Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-4273-2006 http://www.atmos-chem-phys-discuss.net/6/4273/2006/ http://www.atmos-chem-phys-discuss.net/6/4273/2006/acpd-6-4273-2006.html http://www.atmos-chem-phys-discuss.net/6/4273/2006/acpd-6-4273-2006.pdf 4273 4324 2006-05-29 Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N₂O isotope distribution J. Kaiser A. Engel R. Borchers T. Röckmann Atmospheric Physics Division, Max Planck Institute for Nuclear Physics, Heidelberg, Germany Institute for Atmosphere and Environment, J. W. Goethe University, Frankfurt, Germany Planets and Comets Department, Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany now at: School of Environmental Sciences, University of East Anglia, Norwich, UK now at: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands A comprehensive set of stratospheric balloon and aircraft samples was analyzed for the position-dependent isotopic composition of nitrous oxide (N₂O). Results for a total of 220 samples from between 1987 and 2003 are presented, nearly tripling the number of mass-spectrometric N₂O isotope measurements in the stratosphere published to date. Cryogenic balloon samples were obtained at polar (Kiruna/Sweden, 68&deg; N), mid-latitude (southern France, 44&deg; N) and tropical sites (Hyderabad/India, 18&deg; N). Aircraft samples were collected with a newly-developed whole air sampler on board of the high-altitude aircraft M55 Geophysica during the EUPLEX 2003 campaign. All samples were analyzed by laboratory mass spectrometry for their ¹⁸O/¹⁶O and position-dependent ¹⁵N/¹⁴N isotope ratios with very high precision (standard deviation about 0.15 per mil for ¹⁸O/¹⁶O and average ¹⁵N/¹⁴N ratios, about 0.5 per mil for ¹⁵NNO/¹⁴NNO and N¹⁵NO/N¹⁴NO ratios). For mixing ratios above 200 nmol mol^&minus;1, relative isotope enrichments (&delta; values) and mixing ratios display a compact relationship, which is nearly independent of latitude and season and which can be explained equally well by Rayleigh fractionation or mixing. However, for mixing ratios below 200 nmol mol^&minus;1 this compact relationship gives way to meridional, seasonal and interannual variations. A comparison to a previously published mid-latitude balloon profile even shows large zonal variations, justifying the use of three-dimensional models for further data interpretation. <br><br> In general, the magnitude of the apparent fractionation constants (apparent isotope effects) increases continuously with altitude and decreases from the equator to the North pole, which can be qualitatively understood by the interplay between the time-scales of N₂O photochemistry and transport. Deviations from this behavior occur where polar vortex air mixes with nearly N₂O-free upper stratospheric/mesospheric air (e.g., during the boreal winter of 2003 and possibly 1992). Aircraft observations in the polar vortex at mixing ratios below 200 nmol mol^&minus;1 deviate from isotope variations expected for both Rayleigh fractionation and end-member mixing, but could be explained by continuous weak mixing between intravortex and extravortex air (Plumb et al., 2000). Finally, correlations between ¹⁸O/¹⁶O and average ¹⁵N/¹⁴N isotope ratios or between the position-dependent ¹⁵N/¹⁴N isotope ratios show that photo-oxidation makes a large contribution to the total N₂O sink in the lower stratosphere (up to 100%). Towards higher altitudes, the temperature dependence of these isotope correlations becomes visible in the stratospheric observations.