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1991 1992 1993 1994 1995 1996 Year

FIGURE 13.6 Fits to monthly mean tropospheric concentrations of some CFC alternates in the Northern (-) and Southern

(---) Hemispheres (adapted from Montzka et al., 1996a).

Grim (Oram et al., 1995) and at Mace Head. In the latter case, HCFC-141b increased at a rate of 2.49 + 0.03 ppt yr"', compared to a midpoint concentration of 7.38 ppt, and HCFC-f42b increased at a rate of 1.24 + 0.02 ppt yr-1, with a midpoint concentration over the October 1994 to March 1997 time period of 8.78 ppt (Simmonds et al., 1998b). HCFC-141b and -142b have also been detected in the stratosphere (e.g., see Lee et al., 1995).

HCFC-22 (CHC1F2) has been measured continuously from f 978 to 1996 (Miller et al., 1998). its growth rate over this period was 6.0 ppt yr-1, with a 1996 concentration of f f 7 ppt. This concentration and growth rate are similar to those shown in Fig. 13.6 and measured from 1992 to 1997 at La Jolla, California, where the growth rate was 5.5 ppt yr-1.

Figure 13.7 shows the effective total tropospheric concentration of chlorine from halocarbons from 1992 to 1996 (Montzka et al., 1996a). The concentration peaked in 1994 at ~3.0 ppb, but when methyl chloride (CH3C1) and other chlorinated organics are taken into account, the peak was likely ~3.7 ppb. The total tropospheric chlorine concentration in mid-1995 decreased at a rate of approximately 25 ppt per year, in contrast to increases of 110 ppt per year in 1989 (Montzka et al., 1996a; Cunnold et al., 1997). Bromine compounds show the same trend. As a result, the stratospheric levels of chlorine and bromine are expected to peak around the year 2000 (Montzka et al., 1996a; World Meteorological Organization, 1995,1999).

Note, however, that synergistic interactions with the effects of greenhouse gases have the potential to alter the timetable for recovery of stratospheric ozone. For example, while C02 causes warming in the troposphere, it produces cooling in the stratosphere by emitting infrared radiation out to space (see Chapter 14). This cooling may increase the formation of polar stratospheric clouds (PSCs), especially in the Arctic, where temperatures are often not sufficiently low to generate PSCs for extended periods of time. As a result, as discussed in Chapter 14.B.2d, this cooling may delay recovery of stratospheric ozone.

In addition to CFCs and their replacements, there are some fully fluorinated compounds that are emitted to the atmosphere during various industrial processes, including the manufacture of HCFCs and HFCs. Because of the strong C-F bonds, these compounds have long atmospheric lifetimes (e.g., see Cicerone, f979; and Ravishankara et al., 1993) and hence have been used as tracers to determine the age of stratospheric

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