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Published October 21, 2014 | Published + Supplemental Material
Journal Article Open

Neutral molecular cluster formation of sulfuric acid–dimethylamine observed in real time under atmospheric conditions


For atmospheric sulfuric acid (SA) concentrations the presence of dimethylamine (DMA) at mixing ratios of several parts per trillion by volume can explain observed boundary layer new particle formation rates. However, the concentration and molecular composition of the neutral (uncharged) clusters have not been reported so far due to the lack of suitable instrumentation. Here we report on experiments from the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research revealing the formation of neutral particles containing up to 14 SA and 16 DMA molecules, corresponding to a mobility diameter of about 2 nm, under atmospherically relevant conditions. These measurements bridge the gap between the molecular and particle perspectives of nucleation, revealing the fundamental processes involved in particle formation and growth. The neutral clusters are found to form at or close to the kinetic limit where particle formation is limited only by the collision rate of SA molecules. Even though the neutral particles are stable against evaporation from the SA dimer onward, the formation rates of particles at 1.7-nm size, which contain about 10 SA molecules, are up to 4 orders of magnitude smaller compared with those of the dimer due to coagulation and wall loss of particles before they reach 1.7 nm in diameter. This demonstrates that neither the atmospheric particle formation rate nor its dependence on SA can simply be interpreted in terms of cluster evaporation or the molecular composition of a critical nucleus.

Additional Information

© 2014 National Academy of Sciences. Freely available online through the PNAS open access option. Edited by Barbara J. Finlayson-Pitts, University of California, Irvine, CA, and approved September 8, 2014 (received for review March 21, 2014). We thank the European Center for Nuclear Research (CERN) for supporting Cosmics Leaving Outdoor Droplets (CLOUD) with important technical and financial resources and for providing a particle beam from the CERN Proton Synchrotron. We also thank P. Carrie, L.-P. De Menezes, J. Dumollard, K. Ivanova, F. Josa, I. Krasin, R. Kristic, A. Laassiri, O. S. Maksumov, B. Marichy, H. Martinati, S. V. Mizin, R. Sitals, A. Wasem, and M. Wilhelmsson for their important contributions to the experiment and P. H. McMurry for helpful discussion. We thank the tofTools team for providing tools for mass spectrometry analysis. This research has received funding from the European Commission Seventh Framework Programme (Marie Curie Initial Training Network "CLOUD-ITN" 215072, MC-ITN "CLOUD-TRAIN" 316662, European Research Council (ERC)-Starting "MOCAPAF" Grant 57360, and ERC-Advanced "ATMNUCLE" Grant 227463). This research has also received funding from the German Federal Ministry of Education and Research (Projects 01LK0902A and 01LK1222A), the Swiss National Science Foundation (Projects 200020 135307 and 206620 141278), the Academy of Finland (135054, 133872, 251427, 139656, 139995, 137749, 141217, 141451 and Center of Excellence Project 1118615), the Finnish Funding Agency for Technology and Innovation, the Väisälä Foundation, the Nessling Foundation, the Austrian Science Fund (Project J3198-N21), the Portuguese Foundation for Science and Technology (Project CERN/FP/116387/2010), the Swedish Research Council, Vetenskapsrå̊det (Grant 2011-5120), the Presidium of the Russian Academy of Sciences and Russian Foundation for Basic Research (Grants 08-02-91006-CERN and 12-02-91522-CERN), the US National Science Foundation (Grants AGS1136479 and CHE1012293), Pan-European Gas-Aerosols-Climate Interaction Study project [funded by the European Commission under the Framework Program 7 (FP7-ENV-2010-265148)], and the Davidow Foundation.

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