Influence of the NO/NO₂ Ratio on Oxidation Product Distributions under High-NO Conditions

Organic oxidation reactions in the atmosphere can be challenging to parse due to the large number of branching points within each molecule's reaction mechanism. This complexity can complicate the attribution of observed effects to a particular chemical pathway. In this study, we simplify the chemistry of atmospherically relevant systems, and particularly the role of NO, by generating individual alkoxy radicals via alkyl nitrite photolysis (to limit the number of accessible reaction pathways) and measuring their product distributions under different NO/NO₂ ratios. Known concentrations of NO in the classically "high-NO" range are maintained in the chamber, thereby constraining first-generation RO₂ (peroxy radicals) to react nearly exclusively with NO. Products are measured in both the gas phase (with a proton-transfer reaction mass spectrometer) and the particle phase (with an aerosol mass spectrometer). We observe substantial differences in measured products under varying NO/NO₂ ratios (from ∼0.1 to >1); along with modeling simulations using the Master Chemical Mechanism (MCM), these results suggest indirect effects of NO chemistry beyond the commonly cited RO₂ + NO reaction. Specifically, lower-NO/NO₂ ratios foster higher concentrations of secondary OH, higher concentrations of peroxyacyl nitrates (PAN, an atmospheric reservoir species), and a more highly oxidized product distribution that results in more secondary organic aerosol (SOA). The impact of NO concentration beyond simple RO₂ branching must be considered when planning laboratory oxidation experiments and applying their results to atmospheric conditions.