Beryllium-7 Aerosols in Ambient Air

The aerodynamic size distribution of 7Be aerosol particles in ambient air was determined by using 1-ACFM and high-volume (HVI) cascade impactors, 20 m above the ground on the roof of the Faculty of Science building, Aristotle University of Thessaloniki at Thessaloniki Greece (40°38'N, 22°58'E) from November 2006 to June 2008. The activity concentration of 7Be aerosols was determined by gamma spectrometry (Eγ=0.477 MeV). The activity size distribution of 7Be-aerosols was largely associated with submicron aerosol particles in the accumulation mode (0.4-2.0 μm). The activity median aerodynamic diameter, AMAD varied from 0.76-1.18 μm (average 0.90 μm), indicating post-condensation growth either in the upper atmosphere or after mixing into the boundary layer. The geometric standard deviation, σ(subscript g) varied from 1.86-2.77 (average 2.24). The activity size distribution of 7Be-aerosols peaked in the 0.7-1.1 μm size range in the 65% of the measurements carried out by the 1-ACFM cascade impactor. In estimating lifetimes of 7Be-aerosols in ambient air, a mean residence time of about 8 days averaged for atmospheric aerosols of 0.90 μm AMAD size.