officially the Protocol on Substances That Deplete the Ozone Layer, treaty signed on Sept. 16, 1987, at Montreal by 25 nations; 197 nations are now parties to the accord. The protocol set limits on the production of chlorofluorocarbons
(CFCs), halons, and related substances that release chlorine or bromine to the ozone layer
of the atmosphere. On the basis of increasing scientific knowledge about the effects of CFCs and halons on the ozone layer, the original protocol has been amended several times. At subsequent meetings of the signatories to the protocol amendments have been adopted that were designed to speed up the phasing out of or control ozone-depleting substances and aid developing countries in complying; not all parties to the main protocol are parties to these amendments. The production and consumption of halons was phased out by Jan. 1, 1994, and of CFCs, carbon tetrachloride, methyl chloroform, and hydrobromofluorocarbons by Jan. 1, 1996, subject to an exception for agreed essential users. Methyl bromide was to be phased out by 2005 but a number of users of the chemical won temporary exceptions from the ban, and hydrochlorofluorocarbons are to be phased out by 2020. (Phaseout dates were set later for developing countries.)
Under the protocol, the ozone-depleting potential, or ODP, of any substance is measured with respect to an equal mass of CCl 3F, or CFC-11, which is assigned a value of 1.0. Most other CFCs have ODPs that range from about 0.5 to about 1.3. Hydrochlorofluorocarbons, which are being used as transitional replacements (until 2020) for CFCs in refrigeration, have ODPs that are generally less than 0.5. Hydrofluorocarbons, which are also replacing CFCs as refrigerants, have ODPs of zero but are greenhouse gases. Ozone-depleting potentials are based on existing scientific knowledge and are to be reviewed and revised periodically.
See D. E. Newton, The Ozone Dilemma (1995).
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