10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Exploring the Autoxidation Mechanisms of Aromatic VOCs
RUBY MARTEN, Mao Xiao, Lukas Fischer, Bernhard Mentler, Mario Simon, Martin Heinritzi, Olga Garmash, Christopher R. Hoyle, Andrea Baccarini, Chuan Ping Lee, Houssni Lamkaddam, Imad El Haddad, Josef Dommen, Urs Baltensperger, CLOUD Collaboration, Paul Scherrer Institute
Abstract Number: 874 Working Group: Aerosol Chemistry
Abstract It has been shown that both biogenic and anthropogenic aromatic volatile organic compounds (AVOCs) present in the atmosphere are not only important reactants in formation of secondary organic aerosol and ozone, but that they also form highly oxygenated molecules (HOMs) in the gas phase, which in turn will contribute to new particle formation due to their extremely low volatility.1,2 In urban areas, anthropogenic AVOCs are a major atmospheric component and therefore an important part of the chemistry involved. AVOCs become HOMs after addition of OH- to the aromatic ring and a subsequent autoxidation route with multiple additions of O2, leading to a high O:C ratio. Although the oxidation mechanism involved has been studied in depth in many laboratory and theoretical experiments, the complete mechanism of autoxidation is still not clear. Furthermore, mass spectrometry data shows clear evidence of ‘dimers’ of aromatic HOMs formed in the gas phase, another mechanism which remains unexplained.
This work will present results from experiments, focusing on anthropogenic AVOCs, undertaken during the CLOUD 11 (2016) and CLOUD 12 (2017) campaigns at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN. Experiments were performed with toluene, naphthalene, and 1,2,4-tri-methylbenzene under varying atmospherically relevant conditions.
A proposed general mechanism of HOM formation for aromatic VOCs will be presented in order to explain the observed products of autoxidation. This mechanism will be based off of state-of-the-art theories3,4 combined with data collected from the CLOUD campaigns. Reaction rate constants and branching ratios for the different pathways will be proposed, to capture the observed concentrations and time series of HOMs. The general mechanism of aromatic compound oxidation and the associated rates will be presented and the implication of the different pathways in the HOMs production will be discussed.
Acknowledgment: The research is supported by the Swiss National Science Foundation and the MSCA-ITN project CLOUD-MOTION no. 764991. We thank CERN for supporting CLOUD with important technical and financial resources, and for providing a particle beam from the Proton Synchrotron. We acknowledge tofTools software for mass spectrometry analysis.
1. Kirkby, J. et al. (2016). Ion-induced nucleation of pure biogenic particles, Nature, 533(7604), 521–526. 2. Molteni, U. et al. (2016). Formation of highly oxygenated organic molecules from aromatic compounds. Atmospheric Chemistry and Physics Discussions, (December), 1–39. 3. Wang, S. et al. (2017). Formation of Highly Oxidized Radicals and Multifunctional Products from the Atmospheric Oxidation of Alkylbenzenes. Environmental Science and Technology, 51(15), 8442–8449 4. Ji, Y. (2017). Reassessing the atmospheric oxidation mechanism of toluene. Proceedings of the National Academy of Sciences, 114(39), E8314–E8314.