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

Abstract. Formation of secondary organic aerosol (SOA), which affects climate and health, is largely driven by the gas-particle transfer of highly oxygenated organic molecules (HOMs). These HOMs form via autoxidation following reactions of volatile organic compounds (VOCs) with atmospheric oxidants. While the oxidation of monoterpenes, the most important biogenic VOCs for SOA formation, by ozone (O₃) and hydroxyl radicals (OH) is well-studied, the role of the nitrate radical (NO₃), a crucial nighttime oxidant, remains less understood.

This study investigated NO₃-initated oxidation of five monoterpenes: α-pinene (AP), Δ-3-carene, limonene, β-pinene (BP), and β-myrcene. Using a newly built free-jet flow-tube system (8.8 s reaction time) and chemical ionization mass spectrometry (amine/nitrate ionization), we observed a wide range of peroxy radicals and closed-shell products. Product closure was reasonably achieved for AP, limonene, and myrcene (estimated to 50%–70%), but was lower for carene and BP (20%–40%). AP and limonene predominantly yielded C₁₀H₁₆O₂ (molar yields > 50%), while a notably high signal for carene was the peroxy radical C₁₀H₁₆NO₈, for myrcene the radical C₁₀H₁₆NO₇, and for BP the accretion product C₂₀H₃₂N₂O₈. 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). The HOM yields differ from those of ozonolysis, but overall HOM yields from NO₃ oxidation are comparable in magnitude (0–10%). This study provides comprehensive and quantitative distributions of NO₃ oxidation products for the most common monoterpenes, providing important knowledge of their fast (aut)oxidation pathways.