Impact of stratospheric water vapor trends on ozone chemistry

Introduction Conclusions References Tables Figures Back Close Abstract Introduction Conclusions References Tables Figures Back Close Abstract A transient model simulation from 1960 to 2000 with the coupled climate-chemistry model (CCM) ECHAM4.L39(DLR)/CHEM shows a stratospheric water vapor trend during the last two decades of +0.7 ppmv and additionally a short-term increase during volcanic eruptions. At the same time this model simulation shows a long-term decrease 5 in total ozone and a short-term tropical ozone decline after a volcanic eruption. In order to understand the resulting effects of the water vapor changes on stratospheric ozone chemistry, different perturbation simulations have been performed with the CCM ECHAM4.L39(DLR)/CHEM with the water vapor perturbations fed only to the chemistry part. Two different long-term perturbations of stratospheric water vapor, +1 ppmv 10 and +5 ppmv, and a short-term perturbation of +2 ppmv with an e-folding time of two months have been simulated. Since water vapor acts as an in-situ source of odd hydrogen in the stratosphere, the water vapor perturbations affect the gas-phase chemistry of hydrogen oxides. An additional water vapor amount of +1 ppmv results in a 5–10% OH increase. Coupling processes between HO x and NO x /ClO x also affect the ozone 15 destruction by other catalytic reaction cycles. The HO x cycle becomes 6.4% more effective , whereas the NO x cycle is 1.6% less effective. A long-term water vapor increase does not only affect the gas-phase chemistry, but also the heterogeneous ozone chemistry in polar regions. The additional water vapor intensifies the strong denitrification of the Antarctic winter stratosphere caused by an enhanced formation of polar strato-20 spheric clouds. Thus it further facilitates the catalytic ozone removal by the ClO x cycle. The reduction of total column ozone during Antarctic spring peaks at −3%. In contrast, heterogeneous chemistry during Arctic winter is not affected by the water vapor increase. The short-term perturbation studies show similar patterns, but because of the short perturbation time, the chemical effect on ozone is almost negligible. Finally, this 25 study shows that 10% of the simulated long-term ozone decline in the transient model simulation can be explained by the water vapor increase, but the simulated tropical ozone decrease after volcanic eruptions is caused dynamically rather than chemically. Abstract Introduction Conclusions References Tables Figures Back Close