Environmental Engineering Reference
In-Depth Information
Chlorine oxides
released, so rapidly, that the normally inert
atmospheric nitrogen combines with oxygen to
produce quantities of oxides of nitrogen (NO x ).
The rapid heating of the air also sets up strong
convection currents which carry the gases and
other debris into the stratosphere, and it is there
that most of the NO x is deposited. Since natural
NO x is known to destroy ozone, it is only to be
expected that the anthropogenically produced
variety would have the same effect, and one of
the many results of nuclear war might be the large
scale destruction of the ozone layer. Most of the
studies which originally investigated the effects
of nuclear explosions on the atmosphere used
data generated during the nuclear bomb tests of
the 1950s and 1960s. After 1963, when a
moratorium on atmospheric tests of nuclear
devices was declared, information from these
sources was no longer available, and recent
investigations have been based on statistical
models.
The results of the studies of the effects of
nuclear-weapon tests on the ozone layer were not
conclusive. The analysis of data collected during
a period of intense testing in 1961 and 1962
produced no proof that the tests had had any
effect on the ozone (Foley and Ruderman 1973),
although it was estimated that the explosions
should have been sufficient to cause a reduction
of 3 per cent in stratospheric ozone levels
(Crutzen 1974). Techniques for measuring ozone
levels were not particularly sophisticated in the
1960s, and, in addition, a reduction of 3 per cent
is well within the normal annual fluctuation in
levels of atmospheric ozone. Thus, it was not
possible to confirm or refute the predicted effects
of nuclear explosions on the ozone layer by
analysis of the test data. Circumstantial evidence
did indicate a possible link, however. Since ozone
levels are known to fluctuate in phase with
sunspot cycles, it was expected that peak
concentrations of ozone in 1941 and 1952 would
be followed by a similar peak in 1963, in
accordance with the eleven-year cycle. That did
not happen. Instead, the minimum level reached
in 1962 increased only gradually through the
remainder of the decade (Crutzen 1974). The
The extent to which naturally produced chlorine
monoxide (CIO) contributes to the destruction
of the ozone layer is not clear. The most abundant
natural chlorine compound is hydrochloric acid
(HC1). Although it is present in large quantities
in the lower atmosphere, HCl is highly reactive
and soluble in water, and Crutzen (1974)
considered that it was unlikely to diffuse into the
stratosphere in sufficient quantity to have a major
effect on the ozone layer. The addition of large
amounts of chlorine compounds to the
stratosphere during volcanic eruptions was also
proposed as a mechanism for the natural
destruction of the ozone layer (Stolarski and
Cicerone 1974), but observations of the impact
of large volcanic eruptions—such as that of Mt
Agung (see Chapter 5)—on the ozone layer, do
not support that proposition (Crutzen 1974). The
impact of naturally occurring CIO on the ozone
layer was therefore considered relatively
insignificant compared to that of NO x and HO x .
By 1977, however, measurements showed that
the contribution of chlorine to ozone destruction
was growing (Dotto and Schiff 1978), but largely
from human rather than natural sources.
Anthropogenically produced chlorine now poses
a major threat to the ozone layer.
THE HUMAN IMPACT ON THE OZONE
LAYER
It had become clear by the mid-1970s that human
activities had the potential to bring about
sufficient degradation of the ozone layer that it
might never recover. The threat was seen to come
from four main sources, associated with modern
technological developments in warfare, aviation,
life-style and agriculture (see Figure 6.2), and
involving a variety of complex chemical
compounds, both old and new.
Nuclear war and the ozone layer
When a modern thermonuclear device is
exploded in the atmosphere, so much energy is
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