24 Oct 2022
24 Oct 2022
Status: this preprint is open for discussion.

Selective deuteration as a tool for resolving autoxidation mechanisms in α-pinene ozonolysis

Melissa J. A. Meder1, Otso Peräkylä1, Jonathan G. Varelas2, Jenny Luo2, Runlong Cai1, Yanjun Zhang1,4, Theo Kurtén1,3, Matthieu Riva4, Matti P. Rissanen5,3, Franz M. Geiger2, Regan James Thomson2, and Mikael Ehn1 Melissa J. A. Meder et al.
  • 1Institute for Atmospheric and Earth System Research (INAR/physics), University of Helsinki, Helsinki, Finland
  • 2Department of Chemistry, Northwestern University, Illinois, USA
  • 3Department of Chemistry, University of Helsinki, Helsinki, Finland
  • 4Univ Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, Villeurbanne, France
  • 5Aerosol Physics laboratory, Tampere University, Tampere, Finland

Abstract. Highly oxygenated organic molecules (HOM) from α-pinene ozonolysis have been shown to be significant contributors to secondary organic aerosol (SOA), yet our mechanistic understanding of how the peroxy radical-driven autoxidation leads to their formation in this system is still limited. The involved isomerisation reactions such as H-atom abstractions followed by O2 additions can take place on sub-second time-scales in short-lived intermediates, making the process challenging to study. Similarly, while the end products and sometimes radical intermediates can be observed using mass spectrometry, their structures remain elusive. Therefore, we propose a method utilising selective deuterations for unveiling the mechanisms of autoxidation, where the HOM products can be used to infer which C-atoms have taken part in the isomerisation reactions. This relies on the fact that if a C−D bond is broken due to an abstraction by a peroxy group forming a −OOD hydroperoxide, the D-atom will become labile and able to be exchanged with a hydrogen atom in water vapour (H2O), effectively leading to loss of the D-atom from the molecule.

In this study, we test the applicability of this method using three differently deuterated versions of α-pinene with the newly developed chemical ionisation Orbitrap (CI-Orbitrap) mass spectrometer to inspect the oxidation products. The high mass resolving power of the Orbitrap is critical, as it allows the unambiguous separation of molecules with a D-atom (mD=2.0141) from those with two H-atoms (mH2=2.0157). We found that the method worked well and we could deduce that two of the three tested compounds had lost D-atoms during oxidation, suggesting that those deuterated positions were actively involved in the autoxidation process. Surprisingly, the deuterations were not observed to decrease HOM molar yields, as would have been expected due to kinetic isotope effects. This may be an indication that the relevant H (or D) abstractions were fast enough that no competing pathways were of relevance despite slower abstraction rates of the D-atom. We show that selective deuteration can be a very useful method for studying autoxidation on a molecular level, and likely not limited to the system of α-pinene ozonolysis tested here.

Melissa J. A. Meder et al.

Status: open (until 05 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1131', Anonymous Referee #1, 15 Nov 2022 reply
  • RC2: 'Comment on egusphere-2022-1131', Anonymous Referee #2, 27 Nov 2022 reply
  • RC3: 'Comment on egusphere-2022-1131', Anonymous Referee #3, 28 Nov 2022 reply
  • RC4: 'Comment on egusphere-2022-1131', Anonymous Referee #4, 29 Nov 2022 reply

Melissa J. A. Meder et al.

Melissa J. A. Meder et al.


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Short summary
This manuscript discusses the formation pathways of highly oxygenated organic molecules (HOM) from the oxidation of the monoterpene α-pinene. These molecules are very important for secondary organic aerosol (SOA) formation in forested regions, and monoterpenes are the single largest source of SOA globally. The fast reactions leading to HOM remain elusive despite considerable efforts over the last decade. Our focus is on using multiple isotopically labelled precursors to probe these reactions.