Exploring the hydrogen abstraction pathway in HOM formation from α -pinene photooxidation systems under varying NO conditions

Abstract

Abstract. Highly oxygenated organic molecules (HOM) are formed via autoxidation during ⚫OH-initiated oxidation of α-pinene. We investigated the relative contributions of OH-addition and hydrogen (H)-abstraction to HOM formation from α-pinene photooxidation under varying nitrogen oxide (NO) conditions. HOM molecules were detected by a nitrate chemical ionization mass spectrometer (CIMS). In the absence of NO, C10H17Ox⚫ peroxy radicals and related termination products (e.g. C10H18Ox) dominated the HOM spectrum, accounting for > 70 % of total HOM. The presence of NO substantially altered HOM products, particularly by rapid formation of C10H15Ox⚫-related HOM, like C10H15NO8. The ratio of C10H15NOx to C10H17NOx increased from 0.34 to 0.84 as the RO2⚫ loss rate via reaction with NO increased from 0.18 to 1.06 s−1. Under high-NO conditions, C10H15Ox⚫-related HOM contributed up to 34 % to total HOM from α-pinene oxidation systems. The H-abstraction channel proved to be the source of C10H15Ox⚫-related HOM. Fuzzy c-means clustering indicated that C10H15Ox⚫-related HOM exhibited the fastest formation rate among the identified HOM groups, consistent with first-generation products. Comparison with pinonaldehyde oxidation, obtained by normalizing HOM yields to pinonaldehyde turnover, suggests that pinonaldehyde contributed ∼ 5 % of HOM in α-pinene systems, excluding secondary oxidation as the dominant source. Detection of C10H15NO4 under high-NO conditions by propylamine-CIMS indicates the formation of C10H15O3⚫ peroxy radicals, formed by alkoxy radical decomposition and six-membered ring opening in the H-abstraction channel. This study highlights the role of the H-abstraction pathway in ⚫OH-initiated α-pinene oxidation under NO-influenced conditions and provides new constraints on detailed HOM formation mechanisms.