NIWOT RIDGE

Period of Record

Trends

CFC-11 and CFC-12

From January 1977 to December 1992, the monthly average mixing ratio of CFC-11 at Niwot Ridge, Colorado, U.S.A., increased from 151.1 to 277.5 parts per trillion by mole (ppt). During 1977-84, the increase was nearly linear, averaging 8.5 ppt/year (R² = .99). From July 1977 to December 1992, the monthly average mixing ratio of CFC-12 increased from 254.2 to 522.4 ppt. Although gaps occur in the record, the increase was again nearly linear during 1977-84, averaging 16.8 ppt/year (R² = .97). The gap in the CFC-12 record during 1984-85 resulted from a gas chromatograph problem affecting only CFC-12. A surge in CFC production worldwide beginning in 1985 caused a corresponding rise in the atmospheric growth rates at Niwot Ridge; these reached average 1985-88 values of 12.2 ppt/year (R² = .98) for CFC-11 and 22.1 ppt/year (R² = .98) for CFC-12. Since 1988 the growth rates at Niwot Ridge have decreased rapidly, particularly for CFC-11. The mean 1992 growth rate was 12.0 ppt/year for CFC-12, while no statistically significant (at the P = .05 level) trend was found for CFC-11. These decreases in atmospheric CFC mixing ratios reflect the recent worldwide declines in production and emissions of CFCs and are consistent with findings at other sites in the NOAA/CMDL network (Elkins et al. 1993).

HCFC-22

From December 1991 to December 1992, the mixing ratio of HCFC-22 at Niwot Ridge increased from 103.3 to 115.1 ppt, averaging 7.5 ppt/year. This trend is highly significant (P = .02).

The collective results presented here for HCFC-22 from the seven monitoring sites show a mean global mixing ratio of 100 ppt for March 1992, with a mean interhemispheric ratio (Northern hemispheric mixing ratio : Southern hemispheric mixing ratio) of 1.14. The mean global growth rate of HCFC-22 over the period of record was 7% per year (Montzka et al. 1993).

H-1301 and H-1211

From August 1989 to November 1991, the mixing ratio of H-1211 at Niwot Ridge increased from 1.96 to 2.53 ppt, averaging 0.19 ppt/year. Over the same period, the mixing ratio of H-1301 rose from 1.56 to 1.96 ppt, also at an average of 0.19 ppt/year. For both H-1211 and H-1301, the trends are significant (at the P = .05 level). Comparing their data with published results from other investigators (covering the period 1978-92), Butler et al. (1992) concluded that significant declines have occurred in the growth rates of the atmospheric mixing ratios of both halons. These recent drops in growth are supported in general by industry production figures, emission estimates, and current appraisals of atmospheric lifetimes.

Niwot Ridge
Colorado, U.S.A.
Alpine mountain
40° 06' N, 105° 36' W
3472 m above MSL

Niwot Ridge Mixing Ratios Graph

• Monthly atmospheric mixing ratios of CFC-12, CFC-11, HCFC-22, H-1211, and H-1301 at Niwot Ridge