In order to gain a more comprehensive picture on different mechanisms behind atmospheric particle formation, measurement results from QUEST 2-campaign are analyzed with an aid of an aerosol dynamic model. A special emphasis is laid on air ion and charged aerosol dynamics. Conducted model simulations indicate that kinetic 5 nucleation of ammonia and sulphuric acid together with condensation of sulphuric acid and low-volatile organic vapours onto clusters and particles explain basic features of particle formation events as well as ion characteristics. However, observed excess of negative ions in the diameter range 1.5-3 nm and overcharge of 3-5 nm particles demonstrate that ions are also involved in particle formation. These observations can 10 be explained by preferential condensation of sulphuric acid onto negatively charged clusters and particles. According to model simulations, the relative contribution of ionbased particle formation seem to be smaller than kinetic nucleation of neutral clusters. Conducted model simulations also corroborate the recently-presented hypothesis according to which a large number of so-called thermodynamically stable clusters (TSCs) 15 having a diameter between 1-3 nm exist in the atmosphere. TSCs were found to grow to the observable sizes only under favorable conditions, e.g. when the pre-existing particle concentration was low. 25ance of Earth (e.g. Ramanathan et al., 2001). In a view of these uncertainties, detailed 3912 ACPD 4, 2004

Abstract. A modelling study investigating the formation of organic particles from inorganic, thermodynamically stable clusters was carried out. A recently-developed theory, the so-called nano-Köhler theory, which describes a thermodynamic equilibrium between a nanometer-size cluster, water and water-soluble organic compound, was implemented in a dynamical model along with a treatment of the appropriate aerosol and gas-phase processes. The obtained results suggest that both gaseous sulphuric acid and organic vapours contribute to organic particle formation. The initial growth of freshly-nucleated clusters having a diameter around 1 nm is driven by condensation of gaseous sulphuric acid and by a lesser extent cluster self-coagulation. After the clusters have reached sizes of around 2 nm in diameter, low-volatile organic vapours start to condense spontaneously into the clusters, thereby accelerating their growth to detectable sizes. A shortage of gaseous sulphuric acid or organic vapours limit, or suppress altogether, the particle formation, since freshly-nucleated clusters are rapidly coagulated away by pre-existing particles. The obtained modelling results were applied to explaining the observed seasonal cycle in the number of aerosol formation events in a continental forest site.