Since 1959, atmospheric sampling stations of the environmental radioactivity permanent observatory measure bath naturaL and artificial radioactivity in ambient air. Nowadays, Cs-137 (30.07 years), an artificiaL radionuclide mainly issue from atmospheric weapons tests and severaL accidents, is still measured at trace level in the lower atmosphere although there is no significant release anymore. This study has aimed to identify and to characterize the processes which explain this persistence.
Areas highly contaminated by Chernobyl accident are the major contributors to the Cs-137 atmospheric persistence in France. In these areas, wiLdfires are the most significant resuspension processes during the dry season and can lead to long range transport events. Around 1013 Bq of Cs137 are yearly resuspended by this process.
ln connection with significant increases of total suspended particles, Sahara is the second area involved in the atmospheric Cs-137 persistence due to dust transport events. The whole of these events has a mean contribution of around 1/3 of the Cs-137 background Level at the French scale.
The last identified process is the wood burning during winter. Even if its emission factor is Low, spatiaL extent of source areas and quantities used at the season scaLe do of wood burning a signîficant process compared to ambient trace Levels. At the French scale, around 1011 Bq of Cs-137 are yearly resuspended by this process. During this season, the decrease of ventilation conditions in the lower atmosphere leads to an increase of the background level whereas significant increases are due to long range transport events from Eastern Europe.