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Modelling the budget of middle atmospheric water vapour isotopes

by a. Zahn, P. Franz, C. Bechtel, J.-U. Grooß, T. Röckmann
Atmospheric Chemistry and Physics ()
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A one-dimensional chemistry model is applied to study the stable hydrogen ( D) and stable oxygen isotope (O-17, O-18) composition of water vapour in stratosphere and mesosphere. In the troposphere, this isotope composition is determined by "physical" fractionation effects, that are phase changes ( e. g. during cloud formation), diffusion processes ( e. g. during evaporation from the ocean), and mixing of air masses. Due to these processes water vapour entering the stratosphere first shows isotope depletions in D/H relative to ocean water, which are similar to 5 times of those in O-18/O-16, and secondly is mass-dependently fractionated (MDF), i.e. changes in the isotope ratio O-17/O-16 are similar to 0.52 times of those of O-18/O-16. In contrast, in the stratosphere and mesosphere "chemical" fractionation mechanisms, that are the production of H2O due to the oxidation of methane, re-cycling of H2O via the HOx family, and isotope exchange reactions considerably enhance the isotope ratios in the water vapour imported from the troposphere. The model reasonably predicts overall enhancements of the stable isotope ratios in H2O by up to similar to 25% for D/H, similar to 8.5% for O-17/O-16, and similar to 14% for O-18/O-16 in the mesosphere relative to the tropopause values. The O-17/O-16 and O-18/O-16 ratios in H2O are shown to be a measure of the relative fractions of HOx that receive the O atom either from the reservoirs O-2 or O-3. Throughout the middle atmosphere, MDF O-2 is the major donator of oxygen atoms incorporated in OH and HO2 and thus in H2O. In the stratosphere the known mass-independent fractionation (MIF) signal in O-3 is in a first step transferred to the NOx family and only in a second step to HOx and H2O. In contrast to CO2, O(D-1) only plays a minor role in this MIF transfer. The major uncertainty in our calculation arises from poorly quantified isotope exchange reaction rate coefficients and kinetic isotope fractionation factors.

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