Geology Reference
In-Depth Information
A more serious culprit with respect to the depletion of the atmospheric
ozone layer was identified from about 1974 onwards. US scientists
postulated
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that some industrial chemical starting to reach the strato-
sphere might interfere with the mechanisms of ozone formation and
destruction. The chemicals were chlorofluorocarbons, or CFCs, and had
been introduced by General Motors in the 1930s mainly as coolants for
refrigerators and air conditioning systems, aerosol propellants and
blowing agents, etc. The perceived virtue of these compounds was their
physical properties, chemical inertness and lack of toxicity. The poten-
tial problem becomes apparent at higher altitudes where the CFCs (e.g.
CF
2
Cl
2
) are not so inert and can be broken down by UV light
CF
2
Cl
2
þ
hn
-
Cl
þ
CF
2
Cl
(2.100)
releasing a chlorine atom that can enter the previously described cata-
lytic cycles (reactions (2.92 and 2.93)) and destroy ozone. Fortuitously,
as described previously, most of the active chlorine, including that
provided by CFC degradation is tied up in the reservoir compounds.
There are many myths about CFCs and chlorine in the atmosphere, such
as if CFCs are heavier than air how can they make it high up into the
atmosphere? The truth is that thousands of measurements have found
CFCs high in the atmosphere and though heavier than air, the atmos-
phere is not still and the winds can carry and mix gases high into the
stratosphere.
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Figure 26 shows the amount and variety of CFC com-
pounds that can act as source gases for chlorine in the stratosphere.
Based on predictions of the effect of CFCs on the ozone layer, in 1987
a previously unprecedented step was taken when many countries signed
the UN Montreal protocol specifying the control and phase-out of these
ozone-depleting chemicals.
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Since that time the protocol has been
modified in order to speed up the schedule and extend the range of
chemical covered to further lessen the effect of these chemicals (see
Figure 27). One of the factors that lead to more rapid world action on
CFCs was the discovery of the so-called Antarctic ozone hole.
2.10.1 The Antarctic Ozone Hole
British scientists from the British Antarctic Survey (BAS) had been
making measurements of ozone from their base at Halley Bay (761S) for
many years. They detected a decline in the springtime ozone since 1977,
and by October 1984, they had detected a 30% decline in the total
ozone.
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Today, this value has reached 60%. The ozone hole begins to
develop in August, is fully developed by early October and has normally
broken up by early December (see Figure 28).
