Global tropospheric distribution and calibration scale of HCFC‐22

Measurements of atmospheric chlorodifluoro‐methane (HCFC‐22), based upon a new calibration scale developed in this laboratory, suggest a global tropospheric mean that is ∼28% lower than determined previously from surface‐based measurements. A global mean of 101.8 (±1.2, 1 s.d.) ppt and interhemispheric difference of 13 (±1) ppt were determined for HCFC‐22 in 1992 from air collected in flasks from seven remote sites located between 82° N and 90° S. These results are consistent with mixing ratios predicted from recent emission estimates and a lifetime for HCFC‐22 of 13.6 (+1.9, −1.5) yr. Based upon the analysis of flasks and archived air samples from mid‐1987 through 1992, a mean growth rate for HCFC‐22 of 7.3 (±0.3)% yr −1 was estimated for this period.