Observed vertical profile of sulphur hexafluoride (SF6) and its atmospheric applications

The vertical distribution of sulphur hexafluoride (SF6) was measured during a balloon-borne cryogenic air sampler experiment conducted from Hyderabad (17.5 degrees N, 78.6 degrees E) on April 16, 1994, in the altitude region of about 8-37 km, The profile shows a faster decrease in its concentration in the region between the tropopause (similar to 17 km) and about 27 km, which is known as the transition region. Above this region the decrease rate is almost an order of magnitude less than in the transition layer. These results indicate very little loss of SF6 due to photochemistry in the stratosphere. Based on the reported increase rate of SF6 mixing ratio at ground level and assuming that long-lived trace gases are transported to the stratosphere without any measurable loss from the troposphere through the equatorial tropopause, the age of stratospheric air is estimated to vary from 1+/-0.08 years near the tropopause to 4.26+/-0.17 years above the transition layer. Atmospheric lifetime of SF6, inferred from the milling ratio correlations with simultaneous measurements of N2O, and CFC-12, is computed to be about 1937+/-432 years.