Gas-Phase Photolysis and Photo-Oxidation of Isoprene-Derived Atmospherically Relevant Hydroxy Nitrates

Volatile organic compounds (VOCs), released from both natural and anthropogenic activities, undergo oxidation in the atmosphere to form alkyl peroxy radicals (RO2) that, through subsequent chemistry, form a variety of oxygenated species that may impact both air quality and climate. Among the potential oxygenated species formed are organic nitrates (ON) that can be transported over long distances and contribute to ozone production and particle formation. The atmospheric lifetime of an ON is influenced by its molecular structure. The ON structure impacts both further gas phase reaction (e.g., photolysis and photo-oxidation with OH radical reaction) and gas-to-particle partitioning that may result in multiphase chemistry (e.g., hydrolysis) and secondary organic aerosol (SOA) formation. This study investigates the photo-oxidation of two atmospheric-relevant ON, specifically, hydroxy nitrates, 2-methyl-1-nitrooxy-3-buten-2-ol (C5H9NO4, secondary -ONO2, C5-ON), and 3-methyl-2-nitrooxy-4-penten-3-ol (C6H11NO4, tertiary -ONO2, C6-ON). Significant differences in their reactivity are demonstrated, and the C5-ON underwent faster photolysis, while the C6-ON was more resistant to photolysis, whereas the C6-ON underwent faster degradation during OH-initiated photo-oxidation. These differences in reactivity are attributed to the presence of the extra methyl group on C6-ON. Furthermore, oxidation products that demonstrate the complexity of the reaction processes were detected by chemical ionization Orbitrap mass spectrometry operated in negative mode. Notably, cleavage of the O–N bond and subsequent alkoxy radical chemistry is the dominant pathway during photolysis, whereas OH addition followed by RO2 bimolecular reactions (i.e., RO2 + RO2 or HO2) is the dominant pathway during OH oxidation. This research sheds light on the intricate chemistry of organic nitrates in the atmosphere, emphasizing the role of the molecular structure on their fate.