[1] Likely causes of a future increase in stratospheric H 2 O are a rise in tropospheric CH 4 and H 2 leakages from an increased integration of hydrogen into the energy supply system. Here we evaluate the impact of potential future stratospheric H 2 O increases on Arctic ozone loss by comparing ozone loss proxies based on two different mechanisms of chlorine activation. In particular, the H 2 O dependence of the volume of air is analyzed where temperatures are low enough to form nitric acid trihydrate, denoted as V PSC , and for Cl activation on liquid sulfate aerosols, denoted as V ACl . We show that V ACl increases faster than V PSC with increasing H 2 O mixing ratios in the altitude range of 400 K to 550 K potential temperature. As a consequence, the additional ozone column loss is expected to be most pronounced for cold winters and large H 2 O increases and to be significantly higher when V ACl is used as a proxy. Citation: Feck, T., J.-U. Grooß, and M. Riese (2008)

[1] Possible causes of a future increase in stratospheric H 2 O are increasing tropospheric methane levels and a rise in tropospheric H 2 due to leakages from a possible increased integration of hydrogen into the energy supply system. Here we quantify the direct chemical impact of potential future stratospheric H 2 O increases on Arctic ozone loss using the cold Arctic winter 2004/2005 as the basis for our study. We present simulations with the three-dimensional chemistry transport model CLaMS using enhanced stratospheric H 2 O values. Previous studies emphasized that increasing H 2 O concentrations cause stratospheric cooling, and some have suggested that this could significantly increase halogen-induced polar ozone loss. The impact of both increased stratospheric H 2 O values and decreased temperatures on simulated ozone depletion is investigated. Assuming an average increase of water vapor in the lower polar stratosphere of ≈0.58 ppmv (averaged over equivalent latitudes ≥ 65°N, from 400-550 K potential temperature and from December to March) and in addition decreased temperatures (−0.2 K) yields at most 6.8 DU (≈11 %) more accumulated ozone loss in mid-March for the Arctic polar winter 2004/2005 compared to the ozone loss for undisturbed conditions. The assumed H 2 O enhancement in future decades is in the range of current model predictions. Considering in addition the decrease of the future chlorine loading (−40 %) of enhanced H 2 O values (see above) yields at most 3.4 DU (10 %) of accumulated ozone loss in springtime compared to current H 2 O values. The impact of a potential future hydrogen economy alone (assuming an averaged increase of 0.18 ppmv H 2 O in the lower stratosphere) on springtime accumulated ozone loss is found to be negligible (at most 2.5 DU (4 %)) in this study.Citation: Vogel, B., T. Feck, and J.-U. Grooß (2011), Impact of stratospheric water vapor enhancements caused by CH 4 and H 2 O increase on polar ozone loss,

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Understanding the 1% per year increase of stratospheric water vapour from 1954 to 2000 is a great challenge in atmospheric science. The increase is predominantly caused by long-term changes in transport of water vapour into the stratosphere and systematic increases of tropospheric methane levels. This paper gives a review on stratospheric water vapour changes for the 1980 and 2000 time period with emphasis on the contribution of methane oxidation. Predictions for 2050 indicate that likely increases of tropospheric methane levels will lead to an increase of upper stratospheric water vapour values of about 0.4 ppmv. A similar value is predicted as an upper limit of effects of a future hydrogen economy. r