Distribution and reactions of tritiated hydrogen and methane

About 99% of the world's tritium has been converted to HTO and most is in the oceans. Of the sizable fraction in the atmosphere, most is in the stratosphere, again as HTO. However, in that portion of the atmosphere which is in dynamic contact with the earth's surface, the troposphere, most of the tritium is as HT with about 2% as CH3T. The specific activity is high; the T/H ratio is greater than 105 and 103 that of seawater for hydrogen and methane respectively. In the case of hydrogen, this is two or three orders of magnitude higher than before the nuclear era. It is shown that the direct oxidation reactions to form water or the exchange reactions with water are negligible. The remaining mechanisms for conversion of HT and CH3T are biological and photochemical. Both routes are considered to be important. For the former, the contributions from specific processes and their distribution over the earth are unknown. For the latter, the steps are known but the rate constants for many of the reactions in the complex photochemical sequence are not well known. The result is that we cannot accurately fix by direct calculation the lifetime of HT or CH3T in the atmosphere. It is at least several years. In view of possible future accidental releases of tritium which may be large compared with the present world HT inventory, it seems desirable to extend our investigations of these 'inert' species. (author)