estimated to be reduced to (0 to —0.6), 6-7, and —0.7 ppt per year, respectively (Montzka et al., 1996a; Cun-nold et al., 1997). Derwent et al. (1998) report that between 1995 and 1996, CFC-11 decreased by -0.8 ± 0.2 ppt per year, and CFC-113 by about —0.5 ppt per year. That is, the tropospheric concentrations of CFC-11 and CFC-113 have started to decrease in response to the controls. (For a review of the history and atmospheric trends in CFC-11 until 1992, see Khalil and Rasmussen (1993).) The same is seen in Fig. 13.4 to be true for methylchloroform and CC14. The latter is used in the production of CFCs. CC14 peaked at 104 + 3 ppt in 1989-1990 and has been decreasing at 0.7 + 0.1 ppt per year since then (Simmonds et al., 1998a; Derwent et al., 1998). Part of the reason for the rapid response of CH3CC13 concentrations to changes in emissions (Fig. f 3.4), with a decrease of about —18 + 2 ppt per year between mid-1995 and mid-1996 (Derwent et al., 1998), is its short tropospheric lifetime (~5 years) due to the presence of abstractable hydrogen atoms.

From the discussion of the chemistry of CFCs in the stratosphere in the preceding chapter, it might be expected that HC1 and HF from the reactions of the chlorine and fluorine atoms produced on photolysis of the CFCs would also have increased in the stratosphere. This has also been observed, and the magnitude and timing of the increases are clearly consistent with the increase in CFCs (Rinsland et al., 1991; Russell et al., 1996). For example, Wallace et al. (1997) have followed stratospheric HC1 using ground-based infrared spectroscopy with the sun as the source from 1971 through April of 1997. Over this period, HC1 increased by a factor of 3-4. There is an indication in the data that the rate of increase has slowed since about 1990, as expected given the trend in CFCs.

Figure 13.5 shows the measured global mean tropospheric concentrations of the halons H-1301, H-1211, and H-2402 (Butler et al., 1998). At the end of 1996, the mixing ratio of H-f301 was 2.3 + 0.1 ppt, with a growth rate of 0.044 + O.Off ppt yr_l. As seen in Fig. 13.5 and in other analyses of the trend (Montzka et al., f996a), the growth rate appears to be slowing. H-1211 was present at a mixing ratio of 3.5 + 0.1 ppt at the end of 1996, with a growth rate of 0.16 + 0.016 ppt yr-1. Fewer data are available for H-2402, which was present at a concentration of 0.45 + 0.03 ppt at the end of f996 and had a growth rate of 0.009 + 0.001 ppt yr-1.

At the same time that the production and usage of the compounds shown in Figs. 13.4 and 13.5 were decreasing due to the international agreements, the use of alternates (Table 13.1) was increasing (e.g., see Midgley and McCulloch, 1997; and McCulloch and

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