Kinetic nucleation and ions in boreal particle formation events
1Department of Physical Sciences, P.O. Box 64, FIN-00014 University of Helsinki, Finland
2Institute of Environmental Physics, University of Tartu, 18 U¨ likooli Street, Tartu, 50090, Estonia
3Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20E, FIN-00880 Helsinki, Finland
4Max-Planck-Institute f ¨ ur Kernphysik (MPI-K), P.O. Box 103980, D-69029 Heidelberg, Germany
Abstract. 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 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 be explained by preferential condensation of sulphuric acid onto negatively charged clusters and particles. According to model simulations, the relative contribution of ion-based 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) 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.