Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs)
Ozone hole and other CFC environmental effects
Chlorofluorocarbons (CFCs) are artificially made chemical compounds used as refrigerants, cleaning solvents, aerosol propellants, and blowing agents for foam packaging in many commercial applications. They typically consist of chlorine, fluorine, carbon, and hydrogen. Freon®, a trade name, is often used to refer to them. CFCs do not spontaneously occur in nature. They were developed by industrial chemists searching for a safer alternative to refrigerants used until the late 1920s. CFCs are non-toxic, chemically non-reactive, inflammable, and extremely stable near the Earth’s surface. Their apparent safety and commercial effectiveness led to widespread use, and to steadily rising concentrations of CFCs in the atmosphere, throughout the twentieth century.
CFCs are generally non-reactive in the troposphere, the lowest layer of the atmosphere, but intense ultraviolet radiation in the outer layer of the atmosphere, called the stratosphere, decomposes CFCs into component molecules and atoms of chlorine. These subcomponents initiate a chain of chemical reactions that quickly breaks down molecules of radiation-shielding ozone (O3) in the lower stratosphere. The stratospheric ozone layer absorbs ultraviolet radiation and protects the Earth’s surface from destructive biological effects of intense solar radiation, including cancers and cataracts in humans.
CFCs and ozone destruction
Laboratory chemists first recognized CFCs as catalysts for ozone destruction in the 1970s, and atmospheric scientists observed that CFCs and their subcomponents had migrated into the lower stratosphere. When scientists discovered a zone of depleted stratospheric ozone over Antarctica, CFCs were identified as the culprit. Announcement of accelerated loss of stratospheric ozone in 1985 spurred research into the exact chemical and atmospheric processes responsible for the depletion, extensive mapping of the Antarctic and Arctic ozone holes, and confirmation of overall thinning of the ozone layer. These discoveries also precipitated an international regulatory effort to reduce CFC emissions and to replace them with less destructive compounds. The Montreal Protocol of 1987 led to a near-complete ban on CFCs and other long-lived chemicals responsible for stratospheric ozone depletion.
CFCs are halogens, a group of synthetic compounds containing atoms of the elements fluorine, chlorine, bromine and iodine. CFC-11 (CFCl3), CFC-12 (CF2 Cl2), CFC-113 (CF2 ClCFCl2), and CFC-114 (CF2 ClCF2 Cl) are the most common forms. Materials scientists first recognized the utility of CFC-12 in 1928 as a replacement for the extremely toxic sulfur dioxide, methyl-chloride, and ammonia-based refrigerants used in turn-of-the-century appliances. CFC-12 and other CFCs were then rapidly developed for other industrial applications and widely distributed as commercial products. Because of their stability, low toxicity, low surface tension, ease of liquidification, thermodynamic properties, and nonflammability, CFCs were used as refrigerants in heat pumps, refrigerators, freezers, and air conditioners; as propellants in aerosols; as blowing agents in the manufacture of plastic foam products and insulation, such as expanded polystyrene and polyurethane; as cleaning and de-greasing agents for metals and electronic equipment and components, especially circuit boards; as carrier gases for chemicals used in the sterilization of medical instruments; and as dry-cleaning fluids.
CFC-11 and CFC-12 were the most widely-used CFCs. Large industrial air-conditioning equipment and centrifugal systems typically used CFC-11. Residential, automotive, and commercial refrigeration and air-conditioning equipment generally contained CFC-12. Some commercial air-conditioning equipment also contained CFC-113 and CFC-114.
Emissions of CFCs to the atmosphere peaked in 1988, when air conditioners, refrigerators and factories released 690 million lb (315 million kg) of CFC-11, and 860 million lb (392 million kg) of CFC-12. At that time, about 45% of global CFC use was in refrigeration, 38% in the manufacture of foams, 12% in solvents, and 5% in aerosols and other uses. Depending on its size, a typical domestic refrigerator sold in 1988 contained about 0.4–0.6 lb (0.2–0.3 kg) of CFCs, a freezer 0.6–1.1 lb (0.3–0.5 kg), and a central air-conditioning unit about 65.5 lb (13.5 kg). At this time, about 90% of new automobiles sold in the United States and 60% of those in Canada have air conditioning units, and each contained 3–4 lb (1.4–2.0 kg) of CFCs. CFC production and use in the United States in 1988 involved about 5, 000 companies in 375, 000 locations, employing 700, 000 people, and generating $28 billion worth of goods and services. As of the mid-2000s, the production and use of CFCs have declined drastically or been eliminated completely due to increased bans by the Montreal Protocol.
Chemical activity of CFCs
CFCs are highly stable, essentially inert chemicals in the troposphere, with correspondingly long residence times. For example, CFC-11 has an atmospheric lifetime of 60 years, CFC-12 120 years, CFC-113 90 years, and CFC-114 200 years. The atmospheric concentration of total CFCs in the early 1990s was about 0.7 ppb (parts per billion), and was increasing about 5 to 6% per year. Because of continued releases from CFC-containing equipment and products already in use, CFC emissions to the lower atmosphere have continued since their manufacture was banned in 1990. However, CFC concentrations in the troposphere declined in 2000 for the first time since the compounds were introduced. Model calculations show that it will take 20 to 40 years to return to pre-1980 levels.
