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

Abstract

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 x , 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/NO 2 ratios. Known concentrations of NO in the classically "high-NO" range are maintained in the chamber, thereby constraining firstgeneration RO 2 (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/NO 2 ratios (from ∼0.1 to >1); along with modeling simulations using the Master Chemical Mechanism (MCM), these results suggest indirect effects of NO x chemistry beyond the commonly cited RO 2 + NO reaction. Specifically, lower-NO/NO 2 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 x concentration beyond simple RO 2 branching must be considered when planning laboratory oxidation experiments and applying their results to atmospheric conditions.

Reactions of C12–C14 n-Alkylcyclohexanes with Cl Atoms: Kinetics and Secondary Organic Aerosol Formation

“…As the photochemical reactions proceed, the NO/NO 2 ratio rapidly decreased to the typical ratio of ∼0.1, which was similar to the daytime atmosphere level. 88,89 Furthermore, the HO 2 concentrations we simulated by the Master Chemical Mechanism (MCM) were at the atmospheric level (∼10 8 molecules cm −3 ). 90 We put 4 min-average time-dependent data of parallel experiments of the same reaction system together and ran a two-product model 91 (Text S9); the SOA yields of the three nalkylcyclohexanes as a function of SOA mass concentrations are illustrated in Figure 2.…”

Secondary organic aerosol formation from monocyclic aromatic hydrocarbons: insights from laboratory studies

“…Laboratory studies revealed that the [NO]/[NO 2 ] ratio can effectively govern SOA formation in high-NO x experiments. 108,109 Chen et al found that [NO]/[NO 2 ] ratios have a negative relationship with SOA yield from the high-NO x photooxidation of methacrolein and α,β-unsaturated aldehydes. 108 More recently, Nihill et al produced individual RO directly via alkyl nitrite photolysis to rule out the effects of NO x on initiating the oxidation chemistry of SOA precursors.…”

“…RO is converted to RO 2 , which can involve a variety of reactions. 109 In high-NO x environments, higher [NO]/[NO 2 ] ratios lead to the formation of fewer products, whereas lower [NO]/[NO 2 ] ratios can induce the increase in SOA concentration. 109 For m -xylene, Li et al showed that SOA formation has strong correlations with [HO]/[HO 2 ], [HO 2 ]/[RO 2 ], and [NO]/[HO 2 ] in addition to [NO]/[NO 2 ].…”

“…109 In high-NO x environments, higher [NO]/[NO 2 ] ratios lead to the formation of fewer products, whereas lower [NO]/[NO 2 ] ratios can induce the increase in SOA concentration. 109 For m -xylene, Li et al showed that SOA formation has strong correlations with [HO]/[HO 2 ], [HO 2 ]/[RO 2 ], and [NO]/[HO 2 ] in addition to [NO]/[NO 2 ]. 95 Consequently, further studies to identify the fate and lifetime of RO 2 under various NO, NO 2 , RO 2 , and HO 2 levels, and [NO]/[NO 2 ] ratios relevant for the atmosphere are required to better understand the role of NO x in product distribution and aromatic SOA formation.…”

Chemistry of Functionalized Reactive Organic Intermediates in the Earth’s Atmosphere: Impact, Challenges, and Progress

“…68 Beyond this, photoinduced generation of reactive intermediate species without the use of an oxidant, typically performed in flow reactors or in jets under vacuum conditions, has allowed for more selective investigations of particular key reaction steps and the intermediates that lead to them. This includes the photolysis of alkyl iodides for R and RO 2 radicals, 69 of alkyl nitrites to form RO radicals, 70 and of diiodo compounds to form carbonyl oxides. 71 To date, most investigations of this type have been limited to small and/or unfunctionalized reactive intermediates, largely due to the fact that precursors to large, functionalized radicals are not commercially available.…”