Analysis of Factors Contributing to the Increase in 7 Be Activity Concentrations in the Atmosphere.

In March 2013, increased 7 Be activity concentrations in the atmosphere were observed for successive days in Dazaifu, western Japan. The daily 7 Be activity concentration ranged from 0.93 to 14 mBq/m 3 , with a monthly average of 8.3 mBq/m 3 . This average was the highest among the monthly averages observed between 1999 and 2015, and higher than the monthly average over this period (4.7 mBq/m 3 ) plus twice the standard deviation. Also, this exceeded the monthly average (6.0 mBq/m 3 ) only for March 1999-2015 (excluding 2013, when the cosmic-ray intensity, a component producing 7 Be, decreased). Based on the backward trajectory analysis etc. results, the inflow of air from the stratosphere and upper troposphere at high latitudes that frequently occurred in March 2013 was considered the reason for the 7 Be activity concentration increase.