An elevated layer was observed during airborne lidar and in-situ aerosol measurements, leeward of the European Alps and Apennine mountains. The layer was encountered during the Mesoscale Alpine Program (MAP) in autumn 1999, and extended >200 km at an altitude ~4100 m asl over the northern Adriatic sea. Detailed meteorological analysis suggested that mountain venting followed by advection was responsible for formation of the layer. Evidence for particle nucleation was found in six profiles, and was associated with regions of cloud outflow. Despite determination of the average aerosol mass concentration within the layer (~1.8 (SO4) ¿g m-3), an estimate of export to the free troposphere was complicated by the complex structure of the planetary boundary layer.