Airborne measurements of aerosol particles and trace gases were performed at midlatitudes in the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM-96) campaign during late spring in the free troposphere and lower stratosphere. The results show a consistent relation between particle number concentration and water vapor mixing ratio, suggesting a minimum particle number concentration below which nucleation of particles is very likely to occur. Numerical simulations, using a coupled atmospheric chemistry and aerosol nucleation model, support this hypothesis. The model calculations indicate that new particle formation is very sensitive to the preexisting particle number concentration in the tropopause region. ¿ 1998 American Geophysical Union