Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 3 2006 10.5194/acp-6-787-2006 http://www.atmos-chem-phys.net/6/787/2006/ http://www.atmos-chem-phys.net/6/787/2006/acp-6-787-2006.html http://www.atmos-chem-phys.net/6/787/2006/acp-6-787-2006.pdf 787 793 2006-03-10 Cluster activation theory as an explanation of the linear dependence between formation rate of 3nm particles and sulphuric acid concentration M. Kulmala K. E. J. Lehtinen A. Laaksonen University of Helsinki, Department of Physical Sciences, P.O. Box 64, FIN-00014 Helsinki, Finland Finnish Meteorological Institute and University of Kuopio, Department of Applied Physics, P.O.Box 1627, FIN-70211 Kuopio, Finland University of Kuopio, Department of Applied Physics, P.O.Box 1627, FIN-70211 Kuopio, Finland According to atmospheric observations new particle formation seems to be a function of sulphuric acid concentration to the power from one to two. The nucleation theorem then predicts that the critical cluster contains one to two sulphuric acid molecules. However, existing nucleation theories predicts that the power is more (or equal) than 2. Here we present an activation theory, which can explain the observed slope. In cluster activation the clusters containing one sulphuric acid molecule will activate for further growth due to heterogeneous nucleation, heterogeneous chemical reactions including polymerization or activation of soluble clusters. In the activation process organic vapours are typically needed as condensing agents.