Elucidating formation of highly oxygenated organic molecules (HOMs) from α-pinene ozonolysis with isotopic labelling

Upon oxidation, some volatile organic compounds (VOCs) have been shown to go through a rapid process called autoxidation forming highly oxygenated organic molecules (HOMs). The exact autoxidation pathway taken affects the formation rates and the properties of the HOMs, however, a comprehensive step-by-step mechanism of HOM formation has not been described for any system of atmospheric relevance. In the autoxidation process, peroxy radical (RO₂) intermediates undergo intramolecular hydrogen abstractions (H-shifts) followed by oxygen (O₂) additions. This process can be monitored using chemical ionisation mass spectrometry and selective deuteration, where the precursor molecule has had the hydrogen atoms (¹H) of a specific carbon replaced with deuterium atoms (²H). In this work, we studied the initial formation pathways of HOMs in reactions of the monoterpene α-pinene with ozone. We had access to all separately deuterated carbon positions in α-pinene that have hydrogens, i.e. we had in total eight different selectively deuterated α-pinenes. We were able to determine which of the deuterated precursors were prone to losing D during the (aut)oxidation process, which helped us understand the pathways leading to HOM formation.