Chlorofluorocarbons

Chlorofluorocarbons (CFCs) are organic chemical compounds that contain carbon, fluorine, and chlorine atoms. Each CFC is identified by a unique numbering system that describes its structure. The digit in the hundredths place represents the number of carbon atoms in each molecule minus 1, the digit in the tenths place represents the number of hydrogen atoms in each molecule plus 1, and the digit in the units place indicates the number of fluorine atoms. For example, trichlorotrifluoroethane is a CFC that has three fluorine atoms, no hydrogen atoms, and two carbon atoms in each molecule, whence the designation CFC-113, while dichlorodifluoromethane (CFC-12) has one carbon, no hydrogen, and two fluorines.

CFCs are anthropogenic substances and have no natural background level. At room temperature, they are generally volatile, nontoxic, nonflammable, and colorless liquids or gases. These properties make them attractive to industry. Their commercial production, which began in the 1930s and 1940s, was primarily spurred by the need to find safer alternatives to the sulfur dioxide and ammonia refrigerants used at the time. Today, they are used in refrigeration and air conditioning, as cleansing agents for electrical and electronic components, and as foaming agents in shipping-plastics manufacturing. In some parts of the world, they are also used in aerosol propellants. The most common commercial CFCs, registered under the trade name Freon, are trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12).

Most CFC gases have both global-warming impacts and ozone-depleting effects. They are powerful greenhouse gases and contribute to global warming and climate change. Studies conducted by various researchers in the 1970s linked CFCs, along with other chlorine- and bromine-containing compounds, to the destruction of atmospheric ozone. Ozone is a trace gas that occurs in high concentrations in the stratosphere, the layer of the atmosphere that is between 10 and 25 km (kilometers) above the earth's surface. Ozone absorbs harmful ultraviolet (UV) radiation in the wavelengths between 280 and 320 nm (nanometers) of the UV-B band, which can cause biological damage in plants and animals. The loss of stratospheric ozone results in more harmful UV-B radiation reaching the Earth's surface. Because CFCs are chemically inert, when released into the atmosphere, they do not react easily and can exist in the troposphere for several decades, entering the stratosphere through the mixing and transport processes of the atmosphere. When CFCs enter the stratosphere, the intense UV radiation breaks them down, releasing chlorine (Cl) atoms, which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer.

CFC emissions have rapidly declined since the 1980s, largely due to control of their use under the Montreal Protocol, an international treaty designed to implement the 1985 Vienna Convention for the Protection of the Ozone Layer. The Montreal Protocol was originally signed in 1987, entered into force in 1989, and has been amended seven times. It stipulated [Page 401]that the production and consumption of compounds that deplete stratospheric ozone—CFCs, halons, carbon tetrachloride, and methyl chloroform—were to be phased out at various rates, depending on a country's level of economic development. Some substitute compounds have low ozone depletion potential but high global warming potential (e.g., hydrochlorofluorocarbons, or HCFCs), while others do not deplete ozone but have high global warming potential (e.g., hydrofluorocarbons, or HFCs).

...