ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-3999-2009 Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol – Part 2: Theoretical approaches Petters M. D. 1 Wex H. 2 Carrico C. M. 1 Hallbauer E. 2 Massling A. 2 3 McMeeking G. R. 1 4 Poulain L. 2 Wu Z. 2 Kreidenweis S. M. 1 Stratmann F. 2 Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA Institute for Tropospheric Research, Leipzig, Germany now at: National Environmental Research Institute, Aarhus University, Roskilde, Denmark now at: Centre for Atmospheric Science, University of Manchester, Manchester, UK 18 06 2009 9 12 3999 4009 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/9/3999/2009/acp-9-3999-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/3999/2009/acp-9-3999-2009.pdf

We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of α-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.

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