Long-term trends (1987-2020) and seasonal characteristics of atmospheric 7Be activity concentration in Japan based on continuous measurements.

In this study, long-term variation in atmospheric 7Be activity concentrations in Japan was investigated through long-term continuous measurements spanning 1987-2020. A dataset was constructed by integrating sequential observations from Tsukuba in eastern Japan and Dazaifu in western Japan, providing multidecadal (over three decades) data of near-surface 7Be activity concentrations. Measured 7Be activity concentrations ranged from not detected to 18 mBq m-3, with a mean monthly value of 4.6 mBq m-3. Distinct seasonal patterns were observed. In Tsukuba, 7Be activity concentrations exhibited a maximum in spring and a secondary peak in autumn, with lower levels in summer and winter. In contrast, a pronounced decrease occurred only in summer in Dazaifu. Annual 7Be activity concentrations gradually increased, with notable peaks in 1995, 2009, and 2019. These variations corresponded well with increases in cosmic-ray intensity and decreases in sunspot number. This indicates that long-term variation in atmospheric 7Be activity concentrations is primarily controlled by production rate variations associated with the solar modulation of galactic cosmic rays. The multidecadal continuous dataset provides fundamental information on the behavior of atmospheric cosmogenic radionuclides in Japan and offers a valuable basis for evaluating future variations in solar activity and atmospheric transport processes.