The Atmospheric Infrared Sounder (AIRS) is a hyperspectral IR spectrometer orbiting on the EOS/Aqua spacecraft since May 2002. In late October 2002, AIRS detected lower tropospheric sulfur dioxide and ash emitted by an eruption of Mt. Etna (Italy), in plumes which could be tracked over 1000 km from the volcano into north Africa. We report retrievals of SO2 and ash column amounts and ash particle size in the Etna plumes. AIRS total SO2 compares favorably with contemporaneous ground-based correlation spectrometry (COSPEC) measurements. Retrieval of ash cloud parameters in the eruption plumes permits quantitative studies of distal ash fallout. The Etna data demonstrate the potential of AIRS to improve measurements of volcanic SO2 and ash loading in the troposphere, and to refine our understanding of volcanic cloud composition, structure and evolution.