropogenic S Flux Estimates

Natural processes, including volcanoes, emit approximately 24 Tg/a S to the atmosphere [Bates et al., 1992; Spiro et al., 1992]. The volcanic S flux calculated here is 43% of the total natural S flux. Anthropogenic activities emit approximately 79 Tg/a S to the atmosphere [Bates et al., 1992; Spiro et al., 1992; Andreae, 1990]. The volcanic S flux calculated here is 13% of the anthropogenic flux. The bulk of the anthropogenic flux is located in the northern hemisphere while volcanic fluxes occur in more focused belts around the world.

Atmospheric Impact of Volcanic S Emissions

Local effects from volcanic S emissions are primarily limited by dilution with the ambient atmosphere and secondarily by a tropospheric lifetime of a few weeks. Through wet and dry deposition processes, volcanic S can act as a micronutrient for biochemical processes in soils [Murray and Wilson, 1988]. However, too much S can lead to soil and water acidification [e.g., Parnell and Burke, 1990, Stoiber et al., 1986]. While this sometimes can be tolerated, or even welcomed, by local agricultures, increased volcanic activity often stresses these systems into less than optimal production levels [Legge, 1990; Le Guern et al., 1988; Winner and Mooney, 1980]. Arndt et al. [1997] model sources and sinks of S in Asia. Volcanoes in the model domain emit 3.8 Tg/a S which later accounts for 50% of the total S deposition in Indonesia, 30% in Japan and 20% in the Philippines. Such S depositions lead to oxidation (rusting) of various metals [e.g., structural steel in building components) and health problems for livestock and humans due to acidic aerosol inhalation [Le Guern et al., 1980].

Bridging local and global effects, Li et al. [1997] show that five S-rich eruptions over the last century strongly correlate with droughts in Taiwan. The magnitude of the precipitation decrease is two to ten times greater than the effect from El Nino-Southern Oscillation (ENSO) effects. While the exact causal mechanism is unknown, the effect lasts two to three years, which is comparable to the volcanic S lifetime in the stratosphere. Saxena et al. [1997] also show regional effects of volcanic S emissions. In the southeastern United States, maximum daytime temperatures decreased and minimum nighttime temperatures increased due to volcanically-derived S aerosols perturbing the shortwave and longwave radiative forcings, respectively.

Global effects from volcanic S emissions are most readily identified after the emissions are injected into the stratospheric and converted into sulfate aerosol. From 1975 to 1985, at least nine eruptions penetrated the tropopause and deposited S into the stratosphere [McClelland et al., 1989]. Sulfate aerosols change the Earth's albedo; provide surfaces for heterogeneous chemistry; absorb upwelling radiation that warms the stratosphere; and upon sedimentation, alter tropospheric cloud optical properties. Modeling conducted by Bluth et al. [1997] indicates that the important parameters in determining the effect of volcanic injections are the SO₂ flux, volcano latitude and altitude (necessary to determine distance to tropopause). These three parameters are available in the online version of this inventory. The global effects of volcanic emissions that do not penetrate significantly into the stratosphere, or are fully contained in the troposphere, are not as readily identified. This is due to the preponderance of other natural and anthropogenic S in this portion of the atmosphere and to efficient self-removal mechanisms. However, a fraction of this lower atmospheric S enters the stratosphere and contributes to the background sulfate aerosol layer, where the S species total lifetime is 1.9 years. This stratospheric lifetime increases to 2.4 years under heavy volcanic S loadings [Weisenstein et al., 1997].

The effect of large eruptions on this volcanically-enhanced aerosol layer can decrease the solar radiation reaching the Earth's surface. For example, the 1991 Pinatubo eruption caused a 4 W/m²decrease from 40^oS to 40^oN and 8 W/m² from 5^oS to 5^oN. Globally averaged, the Pinatubo eruption caused a negative forcing greater than the positive, anthropogenically enhanced, greenhouse forcing for the latter part of 1991 and most of 1992. This translated to an ENSO-adjusted, global, tropospheric cooling around 0.5^oC in 1992 [McCormick et al., 1995]. On average, the large volcanic eruptions of the last 100 years caused a 0.1 to 0.2^oC cooling for one to two years after the eruption [Mass and Portman, 1989].

