Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 12 2009 10.5194/acp-9-3987-2009 http://www.atmos-chem-phys.net/9/3987/2009/ http://www.atmos-chem-phys.net/9/3987/2009/acp-9-3987-2009.html http://www.atmos-chem-phys.net/9/3987/2009/acp-9-3987-2009.pdf 3987 3997 2009-06-18 Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements H. Wex wex@tropos.de M. D. Petters C. M. Carrico E. Hallbauer A. Massling G. R. McMeeking L. Poulain Z. Wu S. M. Kreidenweis F. Stratmann Institute for Tropospheric Research, Leipzig, Germany Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA now at: National Environmental Research Institute, Aarhus University, Roskilde, Denmark now at: Centre for Atmospheric Science, University of Manchester, Manchester, UK Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of <i>α</i>-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors &le;1.05 was observed at RH<98% RH. At RH>98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4–6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activation is steeper than what would be expected for a constant value of the hygroscopicity. 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