Volcanoes

Volcanoes are another natural source producing both primary and secondary aerosol particles. One of the distinctive aspects of volcanic emissions is that strong eruptions can inject materials directly into the stratosphere where aerosol-induced effects on the balance of solar radiation and ozone depletion, for example, can persist for years (McCormick et al., 1995). Much of the work on volcanic aerosols has, in fact, focused on the stratosphere, but that topic will not be covered in this chapter.

The amounts of material produced by volcanoes can be quite considerable when they are active, but volcanic eruptions are episodic, and most volcanoes exhibit periods of dormancy following the releases of gases, particles, and lava that constitute the active phase. Explosive volcanoes eject primary particles, mainly silicate dust particles and ash, into the atmosphere; but many of the primary particles are so large that they settle out quickly and close to their source. Andreae (1995) observed that during periods of extreme volcanic activity, as much as 10,000 Tg of dust could be produced per year. An annual flux of that magnitude would be larger than that from any other aerosol source, with the possible exception of sea salt. In less active times, the flux of primary particles from volcanoes, —4Tg/yr, would be almost negligible on a global scale. The long-term average production of primary particles from volcanoes has been estimated as 33Tg/yr (Andreae, 1995).

Volcanoes also release water vapor, C02, S02, fluorine, and chlorine into the atmosphere (Lambert et al., 1988; Symonds et al., 1988) both from explosive events and during noneruptive activity. Secondary particles, mainly sulfuric acid droplets, can form from these gaseous emissions but, on a mass basis, the production of secondary particles during periods of high volcanic activity is much smaller than that of primary particles. For nonexplosive, basaltic volcanoes and fissures, the masses of primary and secondary particles produced are more nearly comparable, but the combined production of primary and secondary particles from these sources is considerably smaller than during more active periods. More important, the volca nic emissions of sulfur (9.3 to 11.8 Tg S per year in total) are equivalent to 10 to 30% of the total anthropogenic sulfur flux into the atmosphere; this amount is large enough to significantly affect the chemistry of sulfur in the atmosphere (Berresheim et al., 1995).

Volcanoes inject substantial quantities of trace elements into the atmosphere, and as plumes of volcanic material cool, gaseous species condense and attach to particles. As a result the composition of volcanic aerosols typically is quite different from the parent magma. Compared with crustal material, volcanic particles tend to be enriched with relatively volatile elements, including Zn, Cu, Au, Pb, As, Cd, Sb, and Se (Buat-Menard, 1990). These enrichments vary not only among different volcanoes but also during different eruptive stages for a particular volcano. The elemental composition of the aerosol is particularly sensitive to the amount of nondegassed magmatic material brought to the surface by the volcanic activity. Along this same line, iridium enrichments were found during the eruption of Kilauea (Zoller et al., 1983) but not six other volcanoes, presumably reflecting the different types of magma involved in the eruptions.

Globally volcanoes supply —50% of the 210Po in the atmosphere (Lambert et al., 1982). This nuclide is the last radioactive daughter in the decay series of the naturally occurring radionuclide 238U. In Antarctica, volcanoes are a particularly important source for volatile radionuclides because snow and ice cover minimizes the impact of many other sources (Polian and Lambert, 1979). These authors found that the 210Po/SO2 ratios in the plume from Mt. Erebus were 30-fold higher than those for areas not affected by volcanoes. The recognition of a strong volcanic source for 210Po enabled Lambert et al. (1988) to estimate trace element fluxes from volcanoes by scaling them relative to 210Po. The uncertainties in these figures are estimated to be a factor of 3, but in general the volcanic inputs of trace elements probably are <20% of their total atmospheric inputs (Nriagu, 1989). One exception to this is Bi whose volcanic source strength is perhaps 10 times higher than the inputs from either natural or anthropogenic sources (Lee et al., 1986; Lambert et al., 1988).

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