Volcanic Eruptions And Atmospheric Turbidity

The large volumes of particulate matter thrown into the atmosphere during periods of volcanic activity are gradually carried away from their sources to be redistributed by the wind and pressure patterns of the atmospheric circulation. Dust ejected during the explosive eruption of Krakatoa, in 1883, encircled the earth in about two weeks following the original eruption (Austin 1983), and within 8 to 12 weeks had spread sufficiently to increase atmospheric turbidity between 35°N and 35°S (Lamb 1970). The diffusion of dust from the Mount Agung eruption in 1963 followed a similar pattern (Mossop 1964) and in both cases the debris eventually spread polewards until it formed a complete veil over the entire earth. The cloud of sulphate particles ejected from El Chichon in 1982 was carried around the earth by the tropical easterlies in less than 20 days, and within a year had blanketed the globe (Rampino and Self 1984). Similarly, the volcanic debris injected into the atmosphere during the eruption of Mount Pinatubo in June 1991, circled the earth at the equator in about 23 days (Gobbi et al. 1992). It reached Japan two weeks after the eruption began (Hayashida and Sasano 1993). Within 20 days the edge of the debris cloud had reached southern Europe (Gobbi et al. 1992), and less than 2 months later was recognized in the stratosphere above southern Australia (Barton et al. 1992). By

October of 1991, the cloud was present above Resolute at 74°N in the Canadian Arctic, and by early 1992 had spread worldwide (Rosen et al. 1992). Sulphate aerosols and fine ash particles from Mount Hudson which erupted in Chile in August 1991 were carried by strong zonal westerlies between 45°S and 46°S to pass over southeastern Australia within 5 days of the eruption, and around the earth in just over a week (Barton et al. 1992).

The build up of the dust veil and its eventual dispersal will depend upon the amount of material ejected during the eruption and the height to which the dust is projected into the atmosphere. The eruption of Krakatoa released at least 6 cubic km (and perhaps as much as 18 cubic km) of volcanic debris into the atmosphere (Lamb 1970). In comparison, Mt St Helens produced only about 2.7 cubic km (Burroughs 1981). Neither of these can match the volume of debris from Tambora, an Indonesian volcano which erupted in 1815, producing an estimated 80 cubic km of ejecta (Findley, 1981). More important than the total particulate production, however, is its distribution in the atmosphere. That depends very much on the altitude to which the debris is carried, and whether or not it penetrates beyond the tropopause. The maximum height reached by dust ejected from Krakatoa has been estimated at 50 km and a similar altitude was reached by the dust column from Mount Agung in 1963 (Lamb 1970). A particularly violent eruption at Bezymianny in Kamchatka, in 1956, threw ash and other debris to a height of 45 km (Cronin 1971), but Mt St Helens, despite the explosive nature of its eruption, failed to push dust higher than 20 km, perhaps because the main force of the explosion was directed horizontally rather than vertically (Findley 1981). As a result, it has been estimated that Mt St Helens injected only 5 million tonnes of debris into the stratosphere compared with 10 million tonnes for Mount Agung, and as much as 50 million tonnes for Krakatoa (Burroughs 1981). The eruptions of El Chichón in 1982 and Mount Pinatubo in 1991 injected an estimated 20 and 30 million tonnes of material respectively into the stratosphere, mostly in the form of sulphur dioxide (SO2) which ultimately produced sulphuric acid aerosols (Brasseur and Granier 1992). In the case of Mount Pinatubo, the amount of SO2 injected was probably as much as that from Krakatoa (Groisman 1992), although the total debris production from the latter was higher.

Since the altitude of the tropopause decreases with latitude (see Chapter 2) even relatively minor eruptions may contribute dust to the stratosphere in high latitudes. The dust plume from the Surtsey eruption, off Iceland, in 1963, for example, penetrated the tropopause at 10.5 km (Cronin 1971), whereas the products of a comparable eruption in equatorial regions would have remained entirely within the troposphere. When Nyamuragira in Zaire erupted in 1981, for example, it produced almost as much SO2 as El Chichón, but little of it reached the stratosphere. In contrast, the force of the eruption of Mount Pinatubo penetrated the tropopause at about 14 km, and carried debris up for another 10 to 15 km (Gobbi et al. 1992). Particulate matter which is injected into the stratosphere in high latitudes gradually spreads out from its source, but its distribution remains restricted. Most high latitude volcanoes in the northern hemisphere are located in a belt close to the Arctic Circle, and there is no evidence of dust from an eruption in this belt reaching the southern hemisphere (Cronin 1971). In contrast, products of eruptions in equatorial areas commonly spread to form a world-wide dust veil (Lamb 1970). As a result of this, it might be expected that when volcanoes are active in both regions, turbidity in the northern hemisphere would be greater than in the southern. Atmospheric turbidity patterns in the period between 1963 and 1970, when four volcanic plumes in the Arctic Circle belt and three in equatorial regions penetrated the tropopause, tend to confirm the greater turbidity of the northern stratosphere under such conditions (Cronin 1971). There are fewer volcanoes in high latitudes in the southern hemisphere than in the north, but the same restrictions on the distribution of volcanic debris seem to apply. For example, the volcanic ash and sulphuric acid from the eruption of Mount Hudson in southern Chile in 1991 penetrated the tropopause at about 9 km, and was carried around the world quite rapidly on the upper westerlies. However, simulations carried out to estimate the subsequent spread of the debris cloud suggest that it remained restricted to the area between 70°S and 30°S (Barton et al. 1992).

