Aircraft measurements of sulfur compounds and trace elements including Po 21ø were carried out in the plume of the Mount Etna volcano, at distances 10 to 260 km from the crater. The experiment was performed in September 1983 following the large lava emission of March-August 1983. Trace elements, particularly Po 21ø and Si, were measured with the aim of calculating their atmospheric dilution and evaluating their outputs. This study enabled us to determine the SO 2 to SO,• conversion rates to be between 2.2 x 10-6 and 4.0 x 10-s s-• in a cloud free anticyclonic situation. The main process of SO 2 removal in a volcanic plume should be a heterogeneous process of adsorption of this gas onto large particles. Emission fluxes of SO 2 were found to vary between 27 and 38 kg s-•, leading to a mean Po 2•ø output of 0.4 Ci/d.