ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-12959-2011 Secondary new particle formation in Northern Finland Pallas site between the years 2000 and 2010 Asmi E. 1 2 Kivekäs N. 1 Kerminen V.-M. 1 Komppula M. 1 Hyvärinen A.-P. 1 Hatakka J. 1 Viisanen Y. 1 Lihavainen H. 1 Finnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland currently at: Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, UMR6016, France 20 12 2011 11 24 12959 12972 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/11/12959/2011/acp-11-12959-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/12959/2011/acp-11-12959-2011.pdf

Secondary new particle formation affects atmospheric aerosol and cloud droplet numbers and thereby, the aerosol effects on climate. In this paper, the frequency of nucleation events and the associated particle formation and growth rates, along with their seasonal variation, was analysed based on over ten years of aerosol measurements conducted at the Pallas GAW station in northern Finland. The long-term measurements also allowed a detailed examination of factors possibly favouring or suppressing particle formation. Effects of meteorological parameters and air mass properties as well as vapour sources and sinks for particle formation frequency and event parameters were inspected. In addition, the potential of secondary particle formation to increase the concentration of cloud condensation nuclei (CCN) sized particles was examined. Findings from these long-term measurements confirmed previous observations: event frequency peaked in spring and the highest growth rates were observed in summer, affiliated with increased biogenic activity. Events were almost exclusively observed in marine air masses on sunny cloud-free days. A low vapour sink by the background particle population as well as an elevated sulphuric acid concentration were found to favour particle formation. These were also conditions taking place most likely in marine air masses. Inter-annual trend showed a minimum in event frequency in 2003, when also the smallest annual median of growth rate was observed. This gives further evidence of the importance and sensitivity of particle formation for the condensing vapour concentrations at Pallas site. The particle formation was observed to increase CCN<sub>80</sub> (>80 nm particle number) concentrations especially in summer and autumn seasons when the growth rates were the highest. When the growing mode exceeded the selected 80 nm limit, on average in those cases, 211 ± 114% increase of CCN<sub>80</sub> concentrations was observed.

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