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(K_D) averaged 4.27 ± 0.46 for ⁷Be and 4.79 ± 0.59 for ²¹⁰Pb (±SD, n = 163), with corresponding activity-fractions particulate (f_P) = 24 and 48%. K_D was inversely correlated with particulate mass concentration (p_C), a particle concentration effect (p.c.e.) that indicates that dissolved ⁷Be and ²¹⁰Pb are bound to submicron colloids. Experimental desorption-K_D 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 f^Be7, f^Be10, and f^Pb210 each increased with ⁷Be:¹⁰Be bulk age, a particle-age effect (p.a.e). In multiple regression, f^Be7 was best predicted by N, Mn, Al, and Ni [R² = 0.75, p < 0.0001], whereas f^Pb relied on N, S, Fe, and Mn [R² = 0.69, p < 0.0001]. Despite differences in magnitude and controls on partitioning, the ratio f^Be:f^Pb converged to 1 with p_C 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.