ChloroFluorocarbons
Chlorfluorocarbons
are a family of non-reactive, nonflamable gases and volatile
liquids.
Understanding
the names
- Halocarbons:
Fluorine, chlorine and bromine belong to a chemical family called
the halogens. When attached to carbon they form a group of compounds
called 'halocarbons". The smaller halocarbons turn into a gas
quite easily and are the prime-suspects in ozone-depletion.
- Chlorofluorocarbons
abreviated CFC, are the major category of man-made halocarbons.
Each different CFC is identified by a numbering system which describes
the CFC structure.
-
In the hundreds place, the number represents the number of
carbons, less one.
- In
the tens place the number represents the number of hydrogens,
plus one.
- In
the ones place the number represents the number of fluorines.
For
example CFC-113 has two carbons, no hydrogens and three fluorines.
CFC-12 has one carbon, no hydrogen and two fluorines.
The
CFC-ozone link
The
non-reactivity of CFC's, so desirable to industry, allows them to
drift for years in the environment until they eventually reach the
stratosphere. High in the stratosphere, intense UV solar radiation
severs chlorines off of the CFC's, and it is these unattached chlorines
that are able to catalytically
convert ozone molecules into oxygen molecules.
Different CFC's require different amounts of time to remove from
the stratosphere, times ranging from 50 to over 200 years; so while
it is cheering to see that the growth-rate of Chlorofluorocarbons
in the atmosphere is starting to drop (Elkins et.al._Nature_364:1993),
the impact of chlorofluorcarbons on stratospheric ozone will continue
well into the 22nd century.
CFC's differ widely in their stabilities and in how effectively
they breakup ozone. A rating system called an Ozone
Depletition Potential , ODP, is used to compare compounds by
how much damage they may cause to environmental ozone. The ODP is
arrived at by dividing the cumulative Ozone depletion of the compound
by the ozone depletion caused by the release of an equal amount
of CFC-12, one of the earliest CFC's. For example methyl chloride,
the major naturally-made source of stratospheric chlorine, has a
ODP of less than 0.10. This means that if equal amounts of methyl
chloride and CFC-12 were releaced into the stratosphere, the methyl
chloride would degrade 1/10th as much ozone as the CFC-12 . Halon
1301, a compound that contains bromine as well as chlorine, has
an ODP of 10 so it is 10 times more destructive than CFC-12.
Proposed
replacements for CFC's
As
goverment pressure to ban CFC's increases, chemical companies are
scrambling to find replacements that are effective, nontoxic and have
a low ODP. Industry is currently proposing to substitute HCFC's (Hydro-chloro-fluorcarbons)
for CFC's because they are nearly as effective and nontoxic as CFC's
but with ODP's that are 1/10th to 1/50th that of CFC-11. A big reason
for their lower ODP is that it is predicted, based on lab experiments,
that HCFC's will break-up in the troposphere and therefore not be
able to transport chlorine into the stratosphere. Industry acknowledges
that HCFC's are merely stopgap measures in the ongoing search for
replacement, but many environmental groups are concerned that HCFC's
may relieve pressure on industry to fund research for long-term replacements.
Most
commonly used CFC's
The
size of the square reflects the relative contribution of each compound
to CFC caused ozone depletion(UNEPdata, 1990)
The
times are the lefetimes of the compound in the atmosphere before
it is broken down and/or removed
Author: Brien Sparling
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