Sorption Behavior and Aerosol-Particulate Transitions of 7Be, 10Be, and 210Pb: A Basis for Fallout Radionuclide Chronometry.

We investigated the partitioning of 7Be, 10Be, and 210Pb aerosols between operationally dissolved and >0.5 μm particulate fractions in wet and dry atmospheric deposition. Bulk deposition in situ-log(KD) averaged 4.27 ± 0.46 for 7Be and 4.79 ± 0.59 for 210Pb (±SD, n = 163), with corresponding activity-fractions particulate (fP) = 24 and 48%. KD was inversely correlated with particulate mass concentration (pC), a particle concentration effect (p.c.e.) that indicates that dissolved 7Be and 210Pb are bound to submicron colloids. Experimental desorption-KD was higher than in situ by a factor of 20 for 7Be and 4 for 210Pb (n = 27), indicating that FRN sorption to particulates was irreversible. 7Be:10Be ratios confirmed that colloidal and particulate fractions were geochemically distinct, with corresponding ages of 120 ± 30 and 260 ± 45 days, respectively [mean ± SE, n = 9, p = 0.011]. Fractions particulate fBe7, fBe10, and fPb210 each increased with 7Be:10Be bulk age, a particle-age effect (p.a.e). In multiple regression, fBe7 was best predicted by N, Mn, Al, and Ni [R2 = 0.75, p < 0.0001], whereas fPb relied on N, S, Fe, and Mn [R2 = 0.69, p < 0.0001]. Despite differences in magnitude and controls on partitioning, the ratio fBe:fPb converged to 1 with pC in the range of 10-100 mg L-1. Given sufficient solid surfaces, irreversible sorption and p.a.e. form a basis for 7Be:210Pb chronometry of aerosol biogeochemical cycling.