Aerodynamic size association of 7Be in ambient aerosols

The aerodynamic size distribution of 7Be in ambient aerosol particles was determined by using 1-ACFM cascade impactors. The activity distribution of 7Be measured by γ-spectrometry (Eγ = 447keV), was largely associated with submicron aerosols in the accumulation mode (0.4–2.0 μm). The activity median aerodynamic diameter, AM AD ranged from 0.76 to 1.18μm (average 0.90μm), indicating post-condensation growth either in the upper atmosphere or after mixing into the boundary layer. The geometric standard deviation, gsg ranged from 1.86 to 2.77 (average 2.24). In 65% of the cascade impactor measurements the activity distributions of 7Be showed maxima in the 0.7 to 1.1 μm size range. In estimating lifetimes of 7Be aerosols in ambient air, a mean residence time of about 8 days averaged for tropospheric aerosols of 0.90 μm AM AD size.