Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 4 2003 10.5194/acpd-3-3991-2003 http://www.atmos-chem-phys-discuss.net/3/3991/2003/ http://www.atmos-chem-phys-discuss.net/3/3991/2003/acpd-3-3991-2003.html http://www.atmos-chem-phys-discuss.net/3/3991/2003/acpd-3-3991-2003.pdf 3991 4036 2003-07-28 The isotope composition of water vapour: A powerful tool to study transport and chemistry of middle atmospheric water vapour C. Bechtel A. Zahn Institut für Umweltphysik, University of Heidelberg, Heidelberg, Germany Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe, Germany A one-dimensional chemistry model is applied to study the stable hydrogen (D) and stable oxygen isotope (¹⁷O, ¹⁸O) composition of water vapour in stratosphere and mesosphere. The stable isotope ratios of tropospheric H₂O are determined by "physical'' fractionation effects, i.e. phase changes, diffusion processes, and mixing of air masses. Due to these processes water vapour entering the stratosphere (i) is mass-dependently fractionated (MDF), i.e. shifts in the isotope ratio ¹⁷O/¹⁶O are ~0.52 times of those of ¹⁸O/¹⁶O and (ii) shows isotope shifts in D/H, which are ~5 times of those in ¹⁸O/¹⁶O. In stratosphere and mesosphere "chemical'' fractionation, that are the oxidation of methane, re-cycling of H₂O via the HO_x family, and isotope exchange reactions are shown to considerably enhance the isotope ratios in the imported tropospheric H₂O. Enrichments relative to the isotope ratios at the tropopause are used to derive the partitioning of tropospheric (unmodified), re-cycled and in situ generated H₂O. The model reasonably predicts overall increases of the stable isotope ratios in H₂O by ~23% for D/H, ~8.5% for ¹⁷O/¹⁶O, and ~14% for ¹⁸O/¹⁶O. The¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in H₂O are shown to be a measure of the partitioning of HO_x that receives its O atom either from the reservoirs O₂ or O₃. In the entire middle atmosphere, MDF O₂ is the major donator of oxygen atoms incorporated in OH and HO₂ and thus in H₂O. It is demonstrated that in the stratosphere mass-independent fractionation (MIF) in O₃ in a first step is transferred to the NO_x family and only in a second step to HO_x and H₂O. In contrast to CO₂, O(¹D) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from the many badly quantified isotope exchange reactions and kinetic isotope fractionation factors.