5.2.2.8 Volcanoes

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 SO₄^2- aerosols and H2S. Stoiber et al. (1987) have estimated that the amount of SO₄^2- 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 SO₄^2- 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.

Volcanic aerosols in the troposphere

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.

Table 5.6: Global annual mean sulphur budget (from Graf et al., 1997) and top-of-atmosphere forcing in percentage of the total (102 TgS/yr emission, about 1 TgS burden, –0.65 Wm–2 forcing). Efficiency is relative sulphate burden divided by relative source strength (i.e. column 3 / column 1).
Source	Sulphur emission	SO2 burden	SO42– burden	Efficiency	Direct forcing TOA %
Anthropogenic	66	46	37	0.56	40
Biomass burning	2.5	1.2	1.6	0.64	2
DMS	18	18	25	1.39	26
Volcanoes	14	35	36	2.63	33

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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