Cd

< 1952 1960 1970 1980 Year

FIGURE 12.11 Estimated annual worldwide releases of CFC-11 and CFC-12 from 1952 to 1980. Data from Chemical Manufacturers' Association (adapted from National Research Council, 1984).

from 30°N to 90°N at a variety of locations, including the United States, Canada, and Ireland. The increases seen at the surface in the 1985 to 1994 period have also been observed by remote sensing techniques on the space shuttle (e.g., Zander et al., 1996). The rate of growth has now started to decrease due to the decreased emissions. For example, CFC-11 was growing at about 9-11 ppt/year around 1980; this fell to ~7 ppt/year in 1990, 3 ppt/year by 1993 (WMO, 1995), and, more recently, to slightly negative values (Montzka et al., 1996; Cunnold et al., 1997; Derwent et al., 1998).

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