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 NOx, 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 firstgeneration RO2 (peroxy radicals) to react nearly exclusively with NO. Products are measured in both the gas phase (with a protontransfer 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 similar to 0.1 to >1); along with modeling simulations using the Master Chemical Mechanism (MCM), these results suggest indirect effects of NOx 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 NOx concentration beyond simple RO2 branching must be considered when planning laboratory oxidation experiments and applying their results to atmospheric conditions.

Automated summary

Organic oxidation reactions in the atmosphere can be complex due to branching pathways within each molecule's reaction mechanism. This study found that NOx concentration has indirect effects on product distribution and concentration, impacting the formation of secondary organic aerosol. This highlights the importance of considering NOx concentration beyond simple RO2 branching in atmospheric oxidation experiments.