Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 6 2008 10.5194/acpd-8-21229-2008 http://www.atmos-chem-phys-discuss.net/8/21229/2008/ http://www.atmos-chem-phys-discuss.net/8/21229/2008/acpd-8-21229-2008.html http://www.atmos-chem-phys-discuss.net/8/21229/2008/acpd-8-21229-2008.pdf 21229 21264 2008-12-19 Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF₆ and CO₂ H. Bönisch boenisch@iau.uni-frankfurt.de A. Engel J. Curtius T. Birner P. Hoor Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, Frankfurt am Main, Germany Department of Physics, University of Toronto, Toronto, ON, Canada Max Planck Institute for Chemistry, Atmospheric Chemistry department, Mainz, Germany now at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA The seasonality of transport and mixing of air into the lowermost stratosphere (LMS) is studied using distributions of mean age of air and a~mass balance approach, based on in-situ observations of SF₆ and CO₂ during the SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) aircraft campaigns. Combining the information of the mean age of air and the water vapour distributions we demonstrate that the tropospheric air transported into the LMS above the extratropical tropopause layer (ExTL) originates predominantly from the tropical tropopause layer (TTL). The concept of our mass balance is based on simultaneous measurements of the two passive tracers and the assumption that transport into the LMS can be described by age spectra which are superposition of two different modes. Based on this concept we conclude that the stratospheric influence on LMS composition is strongest in April with tropospheric fractions (&alpha;₁) below 20% and that the strongest tropospheric signatures are found in October with (&alpha;₁ greater than 80%. Beyond the fractions, our mass balance concept allows to calculate the associated transit times for transport of tropospheric air from the tropics into the LMS. The shortest transit times (<0.3 years) are derived for the summer, continuously increasing up to 0.8 years by the end of spring. These findings suggest that strong quasi-horizontal mixing across the weak subtropical jet from summer to mid of autumn and the considerably shorter residual transport time-scales within the lower branch of the Brewer-Dobson circulation in summer than in winter dominates the tropospheric influence in the LMS until the beginning of next year's summer. Andrews,~D G., Holton,~J R., and Leovy,~C B.: Middle Atmosphere Dynamics, Academic Press, 489 pp., 1987. Andrews,~A E., Boering,~K A., Daube,~B C., Wofsy,~S C., Hintsa,~E J., Weinstock,~E M., and Bui,~T P.: Empirical age spectra for the lower tropical stratosphere from in situ observations of \chemCO_2: Implications for stratospheric transport,~J. Geophys. Res., 104, 26581–26595, 1999. 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