Aqueous-phase photo-oxidation of selected green leaf volatiles initiated by ?OH radicals: Products and atmospheric implications

C5- and C6- unsaturated oxygenated organic compounds emitted by plants under stress like cutting, freezing or drying, known as Green Leaf Volatiles (GLVs), may clear some of the existing uncertainties in secondary organic aerosol (SOA) budget. The transformations of GLVs are a potential source of SOA components through photo-oxidation processes oc-curring in the atmospheric aqueous phase. Here, we investigated the aqueous photo-oxidation products from three abundant GLVs (1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al) induced by center dot OH radicals, carried out in a photo-reactor under simulated solar conditions. The aqueous reaction samples were analyzed using advanced hyphen-ated mass spectrometry techniques: capillary gas chromatography mass spectrometry (c-GC-MS); and reversed-phase liquid chromatography high resolution mass spectrometry (LC-HRMS). Using carbonyl-targeted c-GC-MS analysis, we confirmed the presence of propionaldehyde, butyraldehyde, 1-penten-3-one, and 2-hexen-1-al in the reaction samples. The LC-HRMS analysis confirmed the presence of a new carbonyl product with the molecular formula C6H10O2, which probably bears the hydroxyhexenal or hydroxyhexenone structure. Density functional theory (DFT)-based quantum calculations were used to evaluate the experimental data and obtain insight into the formation mechanism and struc-tures of the identified oxidation products via the addition and hydrogen-abstraction pathways. DFT calculations high-lighted the importance of the hydrogen abstraction pathway leading to the new product C6H10O2. Atmospheric relevance of the identified products was evaluated using a set of physical property data like Henry's law constant (HLC) and vapor pressure (VP). The unknown product of molecular formula C6H10O2 has higher HLC and lower VP than the parent GLV and thus has potential to remain in the aqueous phase leading to possible aqueous SOA formation. Other observed carbonyl products are likely first stage oxidation products and precursors of aged SOA.

Automated summary

Plants under stress emit Green Leaf Volatiles (GLVs), which may clarify uncertainties in secondary organic aerosol (SOA) budget. In this study, we investigated the aqueous photo-oxidation products of three abundant GLVs induced by center dot OH radicals. Advanced mass spectrometry techniques were used to analyze the reaction samples. The results suggest the potential formation of aqueous SOA and provide insight into the formation mechanism and structures of the identified oxidation products.