GLOBAL AND HEMISPHERIC

Trends

The most obvious feature is the leveling off of the globally and hemispherically averaged mixing ratios of the CFCs, particularly CFC-11. This decline in growth is consistent with the decreased production and use of CFCs reported by the CFC producers (see AFEAS 1992) and is due primarily to decreased CFC use in aerosol propellants and foam-blowing applications. The leveling off is most significant in the Northern Hemisphere, where most CFCs are used and emitted. Growth in the Southern Hemisphere has decreased less rapidly, however, because growth rates there are determined mainly by interhemispheric transport. Previous reports (Rasmussen and Khalil 1986; Cunnold et al. 1986; Elkins et al. 1988) found that the mean global mixing ratios between 1977 and 1984 were increasing linearly at a constant rate; the collective results presented here for the seven NOAA/CMDL monitoring sites during 1977-84 give global rates of 9±1 (1sigma) parts per trillion by mole per year (ppt/year) for CFC-11 and 17±3 ppt/year for CFC-12. The gap in the data for CFC-12 during 1984-85 resulted from a gas chromatograph problem that affected only CFC-12. A surge in CFC production during 1985-88 caused the observed global growth rates [calculated as average yearly maxima (Elkins et al. 1993)] to climb to 11±1 ppt/year for CFC-11 and 19.5±2 ppt/year for CFC-12. Since 1988, the growth rates have been decreasing rapidly, reaching levels of 2.7±0.7 ppt/year for CFC-11 and 10.5±0.3 ppt/year for CFC-12 by the beginning of 1993 (Elkins et al. 1993). The absolute values (Montzka et al. 1992) presented here for CFC-11 are ~3.2±1.3% lower and for CFC-12 are ~3.4±1.5% higher than those measured by Cunnold et al. (1986), according to a comparison of data from a jointly monitored station at Samoa. Results from a recent laboratory intercomparison of two cylinders showed that this scale is 0.1% higher for CFC-11 and 1.3% higher for CFC-12 than the scale of R.F. Weiss (Bullister and Weiss 1988).

The CFC-11 and CFC-12 records for the Northern and Southern Hemispheres also show a divergence in growth rates during the El Niño-Southern Oscillation (ENSO) event of 1986-87. This period was also coincident with a quasi-biennial oscillation event (QBO), followed by the strongest cold event in 15 years, the La Niña of 1988-89. According to Elkins et al. (1993), all of these processes inhibit interhemispheric transport and therefore slow CFC growth rates in the Southern Hemisphere.

CFC11 and CFC 12 Mixing Ratios Graph

• Monthly global and hemispheric mixing ratios of CFC-11 and CFC-12