Page 163 - Basic physical chemistry for the atmospheric sciences

Photochemistry 1 1 •1

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Other important competitive processes operate in the stratosphcn .
including "reservoir" species, which can divert potential catalysts and
m
other compounds from active to inactive for s , but which "wait in the
wings for their turn on the stage. " Despite these complications, ad­
vanced chemical reaction schemes incorporated into numerical models
can reproduce the vertical profiles of ozone and other chemical species
in the "natural" stratosphere with some fidelity .
The natural concentrations of H , OH, NO, and C l (most of which
originate at the Earth' s surface) in the stratosphere serve to catalyze
Reaction (7.29) and to maintain approximately steady-state concentra­
tions of ozone. However, if the concentrations of the catalysts X in Re­
action (7. 2 8) are increased significantly by anthropogenic activities, the
delicate balance between the sources and sinks of atmospheric ozone
will be disturbed, and stratospheric ozone concentrations can be ex­
pected to decrease. One of the first concerns in this respect were air­
craft flying in the stratosphere (particularly supersonic aircraft) . This is
because aircraft engines emit nitric oxide (NO) which, as shown by Re­
actions (7. 30), can serve as the X in Reaction (7.28) . However, there
are not sufficient numbers of aircraft flying in the stratosphere at the
present time to perturb stratospheric ozone significantly .
Of much greater concern , with already documented impacts, is the
catalytic action of chlorine, from chlorofluorocarbons , in depleting
stratospheric ozone. Chlorofluorocarbons are compounds containing
chlorine, fluorine, carbon, and sometimes hydrogen. They are com­
monly known as "Freons, " of which Freon 1 1 (CFCl3) and Freon 1 2
(CF2Cl2) are the most common. Freons were first synthesized in 1 9 30,
as the result of a search for a nontoxic, nonflammable refrigerant.
Over the next half-century they became widely used, not only as
s
refrigerant , but as propellants in aerosol ca s , in plastic foam, and as
n
solvents and cleansing agents. Concern about their effects on the
atmosphere began in 1 9 73 when it was found that Freons were spread­
ing globally and, because of their inertness , were expected to have
e
lifetimes of up to several hundred years in the tropospher .
Such long-lived compounds eventually find their way into the strato­
sphere. Here they absorb UV radiation in the wavelength interval
0. 1 9 -0.22 J.Lm and photodissociate
CFCl3 + h v � CFC1 + Cl (7.34)
2
and ,