210Pb and 137Cs in air and soils measure the rate and vertical profile of aerosol scavenging

Abstract

We have determined the deposition rates of 210Pb and 137Cs from the atmosphere to the landscape of the eastern and mid‐western United States by measuring their inventories in core samples of undisturbed soils. The mean inventories in lowland sites are 27.7 dpm cm−2 for 210Pb and 21.8 dpm cm−2 for 137Cs as of January 1984. The mean annual concentration of 210Pb in surface air at the 9 sites of the SURE network decreases smoothly from about 0.060 dpm m−3 near the Mississippi River to about 0.030 near the Atlantic coast. The ratio of deposition rate to surface air concentration is 0.62 ± 0.03 cm s−1 for 210Pb and 1.36 ± 0.08 cm s−1 for 137Cs. Using the activity vs. altitude profiles of Moore, et al. [1973], for 210Pb and the fission product nuclide, 90Sr, we calculate tropospheric aerosol mean residence times for 210Pb and 137Cs, of 4.8 and 5.5 days, respectively. This near equality precludes a rapid decrease of the aerosol scavenging rate constant in the mid‐troposphere. The 137Cs/210Pb ratios in soils from elevations above 1000 m in the Appalachian Mountains are lower than those in adjacent lowland soils, indicating the preferential removal of aerosols from the lower layer of the troposphere, presumably by cloud droplet impaction or by seeder‐feeder clouds.