Gas-phase products from nitrate radical oxidation of five monoterpenes: insights from free-jet flow-tube experiments

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and has been widely studied due to its effects on both climate and human health. SOA formation is attributed to the gas-particle transfer of various oxidized products, especially highly oxygenated organic molecules (HOMs), which are formed through autoxidation following the reaction of volatile organic compounds (VOCs) with atmospheric oxidants. Monoterpenes (MTs) are among the most important biogenic VOCs. While their oxidation by ozone and hydroxyl radicals has been extensively studied, the role of nitrate radicals (NO₃) remains less understood, despite it being a crucial nighttime oxidant with non-negligible daytime contributions.

This study utilized a newly built free-jet flow-tube system (at effective reaction time of 8.8 s) and an Eisele-type chemical ionization mass spectrometer (in amine and nitrate modes), to directly investigate the NO₃-initated oxidation of five MTs: α-pinene (AP), Δ-3-carene, limonene, β-pinene (BP), and β-myrcene. We successfully observed a wide range of peroxy radicals and closed-shell products from all five MTs. Product closure was reasonably reached for AP, limonene, and myrcene (estimated to 50%–70%), but the incomplete closure for carene and BP (20%–40%) suggests substantial formation of one-oxygen-containing products that are undetectable by our methods. We found that among the three MTs with an endocyclic double bond, AP and limonene had the dominant product C₁₀H₁₆O₂ with molar yields exceeding 50%, while carene produced much less C₁₀H₁₆O₂. For carene, we instead observed considerably higher amounts of the peroxy radical C₁₀H₁₆NO₈, suggesting that ring-opening processes favoring autoxidation are more common for this MT. For BP, the major species was C₂₀H₃₂N₂O₈, following a quadratic trend with increasing NO₃, suggesting very fast dimer-forming bimolecular reactions of the primary peroxy radical C₁₀H₁₆NO₅. The acyclic structure and three double bonds of myrcene make ring closures (forming C–O–O–C groups) more efficient than in other MTs, resulting in the highest HOM yield out of the studied MTs. The distinct HOM yields further emphasize highly structure-dependent oxidation pathways: 6.5% (myrcene), 6.1% (carene), 1.8% (BP), 1.1% (limonene), and 0.8% (AP). Though the HOM yield from reaction with NO₃ can differ significantly from the ozonolysis HOM yield for a given MT, the overall HOM yields of NO₃ oxidation are comparable in magnitude to ozonolysis, falling in the range of 0–10%. Overall, benefiting from the short reaction times and near-wall-free conditions of the flow-tube, this study provides comprehensive and quantitative distributions of NO₃ oxidation products for the five common MTs, providing important knowledge of their fast (aut)oxidation pathways.