Impact of stratospheric water vapor enhancements caused by CH4 and H2O increase on polar ozone loss

Abstract

Possible causes of a future increase in stratospheric H2O are increasing tropospheric methane levels and a rise in tropospheric H2 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 H2O 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 H2O values. Previous studies emphasized that increasing H2O concentrations cause stratospheric cooling, and some have suggested that this could significantly increase halogen-induced polar ozone loss. The impact of both increased stratospheric H2O 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 65N, 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 H2O 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 H2O values (see above) yields at most 3.4 DU (10 %) of accumulated ozone loss in springtime compared to current H2O values. The impact of a potential future hydrogen economy alone (assuming an averaged increase of 0.18 ppmv H2O in the lower stratosphere) on springtime accumulated ozone loss is found to be negligible (at most 2.5 DU (4 %)) in this study. Copyright 2011 by the American Geophysical Union.