Halogenated fluorocarbons are human-made organic compounds. Since the 1950s, new chemical synthesis techniques have made it possible for chemists to create halogen-containing hydrocarbon gases. These synthetic gases found widespread application as aerosol propellants, foaming agents for plastics, solvents, and coolants for refrigerators and air conditioners. In the mid-1970s, scientists discovered that the halogenated fluorocarbons accumulated in the atmosphere and damaged the protective ozone layer above Earth.
Historical Background and Scientific Foundations
Three generations of halogenated fluorocarbons have been produced. Chlorofluorocarbons (CFCs)—invented in the 1930s—are the first group of halogenated fluorocarbons manufactured. CFCs contain carbon, hydrogen (usually), and either chlorine or fluorine, or both. They are colorless, odorless, nontoxic, and nonflammable.
WORDS TO KNOW
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
FLUOROCARBON: Compounds containing both carbon and fluorine. Such compounds are extremely rare in nature but have many technological uses in refrigeration, microprocessor manufacture, and fire extinguishing. Many fluorocarbons damage the ozone layer and are powerful greenhouse gases as well.
HALOGEN: Any one of the halogen family of elements with similar chemical properties, including fluorine, chlorine, bromine, and iodine.
OZONE LAYER: The layer of ozone that begins approximately 9.3 mi (15 km) above Earth and thins to an almost negligible amount at about 31 mi (50 km) and shields Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 15.5 mi (25 km) above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern.
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.
However, in the mid-1970s, scientists began to determine that CFCs in the stratosphere were decomposed by sunlight. One product of that decomposition, atomic chlorine, reacts with ozone (O3) to form ordinary oxygen (O2). The apparently harmless CFCs turned out, instead, to be a major factor in the loss of ozone from the stratosphere. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation, and shields organisms from its deleterious effects. In addition, CFCs were found to be very persistent chemicals (about 77 years for CFC-11 and 139 years for CFC-12).
Impacts and Issues
Despite the fact that the use of CFCs was phased out following an agreement by many nations in 1987 called the Montreal Protocol, these chemicals will remain in the atmosphere for many years and will continue to pose a threat to the ozone layer.
The next generation of halogenated fluorocarbons, the hydrochlorofluorocarbons (HCFCs), were designed to have hydrogen replace some of the chlorine atoms. As noted earlier, elemental chlorine reacts with ozone to form ordinary oxygen when the gas reaches Earth's upper atmosphere. Although not as damaging as CFCs, HCFCs also deplete the ozone layer and persist for 1–18 years in the atmosphere. HCFCs also are being banned.
A third generation of chemicals that can be used for the same applications as CFCs are hydrofluorocarbons (HFCs). These compounds contain only hydrogen, fluorine, and carbon. Since they do not contain chlorine, HFCs are not ozone-depleting substances, but they are powerful greenhouse gases. The global warming potential of HFC compounds ranges from 140 to 11,700 times greater than the potential of the principal greenhouse gas, carbon dioxide (CO2).
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