Aerosol production and growth in the upper free troposphere

During the Second Aerosol Characterization Experiment (ACE 2), aircraft mea surements of aerosol number concentration and size distribution were perfor med in the upper free troposphere. Temporal changes in aerosol number conce ntration and size distribution were studied by repeatedly probing three dif ferent altitude levels during one single flight. The observations provide i ndirect evidence for particle formation and growth. Simulations with a coup led gas-phase chemistry and aerosol dynamical model, including binary homog eneous nucleation of sulfuric acid and water vapor, suggest that the ultraf ine particles observed in the early morning were most likely formed when th e air mass was lifted over a cold front, one day prior to the measurements. Particle growth was studied using two different aerosol dynamical models a ssuming particle growth by condensation of sulfuric acid and water vapor. T he models were unable to simulate the observed particle growth, which sugge sts that another process might be important for aerosol growth, or multicom ponent condensation involving species not observed and modeled play a key r ole in the evolution of the aerosol size distribution.