Airborne aerosol measurements in the tropopause region and the dependence of new particle formation on preexisting particle number concentration

Airborne measurements of aerosol particles and trace gases were performed a t midlatitudes in the Stratosphere-Troposphere Experiment by Aircraft Measu rements (STREAM-96) campaign during late spring in the free troposphere and lower stratosphere. The results show a consistent relation between particl e number concentration and water vapor mixing ratio, suggesting a minimum p article number concentration below which nucleation of particles is very li kely to occur. Numerical simulations, using a coupled atmospheric chemistry and aerosol nucleation model, support this hypothesis. The model calculati ons indicate that new particle formation is very sensitive to the preexisti ng particle number concentration in the tropopause region.