Selective Deuteration Reveals the Importance of Multiple Branching Pathways in α-Pinene Autoxidation

The oxidation of monoterpenes is one of the largest single sources of atmospheric secondary organic aerosol (SOA) significantly impacting the climate and air quality. Still, the autoxidation mechanisms converting these volatile precursors to low-volatility condensable products remain elusive even for the most abundant monoterpene α-pinene. We studied the ozonolysis of α-pinene by combining advanced isotopic labeling and state-of-the-art chemical ionization mass spectrometry supported by quantum chemical calculations. We reacted a full set of eight selectively deuterated α-pinene analogues separately in a flow reactor to probe the oxidation mechanisms on a molecular level. We found that surprisingly few carbon atoms participate in the autoxidation process when forming even the most oxygenated products. Additionally, prompt pinonic acid formation has likely been greatly overestimated, whereas the α-pinene-derived dioxirane appears more stable than previously thought. Importantly, we reveal that oxidation models should include multiple branching pathways rather than simple linear autoxidation progression from less to more oxygenated species. Correct modeling of the oxidation is crucial to enable accurate predictions in the changing climate and atmospheric conditions.