Study of aerosol deposition at a wind exposed forest edge using 210Pb and 137Cs soil inventories

The radionuclide Pb-210 derived from gaseous Rn-222 present in the atmosphere becomes attached to the same aerosols as the bulk of the main pollutants sulphur and nitrogen. When scavenged from the atmosphere by precipitation, the Pb-210 is readily attached to organic matter in the surface horizons of the soil. Inventories of Pb-210 in soil can thus be used to measure the spatial variations in wet (or cloud) deposition due to orography averaged over many precipitation events (half-life of Pb-210 is 22.3 yr). Similar arguments apply to Cs-137 inventories (half-life 30.0 yr), although more care has to be taken in interpreting the results due to possible resuspension and migration of the Cs-137. The method has been applied to quantify the long-term enhancement in aerosol and cloud water (occult) deposition as a consequence of increased aerodynamic roughness at an exposed forest canopy in the Highlands of Scotland. The specific activities of Pb-210 and Cs-137 in dried soil samples were determined by gamma-spectrometry using HpGe detectors, one of which was fitted with a Compton suppression shield. Measurements revealed an average canopy enhancement in deposition of 36+/-12% relative to the open heathland, in agreement with independent measurements of occult deposition at similar sites and model estimates. Deposition at the exposed edge exceeds that in the open by greater than or similar to50%, and thus provides evidence of a pronounced edge effect.