Measurement and analysis of long-term variation of atmospheric 7Be activity concentrations in Dazaifu (western Japan)

Abstract

Atmospheric ⁷Be activity concentration was continuously measured in Dazaifu (33°30′N, 130°30′E), western Japan, from 1999 to 2020, and its variation was analyzed. Aerosol samples were collected using quartz fiber filters. The filtered samples were punched into circles, pressed, and molded into a 2-mm-thick disk for accurate ⁷Be quantification. Daily ⁷Be data analysis, encompassing an analysis for 7765 days over 22 years, revealed a concentration range of not detected–18 mBq/m³, characterized by substantial monthly variation and smoothed annual variation. An average daily ⁷Be activity concentration of 5.0 ± 2.6mBq/m³ was considered to be representative at the ground-surface-level in 30–40°N.

Results from a two-way Analysis of variance (ANOVA) indicated statistical significance in monthly and annual ⁷Be variabilities. The monthly variability of ⁷Be activity concentration was approximately four times greater than the annual variability. Frequency analysis revealed that the monthly variability comprised major 12-month and minor 6-month periodicities. The substantial decrease in ⁷Be activity concentration during summer, a primary driver of monthly variation, was also observed at other locations in Japan, attributed to a stable high-pressure system in the Pacific Ocean that stalled over Japan’s southern seas, followed by the inflow of air masses containing low ⁷Be activity concentrations. The annual variation was primarily influenced by the 11-year solar activity cycle, which affects the intensity of cosmic rays that produce ⁷Be.