A case study of rapid mixing across the extratropical tropopause based on Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrumented Container (CARIBIC) observations

Using the CARIBIC Boeing 767 aircraft, a suite of trace gases and aerosols was measured between Germany and the Maldives in June 2000 at altitudes between 9.4 and 10 km. In the extratropics, the flight track was located in the tropopause region. A large variability of trace gases and ultrafine aerosol concentrations was observed while the aircraft intercepted air masses from the upper troposphere and the lowermost stratosphere, as well as outflow of deep convection. The correlations of alkanes (C2-C5) observed in the nonconvective areas point to relatively rapid mixing across the tropopause within about a day. Unusually high mixing ratios of short-lived alkanes (C4-C6) in the convective areas indicate rapid transport of boundary layer air masses to cruising altitude. Using the ratios of the mixing ratios of alkanes (C3-C5) observed in the convective and nonconvective areas, we estimate the age of air masses in the tropopause region to be 24(±6) days for this event. This timescale is similar to that of vertical transport within the troposphere. Altogether our observations provide further evidence that the extratropical tropopause is often not a very effective mixing barrier.