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We present measurements of NO, NOy, O3, andN2O within the lowermost stratosphere (LMS) over Europeobtained during the SPURT project. The measurements cover all seasons between November 2001 and July 2003.They span a broad band of latitudes from 30° N to 75° N and a potential temperature range from 290 to 380 K.The measurements representa comprehensive data set of these tracers and reveal atmospherictransport processes that influence tracer distributions in theLMS. Median mixing ratios of stratospheric tracers in equivalentlatitude-potential temperature coordinates show a clear seasonalcycle related to the Brewer-Dobson circulation, with highest valuesin spring and lowest values in autumn. Vertical tracer profiles showstrong gradients at the extratropical tropopause, suggesting thatvertical (cross-isentropic) mixing is reduced above thetropopause. Pronounced meridional gradients in the tracer mixing ratios are found on potential temperature surfaces in the LMS.This suggests strongly reduced mixing along isentropes. Concurrent large gradients in static stability in thevertical direction, and of PV in the meridional direction, suggest thepresence of a mixing barrier. Seasonal cycles were found in thecorrelation slopes ΔO3/ΔN2O andΔNOy/ΔN2O well above the tropopause. Absolute slope values are smallestin spring indicating chemically aged stratospheric airoriginating from high altitudes and latitudes. Larger values were measuredin summer and autumn suggesting that a substantial fractionof air takes a "short-cut" from the tropical tropopause regioninto the extratropical LMS. The seasonal change in the compositionof the LMS has direct implications for the ozone chemistry in this region.Comparisons of measured NOwith the critical NO value at which net ozone productionchanges from negative to positive, imply ozone productionup to 20 K above the local tropopause in spring, up to 30 K in summer, and up to 40 K inautumn. Above these heights, and in winter, net ozone production is negative

atmospheric chemistry and physics

Measurements of NO, NO&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;, N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O, and O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; during SPURT: implications for transport and chemistry in the lowermost stratosphere