Abstract
We report on the secular and annual changes of tritium (3HOH) concentration in precipitation in Tokyo and Tsukuba. The optium residence time in the stratosphere is estimated to be about 1.7 years by using a reservoir model. The average profile of the annual change shows a similar pattern to that of 90Sr, and their concentration of the second half of the year decreases to about one third of that of the first half, when no nuclear weapon test or small scale releases of fission products took place in the preceding year. The difference of fall rate between tritium and 90Sr along the Japan Sea side of Honshu Island of Japan in the winter season is explained by the difference of vertical distribution in the lower troposphere between them, where the mixing ratio of tritium does not change so much with altitude but the concentration of the fission products increases with altitude.
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