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Chlorofluorocarbons (CFCs or chlorofluormethanes) are a group of chemicals containing carbon, fluorine, and chlorine atoms. They were used extensively throughout the 20th century for various applications because of their general nontoxicity and stable chemical properties. However, CFCs are also extremely potent greenhouse gases and are primarily responsible for the destruction of stratospheric ozone (O3). As of 2003, cumulative worldwide production of CFCs was estimated at 24 million metric tons (53 billion pounds), 96 percent of which has been released into the atmosphere.
CFCs were formally introduced to the world in 1928 by Thomas Midgley Jr. and colleagues with the synthesis of dichlorodifluoromethane (now known as CFC-12, CF2Cl2 ). Midgley’s work was the answer to his assignment by General Motor’s Frigidaire di vision to develop a nontoxic, noninflammable, and noncorrosive refrigerant. To this end, CFCs proved to be an excellent, if not perfect product. In 1930, DuPont began commercial production of CFC-12 under the trade name Freon. Soon after, CFC-11 (CFCl3) entered production, and together with CFC-12, formed the bulk of all CFCs ever produced. During World War II, CFCs began to be used as aerosol propellants and as key components in the production of packing, insulating, and buoyancy foams. CFCs were also used as solvents in the dry-cleaning and electronics industries. In particular, CFC-113 (CF2ClCFCl2) was used extensively for these purposes. Additional CFC varieties were produced (most notably CFC-114 and CFC-115) but accounted for only 3 percent of total CFC production.
In 1973, a study by Jim Lovelock and colleagues revealed the existence of CFCs and other halocarbons in the atmosphere over the Atlantic Ocean. The authors highlighted the practical role of these chemicals as inert tracers of atmospheric processes and described them as constituting “no conceivable hazard.” The following year, Mario J. Molina and F.S. Rowland reported that the most important sink for atmospheric CFCs is likely the dissociation of chlorine atoms by ultraviolet light in the stratosphere (Molina and Rowland received the Nobel Prize for their work in 1995). They suggested that free chlorine atoms catalytically destroy ozone molecules through the following reactions:
Cl + O3 → ClO + O2 and ClO + O → Cl + O2
Once stripped from a CFC molecule, a single chlorine atom can catalyze the destruction of up to 100,000 ozone molecules before it is eventually incorporated into “chlorine reservoirs” such as HCl or chlorine nitrate (ClONO2).
Molina and Rowland raised significant concerns about the continuing rise of atmospheric CFCs and the importance of stratospheric ozone. In 1978, as evidence of CFCs’ threat to the ozone layer strengthened, the United States banned the manufacture and sale of CFCs as aerosol propellants in spray cans (by then, the predominant use of CFCs).
By the mid-1980s, extensive research had confirmed a dramatic reduction of the ozone layer overAntarctica and attributed it to CFCs. The 1987 Montreal Protocol and subsequent amendments required industrialized nations to phase out CFC production by 1996, and by 2010 for developing nations. Although worldwide production of CFCs today is a small fraction of what it once was, the long residence times of CFCs in the atmosphere implies a slow recovery of stratospheric ozone.
While CFCs are more widely known as a threat to ozone, they also significantly contribute to the anthropogenic greenhouse effect. Over the course of 100 years, one pound of CFCs emitted to the atmosphere will reradiate as much heat as 4,600 to 14,000 pounds of carbon dioxide (CO ). Because of this property, even the relatively small concentration of CFCs in the atmosphere (about 1 part per billion by volume) is considered to be a major contributor to global warming. Hydrofluorocarbons (HFCs) are replacing CFCs in many applications because of their benign effects on ozone. However, HFCs are also potent greenhouse gases and are recognized as such in the Kyoto Protocol.
- Stephen O. Andersen and K. Madhava Sarma, Protecting the Ozone Layer: The United Nations History (Earthscan Publications, 2004);
- Mark Z. Jacobson, Atmospheric Pollution: History, Science, and Regulation (Cambridge University Press, 2002);
- E. Lovelock, R.J. Maggs, and R.J. Wade, “Halogenated Hydrocarbons in and over the Atlantic,” Nature (v.241, 1973);
- Mario J. Molina and F.S. Rowland, “Stratospheric Sink for Chlorofluoromethanes: Chlorine Atom-Catalysed Destruction of Ozone,” Nature (v.249, 1974);
- Richard P. Wayne, Chemistry of Atmospheres (Oxford University Press, 2000).