Because of their long life spans and resistance to chemical activity, CFCs slowly wend their way into the stratosphere, 5 to 11 mi (8 to 17 km) above the Earth’s surface, where they are exposed to intense ultraviolet and other short-wave radiation. CFCs degrade in the stratosphere by photolytic breakdown, releasing highly reactive atoms of chlorine and fluorine, which then form simple compounds such as chlorine monoxide (ClO). These secondary products of stratospheric CFC decomposition react with ozone (O3), and result in a net consumption of this radiation-shielding gas.
Ozone is naturally present in relatively large concentrations in the stratosphere. Stratospheric O3 concentrations typically average 0.2 to 0.3 ppm, compared with less than 0.02 to 0.03 ppm in the troposphere. (Ozone, ironically, is toxic to humans, and tropospheric O3 is a component of the photochemical smog that pollutes the air in urban areas.) Stratospheric O3 is naturally formed and destroyed during a sequence of photochemical reactions called the Chapman reactions. Ultraviolet radiation decomposes O2 molecules into single oxygen atoms, which then combine with O2 to form O3 . Ultraviolet light then breaks the O3 molecules back into O2 and oxygen atoms by photodissociation. Rates of natural ozone creation and destruction were essentially equal, and the concentration of stratospheric ozone was nearly constant, prior to introduction of ozone-depleting compounds by human activity. Unlike the Chapman reactions, reactions with trace chemicals like ions or simple molecules of chlorine, bromine, and fluorine results in rapid one-way depletion of ozone. CFCs account for at least 80% of the total stratospheric ozone depletion. Other artificially-made chemical compounds, including halogens containing bromide and nitrogen oxides, are responsible for most of the remaining 20%.
The stratospheric O3 layer absorbs incoming solar ultraviolet (UV) radiation, thereby serving as a UV shield that protects organisms on the Earth’s surface from some of the deleterious effects of this high-energy radiation. If the ultraviolet radiation is not intercepted, it disrupts the genetic material, DNA (deoxyribonucleic acid), which is itself an efficient absorber of UV. Damage to human and animal DNA can result in greater incidences of skin cancers, including often-fatal melanomas; cataracts and other eye damage such as snow blindness; and immune system disorders. Potential ecological consequences of excessive UV radiation include inhibition of plant productivity in regions where UV light has damaged pigments, including chlorophyll.
Ozone hole and other CFC environmental effects
British Antarctic Survey scientists first observed a large region of depleted stratospheric O3 over Antarctica during the late 1970s, and dubbed it an ozone hole. Springtime decreases in stratospheric ozone averaged 30 to 40% during the 1980s. By 1987, 50% of the ozone over the Antarctic continent was destroyed during the austral spring (September to December), and 90% was depleted in a 9 to 12 mi (15 to 20 km) wide zone over Queen Maud Land. Atmospheric scientists also observed an Arctic ozone hole, but it was smaller, more variable, and less depleted than its southern hemisphere counterpart. Rapid ozone depletion and the development of seasonal ozone holes is most pronounced at high latitudes where intensely cold, dark conditions and isolating atmospheric circulation promote ozone-destroying chemical reactions and inhibit synthesis of new ozone molecules during the darkest spring months. Stratospheric O3 destruction by the secondary compounds of CFCs also occurs at lower latitudes, but the depletion is much slower because constant light and atmospheric mixing foster ozone regeneration. A seasonal thinning of the stratospheric ozone layer also occurs at lower latitudes when O3-depleted air disperses from the poles in late spring.
In addition to accelerating the loss of stratospheric ozone, CFCs may also contribute to an intensification of the so-called greenhouse effect and to long-term global climate change. The greenhouse effect is a phenomenon by which an envelope of atmospheric gases and vapors like carbon dioxide and water vapor maintains the Earth’s average surface temperature at about 77°F (25°C) by trapping a portion of the heat emitted by the planet’s surface. Insulation by the greenhouse gases keeps the Earth’s temperature approximately 33 degrees warmer than would be possible if the atmosphere was transparent to long-wave infrared energy. The greenhouse effect also permits existence of the large reservoirs of liquid water that sustain biological life on the planet. The pre-industrial concentration of greenhouse gases was chemically balanced to allow global cooling at a rate that maintained temperatures within an acceptable temperature range. However, environmental scientists are concerned that natural and synthetic radioactive gases emitted by human activities will slow the Earth’s cooling rate, and lead to global warming. CFCs are greenhouse gases; they are very efficient absorbers of infrared energy. On a per-molecule basis, CFC-11 is 3, 000 to 12, 000 times as efficient as carbon dioxide as a greenhouse gas, and CFC-12 is 7, 000 to 15, 000 times as effective. Atmospheric concentrations of total CFCs increased from almost zero several decades ago to about 0.5 ppb in 1992. From 1994 to 2006, the ppb rate has been slowly declining and is expected to continue to shrink in the future.
