ALERT

Period of Record

Trends

CFC-11 and CFC-12

From February 1988 to November 1992, the monthly average mixing ratio of CFC-11 at Alert, Northwest Territories, Canada, increased from 256.2 to 283.7 parts per trillion by mole (ppt). Over nearly the same period (February 1988-December 1992), the monthly average mixing ratio of CFC-12 increased from 458.5 to 537.5 ppt. For both CFC-11 and CFC-12, the increases were nearly linear from 1988 through 1989, averaging 10.6 ppt/year (R² = .90) for CFC-11 and 18.1 ppt/year (R² = .91) for CFC-12. Since 1989 the growth rates at Alert have decreased rapidly, and trends for 1992 were not statistically significant (at the P = .05 level) for either CFC-11 or CFC-12. These decreases reflect the recent worldwide declines in production and emissions of CFCs (Elkins et al. 1993). The magnitudes of the declines at Alert are greater than those at low- and middle-latitude sites in the NOAA/CMDL network but are consistent with those at the other high-latitude sites (i.e., Barrow and the South Pole).

The records at Alert also show a seasonal pattern, with peak mixing ratios during the winter months. These maxima reflect the composition of cold air trapped in the polar tropospheric vortex, which forms during the winter months and restricts ventilation with cleaner air from low latitudes (Elkins et al. 1993).

HCFC-22

From November 1991 to November 1992, the mixing ratio of HCFC-22 at Alert increased from 110.2 to 115.0 ppt, averaging 4.3 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 May 1991 to February 1992, the mixing ratio of H-1211 at Alert averaged 2.32 ppt, and the mixing ratio of H-1301 averaged 1.96 ppt. The data do not show a significant (at the P = .05 level) trend for either H-1211 or H-1301. 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.

Alert
Northwest Territories, Canada
Tundra
82° 30' W, 62° 18' W
210 m above MSL

Alert Mixing Ratios Graph

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