Environmental Engineering Reference
In-Depth Information
the stratosphere, mostly in the form of sulphur
dioxide (SO
2
) which ultimately produced
sulphuric acid aerosols (Brasseur and Granier
1992). In the case of Mount Pinatubo, the
amount of SO
2
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 SO
2
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).
