Regional dependence of Be-10 and Cl-36 variations in rainwater at the Sea of Japan and Pacific sides of Japan
Ms Masumi Matsumura1, Dr. Kimikazu Sasa1, Mr. Yuta Ochiai1, Dr. Tetsuya Matsunaka2, Dr. Yuki Tosaki3, Mr. Tsutomu Takahashi1, prof. Keisuke Sueki1
1University Of Tsukuba, Tsukuba, Japan, 2Kanazawa University, Kanazawa, Japan, 3National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
The concentrations of long half lived radionuclides, ¹⁰Be (half-life = 1.36 × 10⁶ years) and ³⁶Cl (half-life = 3.01 × 10⁵ years) in rainwater were measured at the Pacific side and the Sea of Japan side, to discuss the regional dependencies of seasonal variation. This study focused on ¹⁰Be and ³⁶Cl, which are produced in the upper atmosphere via nuclear reactions between cosmic rays and elements in the atmosphere.
We have collected monthly rainwater samples in Tsukuba city, Ibaraki (36°06’N, 140°06’W) at the Pacific side and in Nomi city, Ishikawa (36°25’N, 136°32’W) at the Sea of Japan side. ¹⁰Be and ³⁶Cl were measured with the accelerator mass spectrometry system at UTTAC (K. Sasa et al., 2018). The concentrations of ¹⁰Be in rainwaters ranged from (6.6 ± 0.3) × 10⁶ to (5.9 ± 0.6) × 10⁷ atoms L-¹ at Tsukuba (Jan. 2017 – Dec. 2018), and from (1.9 ± 0.1) × 10⁶ to (4.7 ± 1.2) × 10⁷ atoms L-¹ at Nomi (Jul. 2017 – Sep. 2019). The concentrations of ³⁶Cl in rainwaters ranged from (1.5 ± 0.7) × 10⁵ to (2.0 ± 0.1) × 10⁶ atoms L-¹ at Tsukuba, and from (1.2 ± 0.7) × 10⁵ to (2.6 ± 0.7) × 10⁶ atoms L-¹ at Nomi. The ¹⁰Be and ³⁶Cl concentrations showed insufficient correlation (¹⁰Be; correlation coefficient r = 0.3, p < 0.2, ³⁶Cl; r = 0.2, p < 0.6) at Tsukuba and Nomi, therefore it was confirmed that the variations of ¹⁰Be and ³⁶Cl concentrations had different trend between the Pacific side and the Sea of Japan side.
The seasonal variations were derived from the monthly averages of ¹⁰Be and ³⁶Cl concentrations (Apr. 2004 – Dec. 2018, at Tsukuba and Jul. 2017 – Sep. 2019 at Nomi). In terms of Nomi, the variation of ¹⁰Be had a similar trend with Tsukuba in summer while the ¹⁰Be concentration was higher than Tsukuba in winter. The strong north-west wind blows down around the Sea of Japan side in winter, ¹⁰Be is produced much in polar atmosphere, hence, the strong wind contains huge ¹⁰Be, which makes high ¹⁰Be concentration in rainwater at Nomi, similar as ⁷Be (M. Yamamoto et al., 2006 and J. Masarik et al., 2009). The concentration of ³⁶Cl in winter was also higher than in summer at Nomi, corresponding to the large ³⁶Cl production in the polar regions (J. Masarik et al., 2009). In contrast, ¹⁰Be and ³⁶Cl concentration increased in mid-spring at Tsukuba. The strong stratosphere-troposphere exchange occurs around Japan in spring due to the westerly wind. The air mass has high concentrations of ¹⁰Be or ³⁶Cl in the stratosphere. It is easy to be transported to the boundary layer. Therefore, high concentrations of ¹⁰Be and ³⁶Cl were observed in rainwater at Tsukuba.
Author is engaged in AMS research field at the University of Tsukuba since 2007.