In addition to surface temperature and insolation effects, the enhanced stratospheric aerosol layer profoundly affects the stratosphere, including increasing ozone (O₃) destruction rates by 2 to 3% at tropical latitudes [Rosenfield et al., 1997]. This is due to many factors including an increase in stratospheric warming through absorption, in heterogeneous chemistry catalyzing reactions involving Cl, N, Br and HO_x, and in photolysis rates [Huang and Massey, 1997]. Catalyzing reactions on the volcanically enhanced aerosol layer also deplete Antarctic O₃ [Solomon et al., 1993].

The effects of the stratospheric aerosol layer propagate into the troposphere. For example, tropospheric ultraviolet light (uv) levels decrease when a volcanically enhanced stratospheric aerosol layer is present. This is due to increased uv scattering in the stratosphere by the aerosols and increased stratospheric O₃ photolysis rates. This decrease is important in the troposphere because uv photolyzes tropospheric O₃, a precursor to OH production [Yurganov et al., 1997; Dlugokenchky et al., 1996]. OH is the most important tropospheric oxidizer.

Volcanic S fluxes sometimes concentrate in areas not always strongly affected by other natural and anthropogenic S. Graf et al. [1997] estimate that 64% of quiescently degassed S ultimately transforms into sulfate. Since much of this sulfate forms in areas where anthropogenic sulfate has not reached saturation in radiation balance terms, volcanic S emissions create a sulfate burden with disproportionate radiative effects. General circulation model calculations indicate that direct and indirect radiative forcings due to volcanically-derived sulfate are on par with that due to anthropogenically-derived sulfate. Chin and Jacob [1996] found similar results in that volcanic S composes 20 to 40% of the sulfate in the mid-troposphere globally and 60 to 80% over the North Pacific. It also accounts for a significant percentage of the sulfate burden in the upper troposphere at high latitudes. These disproportionate effects are realized even though volcanic S accounts for only 7% of the total S emitted to the atmosphere in their chemical transport model, yet it accounts for 18% of the column sulfate burden globally. Thornton et al. [1996] show that volcanic SO₂ becomes an important cloud condensation nuclei in the North Pacific. These three studies suggest volcanic S plays a role similar to that of anthropogenic S in direct and indirect radiative effects and cloud lifetime effects [Charlson et al., 1992].

Volcanic S Fluxes - Extrapolated

The GEIA inventory presented here is based upon the early 1970's to 1997 time frame. While sufficient to model average years in this time interval, the inventory structure reveals possible extrapolation methodologies. For example, volcanic S emissions vary in terms of their location, speciation and magnitude.

The location database addresses the possible sites of future volcanic S emissions. While near-future emissions are likely to continue from those volcanoes presently emitting, other volcanoes will also likely become significant S emitters. McClelland et al. [1989] note that over the last two centuries an average of one or two eruptions per year occurs from volcanoes with no previously recorded activity. Most of these volcanoes already occur in the database, but this does not preclude a new volcano emerging in a new location.

The majority of S emitted by volcanoes is as SO₂, although some volcanoes can emit significant quantities of H₂S. However, the warm, moist volcanic plume appears to oxidize H₂S into SO₂rather quickly. Although a variety of SO₂ oxidation rates have been measured, a clear consensus on an appropriate rate has yet to be reached [Bluth et al., 1992; Facchini et al., 1992; Rani et al., 1992; Fung et al., 1991; Grgri et al., 1991; Hansen et al., 1991; Joos and Baltensperger, 1991; Gallagher et al., 1990]. These differences stem from the wide range of conditions that volcanic SO₂encounters in the atmosphere: from stratospheric air to polar or tropical tropospheric air.

The magnitude of volcanic emissions from a single source can be quite variable. Natural variations in activity can increase or decrease the S flux at one volcano. The unpredictable eruption of a Pinatubo-type event can dominate the sulfate aerosol layer in the stratosphere for years. Thus, the variability in one source can drastically alter the annual global SO₂ flux. Over the last 25 years, the global volcanic SO₂ flux has ranged between 10 and 30 Tg/a; the absence or presence of large explosive eruptions explains the threefold range in fluxes.

So how is this inventory to be used? That depends upon a study's objective. The inventory gives volcano locations and altitudes, average SO₂fluxes for individual volcanoes, the observed range of global volcano S output, examples of large volcanic S fluxes, and examples of potential consequences of volcanic S emissions. This information can be used in a variety of ways to extrapolate from the hard database of ~70 volcanoes to the ~300 volcanoes currently active. Among others, methods could include random number generators, VEI-SO₂ relationships [e.g., Bluth et al., 1993], and other activity-SO₂ relationships [e.g., Pyle et al., 1996]. A combination of methods may best achieve the study's objective.

