Measurements of NO, NOy, N2O, and O3 during SPURT: implications for transport and chemistry in the lowermost stratosphere
1Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland
2Max Planck Institute for Chemistry, Air Chemistry, Mainz, Germany
3Institute for Chemistry and Dynamics of the Geosphere: Stratosphere, Research Centre Jülich GmbH, Jülich, Germany
*now at: Atmospheric Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada
Abstract. We present measurements of NO, NOy, O3, and N2O within the lowermost stratosphere (LMS) over Europe obtained during the SPURT project. The measurements cover each of the four seasons during two years between November 2001 and July 2003, and probe the entire altitude and latitude range of the LMS: from 5° N to 85° N equivalent latitude, and from 290 to 375 K potential temperature. The measurements represent a comprehensive data set of these tracers and reveal atmospheric transport processes that influence tracer distributions in the LMS. Mean mixing ratios of stratospheric tracers in equivalent latitude-potential temperature coordinates show a clear seasonal cycle related to the Brewer-Dobson circulation with highest values in spring and lowest values in autumn. Vertical profiles show strong gradients at the extratropical tropopause suggesting that vertical (cross-isentropic) mixing is reduced above the tropopause. Mixing along isentropes is also strongly reduced since pronounced meridional gradients are found on potential temperature surfaces in the LMS. Concurrent large gradients in PV in the vertical and in the meridional direction horizontally suggest the presence of a transport and mixing barrier. Well above the tropopause distinguished seasonal cycles were found in the correlation slopes ΔO3/ΔN2O and ΔNOy/ΔN2O. Smallest slopes found during spring indicate chemically aged stratospheric air originating from high altitudes and latitudes. The slopes are larger in summer and autumn suggesting that a substantial fraction of air takes a 'short-cut' from the tropical tropopause region into the extratropical LMS. The comparison of measured NO with critical NO values at which net ozone production changes from negative to positive implies a net ozone production up to 20 K above the local tropopause in winter, increasing during spring and summer to up to 50 K in autumn. Above this height NO values favor ozone destruction.