During the Second Aerosol Characterization Experiment (ACE 2), aircraft measurements of aerosol number concentration and size distribution were performed in the upper free troposphere. Temporal changes in aerosol number concentration and size distribution were studied by repeatedly probing three different altitude levels during one single flight. The observations provide indirect evidence for particle formation and growth. Simulations with a coupled gas-phase chemistry and aerosol dynamical model, including binary homogeneous nucleation of sulfuric acid and water vapor, suggest that the ultrafine particles observed in the early morning were most likely formed when the air mass was lifted over a cold front, one day prior to the measurements. Particle growth was studied using two different aerosol dynamical models assuming particle growth by condensation of sulfuric acid and water vapor. The models were unable to simulate the observed particle growth, which suggests that another process might be important for aerosol growth, or multicomponent condensation involving species not observed and modeled play a key role in the evolution of the aerosol size distribution. ¿ 2000 American Geophysical Union