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.

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

In March 2013, increased ⁷Be activity concentrations in the atmosphere were observed for successive days in Dazaifu, western Japan. The daily ⁷Be activity concentration ranged from 0.93 to 14 mBq/m³, with a monthly average of 8.3 mBq/m³. 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/m³) plus twice the standard deviation. Also, this exceeded the monthly average (6.0 mBq/m³) only for March 1999-2015 (excluding 2013, when the cosmic-ray intensity, a component producing ⁷Be, 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 ⁷Be activity concentration increase.

Cosmogenic 7Be: Particle size distribution and chemical composition of 7Be-carrying aerosols in the atmosphere in Japan

The particle size distributions of airborne aerosols with ⁷Be were measured using cascade impactors at Dazaifu, a city in western Japan, in 2018 to observe their seasonal variation. Beryllium-7 was found to be adsorbed to aerosols with a particle size of less than 2.1 μm; in general, particles sized 0.43-0.65 μm had the highest ⁷Be activity concentrations. The activity median aerodynamic diameter (AMAD) of ⁷Be fluctuated less over the year within the range of 0.40-0.52 μm, which is the size range of particles that can reach human alveoli, and had an annual mean of 0.43 ± 0.034 μm. The activity concentrations of ⁷Be were significantly lower in summer, which affected ⁷Be activity concentration for each particle size fraction. The particle size distribution of ⁷Be-carrying aerosols was also affected by that of the aerosol particles in the atmosphere. Finally, findings suggest that ⁷Be was mainly adsorbed to sulfate aerosols (particularly ammonium sulfate aerosols).