17 How do CFC's destroy ozone?
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
17 How do CFC's destroy ozone?
CFC's themselves do not destroy ozone; certain of their decay products
do. After CFC's are photolyzed, most of the chlorine eventually ends
up as Hydrogen Chloride, HCl, or Chlorine Nitrate, ClONO2. These are
called "reservoir species" - they do not themselves react with ozone.
However, they do decompose to some extent, giving, among other things,
a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO,
which can catalyze the destruction of ozone by a number of mechanisms.
The simplest is:
Cl + O3 -> ClO + O2
ClO + O -> Cl + O2
Net effect: O3 + O -> 2 O2
Note that the Cl atom is a _catalyst_ - it is not consumed by the
reaction. Each Cl atom introduced into the stratosphere can
destroy thousands of ozone molecules before it is removed.
The process is even more dramatic for Bromine - it has no stable
"reservoirs", so the Br atom is always available to destroy ozone.
On a per-atom basis, Br is 10-100 times as destructive as Cl.
On the other hand, chlorine and bromine concentrations in
the stratosphere are very small in absolute terms. The mixing ratio
of chlorine from all sources in the stratosphere is about 3 parts
per billion, (most of which is in the form of CFC's that have not
yet fully decomposed) whereas ozone mixing ratios are measured in
parts per million. Bromine concentrations are about 100 times
smaller still. (See Part II.)
The complete chemistry is very complicated - more than 100
distinct species are involved. The rate of ozone destruction at any
given time and place depends strongly upon how much Cl is present
as Cl or ClO, and thus upon the rate at which Cl is released from
its reservoirs. This makes quantitative _predictions_ of future
ozone depletion difficult. [Rowland 1989, 1991] [Wayne]
The catalytic destruction of ozone by Cl-containing radicals was first
suggested by Richard Stolarski and Ralph Cicerone in 1973. However,
they were not aware of any large sources of stratospheric chlorine.
In 1974 F. Sherwood Rowland and Mario Molina realized that CFC's
provided such a source. [Molina and Rowland 1974][Rowland and Molina 1975]
For this and for their many subsequent contributions to stratospheric
ozone chemistry Rowland and Molina shared the 1995 Nobel
Prize in Chemistry, together with Paul Crutzen, discoverer of the NOx
cycle. (The official announcement from the Swedish Academy can be found
on the web at http://www.nobel.se/announcement95-chemistry.html .)
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