Fate of long-lived trace species near the Northern Hemispheric tropopause:Carbon dioxide, methane, ozone, and sulfur hexafluoride

The mixing ratios of CO2, CH4, O-3, and SF6 were measured in 118 whole air samples collected onboard a Transall C-160 aircraft around the NH extratrop ical tropopause in the winters 1993-1994 and 1994-1995. The samples origina te mainly from the upper troposphere (similar to 7 km) and partly from the lowermost stratosphere up to altitudes of 12 km. With help of 7-day back tr ajectories the potential temperature theta of an air mass in the free tropo sphere was established to be a reliable measure of the average latitude the air mass spent the last week before sampling, The potential temperature ar e thus interpreted in terms of a "representative latitude" allowing -1- for the creation of representative meridional trace gas profiles in the tropos phere that show a lower scatter than if the data were plotted versus the sa mpling latitude itself and -2- for the distinction of different types of ai r masses, For example, by categorizing the sampled air masses by their pote ntial temperatures and by their trace gas composition, the 300 K theta-surf ace was identified to separate cold polar air from warmer subtropical air. The 300 K theta - surface is thus found to mark the polar front in the wint ertime upper troposphere and can be viewed as the lowest isentrope that all ows quasi-isentropic cross-tropopause transport. The midlatitude upper trop osphere was frequently affected by surface air resulting in a positive corr elation of CH4, SF6, and O-3. In contrast, in the arctic upper troposphere and in the lowermost stratosphere, both CH4 and SF6 were negatively correla ted with O-3. Surprisingly, variable CO2 levels (spanning similar to 14 ppm ) found just above the ozonopause point to intensive mixing of tropospheric air into the lowermost stratosphere. From the high SF6 growth rate of near ly 7% per year a clear aging of the observed air masses since their entry i nto the stratosphere could be inferred. This SF6 age was rather high, for e xample around 2 years at an altitude of 11.5 km and thus just 3.5 km above the dynamical tropopause.