The effects of forests and topography on the deposition of sub-micrometer aerosols measured by lead-210 and cesium-137 in soils

Abstract

210Pb and 137Cs are present in the atmosphere as single atoms residing in sub-micrometer aerosols. When the carrier aerosols are deposited to the landscape, the 210Pb and 137Cs are retained by soils. The inventories of 210Pb and 137Cs in undisturbed soils serve as a measure of the rate of aerosol deposition that is free from experimental artifacts. 210Pb inventories in the lowland soils of New England are ∼25 d.p.m. cm −2. Inventories in soils above 1000 m elevation are a factor of 1.5-3 greater, demonstrating strong orographic enhancement of deposition. 137Cs was injected into the stratosphere by atmospheric testing of nuclear weapons; 210Pb is a decay product of 222Rn which emanates from the earth's surface. As a result of the difference in their sources, the 137 Cs 210 Pb ratio in the atmosphere increases by a factor of 10 with height through the troposphere. The ratio in deposition, as preserved in lowland soils, is a factor of 2.2 greater than that in surface air due to the range of altitudes scavenged by precipitation. Deposition to mountains may be considered to be a mixture of aerosols scavenged by vertical precipitation and from surface air. Since the two sources have differing 137 Cs 210 Pb ratios, the contribution of the two sources to total deposition may be determined by mass-balance calculation. The 137 Cs 210 Pb ratio observed in mountain soils is 82±5% of that in lowland soils. The decrease in 137Cs/ 210Pb is the result of the preferential deposition of near-surface aerosols to mountainous areas. A model based on cautious assumptions indicates that the observed decrease in 137Cs/ 210Pb corresponds to a 24–47% contribution of surface air to the total deposition of surface-source aerosols. The pattern of 210Pb and 137Cs deposition suggests that both the seeder-feeder and the cloud droplet impaction mechanisms are responsible for increased aerosol deposition in mountainous regions. Soils under deciduous forests on mountain flanks which are seldom immersed in cloud show 25% increases in 210Pb inventories and decreased 137Cs/ 210Pb compared to surrounding lowlands. Soils under higher elevation forests that are frequently immersed in cloud show a 2-fold increase in 210Pb inventories and a greater decrease in 137Cs/ 210Pb.