IGACtivities No. 22
December 2000

A time-averaged inventory of subaerial volcanic sulfur emissions prepared for the GEIA

Contributed by:
Robert J. Andres, Space Studies Department,
University of North Dakota, USA

Volcanoes emit gases, aerosols, and fine particulate matter in modes ranging from continuous to ephemeral. From 1975 to 1985, an average of 56 volcanoes erupted yearly. 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].

Sulfur (S) is a common element in many volcanically-emitted compounds. Sulfur dioxide (SO2) and hydrogen sulfide (H2S) are the primary S-containing gases emitted. Other S-containing species usually occur in smaller quantities. 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. Volcanic S sources often occur in areas not strongly affected by other natural and anthropogenic S. Graf et al. [1997] estimated 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 20–40% of the sulfate in the mid-troposphere globally and 60–80% 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 nucleus source in the North Pacific. These three studies suggest that 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]. Some of these effects are due to the ability of volcanoes to loft S high into the troposphere and stratosphere where the S residence times increase significantly.

Due to the importance of volcanic S in the global S cycle, especially the atmospheric portions thereof, a time-averaged inventory of subaerial (directly venting to the atmosphere) volcanic S emissions was compiled. Primarily for the use of global S and sulfate modelers, it incorporates the temporal, chemical and spatial inhomogeneities inherent to the subject that 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 species/S ratios [e.g., Andres et al., 1993; Nriagu, 1989; Symonds et al., 1988; Lantzy and Mackenzie, 1979]. This work was originally reported in Andres and Kasgnoc (1998). More information about the Global Emissions Inventory Activity (GEIA), its products, and the inventory presented here is available at the GEIA website.

This inventory relies upon the 25-year history of S (primarily SO2) measurements at volcanoes. Subaerial volcanic SO2 emissions indicate a 13 Tg/a SO2 time-averaged flux, based upon an early 1970s 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 (Table 1). 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.

Table 1. GEIA volcanic S inventry. Global distribution and elevation of these sources are given in the online version. Mg/d = megagrams (106)/day, Tg/a = teragrams (1012)/annum.

* Total measured SO2 emissions are primarily based upon COSPEC measurements of smaller emission sources (includes 49 continuous and 25 sporadic volcanoes).
# TOMS measurements are satellite-obtained measurements of larger emission sources.
$ Other S species based upon direct samples of volcanic gases (to obtain other S/SO2 ratios) and separately measured SO2 fluxes.

This inventory is based upon an extensive compilation of the available, measured volcanic S fluxes. 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 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, 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 a single index year.

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 [Thor-darson and Self, 1996].

The impact of these larger events has not been as well studied as that of the 1991 Mount Pinatubo eruption which scales between Agung and Laki. Resulting effects of Pinatubo include a 4 W/m2 decrease from 40S to 40N and 8 W/m2 from 5S to 5N. 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 0.5C in 1992 [McCormick et al., 1995].

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.

In summary, Table 1 details a conservative, time-averaged inventory of subaerial volcanic S flux that incorporates the temporal, spatial and chemical inhomogeneities of this source. 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 that predate the modern instrumental record.

References

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