In the mid-1970s, scientists at the Univ. of California, Irvine predicted that CFCs could cause ozone depletion in the upper atmosphere; this was later confirmed by ground-based and satellite studies. When CFCs are released into the atmosphere, they move via air currents to altitudes ranging from 15 to 25 mi (25–40 km). There, they are dissociated by ultraviolet light as given by the reaction: CF2Cl2 → CF2Cl + Cl. The resulting free chlorine atoms (Cl) decompose ozone (O3) into oxygen (O2), Cl + O3 → ClO + O2, and are regenerated by interaction with free oxygen atoms (O), ClO + O → Cl + O2. When chlorine is regenerated, it is free to continue to break down other ozone molecules. This process continues for the atmospheric lifetime of the chlorine atom (one to two years), during which it destroys an average of 100,000 ozone molecules. Chlorine radicals are removed from the stratosphere after forming two compounds that are relatively resistant to dissociation by ultraviolet light: hydrogen chloride (HCl) and chlorine nitrate (ClONO2). Dissociation is slow enough so that these compounds can diffuse down to the troposphere, where they react with water vapor and are removed in rain.
Bromine radicals react like chlorine radicals to remove ozone from the stratosphere and sometimes react in concert with chlorine. Bromine is much more destructive than chlorine because the compounds hydrogen bromide (HBr) and bromine nitrate (BrONO2) are much more susceptible to dissociation by ultraviolet light; thus, many more ozone molecules are destroyed before the bromine molecules can diffuse downward. Fluorine radicals combine to form hydrogen fluoride (HF) and other stable compounds that do not affect the ozone layer.
Ozone is vital to human and animal survival because it is responsible for the absorption of the sun's ultraviolet light. Without this protection, blindness and skin cancers could result from penetrating ultraviolet light. In 1987 an international treaty, the Montreal Protocol, called for reducing CFC use by 50% by 2000. A 1992 amendment to the treaty called for the end of CFC production in industrial countries by 1996, and by 1993 CFC emissions had dropped dramatically.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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