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11 How do we know that CFC's are photolyzed in the stratosphere?




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This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

11 How do we know that CFC's are photolyzed in the stratosphere?

The UV photodissociation cross-sections for the halocarbons have been
measured in the laboratory; these tell us how rapidly they will
dissociate when exposed to light of a given wavelength and intensity.
We can combine this with the measured intensity of radiation in the
stratosphere and deduce the way in which the mixing ratio of a
given halocarbon should depend upon altitude. Since there is almost
no <230 nm radiation in the troposphere or in the lowest parts of
the stratosphere, the mixing ratio should be independent of altitude
there. In the middle stratosphere the mixing ratio should drop off
quickly, at a rate which is determined by the photodissociation
cross-section. Thus each halocarbon has a characteristic signature
in its mixing ratio profile, which can be calculated. Such calculations
(first carried out in the mid 1970's) agree well with the distributions
presented in the next section.

There is direct evidence as well. Photolysis removes a chlorine
atom, leaving behind a reactive halocarbon radical. The most likely
fate of this radical is reaction with oxygen, which starts a long
chain of reactions that eventually remove all the chlorine and
fluorine. Most of the intermediates are reactive free radicals, but
two of them, COF2 and COFCl, are fairly stable and live long enough
to be detected - and have been. [Zander et al. 1992, 1994].

 

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