Page 162 - Basic physical chemistry for the atmospheric sciences

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reaction cycle (7 3 0) , derive an expression, in terms of rate coefficients
.
and the concentration of ozone, for the concentrations that N02 must
exceed if the reaction cycle (7.30) is to destroy ozone faster than
Reaction (7.27).
Solution. Let k be the rate coefficient for Reaction (7.30b). Since
this is the rate-determining step, the net Reaction (7. 3 1 ) cannot pro­
ceed faster than the rate at which Reaction (7.30b) destroys atomic
oxygen. This rate is given by

(7. 3 2)

The rate at which atomic oxygen is destroyed in Reaction (7. 2 7) is

(7.33)
Therefore, if atomic oxygen, and therefore ozone, is to be destroyed
faster by the reaction cycle (7. 3 0) than by Reaction (7. 2 7), the right
side of Eq. (7. 3 2) must exceed the right reaction side of Eq. (7.33),
that is

or

When the appropriate rate coefficients and concentrations for the
various reactions and species in the stratosphere are taken into ac­
count, it appears that the catalytic cycles involving H , OH , NO, and Cl
all make major contributions to the destruction of ozone in the strato­
sphere. Reaction cycle (7.30) dominates in the lower stratosphere ; the
u
cycles involving H and OH dominate in the p per stratosphere ; and the
cycle involving Cl is important n the middle stratosphere. However,
i
the destruction of ozone by the various catalytic cycles is not simply an
additive process . This is because the species in one cycle can react with
those in another cyc e . For example, two important interactions that af­
l
fect the contributions of the OH and Cl cycles are
H02 + NO� OH + N02

and ,
ClO + NO� Cl + N02