Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 6 2003 10.5194/acpd-3-6147-2003 http://www.atmos-chem-phys-discuss.net/3/6147/2003/ http://www.atmos-chem-phys-discuss.net/3/6147/2003/acpd-3-6147-2003.html http://www.atmos-chem-phys-discuss.net/3/6147/2003/acpd-3-6147-2003.pdf 6147 6178 2003-12-08 Modelling the formation of organic particles in the atmosphere T. Anttila V.-M. Kerminen M. Kulmala A. Laaksonen C. O’Dowd Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20E, FIN-00880 Helsinki, Finland University of Helsinki, Department of Physical Sciences, P.O. Box 6, FIN-00014 University of Helsinki, Helsinki, Finland University of Kuopio, Department of Applied Physics, P.O. Box 1627, FIN-70211, Kuopio, Finland 4Department of Experimental Physics, National University of Ireland, Galway, University Road, Galway, Ireland 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.