The dust veil index

Individual volcanic eruptions differ from each other in such properties as the amount of dust ejected, the geographical extent of its diffusion and the length of time it remains in the atmosphere. Comparison is possible using these elements, but to simplify the process, and to make it easier to compare the effects of different eruptions on weather and climate, Lamb (1970) developed a rating system which he called a dust veil index (DVI). It was derived using formulae which took into account such parameters as radiation depletion, the estimated lowering of average temperatures, the volume of dust ejected and the extent and duration of the veil. The final scale of values was adjusted so that the DVI for the 1883 eruption of Krakatoa had a value of 1,000. Other eruptions were then compared to that base. The 1963 eruption of Mount Agung was rated at 800, for example, whereas the DVI for Tambora in 1815 was 3,000 (Lamb 1972).

Individual dust veils may combine to produce a cumulative effect when eruptions are frequent (see Figure 5.4). The 1815 eruption of Tambora, for example, was only the worst of several between 1811 and 1818. The net DVI for that period was therefore 4,400. Similarly, Lamb (1972) estimated that between 1694 and 1698, the world DVI was 3,000 to 3,500. At times when volcanic activity is infrequent, the DVI is low, as it was between 1956 and 1963 when no eruptions injected debris into the stratosphere (Lockwood 1979).

Figure 5.4 Cumulative DVI for the northern hemisphere: dust from an individual eruption is apportioned over 4 years-40% to year 1; 30% to year 2; 20% to year 3 and 10% to year 4

Figure 5.4 Cumulative DVI for the northern hemisphere: dust from an individual eruption is apportioned over 4 years-40% to year 1; 30% to year 2; 20% to year 3 and 10% to year 4

Source: From Bradley and Jones (1992)

The DVI provides an indication of the potential disruption of weather and climate by volcanic activity. Dust in the atmosphere reduces the amount of solar radiation reaching the earth's surface, and at high index levels that reduction can be considerable. This is particularly so in higher latitudes where the sun's rays follow a longer path through the atmosphere, and are therefore more likely to be scattered. Major eruptions, producing a DVI in excess of 1,000, have caused reductions in direct beam solar radiation of between 20 and 30 per cent for several months (Lamb 1972). The effect is diminished to some extent by an increase in diffuse radiation, but the impact on net radiation is negative. Observations in Australia, following the eruption of Mount Agung in 1963, showed a maximum reduction of 24 per cent in direct beam solar radiation, yet, because of the increase in diffuse radiation, net radiation fell by only 6 per cent (Dyer and Hicks 1965). Similar values were recorded at the Mauna Loa Observatory in Hawaii at that time (Ellis and Pueschel 1971). El Chichón reduced net radiation by 2-3 per cent at ground level (Pollack and Ackerman 1983), while the reduction caused by Pinatubo averaged 2.7 per cent over a 10-month period following the eruption, with individual monthly values reaching as high as 5 per cent (Dutton and Christy 1992).

A number of problems with the DVI have been identified since it was first developed, and these have been summarized by Chester (1988). The DVI was based solely on dust and did not include the measurement of sulphates or sulphuric acid aerosols, which are now recognized as being very effective at scattering solar radiation. As a result, the impact of sulphur-rich eruptions such as El Chichón or Mount Pinatubo would be underestimated. At the same time, there is no way of preventing non-volcanic sources of dust from being included in the index. The use of climatic parameters in the calculation of certain index values may introduce the possibility of circular reasoning. For example, falling temperatures are taken as an indication of an increasing DVI, yet a high DVI may also be used to postulate or confirm falling temperatures.

In an attempt to deal with some of these problems a number of researchers reassessed Lamb's index, but introduced only limited refinement and modification (Mitchell 1970; Robock 1981). Other indices have also been proposed, although none has been as widely used as the original DVI. One example is the volcanic explosivity index (VEI), developed as a result of research sponsored by the Smithsonian Institute into historic eruptions (Chester 1988). It is based only on volcanological criteria, such as the intensity, dispersive power and destructive potential of the eruption, as well as the volume of material ejected. It also includes a means of differentiating between instantaneous and sustained eruptions (Newhall and Self 1982). Being derived entirely from volcanological criteria, it eliminates some of the problems of the DVI—such as circular reasoning—for example, but it does not differentiate between sulphates and dust, nor does it include corrections for the latitude or altitude of the volcanoes (Chester 1988). Thus, although the VEI is considered by many to be the best index of explosive volcanism, it is not without its problems when used as a tool in the study of climatic change.

A glaciological volcanic index (GVI) has also been proposed (Legrand and Delmas 1987). Based on ice cores, it would reveal acidity levels in glacial ice, and therefore give an indication of the SO2 levels associated with past volcanic eruptions. Since neither the DVI nor the VEI deal adequately with volcanic SO2 emissions, the GVI has the potential to improve knowledge of the composition of volcanic debris. However, current ice core availability is limited, and the GVI adds little to the information available from established indices (Bradley and Jones 1992).

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  • yohannes kiros
    How much material was ejected from el chichon?
    5 years ago

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