Spatial and Temporal Variations of Atmospheric ⁸⁵Kr Observed During 1995-2001 in Japan: Estimation of Atmospheric ⁸⁵Kr Inventory in the Northern Hemisphere

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Atmospheric ⁸⁵Kr concentrations have been continuously monitored since 1995 at the Meteorological Research Institute (MRI) in Tsukuba, Japan. They have also been observed once a year at several stations over the Japanese islands since 1995. The annual growth rate of the background atmospheric ⁸⁵Kr concentrations in Tsukuba was 0.03 Bq·m^-3·yr^-1 during 1996-2001. The atmospheric ⁸⁵Kr concentrations at several stations over Japan were within the range of the annual variations in Tsukuba. However, higher and lower ⁸⁵Kr concentrations in early winter, compared with those in Tsukuba (36.1 °N, 140.1°E), occurred in Sapporo (43.1°N, 141.3°E) and Ishigaki (24.3°N, 124.2°E), respectively. The reason for this is that Sapporo is covered by a continental air mass, some from European sources, whereas Ishigaki is still covered by a subtropical air mass. The Northern Hemispheric background ⁸⁵Kr concentrations from 1994 to 2001 was calculated from the ⁸⁵Kr inventory and the release rate of ⁸⁵Kr from the nuclear fuel reprocessing plants in Europe. Calculated ⁸⁵Kr concentrations in surface air were in good agreement with annual average observed values at the MRI, Tsukuba. The global atmospheric inventory of ⁸⁵Kr in December 2001 was also estimated to be approximately 5 EBq by using observed data in Tsukuba.