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Enhancing Nuclear Safety


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Relationship between origin of air masses and 137Cs activities in aerosols sampled in France between 2000 and 2006


Congress title :EAC 2007 - European Aerosol Conference
Congress town :Salzbourg
Congress date :09/09/2007

Summary

Atmospheric 137Cs activities are no more largely decreasing in collected French aerosols since 2000, with actual values between 0.2 et 0.3 µBq.m-3. After the large decrease of purely atmospheric, tropospheric or/and stratospheric, inputs of artificial radionuclides, variability of 137Cs in air is now explained by re-suspension (i.e. soil erosion), re-emission (i.e. fires) of contaminated soils and vegetation coming from more or less remote areas. Thanks to the sampling stations of the network OPERA ("Observatoire PErmanent de la Radioactivité": Permanent Observatory of the Radioactivity), even small inputs of 137Cs due to local re-suspension of low-contaminated French soils (1000 to 10 000 Bq.m-2) or incoming of contaminated aerosols transported on a continental scale can be detected. Our aim was to precise the origin of the past 6y. variability of 137Cs in the air using a combination of measurements in 6 stations in France and modelling of back-trajectories using Hyslpit. More than 14 000 daily back-trajectories were calculated in order to establish a relationship between 137Cs in the air and the origin of the air masses. Analysis of back-trajectories highlights quasi-systematic correlations between high 137Cs in air and continental origin of air masses (Fig.1). Only exceptions are Saharan-dust events increasing aerosol load and importing aerosols from African contaminated soils by global fallout of Nuclear Weapons Tests. This is a combination of re-suspension of contaminated soil and long-distance transport, which can explain the largest observed variability. Oceanic air masses are, them, associated with lower 137Cs activities (Fig.2). Combination of our measurements of 137Cs in aerosols, modelled back-trajectories and also our knowledge of the repartition of the contamination of soils in France and more remote areas, but largely contaminated areas, allows us to calculate the parts of local and long-distance re-suspension and transport in observed 137Cs activities in the air.

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