Page 161 - Basic physical chemistry for the atmospheric sciences

Photochemistry 1 4 7

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chemica s , represents one of the most exciting and important areas or
research in the environmental sciences over the past several decades.
A brief account of current views on this subject is given below. 6
Ozone is continually generated in the stratosphere by Reactions
(7. 2 4) and (7. 2 5). Therefore, if the trace chemicals responsible for the
removal of stratospheric ozone are not to be rapidly depleted, they
must serve as a catalyst for the removal of ozone and/or atomic
oxygen [note that the removal of atomic oxygen will slow down the
)
production of ozone by Reaction (7. 2 5 ) . Most of the catalytic reac­
tions that have been proposed for this purpose are of the form
x + o 3 � x o + o 2 (7.28a)
xo + o � x + o 2 (7.28b)

Net: (7.29)
where X represents the catalyst. Reactions (7.28) form a cycle in
which X is consumed in (7.28a) and reformed in (7. 2 8b), with XO
acting as an intermediate. Provided that both Reactions (7.28a) and
(7.28b) are fast, Reaction (7.29) can proceed much faster than it would
by the direct route proposed by Chapman [i . e . , faster than Reaction
(7.27)) . Under these conditions, and provided there is no appreciable
sink for X, just a few molecules of X have the potential to eliminate
indefinite numbers of ozone molecules and atomic oxygen.
In the natural (i.e. , anthropogenically undisturbed) stratosphere, the
most important contenders for the catalyst X are H, OH , NO, and Cl.
For example, n the case of NO
i
NO + 03� N02 + 02 (7.30a)
N02 + 0 � N 0 + 0 2 (7. 3 0b)

Net: (7. 3 1 )
i
At a temperature of - 53°C (which s typical of the stratosphere), the
rate coefficients for Reactions (7. 3 0a) and (7.30b) are 3 . 5 x 1 0 - 1 5 and
2
3
-
9 . 3 x 1 0 - 1 cm molecu1e - 1 s 1 , respectively, compared to . 8 x 1 0 - 1 6
6
3
cm molecule - 1 s - 1 for kd in Reaction (7 .27). Hence, the rate con­
stants for Reactions (7 3 0) are greater than those for the direct Reac­
.
tion (7. 2 7). However, whether or not Reactions (7.30) will destroy
ozone faster than Reaction (7 2 7) will depend on the concentrations of
.
N02 and 03 , as illustrated in the following exercise.
Exercise 7.5 . If Reaction (7.30b) is the rate-determining step in the