Environmental Engineering Reference
In-Depth Information
elevated levels of ultraviolet radiation at the
earth's surface. The depletion of the ozone
layer became a major environmental
controversy by the middle of the decade. Its
technological complexity caused dissension in
scientific and political arenas, and—with more
than a hint of science fiction in its make-up—it
garnered lots of popular attention. In common
with many environmental concerns of that era,
however, interest waned in the late 1970s and
early 1980s, only to be revived again with the
discovery in 1985 of what has come to be called
the Antarctic ozone hole.
The role of ultraviolet radiation and
molecular oxygen in the formation of the ozone
layer was first explained by Chapman in 1930.
Later measurement indicated that the basic
theory was valid, but observed levels of ozone
were much less than expected, given the rate of
decay possible through natural processes. Since
none of the other normal constituents of the
atmosphere, such as molecular oxygen,
nitrogen, water vapour or carbon dioxide was
considered capable of destroying the ozone,
attention was eventually attracted to trace
elements in the stratosphere. Initially, it seemed
that these were present in insufficient quantity
to have the necessary effect, but the problem
was solved with the discovery of catalytic chain
reactions in the atmosphere (Dotto and Schiff
1978). A catalyst is a substance which
facilitates a chemical reaction, yet remains itself
unchanged when the reaction is over. Being
unchanged, it can go on to promote the same
reaction again and again, as long as the reagents
are available, or until the catalyst itself is
removed. In this form of chain reaction, a
catalyst in the stratosphere may destroy
thousands of ozone molecules before it is finally
removed. The ozone layer is capable of dealing
with the relatively small amounts of naturally
occurring catalysts. Recent concern over the
thinning of the ozone layer has focused on
anthropogenically produced catalysts (see
Figure 6.2), which were recognized in the
stratosphere in the early 1970s, and which have
now accumulated in quantities well beyond the
system's ability to cope.
THE PHYSICAL CHEMISTRY OF THE
OZONE LAYER
Ozone owes its existence to the impact of
ultraviolet radiation on oxygen molecules in the
stratosphere, with the main production taking
place in tropical regions where radiation levels
are high (Rodriguez 1993). Oxygen molecules
normally consist of two atoms, and in the lower
atmosphere they retain that configuration. At the
high energy levels associated with ultraviolet
radiation in the upper atmosphere, however, these
molecules split apart to produce atomic oxygen
(see Figure 6.1). Before long, these free atoms
combine with the available molecular oxygen to
create triatomic oxygen or ozone. That reaction
is reversible. The ozone molecule may break
down again into its original components,
molecular oxygen and atomic oxygen, as a result
of further absorption of ultraviolet radiation, or
it may combine with atomic oxygen to be
reconverted to the molecular form (Crutzen
1974). The total amount of ozone in the
stratosphere at any given time represents a
balance between the rate at which the gas is being
produced and the rate at which it is being
destroyed. These rates are directly linked; any
fluctuation in the rate of production will be
matched by changes in the rate of decay until
some degree of equilibrium is attained (Dotto and
Schiff 1978). Thus, the ozone layer is in a
constant state of flux as the molecular structure
of its constituents changes.
NATURALLY OCCURRING, OZONE
DESTROYING CATALYSTS
Natural catalysts have probably always been part
of the atmospheric system, and many—such as
hydrogen, nitrogen and chlorine oxides—are
similar to those now being added to the
atmosphere by human activities. The main
difference is in production and accumulation. The
natural catalysts tend to be produced in smaller
quantities and remain in the atmosphere for a
