Argon-37 variability in the low troposphere

Purtschert, R.*; Kalinowski, M.*; Riedmann, R.*; Fontaine-Lagrand, J.-E.*; Kusmierczyk-Michulec, J.*; Gheddou, A.*; Bourgouin, P.*; Tomita, Yutaka

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Argon-37 (Ar) is produced in the high troposphere by spallation of argon through Ar(n,4n)Ar and by neutron capture, Ar(n,)Ar. The resulting natural equilibrium concentration of Ar in the mixed troposphere is about 0.5-1 mBq/m. This value may define the background level for the use of Ar for search area reduction by means of atmospheric Ar measurements in downwind direction of a potential test area in the course of an OSI. In order to investigate long term atmospheric activity levels of Ar, bulk air samples were collected close to the CTBTO IMS Radionuclide station located in Takasaki, Japan. In the years 2016-2018 in total 105 air samples were taken and analyzed for their Ar activity concentrations with no value exceeding 10 mBq/m.