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European Geosciences Union, Atmospheric Chemistry and Physics, 21(16), p. 13601-13618, 2016

DOI: 10.5194/acp-16-13601-2016

European Geosciences Union, Atmospheric Chemistry and Physics Discussions, p. 1-35

DOI: 10.5194/acp-2016-361

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

Journal article published in 2016 by Michael J. Lawler ORCID, Paul M. Winkler, Jaeseok Kim, Lars Ahlm, Jasmin Tröstl ORCID, Arnaud P. Praplan, Siegfried Schobesberger ORCID, Andreas Kuerten, Andreas Kürten, Jasper Kirkby ORCID, Federico Bianchi ORCID, Jonathan Duplissy, Armin Hansel ORCID, Tuija Jokinen ORCID, Helmi Keskinen and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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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 physico-chemical 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 that suggests a change to a mixed-phase state as the particles grew beyond this size. The reasons for the very acidic composition remain uncertain, but a possible explanation is that the particles did not reach thermodynamic equilibrium with respect to the bases due to rapid heterogeneous conversion of SO 2 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.