Page 166 - Basic physical chemistry for the atmospheric sciences

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of active ClO and C l . However, Reactions (7.36) cannot proceed with
u
full vigor n til enough sunlight appears to release both sufficient free
chlorine atoms [by Reaction (7.41 ) ] and sufficient quantities of atomic
oxygen [by Reaction (7 .26 ] . Since the latter requirement is not met in
)
.
early spring in the Antarctic stratosphere, Reactions (7 3 6) cannot
explain the very large depletions of ozone that produce the Antarctic
ozone hole, although they probably contribute to it.
A cycle, catalyzed by CIO, that does appear capable of explaining
the Antarctic ozone hole s
i
CIO + CIO + M-? (CIO)z + M (7.42a)
(CIO)z + h v -? CJ + ClOO (7.42b)
CIOO + M -? C l + 0 2 + M (7.42c)
(7.42d)
2Cl + 203-? 2CIO + 202

Net: (7.43)
The following points should be noted about this reaction cycle:

i
I . Reactions (7.42) form a catalytic cycle in which CIO s the catalyst,
because two CIO molecules are regenerated for every two CIO
molecules that are consumed.
2. The cycle does not depend on atomic oxygen (which is in short
supply).
3 . The chlorine atom in the CIO on the left side of Reaction (7.42a)
derives from Cl released from Freons via Reactions (7 3 4) and
.
(7. 3 5 ) . However, as we have seen, the chlorine atom is then quickly
tied up as HCI and CION02 by Reactions (7. 3 8) and (7. 3 9). But, in
the presence of PS s , chlorine gas (Cl2) is released by Reaction
C
(7.40) and, as soon as the solar radiation reaches sufficient intensity
in early spring, Reaction (7. 4 1 ) releases atomic chlorine. Reaction
(7.36a) converts this into CIO, which is then available for the first
step in the reaction cycle (7 .42) that leads to the rapid depletion of
ozone in the Antarctic stratosphere.
4. The dimer (CIO)z is formed by Reaction (7.42a) only at low temper­
e
atur s . Low-enough temperatures are present in the Antarctic
stratosphere, where there are also large concentrations of CIO.
Therefore, on both counts, the Antarctic stratosphere in spring is a
region in which the reaction cycle (7.42) can destroy large quanti­
ties of ozone.