Open Access

Helmi Keskinen, Annele Virtanen, Jorma Joutsensaari, Georgios Tsagkogeorgas, Jonathan Duplissy, Siegfried Schobesberger, Martin Gysel, Francesco Riccobono, Jay G. Slowik, Federico Bianchi, Taina Yli-Juuti, Katrianne Lehtipalo, Linda Rondo, Martin Breitenlechner, Agnieszka Kupc, Joao Almeida, António Amorin, Eimear M. Dunne, Andrew J. Downward, Sebastian Ehrhart, Alessandro Franchin, Maija K. Kajos, Jasper Kirkby, Christoph Andreas Kürten, Tuomo Nieminen, Vladimir Makhmutov, Serge Mathot, Pasi Miettinen, Antti Onnela, Tuukka Petäjä, Arnaud Patrick Praplan, Filipe Duarte Santos, Simon Schallhart, Mikko Sipilä, Yuri Stozhkov, Antonio Tomé, Petri Vaattovaara, Daniela Wimmer, André Stephan Henry Prévôt, Josef Dommen, Neil McPherson Donahue, Richard C. Flagan, Ernest Weingartner, Yrjö Viisanen, Ilona Riipinen, Armin Hansel, Joachim Curtius, Markku Kulmala, Douglas R. Worsnop, Urs Baltensperger, Heike Wex, Frank Stratmann, Ari Laaksonen

• Sulphuric acid, ammonia, amines, and oxidised organics play a crucial role in nanoparticle formation in the atmosphere. In this study, we investigate the composition of nucleated nanoparticles formed from these compounds in the CLOUD chamber experiments at CERN. The investigation is carried out via analysis of the particle hygroscopicity, ethanol affinity, oxidation state, and ion composition. Hygroscopicity was studied by a hygroscopic tandem differential mobility analyser and a cloud condensation nuclei counter, ethanol affinity by an organic differential mobility analyser and particle oxidation level by a high-resolution time-of-flight aerosol mass spectrometer. The ion composition was studied by an atmospheric pressure interface time-of-flight mass spectrometer. The volume fraction of the organics in the particles during their growth from sizes of a few nanometers to tens of nanometers was derived from measured hygroscopicity assuming the Zdanovski-Stokes-Robinson relationship, and compared to values gained from the spectrometers. The ZSR-relationship was also applied to obtain the measured ethanol affinities during the particle growth, which were used to derive the volume fractions of sulphuric acid and the other inorganics (e.g. ammonium salts). In the presence of sulphuric acid and ammonia, particles with a mobility diameter of 150 nm were chemically neutralised to ammonium sulphate. In the presence of oxidation products of pinanediol, the organic volume fraction of freshly nucleated particles increased from 0.4 to ∼0.9, with an increase in diameter from 2 to 63 nm. Conversely, the sulphuric acid volume fraction decreased from 0.6 to 0.1 when the particle diameter increased from 2 to 50 nm. The results provide information on the composition of nucleated aerosol particles during their growth in the presence of various combinations of sulphuric acid, ammonia, dimethylamine and organic oxidation products.