|Working Group I: The Scientific Basis|
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Two components of volcanic emissions are of most significance for aerosols:
primary dust and gaseous sulphur. The estimated dust flux reported in Jones
et al., (1994a) for the1980s ranges from 4 to 10,000 Tg/yr, with a “best”
estimate of 33 Tg/yr (Andreae, 1995). The lower limit represents continuous
eruptive activity, and is about two orders of magnitude smaller than soil dust
emission. The upper value, on the other hand, is the order of magnitude of volcanic
dust mass emitted during large explosive eruptions. However, the stratospheric
lifetime of these coarse particles is only about 1 to 2 months (NASA, 1992),
due to the efficient removal by settling.
Sulphur emissions occur mainly in the form of SO2, even though
other sulphur species may be present in the volcanic plume, predominantly SO42-
aerosols and H2S. Stoiber et al. (1987) have estimated that the amount of SO42-
and H2S is commonly less than 1% of the total, although it may in some cases
reach 10%. Graf et al. (1998), on the other hand, have estimated the fraction
of H2S and SO42- to be about 20% of the total.
Nevertheless, the error made in considering all the emitted sulphur as SO2 is
likely to be a small one, since H2S oxidises to SO2 in
about 2 days in the troposphere or 10 days in the stratosphere. Estimates of
the emission of sulphur containing species from quiescent degassing and eruptions
range from 7.2 TgS/yr to 14 ± 6 TgS/yr (Stoiber et al., 1987; Spiro et
al., 1992; Graf et al., 1997; Andres and Kasgnoc, 1998). These estimates are
highly uncertain because only very few of the potential sources have ever been
measured and the variability between sources and between different stages of
activity of the sources is considerable.
Graf et al. (1997) suggest that volcanic sources are important to the sulphate
aerosol burden in the upper troposphere, where they might contribute to the
formation of ice particles and thus represent a potential for a large indirect
radiative effect (see Section 5.3.6). Sassen (1992) and
Sassen et al. (1995) have presented evidence of cirrus cloud formation from
volcanic aerosols and Song et al. (1996) suggest that the interannual variability
of high level clouds is associated with explosive volcanoes.
Calculations using a global climate model (Graf et al., 1997) have reached the “surprising” conclusion that the radiative effect of volcanic sulphate is only slightly smaller than that of anthropogenic sulphate, even though the anthropogenic SO2 source strength is about five times larger. Table 5.6 shows that the calculated efficiency of volcanic sulphur in producing sulphate aerosols is about 4.5 times larger than that of anthropogenic sulphur. The main reason is that SO2 released from volcanoes at higher altitudes has a longer residence time, mainly due to lower dry deposition rates than those calculated for surface emissions of SO2 (cf.B,enkovitz et al., 1994). On the other hand, because different models show major discrepancies in vertical sulphur transport and in upper tropospheric aerosol concentrations, the above result could be very model- dependent.
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