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Formation of Highly Oxygenated Organic Molecules from α‑Pinene Ozonolysis: Chemical Characteristics, Mechanism, and Kinetic Model Development

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journal contribution
posted on 03.04.2019, 00:00 authored by Ugo Molteni, Mario Simon, Martin Heinritzi, Christopher R. Hoyle, Anne-Kathrin Bernhammer, Federico Bianchi, Martin Breitenlechner, Sophia Brilke, António Dias, Jonathan Duplissy, Carla Frege, Hamish Gordon, Claudia Heyn, Tuija Jokinen, Andreas Kürten, Katrianne Lehtipalo, Vladimir Makhmutov, Tuukka Petäjä, Simone M. Pieber, Arnaud P. Praplan, Siegfried Schobesberger, Gerhard Steiner, Yuri Stozhkov, António Tomé, Jasmin Tröstl, Andrea C. Wagner, Robert Wagner, Christina Williamson, Chao Yan, Urs Baltensperger, Joachim Curtius, Neil M. Donahue, Armin Hansel, Jasper Kirkby, Markku Kulmala, Douglas R. Worsnop, Josef Dommen
Terpenes are emitted by vegetation, and their oxidation in the atmosphere is an important source of secondary organic aerosol (SOA). A part of this oxidation can proceed through an autoxidation process, yielding highly oxygenated organic molecules (HOMs) with low saturation vapor pressure. They can therefore contribute, even in the absence of sulfuric acid, to new particle formation (NPF). The understanding of the autoxidation mechanism and its kinetics is still far from complete. Here, we present a mechanistic and kinetic analysis of mass spectrometry data from α-pinene (AP) ozonolysis experiments performed during the CLOUD 8 campaign at CERN. We grouped HOMs in classes according to their identified chemical composition and investigated the relative changes of these groups and their components as a function of the reagent concentration. We determined reaction rate constants for the different HOM peroxy radical reaction pathways. The accretion reaction between HOM peroxy radicals was found to be extremely fast. We developed a pseudo-mechanism for HOM formation and added it to the AP oxidation scheme of the Master Chemical Mechanism (MCM). With this extended model, the observed concentrations and trends in HOM formation were successfully simulated.