Seasonal variation for 7Be and 10Be concentrations in the atmosphere at Tokyo and Hachijo-Island during the period of 2002 to 2003

Thursday, 8 September 2005

This presentation is part of: Poster Session II

Seasonal variation for ⁷Be and ¹⁰Be concentrations in the atmosphere at Tokyo and Hachijo-Island during the period of 2002 to 2003

Takeyasu YAMAGATA¹, Takasi SAITO², Hisao NAGAI², and Hiroyuki MATSUZAKI³. (1) Graduate School of Integrated Basic Sciences, Nihon University, 3-25-40 Sakura-Josui Setagaya-ku, Tokyo 156-8550, JAPAN, Tokyo, Japan, (2) Department of Chemistry, College of Humanities and Sciences, Nihon University, 3-25-40 Sakura-Josui Setagaya-ku, Tokyo 156-8550, JAPAN, Tokyo, Japan, Tokyo, Japan, (3) Department of Nuclear Engineering and Managemant, School of Engineering, The University of Tokyo, Tokyo, Japan

Radioisotopes of Be, ⁷Be(T_1/2=53.3d) and ¹⁰Be(T_1/2=1.5x10⁶y), are produced by cosmic ray in the stratosphere and upper troposphere. Since ⁷Be and ¹⁰Be attached to aerosols and were transported together after the production, concentration of Be radioisotopes can be regarded as indicator of upper atmospheric aerosols. The atomic ratio of ¹⁰Be/⁷Be changes by residence time in the atmosphere, because the half-life of ¹⁰Be is much longer than half-life of ⁷Be and mean residence time in the atmosphere. When stratosphere-troposphere exchange occurs, the atomic ratio of ¹⁰Be/⁷Be rises in the lower troposphere. However, observation on land supposed to be over-estimation for the atomic ratio ¹⁰Be/⁷Be, because ¹⁰Be concentration in the atmosphere rises by re-suspended dust particle. It is necessary to divide two sources to observe the influence of the stratospheric aerosols. For this purpose, open ocean is ideal location, but continuous observation above the ocean is very difficult. So we started continuous observation at Hachijo-Island (33.1°N 139.9°E) that is a remote island of Pacific Ocean as a quasi-ocean station, 300 km south from Tokyo, and at Tokyo, Japan (35.6 °N 139.6 °E) as an overland station. The aerosols were collected weekly on paper filters by a high volume air sampler (30 m³/h). Concentrations for ⁷Be, ¹⁰Be in samples were determined by gamma-ray spectrometry (E_γ=477.6 keV) using a HPGe detector and AMS using 5 UD Pelletron tandem accelerator at MALT, Univ. of Tokyo, respectively, and major element in aerosol samples were determined by NAA. Seasonal variation for ⁷Be and ¹⁰Be concentration in Tokyo and Hachijo-Island were similar to each other. Peak value for ⁷Be and ¹⁰Be concentration appeared in April and October. Especially in April when stratosphere-troposphere exchange occurs, peak value for the atomic ratio ¹⁰Be/⁷Be appeared. Low ⁷Be and ¹⁰Be concentrations and the atomic ratio of ¹⁰Be/⁷Be appeared in summer, July to August. Because the composition of the aerosol of Tokyo was almost sameto the nearby soil, it is considered that Tokyo was strongly influenced by re-suspended soil contamination. Using Al concentration in the aerosols, the enrichment of ¹⁰Be concentration by re-suspended soil contamination was estimated to about 20%.

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