Variation analysis of atmospheric ⁷Be activity concentrations with respect to precipitation

Beryllium-7 activity concentrations in the atmosphere and precipitation were continuously measured every day between April 2011 and December 2015 in Dazaifu, western Japan. The measured data were quantitatively analyzed to determine the precipitation-induced variation in ⁷Be activity concentrations. The average concentrations on nonprecipitation and precipitation days were 5.5 and 3.8 mBq/m³, respectively. This difference of 31% (1.7 mBq/m³) on average, was attributable to the washout effect, which was more significant in the summer. Regarding the association between ⁷Be activity concentration and precipitation, the concentration remained at a similar level for the small precipitation amount of <5.0 mm/day and showed a decreasing trend (but was insignificant) for the precipitation of 5.0-10.0 mm/day. A significant decrease in the concentration was observed for ≥10 mm/day. Furthermore, when precipitation occurred on two successive days, the ⁷Be activity concentrations on the second day significantly decreased regardless of precipitation.