Recent trends and explanation for airborne ¹³⁷Cs activity level increases in France

¹ IRSN/DEI/SESURE/LERCM, Centre de Cadarache, Bâtiment 153, BP. 3, 13115 Saint Paul-lez-Durance Cedex, France
² IRSN/DEI/STEME/LMRE, Bâtiment 501, le Bois des Rames, 91400 Orsay, France

Corresponding author: olivier.masson@irsn.fr

Abstract

Airborne ¹³⁷Cs level in France is not decreasing significantly anymore (mean yearly value around 0,25 µ Bq.m^-3 contrarily to what was noticed in the past decades. This observation points out the role of processes that delay the atmospheric cleaning and participate to the persistence of radionuclide in the air at ground-level after a deposit, in the frame of medium as well as long-dated post-accidental contexts. The current background level also yields to consider ¹³⁷Cs in the atmosphere as a tracer of atmospheric processes like resuspension and re-emission from biomass burnings. This allows us to explain 2/3 of the peaks observed over the last six years. The remaining 1/3 is mainly noticed in winter when spreading of pollutants in the atmosphere is often weak due to temperature gradient inversion. On average, continental air masses are responsible for increases by a factor of 3 while oceanic air masses are characterised by levels 3 times lower, compared to the mean value. Feeding of ¹³⁷Cs in air at ground-level is the result of both local resuspension that signs ¹³⁷Cs activity levels in soils to which is added a remote contribution from time to time (resuspension of Saharan dust or re-emission from fires occurring in eastern territories with high ¹³⁷Cs deposition level). Finally, ¹³⁷Cs activity levels in air masses crossing over France can be described on average by a longitudinal gradient.