Open Access

Christopher Robert Hoyle, Claudia Fuchs, Emma Järvinen, Harald Saathoff, Antonio Dias, Imad El Haddad, Martin Gysel, Sean C. Coburn, Jasmin Tröstl, Anne-Kathrin Bernhammer, Federico Bianchi, Martin Breitenlechner, Joel C. Corbin, Jill Craven, Neil McPherson Donahue, Jonathan Duplissy, Sebastian Ehrhart, Carla Frege, Hamish Gordon, Niko Florian Höppel, Martin Heinritzi, Thomas Bjerring Kristensen, Ugo Molteni, Leonid Nichman, Tamara Pinterich, André Stephan Henry Prévôt, Mario Simon, Jay G. Slowik, Gerhard Steiner, Antonio Tomé, Alexander L. Vogel, Rainer Volkamer, Andrea Christine Wagner, Robert Wagner, Anthony S. Wexler, Christina Williamson, Paul M. Winkler, Chao Yan, Antonio Amorim, Josef Dommen, Joachim Curtius, Martin William Gallagher, Richard C. Flagan, Armin Hansel, Jasper Kirkby, Markku Kulmala, Ottmar Möhler, Frank Stratmann, Douglas R. Worsnop, Urs Baltensperger

• The growth of aerosol due to the aqueous phase oxidation of sulfur dioxide by ozone was measured in laboratory-generated clouds created in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). Experiments were performed at 10 and −10 °C, on acidic (sulfuric acid) and on partially to fully neutralised (ammonium sulfate) seed aerosol. Clouds were generated by performing an adiabatic expansion – pressurising the chamber to 220 hPa above atmospheric pressure, and then rapidly releasing the excess pressure, resulting in a cooling, condensation of water on the aerosol and a cloud lifetime of approximately 6 min. A model was developed to compare the observed aerosol growth with that predicted using oxidation rate constants previously measured in bulk solutions. The model captured the measured aerosol growth very well for experiments performed at 10 and −10 °C, indicating that, in contrast to some previous studies, the oxidation rates of SO2 in a dispersed aqueous system can be well represented by using accepted rate constants, based on bulk measurements. To the best of our knowledge, these are the first laboratory-based measurements of aqueous phase oxidation in a dispersed, super-cooled population of droplets. The measurements are therefore important in confirming that the extrapolation of currently accepted reaction rate constants to temperatures below 0 °C is correct.