DEUTERIUM, O-18, AND TRITIUM AS TRACERS FOR WATER-VAPOR TRANSPORT IN THE LOWER STRATOSPHERE AND TROPOPAUSE REGION

Simultaneous measurements of the three rare isotopes Deuterium (D), Tr itium (T), and Oxygen-18 (O-18) in water vapour were made for the firs t time in the vicinity of the northern hemisphere tropopause. In contr ast to expectation, high D/H and O-18/O-16 ratios, but relatively low T/H ratios, were found within the lowermost stratosphere. Since water vapour in the low-latitude upper troposphere shows a similar isotopic signature, we conclude that in the mid-latitudes considerable amounts of tropospheric water vapour are injected into the lowermost stratosph ere, probably resulting in a hydration of the lower stratosphere. In a ddition, T can serve as tracer for precipitation of water containing s tratospheric aerosol particles, because the T/H ratio in stratospheric water vapour is orders of magnitude higher than in the upper troposph ere. Thus, even a small contribution of water of stratospheric origin should be detectable in the tropopause region. In our measurements per formed in the Arctic we did not find isotopic evidence for sedimentati on of PSC particles down to the tropopause. This may be caused by the low spatial and temporal coverage of our observations; however, it may also be due to the much weaker wintertime dehydration of the Arctic v ortex compared to the Antarctic.