Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 2 2004 10.5194/acpd-4-1691-2004 http://www.atmos-chem-phys-discuss.net/4/1691/2004/ http://www.atmos-chem-phys-discuss.net/4/1691/2004/acpd-4-1691-2004.html http://www.atmos-chem-phys-discuss.net/4/1691/2004/acpd-4-1691-2004.pdf 1691 1726 2004-03-16 Seasonality and extent of extratropical TST derived from in-situ CO measurements during SPURT P. Hoor peter.hoor@env.ethz.ch C. Gurk D. Brunner M. I. Hegglin H. Wernli H. Fischer Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zürich, Switzerland Max Planck Institute for Chemistry, Air Chemistry, Mainz, Germany Institute for Atmospheric Physics, University of Mainz, Germany We present airborne in-situ trace gas measurements which were performed on eight campaigns between November 2001 and July 2003 during the SPURT-project (SPURenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region). The measurements on a quasi regular basis allowed an overview on the seasonal variations of the trace gas distribution in the tropopause region over Europe from 35&deg;&ndash;75&deg; N to investigate the influence of transport and mixing across the extratropical tropopause on the lowermost stratosphere. <br><br> From the correlation of CO and O₃ irreversible mixing of tropospheric air into the lowermost stratosphere is identified. The CO distribution indicates that transport and subsequent mixing of tropospheric air across the extratropical tropopause predominantely affects a layer, which closely follows the shape of the local tropopause. In addition the seasonal cycle of CO₂ illustrates the strong coupling of that layer to the extratropical troposphere. Both, horizontal gradients of CO on isentropes as well as the CO-O₃-distribution in the lowermost stratosphere reveal that the influence of quasi-horizontal transport and subsequent mixing weakens with distance from the local tropopause. <br><br> However, at large distances from the tropopause a significant influence of tropospheric air is still evident. The relation between N₂O and CO₂ indicates that a significant contribution of air originating from the tropical tropopause contributes to the background air in the extratropical lowermost stratosphere.