7Be, excess 210Pb and artificial radionuclides as tracers of orographic deposition of aerosols

Titre du congrès :Goldschmidt 2007 Conference, Cologne, Germany
Ville du congrès :Köln
Date du congrès :19/08/2007

Titre de la revue : Geochimica Cosmochimica Acta 71 issue 15 supplement 1 Page: A562

Aerosols play a significant role in global biogeochemical cycles and climate system. In mountainous sites, atmospheric deposition of pollutants and nutriments is enhanced by different mechanisms including feeder-seeder effect or cloud-interception. Understanding these mechanisms is necessary to improve deposition models and evaluate the sensitivity of these fragile ecosystems. We use inventories of 210Pb, 7Be, both natural aerosol-bound radionuclides and artificial radionuclides (137Cs and Pu) in undisturbed soils to quantify mechanisms influencing aerosol deposition along the different slopes of the Puy de Dôme (1465m asl), an unactive volcanoe in Massif Central. We combine these results with 2-years measurements of aerosols and atmospheric deposition at the top and the base of Puy de Dôme. Figure 1: 210Pb inventories in soils vs. altitude along different slopes of the Puy de Dôme The figure 1 shows that 210Pb inventories are increasing with altitude. Using also 7Be and 137Cs inventories, we are able to demonstrate that this increase of 210Pb and therefore aerosol deposition is due to orographic effects. It seems that NO and NE slopes are more affected by orographic deposition Other mechanisms increasing the deposition of "surface air" enriched in 210Pb like the feeder-seeder mechanism can also explain these results. 7Be gives us insights of aerosol origin from the free troposphere and the stratosphere. Despite shorter representativity of 7Be values (1/2life: 50d), measured flux ratios of 210Pb/7Be (0.04-0.016) in soils are in good agreement with 210Pb/7Be ratio measured in aerosol and precipitations (0.1-0.16). It will be discussed how the use of both radionuclides can offer a way to quantify wet deposition mechanisms and validate 3-D conceptual models of atmospheric deposition.