GEIA Repository

The GEIA repository for this inventory contains nine files. It can be accessed at http://www.geiacenter.org/emits/volcano.html. The file, volcanos.txt, is a copy of this inventory description. Volcano altitudes are in volcalt.txt; references used to construct the inventory are in bibliography.txt. Volcano locations are stored in LOCavg1.1a and plotted in LOCavg1.ps. SO₂ data from continuously emitting volcanoes are stored in COVavg1.1a and plotted in COVavg1.ps. SO₂ data from sporadically emitting volcanoes are stored in SPVmax1.1a and plotted in SPVmax1.ps.

CONCLUSIONS

Comparing volcanic emissions with natural and anthropogenic emissions is necessary to better understand the climate that sustains life on Earth. To discern effects of increasing industrialization and of human population on the global climate requires a better understanding of present and past climates. One component of this understanding rests on determining material inputs into the atmosphere, both in terms of their mass per time and global distribution.

Actual measurements of subaerial volcanic SO₂ fluxes indicate a time-averaged flux of 13 Tg/a SO₂. While this flux is based upon an early 1970's to 1997 time frame, it likely can be extended into the future with the understanding of the heterogeneity inherent in volcanic emissions. More realistic inventories for the future would include the semi-periodic, but infrequent, large eruptions and their associated SO₂ fluxes.

SO₂ contributes 64% to the global volcanic S flux of 10.4 Tg/a S. This conservative, time-averaged inventory of subaerial volcanic S flux includes other S species present in volcanic emissions. While this average flux is based upon the actual, modern instrumental record, TOMS measurements indicate that the 1991 Pinatubo eruption released an equivalent amount of S in one event. Even larger contributions are indicated for eruptions which predate the modern instrumental record.

ACKNOWLEDGMENTS

RJA was partially supported by a National Science Foundation subcontract (ATM 9629353). ADK was supported by an EPA summer fellowship program. R Pool provided an early version of the Smithsonian database used in this inventory. N Mead, J Harikumar and the Arctic Region Supercomputing Center provided computer support. Five anonymous referees provided critical and helpful comments.

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FIGURE CAPTIONS

Figure 1. The locations of 218 known subaerial volcanoes that erupted at least once since 1 January 1964 are depicted by black cells. These volcanoes are considered the most likely sites of future eruptions. Twenty-two other volcanoes with eruptions in this time period are not depicted in black since they peak below sea level. Shown in dark gray are the locations of 1269 volcanoes which were active over the last 10,000 years. Note that many cells contain more than one volcano. Land mass outlines appear in light gray and water bodies in white. The scale, one degree latitude by one degree longitude, is used throughout this manuscript. Volcano locations were compiled from Smithsonian [1997]. This figure highlights the importance of the volcanic S inputs. While volcanoes produce only 13% of the anthropogenic S flux, they emit this flux from less than 0.1% of the available cells. Many of these cells occur in areas not already unduly saturated by anthropogenic S.

Figure 2. The locations of 49 continuously active volcanoes. These are the persistently degassing volcanoes on which SO₂ flux measurements were made. Only 43 cells appear in black since some volcanoes share cells.

Figure 3. The locations of 25 sporadically active volcanoes. These are the occasionally erupting volcanoes on which SO₂ flux measurements were made. Only 22 cells appear in black since some volcanoes share cells.

Table 1. Forty-nine volcanoes and their continuously erupting SO₂ fluxes. Volcanoes are listed in decreasing flux order.

Table 2. Twenty-five volcanoes and their sporadically erupting SO₂ fluxes. Volcanoes are listed in decreasing flux order.

Table 3. Flux measurement summary for volcanic S species other than SO₂. Ratios given are an average (minimum, maximum, number of determinations used for average). The first entry is for 12 S-containing species and the ratio given is S/SO₂ where S is the sum of the mass of S in each specie. Ten H₂S/SO₂ ratios were excluded because they were beyond two standard deviations from the mean. Likewise, three SO₄^2-/SO₂ ratios were excluded because they were beyond two standard deviations from the mean. OCS is carbonyl sulfide.

Table 4. A partial summary of TOMS-measured volcanic SO₂ emissions [Bluth et al., 1997]. See Bluth et al. [1997] for measurement dates. These masses are included here to give a sense of the large atmospheric injections not normally seen by COSPEC. Masses are arranged in decreasing amounts.

