Understanding the Importance of the H-Abstraction Channel in HOM Formation from OH Oxidation of α-pinene

Secondary organic aerosols (SOA) can affect global climate change and air quality. Explaining the formation of highly oxygenated organic molecules (HOM) is important due to their vital role in SOA formation. α-pinene as the most abundant monoterpene can react rapidly with oxidants (e.g. OH radicals and O₃) and peroxy radicals (RO₂) undergo fast unimolecular reactions to form HOM. For the reaction with the important daytime hydroxyl radical, previous studies (Shen et al., 2022; Luo et al., 2023) have shown that the H-abstraction pathway, which initially appears to be a minor reaction channel (~10%), contributes significantly to the HOM formation during the early stages of monoterpene oxidation reactions. However, the importance of the H-abstraction channel under different environmentally relevant conditions is unknown.

Our study focused on the OH oxidation reactions of a-pinene under different NOx and OH exposure conditions. The experiments were conducted in the Jülich Saphir STAR (Stirred atmospheric tank Reactor) chamber. The photolysis of hydrogen peroxide was used as OH source to ensure pure OH radical reactions without interference of ozone reactions. A multi-scheme chemical ionization inlet (MION) was coupled to an APi-Long-TOF-MS to characterize HOM. An increased mass fraction of H-abstraction pathway-related HOM (C₁₀H₁₅O_x and C₁₀H₁₅NO_x) were observed among all HOM containing 10 C-atoms, with 0.7% at NO levels of ~0 ppb, 6% at 0.03 ppb NO, 22% at 1.0 ppb NO, and 31% at 2.2 ppb NO. Time series of these H-abstraction related HOM show a fast increase within the first minute after initiating reactions, which corresponds to direct formation from H-abstraction instead of secondary oxidation of accumulated pinonaldehyde. This could be explained by accelerated formation of alkoxy radicals promoted by RO₂ radicals and NO reactions. Similar results were observed under OH exposure ranging from 1×10⁶ to 1.3×10⁷ molecule cm^-3. Our study here shows the importance of the H-abstraction channel for the formation of HOM from OH oxidation of a-pinene, further emphasizing the role of NOx.

  Luo, H., Vereecken, L., Shen, H., Kang, S., Pullinen, I., Hallquist, M., Fuchs, H., Wahner, A., Kiendler-Scharr, A., Mentel, T. F., and Zhao, D.: Formation of highly oxygenated organic molecules from the oxidation of limonene by OH radical: significant contribution of H-abstraction pathway, Atmospheric Chemistry and Physics, 23, 7297-7319, 10.5194/acp-23-7297-2023, 2023.

  Shen, H. A.-O., Vereecken, L. A.-O. X., Kang, S. A.-O., Pullinen, I. A.-O., Fuchs, H. A.-O., Zhao, D. A.-O., and Mentel, T. A.-O.: Unexpected significance of a minor reaction pathway in daytime formation of biogenic highly oxygenated organic compounds, 2022.