ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-7255-2008 SO<sub>2</sub> oxidation products other than H<sub>2</sub>SO<sub>4</sub> as a trigger of new particle formation. Part 2: Comparison of ambient and laboratory measurements, and atmospheric implications Laaksonen A. 1 7 Kulmala M. 2 Berndt T. 3 Stratmann F. 3 Mikkonen S. 1 Ruuskanen A. 1 Lehtinen K. E. J. 1 7 Dal Maso M. 2 Aalto P. 2 Petäjä T. 2 Riipinen I. 2 Sihto S.-L. 2 Janson R. 4 Arnold F. 5 Hanke M. 5 Ücker J. 5 Umann B. 5 Sellegri K. 5 8 O'Dowd C. D. 6 Viisanen Y. 7 University of Kuopio, Department of Physics, POB 1627, 70211 Kuopio, Finland University of Helsinki, Department of Physical Sciences, Helsinki, Finland Leibniz-Institut für Troposphärenforschung e.V., Permoserstraße 15, 04318 Leipzig, Germany Stockholm University, Department of Applied Environmental Science (ITM), Atmospheric Science Unit, 10691, Stockholm, Sweden Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany National University of Ireland, Galway, Department of Physics Finnish Meteorological Institute, POB 503, 00101 Helsinki, Finland now at: Laboratoire de Météorologie Physique (LaMP), Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), UMR 6016 CNRS, France 10 12 2008 8 23 7255 7264 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/8/7255/2008/acp-8-7255-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/7255/2008/acp-8-7255-2008.pdf

Atmospheric new particle formation is generally thought to occur due to homogeneous or ion-induced nucleation of sulphuric acid. We compare ambient nucleation rates with laboratory data from nucleation experiments involving either sulphuric acid or oxidized SO<sub>2</sub>. Atmospheric nucleation occurs at H<sub>2</sub>SO<sub>4</sub> concentrations 2–4 orders of magnitude lower than binary or ternary nucleation rates of H<sub>2</sub>SO<sub>4</sub> produced from a liquid reservoir, and atmospheric H<sub>2</sub>SO<sub>4</sub> concentrations are very well replicated in the SO<sub>2</sub> oxidation experiments. We hypothesize these features to be due to the formation of free HSO<sub>5</sub> radicals in pace with H<sub>2</sub>SO<sub>4</sub> during the SO<sub>2</sub> oxidation. We suggest that at temperatures above ~250 K these radicals produce nuclei of new aerosols much more efficiently than H<sub>2</sub>SO<sub>4</sub>. These nuclei are activated to further growth by H<sub>2</sub>SO<sub>4</sub> and possibly other trace species. However, at lower temperatures the atmospheric relative acidity is high enough for the H<sub>2</sub>SO<sub>4</sub>–H<sub>2</sub>O nucleation to dominate.

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