Table 5. Previous estimates of the global, volcanic SO₂ flux. Fluxes arranged by increasing magnitude.

Table 6. Global fluxes of volcanic S species. The 12 S species are already in an S/SO₂ ratio. Species are arranged in decreasing equivalent S fluxes.

Volcano                  Sources

Arenal                   apr 95 gvn, gvn 7(11), may 96 gvn, nov 87 sean,
                         35, 57, 74 (509)
Asama                    8, 10, 21, 56, 111
Aso                      K Kazahaya eml 23 may 97, 21, 111
Augustine                sep 89 sean, 6, 38, 52, 58, 97
Bagana                   jul 89 sean, 74 (152)
Bulusan                  nov 94 gvn
Cameroon, Mt.            74 (90)
Chichon, El              103
Colima Volc Complex      J Gavilanes eml 19 may 97, apr 91 gvn, apr 96
                         gvn, aug 94 gvn, feb 94 gvn, jul 95 gvn, jun 94
                         gvn, jun 95 gvn, nov 86 sean, 57, 74 (466)
Erebus                   RX Faivre-Pierret unpublished data, 3, 7, 10,
                         61, 85, 100
Erta Ale                 10
Etna                     P Allard personal communication, 1, 2, 8, 10,
                         11, 14, 18, 20, 26, 27, 32, 63, 92
Fuego                    16, 19, 21, 46
Galeras                  F Goff 1997 IAVCEI short course, apr 89 sean,
                         apr 90 gvn, apr 96 gvn, aug 89 sean, aug 90
                         gvn, aug 91 gvn, aug 92 gvn, aug 93 gvn, dec
                         91 gvn, dec 92 gvn, dec 94 gvn, feb 90 gvn,
                         feb 91 gvn, feb 94 gvn, feb 95 gvn, feb 96 gvn,
                         jan 93 gvn, jul 89 sean, jul 90 gvn, jul 94 gvn,
                         jul 95 gvn, jun 92 gvn, jun 96 gvn, mar 89 sean,
                         mar 90 gvn, mar 91 gvn, mar 92 gvn, mar 93
                         gvn, mar 94 gvn, mar 95 gvn, may 89 sean,
                         may 92 gvn, may 93 gvn, may 94 gvn, nov 89
                         sean, nov 92 gvn, nov 93 gvn, oct 89 sean, oct
                         90 gvn, oct 91 gvn, oct 94 gvn, oct 95 gvn, sep
                         89 sean, sep 91, sep 92 gvn, 115, 116
Galunggung               33, 87
Iliamna                  AVO eml 30 aug 96, 97
Izalco                   46
Iztaccihuatl             H Delgado 1997 personal communication
Karymsky                 19, 109
Kikai (Satsuma Iwojima)  F Goff 1997 IAVCEI short course, K Kazahaya
                         eml 23 may 97, oct 91 gvn, 117
Kilauea                  T Elias 30 may 97 eml, JB Stokes unpublished
                         data, 8, 10, 14, 42, 51, 64, 65, 66, 67, 74 (410), 74
                         (433), 106, 118
Kilauea East Rift Zone   T Elias 30 may 97 eml, JB Stokes unpublished
                         data, jan 96 gvn, 9, 42, 65, 66, 67, 74 (426), 74
                         (433), 74 (442), 99, 107
Kilauea Pauahi Crater    65
Kilauea Sulfur Bank      8, 10, 51
Kuju Group               K Kazahaya eml 23 may 97
Kverkfjoll               100
Langila                  74 (152)
Lascar                   28
Lengai, Ol Doinyo        98a, 98b
Lonquimay                28
Mageik                   52
Manam                    35, 74 (152)
Martin                   52
Masaya                   W Strauch 17 oct 96 eml, apr 92 gvn, apr 96
                         gvn, mar 97 gvn, 8, 10, 19, 21, 41, 46, 68, 73, 74
                         (501), 105
Mauna Ulu                8, 10, 51
Mayon                    aug 95 gvn, feb 93 gvn, jan 93 gvn, mar 93
                         gvn, 35
Medvezhia                oct 95 gvn
Merapi                   dec 94 gvn, jan 96 gvn, jun 91 gvn, mar 92 gvn,
                         mar 94 gvn, nov 89 sean, nov 91 gvn, nov 92
                         gvn, oct 86 sean, oct 95 gvn, 10, 18, 23, 24, 73,
