Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 4 4 2004 10.5194/acp-4-1071-2004 http://www.atmos-chem-phys.net/4/1071/2004/ http://www.atmos-chem-phys.net/4/1071/2004/acp-4-1071-2004.html http://www.atmos-chem-phys.net/4/1071/2004/acp-4-1071-2004.pdf 1071 1083 2004-07-06 Modelling the formation of organic particles in the atmosphere T. Anttila V.-M. Kerminen M. Kulmala A. Laaksonen C. D. O'Dowd Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20E, FIN-00880 Helsinki, Finland University of Helsinki, Department of Physical Sciences, P.O. Box 64, FIN-00014 University of Helsinki, Helsinki, Finland University of Kuopio, Department of Applied Physics, P.O. Box 1627, FIN-70211, Kuopio, Finland Department of Experimental Physics, National University of Ireland, Galway, University Road, Galway, Ireland Particle formation resulting from activation of inorganic stable clusters by a supersaturated organic vapour was investigated using a numerical model. The applied aerosol dynamic model included a detailed description of the activation process 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 in continental background areas. The initial growth of freshly-nucleated clusters is driven mainly by condensation of gaseous sulphuric acid and by a lesser extent 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.