Exploring the Hydrogen Abstraction Pathway in HOM Formation from α-pinene Photooxidation Systems under Varying NO Conditions

Abstract. Highly oxygenated organic molecules (HOM) are formed via autoxidation during ∙OH initiated oxidation of α-pinene. We investigated the importance of ∙OH addition and hydrogen (H)-abstraction in HOM formation from α-pinene photooxidation under varying NO concentrations. HOM-RO2∙ and subsequent termination products were detected by chemical ionization mass spectrometry. In the absence of NO, C10H17Ox∙ peroxy radicals and related products dominated the HOM spectrum, contributing >70 %. In contrast, the presence of NO induced substantial changes in HOM products, particularly the rapid formation of C10H15Ox∙-related HOM, such as C10H15NO8. This indicates an enhanced contribution from the H-abstraction pathway. The ratio of C10H15NOx to C10H17NOx also increased significantly from 0.34 to 0.84 as the initial loss rate of RO2∙ via reaction with NO rose from 0.18 s-1 to 1.7 s-1. Under high NO conditions (7.4 ppbv), major C10H15Ox∙-related closed-shell HOM (C10H14Ox and C10H15NOx) contributed ~30 % to the total HOM. Fuzzy c‑means clustering identified C10H15Ox∙-related HOM, thought to be second generation products via pinonaldehyde formation, as the cluster with the fastest formation rate, consistent with first-generation products. Subsequent pinonaldehyde oxidation experiments under comparable conditions showed significantly different product distributions. Formation of C10H15Ox∙-related HOM in α‑pinene experiments was more than two times higher than in the pinonaldehyde experiments despite the pinonaldehyde turnover being more than ten times lower. This study highlights the significance of the H-abstraction pathway for ∙OH initiated α-pinene photooxidation in the presence of NO, exploring detailed product distributions formed via this pathway.