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

Abstract

The mixing ratios of CO2, CH4, O3, and SF6 were measured in 118 whole air samples collected onboard a Transall C‐160 aircraft around the NH extratropical tropopause in the winters 1993–1994 and 1994–1995. The samples originate mainly from the upper troposphere (∼7 km) and partly from the lowermost stratosphere up to altitudes of 12 km. With help of 7‐day back trajectories the potential temperature θ of an air mass in the free troposphere was established to be a reliable measure of the average latitude the air mass spent the last week before sampling. The potential temperature are thus interpreted in terms of a “representative latitude” allowing −1‐ for the creation of representative meridional trace gas profiles in the troposphere that show a lower scatter than if the data were plotted versus the sampling latitude itself and‐2‐ for the distinction of different types of air masses. For example, by categorizing the sampled air masses by their potential temperatures and by their trace gas composition, the 300 K θ ‐ surface was identified to separate cold polar air from warmer subtropical air. The 300 K θ ‐ surface is thus found to mark the polar front in the wintertime upper troposphere and can be viewed as the lowest isentrope that allows quasi‐isentropic cross‐tropopause transport. The midlatitude upper troposphere was frequently affected by surface air resulting in a positive correlation of CH4, SF6, and O3. In contrast, in the arctic upper troposphere and in the lowermost stratosphere, both CH4 and SF6 were negatively correlated with O3. Surprisingly, variable CO2 levels (spanning ∼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 nearly 7% per year a clear aging of the observed air masses since their entry into the stratosphere could be inferred. This SF6 age was rather high, for example around 2 years at an altitude of 11.5 km and thus just 3.5 km above the dynamical tropopause.