Ozone Layer Depletion
Stratosphere And Chlorofluorocarbons
Chlorofluorocarbons (CFCs) are compounds that contain atoms of carbon, chlorine, and fluorine. CFCs are very stable chemicals, and they are easily liquefied and gasified, are non-flammable, and are of low toxicity. CFCs have had many industrial uses, especially in refrigeration, as propellants in aerosol sprays, as blowing agents used to manufacture synthetic foams and insulation, as cleaning agents for electronic components, as carrier gases for sterilizers of medical instruments, and as dry-cleaning fluids. After most of these uses, CFCs are emitted to the lower atmosphere, where they are very persistent. The CFCs slowly penetrate into the stratosphere, where exposures to highly energetic, short wave solar radiation are intense, causing the CFCs to degrade. This releases chlorine and fluorine atoms, which are then available to consume ozone molecules in secondary reactions. It has been estimated that CFCs account for at least 80% of the depletion of stratospheric ozone worldwide. It has been estimated that the ozone hole over Antarctica is almost 100% a result of CFCs.
As a result of widespread awareness and concerns about the role of CFCs in the depletion of stratospheric ozone, the uses and emissions of these chemicals are being rapidly diminished. Some uses were widely banned as early as the 1980s, for example, the use of CFCs as propellants in aerosol spray cans. A conference sponsored by the United Nations Environment Programme in 1987 resulted in the so-called Montreal Protocol, which was subsequently revised and made more stringent in 1990, when it called for a complete phase out of global CFC use by the year 2000. One hundred and sixty five nations signed this document. The United States, Australia, and other developed countries have completely phased out the production of CFCs. According to the Montreal Protocol, developing countries have until the year 2010 to complete their phase out. Since the Montreal Protocol, there has been a 3% overall decline in ozone-depleting substances, including CFCs as well as methyl chloroform and halons from fire extinguishers. There has been a relatively rapid and effective international response to CFC emissions.
Other substances that can find their way into the stratosphere can increase the rate of ozone depletion as well. Halons, which are compounds consisting of bromine, fluorine, and carbon, can end up in the upper atmosphere where the halogens found in the compounds catalyze the ozone consuming reactions. Methyl bromide provides the catalyst bromine. Hydrochlorofluorocarbons (HCFCs, compounds consisting of hydrogen, chlorine, fluorine, and carbon) and carbon tetrachloride release chlorine and fluorine into the upper atmosphere.
Recent data from both ground-based and satellite measurements indicate that after decades of continual increase, concentrations of chlorine in the stratosphere are starting to plateau. The concentrations of bromide, however are continuing to increase and the depletion of ozone as a result of halides has been as great as 30% over the last decade. However, if halide and CFC concentrations continue to fall as directed by the Montreal Protocol, computer models predict that Antartic ozone concentrations should begin to increase in 2010.
See also Greenhouse effect.
Benedick, R.E. Ozone Diplomacy: New Directions in Safeguarding the Planet. Washington, DC: World Wildlife Fund, 1991.
Zurer, P.S. "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists." Chemical and Engineering News (May 24, 1993): 8–18.
Executive Summary. Scientific Assessment of Ozone Depletion: 2002. July, 31 2002.