Because of the recognition in the mid-f970s that CFCs were a potential threat to stratospheric ozone, their use in aerosol sprays was banned in the United States in f978. On an international level, the Vienna Convention for the Protection of the Ozone Layer agreed in March 1985 on the need to control emissions of CFCs and other chlorine-containing substances. This was followed by the Montreal Protocol in September f 987, which limited the production and import internationally of CFCs as well as halons. Specifically, the consumption of CFC-11, -12, -113, -114, and -115 was to be reduced to their 1986 levels by 1990, to 80% of these levels by 1994, and to 50% by 1999. The Halons-1211, -1301, and -2402 were specified to return to the 1986 levels by 1992. For developing countries, the target for phaseout was the year 20f0 (see International Legal Materials, 1987a,b, for details).
However, about this time, a variety of research indicated that even with full implementation of the Montreal Protocol, the atmospheric abundance of chlorine could reach as much as 6-9 ppb between the years 2050 and 2075. This delay is due to the relatively long time between emission of these compounds into the troposphere and when they reach the stratosphere and photolyze to produce an active chlorine atom. Figure 13.1, for example, compares the estimated equivalent effective stratospheric chlorine from 1960 until the year 2100 with no controls and a 3% increase per year in CFC and methylchloroform emissions to those with the controls agreed to in the Montreal Protocol. Equivalent effective stratospheric chlorine loading depends on emissions as well as removal processes, which determine what fraction of the CFCs emitted at the earth's
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