| Themes > Science > Earth Sciences > Hydrology, Meteorology, Climatology > Meteorology / Climatology > The Ozone Layer > 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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