Seasonal variability of tritium and ion concentrations in rain at Kumamoto, Japan and back-trajectory analysis of air mass

Tritium and major ion concentrations in rain were analyzed in Kumamoto, Japan between 2001 and 2006 to examine present tritium concentration and seasonal variation. The average tritium concentration was 0.36 ± 0.19 Bq/L (n=104) and higher tritium concentrations were observed in spring than the other seasons. Among the ions, non-sea-salt (nss) SO₄^2- showed higher concentration in winter while other ions did not show marked increase in winter. Based on the back-trajectory analyses of air masses, the increase in tritium concentrations in spring arises from downward movement of naturally produced tritium from stratosphere to troposphere, while the increase of the nss-SO₄^2- concentrations in winter is due to long range transport of pollutants from China to Japan.