Articles | Volume 16, issue 21
Atmos. Chem. Phys., 16, 13601–13618, 2016

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

Atmos. Chem. Phys., 16, 13601–13618, 2016

Research article 03 Nov 2016

Research article | 03 Nov 2016

Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUD

Michael J. Lawler1,a,b, Paul M. Winkler2, Jaeseok Kim3,4, Lars Ahlm5, Jasmin Tröstl6, Arnaud P. Praplan7,8, Siegfried Schobesberger7,9, Andreas Kürten10, Jasper Kirkby10,11, Federico Bianchi7, Jonathan Duplissy7, Armin Hansel12, Tuija Jokinen7, Helmi Keskinen3,7, Katrianne Lehtipalo7,6, Markus Leiminger12, Tuukka Petäjä7, Matti Rissanen7, Linda Rondo10, Mario Simon10, Mikko Sipilä7, Christina Williamson10,13,14, Daniela Wimmer7,10, Ilona Riipinen5, Annele Virtanen2, and James N. Smith1,b Michael J. Lawler et al.
  • 1Department of Chemistry, University of California, Irvine, Irvine, CA, 92697, USA
  • 2Faculty of Physics, University of Vienna, 1090 Vienna, Austria
  • 3Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
  • 4Arctic Research Center, Korea Polar Research Institute, Yeonsu-gu, Incheon 21990, Republic of Korea
  • 5Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden
  • 6Paul Scherrer Institute, Villigen, Switzerland
  • 7Department of Physics, University of Helsinki, 00014 Helsinki, Finland
  • 8Finnish Meteorological Institute, 00101 Helsinki, Finland
  • 9Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • 10Institute for Atmospheric and Environmental Sciences, Goethe University of Frankfurt, 60438 Frankfurt am Main, Germany
  • 11European Organization for Nuclear Research (CERN), Geneva, Switzerland
  • 12Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria
  • 13Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 14Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA
  • avisitor at: National Center for Atmospheric Research, Atmospheric Chemistry Observations and Modeling Lab, Boulder, CO, 80301, USA
  • bformerly at: University of Eastern Finland, Department of Applied Physics, Kuopio, Finland

Abstract. New particle formation driven by acid–base chemistry was initiated in the CLOUD chamber at CERN by introducing atmospherically relevant levels of gas-phase sulfuric acid and dimethylamine (DMA). Ammonia was also present in the chamber as a gas-phase contaminant from earlier experiments. The composition of particles with volume median diameters (VMDs) as small as 10 nm was measured by the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS). Particulate ammonium-to-dimethylaminium ratios were higher than the gas-phase ammonia-to-DMA ratios, suggesting preferential uptake of ammonia over DMA for the collected 10–30 nm VMD particles. This behavior is not consistent with present nanoparticle physicochemical models, which predict a higher dimethylaminium fraction when NH3 and DMA are present at similar gas-phase concentrations. Despite the presence in the gas phase of at least 100 times higher base concentrations than sulfuric acid, the recently formed particles always had measured base : acid ratios lower than 1 : 1. The lowest base fractions were found in particles below 15 nm VMD, with a strong size-dependent composition gradient. The reasons for the very acidic composition remain uncertain, but a plausible explanation is that the particles did not reach thermodynamic equilibrium with respect to the bases due to rapid heterogeneous conversion of SO2 to sulfate. These results indicate that sulfuric acid does not require stabilization by ammonium or dimethylaminium as acid–base pairs in particles as small as 10 nm.

Short summary
We present chemical observations of newly formed particles as small as ~ 10 nm from new particle formation experiments using sulfuric acid, dimethylamine, ammonia, and water vapor as gas phase reactants. The nanoparticles were more acidic than expected based on thermodynamic expectations, particularly at the smallest measured sizes. The results suggest rapid surface conversion of SO2 to sulfate and show a marked composition change between 10 and 15 nm, possibly indicating a phase change.
Final-revised paper