Chlorofluorocarbons and Related Compounds
Chlorofluorocarbons and Related Compounds
Chlorofluorocarbons (CFCs) are greenhouse gas compounds that contain only carbon, fluorine, and chlorine. Chlorofluorocarbons and related compounds, including halons and hydrobromofluorocarbons, destroy ozone molecules in the stratosphere and cause ozone depletion. The ozone layer is a layer in Earth's stratosphere that absorbs most of the ultraviolet radiation entering Earth's atmosphere.
The greatest concern is that ozone is the only absorber of UVB (medium-wave ultraviolet) radiation, which can cause skin cancer. As ozone is depleted, a greater amount of UVB radiation reaches the surface of Earth and the risk of skin cancer increases. Concern over the depletion of the ozone layer has led to successful international action to reduce the use of CFCs and related compounds.
Historical Background and Scientific Foundations
In 1974, Mexican-American chemist Mario Molina and American chemist F. Sherwood Rowland published a
paper in the journal Nature stating that CFCs were depleting stratospheric ozone. The hypothesis was initially controversial, but was confirmed by later research, including an alarm sounded by the British scientist Joseph Farman, who had studied atmospheric chemistry near the South Pole since 1957, and reported in 1985 that ozone levels above Antarctica had reduced by more than one third. Satellite observation of the increase in the size of the Antarctic ozone hole was then confirmed by the National Aeronautics and Space Administration (NASA) satellite Nimbus-7.
CFCs were being released into the atmosphere because of their use in refrigeration and air conditioning as well as their use as solvents, fire extinguishers, and as aerosol propellants in spray cans. Molina and Rowland described how CFCs are extremely stable in the lower atmosphere and remain in the atmosphere for about 100 years. They also described how they deplete stratospheric ozone in a chain reaction that results in the production of molecules that destroy additional molecules.
In a paper in American Scientist, Rowland stated that one CFC molecule can destroy around 100,000 ozone molecules. For their research into CFCs, Molina and Rowland were awarded a share in the 1995 Nobel Prize in chemistry.
Impacts and Issues
In 1981, action began on developing an international agreement to reduce CFC production and consumption. This led to the Montreal Protocol on Substances that Deplete the Ozone Layer, an international treaty that went into effect on January 1, 1989. The Montreal Protocol originally called for a reduction of CFC production and consumption to 50% of 1986 levels by 1999, then 0% as of 1996. By September 2007, 190 countries had signed the treaty.
In 2007, the EPA reported that the thinning of the ozone layer had slowed and that it could recover to its pre-1980 level between the years 2060 and 2075. It also reported that this could result in more than $4 trillion in societal health benefits in the United States alone, while preventing more than 6 million premature skin cancer deaths within the following century.
WORDS TO KNOW
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
ATMOSPHERIC CHEMISTRY: Study of the chemistry of planetary atmospheres. The interactions of pollutants, greenhouse gases, Earth's natural atmosphere, solar radiation, and other factors are all studied under the aegis of atmospheric chemistry.
OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.
STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth's surface.
ULTRAVIOLET RADIATION: The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5% of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.
Although CFCs do contribute to the greenhouse effect, their properties also cause cooling during ozone destruction. So far, scientists estimate that the cumulative effect of warming versus cooling resulting from CFC release into the atmosphere has been about equal. Ozone destruction, therefore, not climate change itself, remains the largest problem associated with CFCs.
Molina, M. J., and F. S. Rowland. “Stratospheric Sink for Chlorofluoromethanes: Chlorine-atom Catalyzed Distribution of Ozone.” Nature 249 (1974): 810-812.
Rowland, F. S. “Chlorofluorocarbons and the Depletion of Stratospheric Ozone.” American Scientist 77 (1989): 36-45.
“Achievements in Stratospheric Ozone Protection Progress Report.” Environmental Protection Agency (EPA), 2007. < http://www.epa.gov/ozone/pdffile/spd-annual-report_final_lowres_4-25-07.pdf> (accessed October 26, 2007).