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

Organic peroxy radicals (RO) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO 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 RO and gas-phase dimers from O-initiated oxidation of α-pinene, that ∼150 gaseous dimers (CHO) are primarily formed through RO cross-reactions, with a typical rate constant of 0.75-2 × 10 cm molecule s 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 C 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 RO, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO from OH reaction and 10% from ozonolysis autoxidize at 3-10 s and ≥1 s, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.