ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-767-2005 Effects of SO<sub>2</sub> oxidation on ambient aerosol growth in water and ethanol vapours Petäjä T. 1 Kerminen V.-M. 2 Hämeri K. 1 Vaattovaara P. 3 Joutsensaari J. 3 Junkermann W. 4 Laaksonen A. 3 Kulmala M. 1 Division of Atmospheric Sciences, Department of Physical Sciences, University of Helsinki, Finland Climate and Global Change, Finnish Meteorological Institute, Finland Department of Applied Physics, University of Kuopio, Finland Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe, Germany 10 03 2005 5 3 767 779 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/5/767/2005/acp-5-767-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/767/2005/acp-5-767-2005.pdf

Hygroscopicity (i.e. water vapour affinity) of atmospheric aerosol particles is one of the key factors in defining their impacts on climate. Condensation of sulphuric acid onto less hygroscopic particles is expected to increase their hygrocopicity and hence their cloud condensation nuclei formation potential. In this study, differences in the hygroscopic and ethanol uptake properties of ultrafine aerosol particles in the Arctic air masses with a different exposure to anthropogenic sulfur pollution were examined. The main discovery was that Aitken mode particles having been exposed to polluted air were more hygroscopic and less soluble to ethanol than after transport in clean air. This aging process was attributed to sulphur dioxide oxidation and subsequent condensation during the transport of these particle to our measurement site. The hygroscopicity of nucleation mode aerosol particles, on the other hand, was approximately the same in all the cases, being indicative of a relatively similar chemical composition despite the differences in air mass transport routes. These particles had also been produced closer to the observation site typically 3&ndash;8 h prior to sampling. Apparently, these particles did not have an opportunity to accumulate sulphuric acid on their way to the site, but instead their chemical composition (hygroscopicity and ethanol solubility) resembled that of particles produced in the local or semi-regional ambient conditions.