Formation of highly oxygenated organic molecules from the oxidation of limonene by OH radical: significant contribution of H-abstraction pathway

Abstract. Highly oxygenated organic molecules (HOM) play a pivotal role in the formation of secondary organic aerosol (SOA). Therefore, the distribution and yields of HOM are fundamental to understand their fate and chemical evolution in the atmosphere, and it is conducive to ultimately assess the impact of SOA on air quality and climate change. In this study, gas-phase HOM formed from the reaction of limonene with OH radical in photooxidation were investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber) using a time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO₃⁻-CIMS). A large number of HOM, including monomers (C_9–10) and dimers (C_17–20), were detected and classified into various families. Both closed-shell products and open-shell peroxy radicals (RO₂), were identified under low NO (0.1 ppt–~0.2 ppb) and high NO conditions (17 ppb). C₁₀ monomers are the most abundant HOM products and account for over 80 % total HOM. Closed-shell C₁₀ monomers were formed from two peroxy radical familie, C₁₀H₁₅O_x•(x=7–12) and C₁₀H₁₇O_x•(x=8–13), and their respective termination reactions with NO, RO₂, and HO₂. While C₁₀H₁₇O_x• is likely formed by OH addition to C₁₀H₁₆, the dominant initial step of limonene+OH, C₁₀H₁₅O_x•, is likely formed via H-abstraction by OH. C₁₀H₁₅O_x• and related products contributed 43 % and 46 % of C₁₀-HOM at low and high NO, demonstrating that H-abstraction pathways play a significant role in HOM formation in the reaction of limonene+OH. Combining theoretical kinetic calculations, structure activity relationships (SARs), literature data, and the observed RO₂ intensities, we proposed tentative mechanisms of HOM formation from both pathways. We further estimated the molar yields of HOM to be 3.04_−1.64^+3.89 % and 0.82_−0.44^+1.05 % at low and high NO, respectively. Our study highlights the importance of H-abstraction by OH and provides yield and tentative pathways in the OH oxidation of limonene to simulate the HOM formation and assess their role in SOA formation.