Preprints
https://doi.org/10.5194/acp-2021-556
https://doi.org/10.5194/acp-2021-556

  20 Jul 2021

20 Jul 2021

Review status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas phase oxidation products and radical budgets

Zhaofeng Tan1, Luisa Hantschke1, Martin Kaminski1,a, Ismail-Hakki Acir1,b, Birger Bohn1, Changmin Cho1, Hans-Peter Dorn1, Xin Li1,c, Anna Novelli1, Sascha Nehr1,d, Franz Rohrer1, Ralf Tillmann1, Robert Wegener1, Andreas Hofzumahaus1, Astrid Kiendler-Scharr1, Andreas Wahner1, and Hendrik Fuchs1 Zhaofeng Tan et al.
  • 1Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, Jülich, Germany
  • anow at: Federal Office of Consumer Protection and Food Safety, Department 5: Method Standardisation, Reference Laboratories, Resistance to Antibiotics, Berlin, Germany
  • bnow at: Institute of Nutrition and Food Sciences, Food Science, University of Bonn, Bonn, Germany
  • cnow at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
  • dnow at: European University of Applied Sciences, Brühl, Germany

Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR. The chemical structure of myrcene consists of one moiety that is a conjugated π-system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals RO2 formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure-activity relationships (SAR) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bi-molecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 s−1 and 0.097 s−1 (T = 298 K) for the 3 isomers resulting from the 3'-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed to calculate radical production and destruction rates, which are expected to be balanced. Largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx = OH+HO2+RO2) production rates were high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced, if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6 × 10−11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical terminating organic peroxides. Experiments also allowed to determine the yields of organic oxidation products acetone (yield: 0.45 ± 0.08) and formaldehyde (yield: 0.35 ± 0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber giving a value of 0.13 ± 0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration yielding a value of (2.3 ± 0.3) × 10−10 cm3 s−1.

Zhaofeng Tan et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-556', Anonymous Referee #1, 24 Aug 2021
    • AC1: 'Reply on RC1', Hendrik Fuchs, 27 Sep 2021
  • RC2: 'Review of Tan et al.', Anonymous Referee #2, 31 Aug 2021
    • AC2: 'Reply on RC2', Hendrik Fuchs, 27 Sep 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-556', Anonymous Referee #1, 24 Aug 2021
    • AC1: 'Reply on RC1', Hendrik Fuchs, 27 Sep 2021
  • RC2: 'Review of Tan et al.', Anonymous Referee #2, 31 Aug 2021
    • AC2: 'Reply on RC2', Hendrik Fuchs, 27 Sep 2021

Zhaofeng Tan et al.

Zhaofeng Tan et al.

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Short summary
The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR. The chemical structure of myrcene is partly similar to isoprene. Therefore, it can be expected that hydrogen shift reactions could play a role as observed for isoprene. In this work, their potential impact on the regeneration efficiency of hydroxyl radicals is investigated.
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