Seasonal Contrasts of Air Mass Composition in the Extratropical UTLS

We present tracer measurements from the German research aircraft HALO, which were obtained during the TACTS/ESMVal-project (Transport and Composition in the UT/LMS and Earth System Model Validation) in September 2012 and during the POLSTRACC (POLar STRAtosphere in a Changing Climate) mission in winter 2015/2016. We will focus on the distribution of CO, N2O and ozone in the extratropics between potential temperatures of 360 K and 410 K and their changes with season. Based on the distribution of N2O and CO, which constitute two tropospheric tracers of different lifetime one can quantify time scales of transport and chemical ageing of air masses. In the absence of tropospheric transport (e.g. vortex conditions) an equilibrium value of CO of 15 ppbv establishes, which is the result of methane oxidation and CO degradation. Any additional CO can be related to mixing with tropospheric air. The lower stratosphere up to 410 K shows a clear tropospheric signature in late September, but less efficient mixing according to the N2O CO correlation. This signature changes rapidly from December to March. During winter 2015/2016 CO equilibrium values and low decreasing N2O down to 380K indicates the influence of air from the polar vortex. The increasing CO relative to N2O from February to March indicates a substantial influence of tropospheric air and efficient mixing with tropical air until March. This leads to a mean increase of the tropospheric fraction relative to N2O by 20 %.