Spatial variations of wet deposition rates in an extended region of complex topography deduced from measurements of Pb-210 soil inventories

The radionuclide 210Pb derived from gaseous 222Rn 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 210Pb is readily attached to organic matter in the surface horizons of the soil. Inventories of 210Pb 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 210Pb is 22·3 year). Measurements of soil 210Pb inventories were made along a transect through complex terrain in the Scottish Highlands to quantify the orographic enhancement of wet deposition near the summits of the three mountains Ben Cruachan, Beinn Dorain and Ben Lawers, which, respectively, lie at distances of approximately 30, 55 and 80 km from the coast in the direction of the prevailing wind. The inventory of 210Pb on the wind-facing slopes of Ben Cruachan shows an increase with altitude that rises faster than the precipitation rate, which is indicative of seeder-feeder scavenging of orographic cloud occurring around the summit. Results for Beinn Dorain show a smaller rise with altitude whereas those for Ben Lawers give no indication of a rise. It is concluded that the seeder–feeder mechanism in regions of complex topology decreases in effectiveness as a function of distance inland along the direction of the prevailing wind.