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Conclusions

Mesospheric loss of could not be identified uniqueley in the observations made so far in comparison with our simplified chemistry scheme. Our findings contradict loss without a stabilization process. The comparisons with the model simulations favor a longer lifetime of in the middle atmosphere, which is caused by charge exchange reactions of and its photodetachment.

The atmospheric lifetimes found for the different chemistry scenarios differ by more than an order of magnitude. Stratospheric loss may be important for lifetime, without considerably affecting the profiles. Therefore, in order to estimate the possible contribution of to global warming, studies with a more detailed stratopsheric ion chemistry, including the energy spectrum of free electrons are necessary.

The transport properties of 3D models are also an important parameter when studying chemical loss of . Correlation studies with other quasi-inert tracers, such as , and would allow to separate more clearly the dynamically and chemically caused transformations of the mixing ratios. In addition, a higher horizontal resolution seems to be necessary to reproduce the observations taken near the vortex boundary or in highly inhomogeneous air masses, as found in June 1997 in midlatitudes.

On the other hand, winter/spring polar profiles taken inside the polar vortex at heights above 35 km should definitely show mesospheric loss effects. Unfortunately, accurate in-situ observations of in this region of the stratosphere have only been sparse and often have been taken near the vortex boundary. Therefore observations at higher altitude in the polar night are highly desirable to confirm chemical loss effects of and to specify the reactions involved.

We would like to thank I. Langbein for preparing the ECMWF data, A. Engel and M. Strunk for providing their data prior publication, J. Harnisch for sending us the data in electronic form, and B. Fichtelmann for providing UV solar flux data. We also thank ECMWF for permitting us to use their data and the DKRZ in Hamburg for their kind assistance. U. Schurath gave helpful comments during the preparation of the manuscript. The very detailed report of an anonymous referee is highly appreciated.