CFC reduction efforts
Concerns about the environmental effects of CFCs led to partial restrictions on their use in the early 1980s, when they were prohibited as propellants in aerosol cans. Industrial chemists also began a search for chemical compounds to replace CFCs in refrigerators, air conditioners, manufacturing processes and aerosol generators. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) can replace CFCs for many purposes, and ammonia functions well as a refrigerant in modern cooling units. HCFCs and HFCs are much less durable than CFCs in the lower atmosphere because they contain hydrogen atoms in their molecular structure, are thus less likely to persist and carry their ozone-destroying chlorine and fluorine atoms into the stratosphere. However, these materials are also not free from controversy. In the 2000s, some HCFCs have been phased out of the Montreal Protocol. The United States has eliminated the production of one type of HCFC as of 2002.
The United Nations Environment Program (UNEP) convened the Montreal Protocol on Substances that
KEY TERMS
Greenhouse effect— The physical process that allows the Earth’s surface to be maintained at an average of 77°F (25°C), about 33° warmer than would otherwise be possible if certain gases and vapors, especially carbon dioxide and water, did not interfere with the rate of dissipation of absorbed solar radiation.
Ozone holes— Decreased concentrations of stratospheric ozone, occurring at high latitudes during the early springtime. Ozone holes are most apparent over Antarctica, where they develop under intensely cold conditions during September and November, allowing a greater penetration of deleterious solar ultraviolet radiation to the Earth’s surface.
Stratosphere— A layer of the upper atmosphere above an altitude of 5 to 11 mi (8 to 17 km) and extending to about 31 mi (50 km), depending on season and latitude. Within the stratosphere, air temperature changes little with altitude, and there are few convective air currents.
Troposphere— The layer of air up to 15 mi (24 km) above the surface of the Earth, also known as the lower atmosphere.
Deplete the Ozone Layer to regulate production, use, and emission of CFCs in 1987. The Montreal Protocol was a comprehensive, international agreement designed to slow and eventually reverse stratospheric ozone depletion. The 1987 protocol called for a 50% reduction of CFC emissions by 2000. Scientific advances prompted amendments to the protocol in 1990 and 1992, the most recent of which required signatories to cease production of the main CFCs by 1995. (Exceptions were allowed for limited essential uses, including medical sprays.) Since then, three other revisions have been enacted—in 1995, 1997, and 1999. Some major industrial users of CFCs committed to phasing out CFCs earlier, and the European community agreed to an even stricter set of regulations that requires even tighter restrictions on CFCs and other ozone-depleting chemicals. With the success of the initial signatory countries at reducing CFC usage, over 150 additional countries also signed the Montreal Protocol, which has been modified to accommodate new information and changing circumstances. As of March 2005, 189 countries have signed the Montreal Protocol. The manufacture of CFCs has since been banned in most industrialized countries. Developing countries who have signed the Protocol have restricted CFC consumption as of 1999 and will reduce this consumption further as of 2005 with the promise to eliminate CFCs totally by 2010. The Montreal Protocol allows each country to develop its own economically favorable but still effective way to limit its emissions of ODSs and then eliminate them.
Because of these regulatory measures, CFC concentrations declined in lower atmosphere, and remained constant in the upper atmosphere starting from 2000 to 2006. Atmospheric scientists predict that the phase-out of CFCs and other ozone-destroying chemicals should result in disappearance of the Antarctic ozone hole by about 2050. Because of the unusually rapid and effective international response to the problem of stratospheric ozone depletion caused by emissions of CFCs, the Montreal Protocol and subsequent agreements on CFCs have been described as an environmental success story.
See also Air pollution; Halogenated hydrocarbons; Ozone layer depletion.
Resources
BOOKS
Arieti, David F. The Earth is my Patient. Bloomington, IN: Authorhouse, 2005.
Hester, R. E., and R. M. Harrison. Global Environmental Change. Cambridge, UK: Royal Society of Chemistry, 2002.
Metcalfe, Sarah E. Atmospheric Pollution and Environmental Change. London, UK: Hodder Arnold; New York: Oxford University Press, 2005.
Tinsley, Ian J. Chemical Concepts in Pollutant Behavior. Hoboken, NJ: Wiley-Interscience, 2004.
PERIODICALS
“For the Ozone Layer, a New Look.” New York Times (October 8, 2002): D1.
OTHER
CEISN thematic guides. “Chlorofluorocarbons and Ozone Depletion” <http://www.ciesin.org/TG/OZ/cfcozn.html> (accessed October 4, 2006).
National Oceanographic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA). “Chlorofluorocarbons (CFCs)” <http://www.cmdl.noaa.gov/noah/publictn/elkins/cfcs.html> (accessed October 4, 2006).
University of Cambridge Centre for Atmospheric Science. “The Ozone Hole Tour.” Cambridge University. <http://www.atm.ch.cam.ac.uk/tour/index.html> (accessed October 4, 2006).
Bill Freedman
Laurie Duncan