16 What are CFC's?
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
16 What are CFC's?
CFC's - ChloroFluoroCarbons - are a class of volatile organic compounds
that have been used as refrigerants, aerosol propellants, foam blowing
agents, and as solvents in the electronic industry. They are chemically
very unreactive, and hence safe to work with. In fact, they are so inert
that the natural reagents that remove most atmospheric pollutants do not
react with them, so after many years they drift up to the stratosphere
where short-wave UV light dissociates them. CFC's were invented in 1928,
but only came into large-scale production after ~1950. Since that year,
the total amount of chlorine in the stratosphere has increased by
a factor of 4. [Solomon]
The most important CFC's for ozone depletion are:
Trichlorofluoromethane, CFCl3 (usually called CFC-11 or R-11);
Dichlorodifluoromethane, CF2Cl2 (CFC-12 or R-12); and
1,1,2 Trichlorotrifluoroethane, CF2ClCFCl2 (CFC-113 or R-113).
"R" stands for "refrigerant". One occasionally sees CFC-12 referred
to as "F-12", and so forth; the"F" stands for "Freon", DuPont's trade
name for these compounds.
In discussing ozone depletion, "CFC" is occasionally used to
describe a somewhat broader class of chlorine-containing organic
compounds that have similar properties - unreactive in the
troposphere, but readily photolyzed in the stratosphere. These include:
HydroChloroFluoroCarbons such as CHClF2 (HCFC-22, R-22);
Carbon Tetrachloride (tetrachloromethane), CCl4;
Methyl Chloroform (1,1,1 trichloroethane), CH3CCl3 (R-140a);
and Methyl Chloride (chloromethane), CH3Cl.
(The more careful publications always use phrases like "CFC's and
related compounds", but this gets tedious.)
Only methyl chloride has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning.
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years. Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a substantia fraction to reach the stratosphere.
Most of Part II is devoted to stratospheric chlorine chemistry;
look there for more detail.
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