Aircraft measurements of aerosol particles and trace gases were performed in the upper free troposphere and lower stratosphere during the Stratosphere and Troposphere Experiment by Aircraft Measurements (STREAM-96) campaign from Shannon airport, Ireland. During one measurement flight, ultrafine particle number densities up to 104 cm-3 (STP) were observed in the lowermost stratosphere. Concurrent with these very high number densities of ultrafine particles, high accumulation mode particle number densities were observed over the same geographical location in the free troposphere, which were attributed to convective transport in the troposphere. The observations suggest that adiabatic cooling of the stratospheric air, as a result of the convective transport in the troposphere that lifted the tropopause and the air in the lowermost stratosphere, was responsible for triggering the formation of new particles. However, also aircraft emissions could have contributed to the enhancement in ultrafine particles. ¿ 1999 American Geophysical Union