Particle production in the lowermost stratosphere by convective lifting ofthe tropopause

Aircraft measurements of aerosol particles and trace gases were performed i n the upper free troposphere and lower stratosphere during the Stratosphere and Troposphere Experiment by Aircraft Measurements (STREAM-96) campaign f rom Shannon airport, Ireland. During one measurement flight, ultrafine part icle number densities up to 10(4) cm(-3) (STP) were observed in the lowermo st stratosphere. Concurrent with these very high number densities of ultraf ine particles, high accumulation mode particle number densities were observ ed over the same geographical location in the free troposphere, which were attributed to convective transport in the troposphere. The observations sug gest that adiabatic cooling of the stratospheric air, as a result of the co nvective transport in the troposphere that lifted the tropopause and the ai r in the lowermost stratosphere, was responsible for triggering the formati on of new particles. However, also aircraft emissions could have contribute d to the enhancement in ultrafine particles.