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19 What about HCFC's and HFC's? Do they destroy ozone?




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

19 What about HCFC's and HFC's? Do they destroy ozone?

HCFC's (hydrochlorofluorocarbons) differ from CFC's in that only
some, rather than all, of the hydrogen in the parent hydrocarbon
has been replaced by chlorine or fluorine. The most familiar
example is CHClF2, known as "HCFC-22", used as a refrigerant and
in many home air conditioners (auto air conditioners use CFC-12).
The hydrogen atom makes the molecule susceptible to attack by the
hydroxyl (OH) radical, so a large fraction of the HCFC's are
destroyed before they reach the stratosphere. Molecule for
molecule, then, HCFC's destroy much less ozone than CFC's, and
they were suggested as CFC substitutes as long ago as 1976.

Most HCFC's have ozone depletion potentials around 0.01-0.1, so that
during its lifetime a typical HCFC will have destroyed 1-10% as
much ozone as the same amount of CFC-12. Since the HCFC's are more
reactive in the troposphere, fewer of them reach the stratosphere.
However, they are also more reactive in the stratosphere, so they
release chlorine more quickly. The short-term effects are therefore
larger than one would predict from the steady-state ozone depletion
potential. When evaluating substitutes for CFC's, the "time-dependent
ozone depletion potential", discussed in the preceding section,
is more useful than the steady-state ODP. [Solomon and Albritton]

HFC's, hydrofluorocarbons, contain no chlorine at all, and hence
have an ozone depletion potential of zero. (In 1993 there were
tentative reports that the fluorocarbon radicals produced by
photolysis of HFC's could catalyze ozone loss, but this has now
been shown to be negligible [Ravishankara et al. 1994]) A familiar
example is CF3CH2F, known as HFC-134a, which is being used in some
automobile air conditioners and refrigerators. HFC-134a is more
expensive and more difficult to work with than CFC's, and while it
has no effect on stratospheric ozone it is a greenhouse gas (though
somewhat less potent than the CFC's). Some engineers have argued
that non-CFC fluids, such as propane-isobutane mixtures, are better
substitutes for CFC-12 in auto air conditioners than HFC-134a.

 

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