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

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 fo
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.
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Author:Siegfried Schobesberger, Alessandro Franchin, Federico Bianchi, Linda Rondo, Jonathan Duplissy, Andreas Kürten, Ismael K. Ortega, Axel Metzger, Ralf Schnitzhofer, Joao Almeida, Antonio Amorim, Josef Dommen, Eimear M. Dunne, Mikael Ehn, Stéphanie Gagné, Luisa Ickes, Heikki Junninen, Armin Hansel, Veli-Matti Kerminen, Jasper Kirkby, Agnieszka Kupc, Ari Laaksonen, Katrianne Lehtipalo, Serge Mathot, Antti Onnela, Tuukka Petäjä, Francesco Riccobono, Filipe Duarte Santos, Mikko Sipilä, Antonio Tomé, Georgios Tsagkogeorgas, Yrjo Viisanen, Paul E. Wagner, Daniela Wimmer, Joachim Curtius, Neil McPherson Donahue, Urs Baltensperger, Markku Kulmala, Douglas R. Worsnop
Parent Title (English):Atmospheric chemistry and physics
Publisher:European Geosciences Union
Place of publication:Katlenburg-Lindau
Document Type:Article
Date of Publication (online):2015/01/07
Date of first Publication:2015/01/07
Publishing Institution:Universitätsbibliothek Johann Christian Senckenberg
Release Date:2015/04/22
First Page:55
Last Page:78
© Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License.
HeBIS PPN:369048377
Dewey Decimal Classification:550 Geowissenschaften
Licence (German):License LogoCreative Commons - Namensnennung 3.0

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