                         113, 119
Momotombo                8, 10, 19, 41, 46, 68, 120
Mount Baker              52
Negro, Cerro             apr 92 gvn, 120
Nyiragongo               10
Oshima (Mihara, Izu)     K Kazahaya eml 23 may 97, 8, 10, 21, 46, 55,
                         101, 111
Pacaya                   16, 21, 35, 46
Pinatubo                 89
Poas                     57, 68, 70, 74 (514), 96, 120
Popocatepetl             H Delgado eml 13 jun 97, H Delgado eml 13
                         may 97, H Delgado eml 24 apr 97, H Delgado
                         eml 29 oct 96, I Galindo 1997 personal
                         communication, S Williams IAVCEI short
                         course, apr 94 gvn, aug 94 gvn, dec 94 gvn, jan
                         94 gvn, mar 95 gvn, mar 96 gvn, oct 94 gvn,
                         oct 96 gvn
Rabaul                   B Talai fax 5 may 97, jan 97 gvn, mar 97 gvn,
                         may 96 gvn, sep 96 gvn, 112
Redoubt                  71, 97
Ruapehu                  F Goff eml 23 aug 96-revised 16 sep 96, jan 96
                         gvn, jun 96 gvn, nov 96 gvn, sep 95 gvn,
                         science alert bulletin v96/14, science alert
                         bulletin v96/27, science alert bulletin v96/33,
                         science alert bulletin v96/50, 39
Ruiz                     apr 89 sean, apr 90 gvn, apr 91 gvn, aug 86
                         sean, aug 89 sean, aug 90 gvn, aug 91 gvn, dec
                         86 sean, dec 89 sean, dec 90 gvn, feb 86 sean,
                         feb 87 sean, feb 89 sean, feb 91 gvn, jan 87
                         sean, jan 88 sean, jan 90 gvn, jan 91 gvn, jul 87
                         sean, jul 89 sean, jul 91 gvn, jun 87 sean, jun 90
                         gvn, jun 91 gvn, mar 86 sean, mar 87 sean,
                         mar 88 sean, mar 89 sean, mar 90 gvn, mar 91
                         gvn, may 86 sean, may 89 sean, may 91 gvn,
                         nov 86 sean, nov 87 sean, nov 89 sean, nov 90
                         gvn, oct 86 sean, oct 87 sean, oct 89 sean, oct
                         90 gvn, sep 86 sean, sep 87 sean, sep 89 sean,
                         sep 90 gvn, sep 91 gvn, 29, 74 (522), 121
Sakura-jima              86, 101, 110, 111
San Cristobal (Nicaragua)apr 92 gvn, 8, 10, 14, 19, 21, 41, 46, 74 (493-5)
Santa Ana                46
Santa Maria (Santiguito) 16, 21, 35, 46, 74 (476)
Slamet                   jun 91 gvn, 113
Soufriere Hills          S Young eml 28 may 97, 8 jun 96 daily report,
                         aug 95 gvn, aug 96 gvn, dec 96 gvn, feb 97
                         gvn, jan 97 gvn, jul 95 gvn, jun 96 gvn, mar 97
                         gvn, may 96 gvn, nov 96 gvn, oct 96 gvn
Soufriere St. Vincent    36
Spurr                    97
Stromboli                aug 91 gvn, 17, 26, 35, 92, 94
St. Helens               F Goff eml 23 aug 96, 22, 54, 74 (371), 74 (372),
                         74 (382), 74 (384), 74 (389), 95, 102, 103
Tangkubanparahu          74 (203)
Telica                   apr 96 gvn, 8, 10, 19, 41, 46, 68, 74 (496), 120
Tengger Caldera          mar 95 gvn
Ugashik-Peulik           52
Ulawun                   jul 89 sean, mar 95 gvn, 74 (152)
Unzen                    nov 94 gvn, oct 94 gvn, 23
Usu                      K Kazahaya eml 23 may 97, 21, 111
Vulcano                  F Italiano personal communication, 26, 88, 93,
                         94, 104
White Island             F Goff eml 23 aug 96, F Goff eml 23 aug 96-
                         revised 16 sep 96, P Kyle eml 9 Nov 96, dec 93
                         gvn, may 92 gvn, 72, 74 (121), 100
Yasur                    J Eissen eml 27 may 97, dec 88 sean, nov 90
                         gvn