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Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry
Citation:
Zhao, Y., J. Thornton, AND H. Pye. Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, 115(48):12142-12147, (2018). https://doi.org/10.1073/pnas.1812147115
Impact/Purpose:
Highly oxygenated multifunctional compounds and large-molecular-mass products from the oxidation of forest emissions can drive the formation and growth of atmospheric particles, which contribute significantly to degraded air quality and Earth’s energy balance. Yet, our knowledge of the reaction rates and formation mechanisms of these low-volatility compounds remains poor. Here we provide direct observations of a large suite of organic peroxy radicals (RO2) and related products from O3-initiated oxidation of α-pinene. We observe rapid RO2 autoxidation at room temperature and efficient RO2 cross-reactions forming gaseous dimers (ROOR′), and constrain the role of these processes in secondary organic aerosol mass formation. Our findings shed important light on key monoterpene-derived RO2 chemistry governing particle formation and growth in regions impacted by forest emissions.
Description:
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 organic medium 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.
URLs/Downloads:
DOI: Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry