Atmospheric aerosols exert an important influence on climate1 through their effects on stratiform cloud albedo and lifetime2 and the invigoration of convective storms3. Model calculations suggest that almost half of the global cloud condensation nuclei in the atmospheric boundary layer may originate from the nucleation of aerosols from trace condensable vapours4, although the sensitivity of the number of cloud condensation nuclei to changes of nucleation rate may be small5,6. Despite extensive research, fundamental questions remain about the nucleation rate of sulphuric acid particles and the mechanisms responsible, including the roles of galactic cosmic rays and other chemical species such as ammonia7. Here we present the first results from the CLOUD experiment at CERN. We find that atmospherically relevant ammonia mixing ratios of 100 parts per trillion by volume, or less, increase the nucleation rate of sulphuric acid particles more than 100–1,000-fold. Time-resolved molecular measurements reveal that nucleation proceeds by a base-stabilization mechanism involving the stepwise accretion of ammonia molecules. Ions increase the nucleation rate by an additional factor of between two and more than ten at ground-level galactic-cosmic-ray intensities, provided that the nucleation rate lies below the limiting ion-pair production rate. We find that ion-induced binary nucleation of H2SO4–H2O can occur in the mid-troposphere but is negligible in the boundary layer. However, even with the large enhancements in rate due to ammonia and ions, atmospheric concentrations of ammonia and sulphuric acid are insufficient to account for observed boundary-layer nucleation.

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We thank CERN for supporting CLOUD with important technical and financial resources, and for providing a particle beam from the CERN Proton Synchrotron. We also thank J.-L. Agostini, S. Atieh, J. Baechler, D. Bloess, G. Bowden, A. Braem, T. Callamand, A. Castel, L.-P. De Menezes, G. Favre, L. Ferreira, L. Gatignon, D. Gregorio, M. Guinchard, E. Ivanova, F. Josa, I. Krasin, R. Kristic, A. Kuzmin, O. Maksumov, S. Mizin, R. Richter, R. Sitals, A. Vacca, R. Veenhof, A. Wasem and M. Wilhelmsson for their contributions to the experiment. This research has received funding from the EC Seventh Framework Programme under grant agreement no. 215072 (Marie Curie Initial Training Network, ‘CLOUD-ITN’) and ERC-Advanced Grant ‘ATMNUCLE’ no. 227463, the German Federal Ministry of Education and Research (project no. 01LK0902A), the Swiss National Science Foundation (project nos 206621_125025 and 206620_130527), the Academy of Finland Center of Excellence program (project no. 1118615), the Austrian Science Fund (FWF; project nos P19546 and L593), and the Russian Academy of Sciences and Russian Foundation for Basic Research (grant N08-02-91006-CERN).

Author information


  1. CERN, CH-1211 Geneva, Switzerland

    • Jasper Kirkby
    • , Jonathan Duplissy
    • , André David
    • , Stefan Haider
    • , Serge Mathot
    • , Pierre Minginette
    •  & Antti Onnela
  2. Goethe-University of Frankfurt, Institute for Atmospheric and Environmental Sciences, 60438 Frankfurt am Main, Germany

    • Joachim Curtius
    • , João Almeida
    • , Sebastian Ehrhart
    • , Luisa Ickes
    • , Andreas Kürten
    • , Linda Rondo
    • , Daniela Wimmer
    •  & Fabian Kreissl
  3. SIM, University of Lisbon and University of Beira Interior, 1749-016 Lisbon, Portugal

    • João Almeida
    • , Antonio Amorim
    • , Jorge Lima
    • , Sandra Mogo
    • , Paulo Pereira
    •  & Antonio Tomé
  4. University of Leeds, School of Earth and Environment, LS2-9JT Leeds, United Kingdom

    • Eimear Dunne
    •  & Kenneth S. Carslaw
  5. University of Helsinki, Department of Physics, FI-00014 Helsinki, Finland

    • Jonathan Duplissy
    • , Alessandro Franchin
    • , Stéphanie Gagné
    • , Siegfried Schobesberger
    • , Mikael Ehn
    • , Heikki Junninen
    • , Katrianne Lehtipalo
    • , Jyri Mikkilä
    • , Tuomo Nieminen
    • , Tuukka Petäjä
    • , Mikko Sipilä
    • , Joonas Vanhanen
    • , Douglas R. Worsnop
    •  & Markku Kulmala
  6. Helsinki Institute of Physics, University of Helsinki, FI-00014 Helsinki, Finland

    • Jonathan Duplissy
    • , Stéphanie Gagné
    •  & Mikko Sipilä
  7. University of Vienna, Faculty of Physics, 1090 Vienna, Austria

    • Agnieszka Kupc
    • , Aron Vrtala
    • , Paul E. Wagner
    •  & Paul M. Winkler
  8. Ionicon Analytik GmbH and University of Innsbruck, Institute for Ion and Applied Physics, 6020 Innsbruck, Austria

    • Axel Metzger
    • , Martin Breitenlechner
    • , Armin Hansel
    • , Daniel Hauser
    • , Werner Jud
    •  & Ralf Schnitzhofer
  9. Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, CH-5232 Villigen, Switzerland

    • Francesco Riccobono
    • , Federico Bianchi
    • , Josef Dommen
    • , Hansueli Walther
    • , Ernest Weingartner
    •  & Urs Baltensperger
  10. Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany

    • Georgios Tsagkogeorgas
    • , Frank Stratmann
    •  & Heike Wex
  11. University of Milan, Department of Inorganic, Metallorganic, and Analytical Chemistry, 20133 Milan, Italy

    • Federico Bianchi
  12. California Institute of Technology, Division of Chemistry and Chemical Engineering, Pasadena, California 91125, USA

    • Andrew Downard
    • , Richard C. Flagan
    •  & John H. Seinfeld
  13. Lebedev Physical Institute, Solar and Cosmic Ray Research Laboratory, 119991 Moscow, Russia

    • Alexander Kvashin
    • , Vladimir Makhmutov
    •  & Yuri Stozhkov
  14. University of Eastern Finland, FI-70211 Kuopio, Finland

    • Ari Laaksonen
  15. NOAA Earth System Research Laboratory, Boulder, Colorado 80305, USA

    • Edward R. Lovejoy
  16. Finnish Meteorological Institute, FI-00101 Helsinki, Finland

    • Yrjo Viisanen
  17. Aerodyne Research Inc., Billerica, Massachusetts 01821, USA

    • Douglas R. Worsnop


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J.A. performed the nucleation rate analysis. S.S. conducted the APi-TOF analysis. J.A., F.B., M.B., A. Downard, E.D., J. Duplissy, S.E., A.F., S.G., D.H., L.I., W.J., J.K., F.K., A. Kürten, A. Kupc, K.L., V.M., A.M., T.N., F.R., L.R., R.S., S.S., Y.S., G.T. and D.W. conducted the data collection and analysis. J.A., K.S.C., J.C., E.D., S.E., L.I., E.R.L. and F.S. performed the modelling. J.K. wrote the manuscript. U.B., K.S.C., J.C., J.K., M.K., J.H.S. and D.R.W. did data interpretation and editing of the manuscript. All authors contributed to the development of the CLOUD facility and analysis instruments, and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Jasper Kirkby.

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