The concentrations of tritium have been determined in wet precipitations occurring over the Indian subcontinent during 1961–64, using a sensitive method for counting of tritium activity discussed in Part I* of this paper.
The tritium concentrations varied significantly during the period of observation; highest concentrations were observed during 1963. An analysis of the data reported here, in conjunction with those available for concentrations of H3 and Sr90 in rains at higher latitudes, reveals that these nuclides which were originally placed at high altitudes in the polar regions during late 1962, were deposited chiefly at 30°–90° latitudes during 1963 and 1964 respectively in relative proportions of 1 and 0·6. The data show that the largest gradients in their zonal deposition occur at about 35°–40° N latitude and that to a first approximation, their deposition per unit area in 1963 or 1964 was practically uniform, separately in the 30°–90° and 0°–30° latitude regions. This observation suggests the existence of two well-defined cells, which are internally well mixed: the meridional transport to low latitudes occurs as a result of interaction between these cells. The annual deposition rates of Sr90 as observed during 1963 and 1964 suggest a mean time of 3 months for exchange of air between the two cells, in good agreement with the values deduced for mid-months of the year on the basis of analysis of bomb produced C14 data.
The tritium and strontium data for the inland, coastal and island stations are analysed to evaluate the importance of (i) the re-evaporation of tritium from continents, and (ii) the molecular exchange of atmospheric tritium with oceanic water. Process (i) probably plays a significant role over the continents throughout the year; its effect, however, is experimentally visible only during June to September. The estimated concentration of H3 in evaporated water suggests that the precipitated water mixes very slowly with that in the soil; limits on the equivalent amount of exchangeable soil water are given.
It is shown that the relative wet deposition of H3 and Sr90 atisland andcoastal stations is similar to their estimated concentration ratio in upper level tropospheric air. Furthermore, the relative concentrations of H3 and Sr90 at continental and occanic stations differ only to the extent expected due to reinjection of H3 over continents. Thus, if one takes into account the recycling of H3 at continental stations (which results in about a 50% higher apparent deposition on an annual basis), one is led to the conclusion that process (ii) is rather unimportant; an upper limit of 30% on the fraction of tritium removed over oceans by molecular exchange is deduced.
The mean annual concentration of Sr90 in wet precipitation is lower at oceanic stations compared to that at continental stations. This could be due to meteorological effects peculiar to oceanic areas,e.g., higher rainfall and quick recycling of evaporated water. Otherwise, one must postulate a significant removal of Sr90 (and H3) by ocean spray and jet action.