High-precision isotope measurements of H216O, H217O, H218O, and the Δ 17O-anomaly of water vapor in the southern lowermost stratosphere P. Franz and T. Röckmann Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany
Abstract. We report the first high-precision measurements of δ 18O and Δ 17O at high
southern latitudes that can resolve changes in the isotopic composition of
water vapor in the lowermost stratosphere and upper troposphere. A strong
increase of δ 18O with decreasing mixing ratio above the tropopause is
evident in the data. Since also the water vapor mixing ratio decreases above
the tropopause, the effect seen in the isotope data can be explained by
mixing of moist air from the tropopause with dry stratospheric air. However,
the source of this dry stratospheric air is not known; both fast transport
from the extratropical tropopause or mixing with air from the dehydrated
polar vortex are likely. The magnitude of the Δ 17O-anomaly (departure from
mass-dependent fractionation (MDF)) was below 2 per mil for each datapoint,
and a zero anomaly in lower level stratospheric water vapor is possible.
Various transport histories for the stratospheric data are discussed based on
the mixing ratio and isotope data.
Citation: Franz, P. and Röckmann, T.: High-precision isotope measurements of H216O, H217O, H218O, and the Δ 17O-anomaly of water vapor in the southern lowermost stratosphere, Atmos. Chem. Phys., 5, 2949-2959, doi:10.5194/acp-5-2949-2005, 2005.