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We present airborne in-situ trace gas measurements which were performedon eight campaigns between November 2001 and July 2003during the SPURT-project (SPURenstofftransport in derTropopausenregion, trace gas transport in the tropopause region).The measurements on a quasi regular basis allowed an overview of the seasonalvariations of the trace gas distribution in the tropopause region overEurope from 35°-75°N to investigate the influence oftransport and mixing across the extratropical tropopause on thelowermost stratosphere.From the correlation of CO and O3 irreversible mixing of tropospheric airinto the lowermost stratosphere is identified.The CO distribution indicates that transport and subsequent mixing oftropospheric air across theextratropical tropopause predominantly affects alayer, which closely follows the shape of the local tropopause.In addition, the seasonal cycle of CO2 illustrates the strong couplingof that layer to the extratropical troposphere.Both, horizontal gradients of CO on isentropes as well as theCO-O3-distribution in the lowermost stratospherereveal that the influence of quasi-horizontal transport and subsequentmixing weakens with distance from the local tropopause.The mixing layer extends to about 25 K in potential temperature abovethe local tropopause exhibiting only a weak seasonality.However, at large distances from the tropopause a significant influenceof tropospheric air is still evident.The relation between N2O and CO2 indicates that asignificant contribution of air originating from the tropical tropopausecontributes to the background air in the extratropical lowermost stratosphere

Seasonality and extent of extratropical TST derived from in-situ CO measurements during SPURT