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Molecular understanding of atmospheric particle formation from sulfuric acid and large oxidized organic molecules.

Proceedings of the National Academy of Sciences of the United States of America (2013-10-09)
Siegfried Schobesberger, Heikki Junninen, Federico Bianchi, Gustaf Lönn, Mikael Ehn, Katrianne Lehtipalo, Josef Dommen, Sebastian Ehrhart, Ismael K Ortega, Alessandro Franchin, Tuomo Nieminen, Francesco Riccobono, Manuel Hutterli, Jonathan Duplissy, João Almeida, Antonio Amorim, Martin Breitenlechner, Andrew J Downard, Eimear M Dunne, Richard C Flagan, Maija Kajos, Helmi Keskinen, Jasper Kirkby, Agnieszka Kupc, Andreas Kürten, Theo Kurtén, Ari Laaksonen, Serge Mathot, Antti Onnela, Arnaud P Praplan, Linda Rondo, Filipe D Santos, Simon Schallhart, Ralf Schnitzhofer, Mikko Sipilä, António Tomé, Georgios Tsagkogeorgas, Hanna Vehkamäki, Daniela Wimmer, Urs Baltensperger, Kenneth S Carslaw, Joachim Curtius, Armin Hansel, Tuukka Petäjä, Markku Kulmala, Neil M Donahue, Douglas R Worsnop
ABSTRACT

Atmospheric aerosols formed by nucleation of vapors affect radiative forcing and therefore climate. However, the underlying mechanisms of nucleation remain unclear, particularly the involvement of organic compounds. Here, we present high-resolution mass spectra of ion clusters observed during new particle formation experiments performed at the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research. The experiments involved sulfuric acid vapor and different stabilizing species, including ammonia and dimethylamine, as well as oxidation products of pinanediol, a surrogate for organic vapors formed from monoterpenes. A striking resemblance is revealed between the mass spectra from the chamber experiments with oxidized organics and ambient data obtained during new particle formation events at the Hyytiälä boreal forest research station. We observe that large oxidized organic compounds, arising from the oxidation of monoterpenes, cluster directly with single sulfuric acid molecules and then form growing clusters of one to three sulfuric acid molecules plus one to four oxidized organics. Most of these organic compounds retain 10 carbon atoms, and some of them are remarkably highly oxidized (oxygen-to-carbon ratios up to 1.2). The average degree of oxygenation of the organic compounds decreases while the clusters are growing. Our measurements therefore connect oxidized organics directly, and in detail, with the very first steps of new particle formation and their growth between 1 and 2 nm in a controlled environment. Thus, they confirm that oxidized organics are involved in both the formation and growth of particles under ambient conditions.

MATERIALS
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