Articles | Volume 15, issue 1
Atmos. Chem. Phys., 15, 55–78, 2015

Special issue: The CERN CLOUD experiment (ACP/AMT inter-journal SI)

Atmos. Chem. Phys., 15, 55–78, 2015

Research article 07 Jan 2015

Research article | 07 Jan 2015

On the composition of ammonia–sulfuric-acid ion clusters during aerosol particle formation

S. Schobesberger1, A. Franchin1, F. Bianchi2, L. Rondo3, J. Duplissy1,4,5, A. Kürten3, I. K. Ortega1,6, A. Metzger7, R. Schnitzhofer8, J. Almeida5, A. Amorim9, J. Dommen2, E. M. Dunne10,11, M. Ehn1, S. Gagné1,4,*, L. Ickes3,**, H. Junninen1, A. Hansel7,8, V.-M. Kerminen1, J. Kirkby3,5, A. Kupc12, A. Laaksonen13,14, K. Lehtipalo1, S. Mathot5, A. Onnela5, T. Petäjä1, F. Riccobono2, F. D. Santos9, M. Sipilä1,4, A. Tomé9, G. Tsagkogeorgas15, Y. Viisanen13, P. E. Wagner12, D. Wimmer1,3, J. Curtius3, N. M. Donahue16, U. Baltensperger2, M. Kulmala1, and D. R. Worsnop1,13,14,17 S. Schobesberger et al.
  • 1Department of Physics, University of Helsinki, 00014 Helsinki, Finland
  • 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
  • 3Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
  • 4Helsinki Institute of Physics, University of Helsinki, 00014 Helsinki, Finland
  • 5CERN, 1211 Geneva, Switzerland
  • 6Laboratoire de Physique des Lasers, Atomes et Molécules, Université de Lille 1, 59655 Villeneuve d'Ascq, France
  • 7Ionicon Analytik GmbH, 6020 Innsbruck, Austria
  • 8Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria
  • 9SIM, University of Lisbon and University of Beira Interior, 1749-016 Lisbon, Portugal
  • 10School of Earth and Environment, University of Leeds, LS2 9JT Leeds, UK
  • 11Finnish Meteorological Institute, Atmospheric Research Centre of Eastern Finland, 70211 Kuopio, Finland
  • 12Faculty of Physics, University of Vienna, 1090 Vienna, Austria
  • 13Finnish Meteorological Institute, 00101 Helsinki, Finland
  • 14Department of Applied Physics, University of Eastern Finland, 70211 Kuopio, Finland
  • 15Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany
  • 16Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA 15213, USA
  • 17Aerodyne Research, Inc., Billerica, MA 01821, USA
  • *now at: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, B3H 3J5, Canada, and Environment Canada, Downsview, Toronto, M3H 5T4, Canada
  • **now at: Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland

Abstract. The formation of particles from precursor vapors is an important source of atmospheric aerosol. Research at the Cosmics Leaving OUtdoor Droplets (CLOUD) facility at CERN tries to elucidate which vapors are responsible for this new-particle formation, and how in detail it proceeds. Initial measurement campaigns at the CLOUD stainless-steel aerosol chamber focused on investigating particle formation from ammonia (NH3) and sulfuric acid (H2SO4). Experiments were conducted in the presence of water, ozone and sulfur dioxide. Contaminant trace gases were suppressed at the technological limit. For this study, we mapped out the compositions of small NH3–H2SO4 clusters over a wide range of atmospherically relevant environmental conditions. We covered [NH3] in the range from < 2 to 1400 pptv, [H2SO4] from 3.3 × 106 to 1.4 × 109 cm−3 (0.1 to 56 pptv), and a temperature range from −25 to +20 °C. Negatively and positively charged clusters were directly measured by an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer, as they initially formed from gas-phase NH3 and H2SO4, and then grew to larger clusters containing more than 50 molecules of NH3 and H2SO4, corresponding to mobility-equivalent diameters greater than 2 nm. Water molecules evaporate from these clusters during sampling and are not observed. We found that the composition of the NH3–H2SO4 clusters is primarily determined by the ratio of gas-phase concentrations [NH3] / [H2SO4], as well as by temperature. Pure binary H2O–H2SO4 clusters (observed as clusters of only H2SO4) only form at [NH3] / [H2SO4] < 0.1 to 1. For larger values of [NH3] / [H2SO4], the composition of NH3–H2SO4 clusters was characterized by the number of NH3 molecules m added for each added H2SO4 molecule n (Δm/Δ n), where n is in the range 4–18 (negatively charged clusters) or 1–17 (positively charged clusters). For negatively charged clusters, Δ m/Δn saturated between 1 and 1.4 for [NH3] / [H2SO4] > 10. Positively charged clusters grew on average by Δm/Δn = 1.05 and were only observed at sufficiently high [NH3] / [H2SO4]. The H2SO4 molecules of these clusters are partially neutralized by NH3, in close resemblance to the acid–base bindings of ammonium bisulfate. Supported by model simulations, we substantiate previous evidence for acid–base reactions being the essential mechanism behind the formation of these clusters under atmospheric conditions and up to sizes of at least 2 nm. Our results also suggest that electrically neutral NH3–H2SO4 clusters, unobservable in this study, have generally the same composition as ionic clusters for [NH3] / [H2SO4] > 10. We expect that NH3–H2SO4 clusters form and grow also mostly by Δm/Δn > 1 in the atmosphere's boundary layer, as [NH3] / [H2SO4] is mostly larger than 10. We compared our results from CLOUD with APi-TOF measurements of NH3–H2SO4 anion clusters during new-particle formation in the Finnish boreal forest. However, the exact role of NH3–H2SO4 clusters in boundary layer particle formation remains to be resolved.

Short summary
We used an ion mass spectrometer at CERN's CLOUD chamber to investigate the detailed composition of ammonia--sulfuric acid ion clusters (of both polarities) as they initially form and then grow into aerosol particles, at atmospherically relevant conditions. We found that these clusters’ composition is mainly determined by the ratio of the precursor vapors and ranges from ammonia-free clusters to clusters containing > 1 ammonia per sulfuric acid. Acid--base bindings are a key formation mechanism.
Final-revised paper