Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 115; no. 48; pp. 12142 - 12147
• Main Authors: Zhao, Yue, Thornton, Joel A., Pye, Havala O. T.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 27.11.2018
• Subjects: Aerosols; Anthropogenic factors; Autoxidation; Dimers; Emissions; Fuel combustion; Human influences; Intermediates; Nitric oxide; Organic chemistry; Organic compounds; Organic matter; Oxidation; Particle mass; Peroxy radicals; Physical Sciences; Urban areas; Volatility; α-Pinene; dimers; autoxidation; peroxy radicals; monoterpenes; particle formation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1812147115

Organic peroxy radicals (RO₂) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO₂ in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO₂ and gas-phase dimers from O₃-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C16–20H24–34O4–13) are primarily formed through RO₂ cross-reactions, with a typical rate constant of 0.75–2 × 10−12 cm³ molecule−1 s−1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2–2.5% by mole (0.5–6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C16–20 dimers provide a potentially important organicmedium for initial particle formation, and alone can explain 5–60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO₂, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO₂ from OH reaction and 10% from ozonolysis autoxidize at 3–10 s−1 and ≥1 s−1, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO₂ autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.