The presence of Caesium-137 (137Cs) in the environment is mainly due to past nuclear tests and accidental reactor releases. Due to the half-life of 137Cs (30.2 y), amounts of this radionuclide releases are in fact still detectable in soils, and at trace levels in the vegetation and the atmosphere. Since the middle of the 1990’s, the presence of 137Cs in the atmosphere has long been attributed to the resuspension of terrestrial dust. Recently, modelling studies have demonstrated that an additional and possibly dominant source of this anthropogenic radionuclide is biomass burning. Here, we report the variations of atmospheric 137Cs activity levels over a 2-year period at the puy de Dôme (1465 m a.s.l.), France in combination with measurements of the aerosol chemical composition, in particular with indicators for biomass burning (levoglucosan and potassium) and soil dust (calcium). Temporal co-variations of these chemical compounds in addition to back-trajectories are used to identify common source emissions. Significant correlation is found between these compounds. Hence, we experimentally confirm the modelling study highlighting the fact that the atmospheric 137Cs is partly released by biomass burning. In addition, we observed that the correlations between the 137Cs concentrations and levoglucosan and non-sea-salt non-mineral K+ differ according to the season, indicating that the biomass burning source is season-dependant.