The application of²¹⁰Pb inventories in soil to measure long-term average wet deposition of pollutants in complex terrain

Abstract

The radionuclide²¹⁰Pb derived from gaseous²²²Rn is present in particle form in the atmosphere attached to the same aerosols which contain the bulk of the pollutant sulphur and nitrogen. When scavenged from the atmosphere by precipitation, the²¹⁰Pb is readily attached to organic matter in the surface horizons of soil. The inventory of²¹⁰Pb in soil can be used to measure the spatial variation in wet (or cloud) deposition within a region due to orography or land use, averaged over several decades (half life of²¹⁰Pb is 22.3 years). Measurements of soil²¹⁰Pb inventories along a transect through complex terrain in north Britain were made to quantify the orographic enhancement of wet deposition, at Great Dun Fell in Cumbria. At the hill summit (∼800m asl) precipitation of approximately 2000 mm year⁻¹ exceeds that on the low ground upwind by a factor of 2.0. The inventory of²¹⁰Pb increases along the same transect by a factor of 3.3, due to seeder-feeder scavenging of orographic cloud. The measurements show that the average ratio of concentrations in scavenged orographic cloud to rain upwind of the hills is 2.2. These data are entirely consistent with the studies of the variation in major ion concentration with altitude at Great Dun Fell and elsewhere.

The modelling procedures developed to provide estimates of wet deposition throughout the UK uplands are shown to be consistent with these new field data.