Formation of highly oxygenated organic molecules from α-pinene photooxidation: evidence for the importance of highly oxygenated alkoxy radicals

Abstract. Highly oxygenated organic compounds (HOMs) from α-pinene oxidation are of great interest because of their importance in secondary organic aerosol (SOA) formation. Despite intensive investigations, the mechanisms of HOM formation from first-generation peroxy radicals to HOM-peroxy radicals (HOM-RO₂·) and to HOM-closed shell products are not well understood. One reason is that HOM-alkoxy radicals (HOM-RO·) are likely to contribute to the propagation of oxidative radical chains (alkoxy-peroxy pathway) because isomerization of functionalized alkoxy radicals can compete with their fragmentation (and reaction with O₂), as shown by theoretical kinetics. However, HOM-RO· reaction steps are difficult to verify in mechanisms. In this work, we have investigated HOM formation by varying the significance of the alkoxy-peroxy pathway as a function of NO_X, OH·, and CO. HOM-RO· are likely formed with high branching ratios in reactions of HOM-RO₂· with peroxy radicals (0.6) and NO (0.64) in analogy to simpler alkoxy radicals. We provide experimental evidence that for HOM-RO· the branching into isomerization is about 50 % (±14 %). Thus, HOM-RO· can play a central role in HOM formation, since alkoxy-peroxy pathways can compete with direct autoxidation. We observed significant concentrations of HOM-RO₂·, despite fast termination by NO, and shifts to higher O/C for HOM-RO₂· and termination products with increasing NO. At NO concentrations >1.5 ppb, the alkoxy-peroxy pathway may even prevail in propagating the oxidative radical chain leading to HOM formation. The increasing sink of HOM-RO₂· with increasing concentration of peroxy radicals and NO is compensated by an increasing source via the alkoxy-peroxy pathway.