30TH INTERNATIONAL COSMIC RAY CONFERENCE Atmospheric Transport Inferred from Seasonal Variations in Cosmogenic Be-7 Concentrations

A cosmogenic radionuclide Be-7 (half life 53.3 days) results from nuclear interactions of galactic cosmic rays in the Earth’s atmosphere. Most of Be-7 is produced in the lower stratosphere and slowly transported to the surface. Be-7 plays a role of atmospheric tracer and its measurements provide an important clue on atmospheric air mass motions. We have continuously measured the Be-7 concentrations in Tokyo since 2002. The data in 2002-2006 indicated clear enhancements of Be-7 concentrations in spring and autumn every year. The seasonal variations are not associated with scavenging by precipitation because the surface Be-7 concentrations do not correlate with rainfall. We suggested a possibility that the stratospheric Be-7 are brought to the upper troposphere through a large-scale air mass exchange between the stratosphere and troposphere. The air mass exchange is inferred to occur in association with a periodic passage (a period of a few days) of a pair of traveling high pressure and extratropical low pressure over Japan in spring and autumn.