Unexpected significance of a minor reaction pathway in daytime formation of biogenic highly oxygenated organic compounds

Abstract

Secondary organic aerosol (SOA), formed by oxidation of volatile organic compounds, substantially influence air quality and climate. Highly oxygenated organic molecules (HOMs), particularly those formed from biogenic monoterpenes, contribute a large fraction of SOA. During daytime, hydroxyl radicals initiate monoterpene oxidation, mainly by hydroxyl addition to monoterpene double bonds. Naturally, related HOM formation mechanisms should be induced by that reaction route, too. However, for α-pinene, the most abundant atmospheric monoterpene, we find a previously unidentified competitive pathway under atmospherically relevant conditions: HOM formation is predominately induced via hydrogen abstraction by hydroxyl radicals, a generally minor reaction pathway. We show by observations and theoretical calculations that hydrogen abstraction followed by formation and rearrangement of alkoxy radicals is a prerequisite for fast daytime HOM formation. Our analysis provides an accurate mechanism and yield, demonstrating that minor reaction pathways can become major, here for SOA formation and growth and related impacts on air quality and climate.

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

“…This is as expected given that the fraction of RO2 terminated by NO forming organic nitrate was higher at high NO. Higher fractions of organic nitrate have also been observed by the previous studies in the photo-oxidation of other monoterpenes at high NO (Pullinen et al, 2020;Shen et al, 2022).…”

“…Interestingly, in our previous study, we found that hydrogen abstraction is important for HOM formation in the reaction of α-pinene+OH (Shen et al, 2022), where intermediate alkoxy radicals oxidation steps aid autoxidation by breaking the sixmembered and four-membered rings. As limonene+OH is known to have a high H-abstraction contribution of ~34% (Rio et al, 2010;Dash and Rajakumar, 2015;Braure et al, 2014), it is interesting to compare these two systems due to the similar but subtly different structures of α-pinene and limonene, with both containing a six-membered ring whereas limonene does not 100 contain a four-membered ring.…”

“…HOM formation in OH-initiated oxidation of monoterpenes such as α-pinene and β-pinene has been studied in the laboratory 80 (Berndt, 2021;Xu et al, 2019;Berndt et al, 2016;Kirkby et al, 2016;Shen et al, 2022), and is assumed to mainly start with the OH addition to C=C double bond. The alkyl radical adduct can rapidly add O2 to the radical site to form a peroxy radical (Finlayson-Pitts and Pitts, 2000;Ziemann and Atkinson, 2012).…”

“…CC BY 4.0 License. from several monoterpenes following the reaction with hydroxyl radical (OH) were observed, such as C10H14-16O7-11 and C19-20H28-32O10-18 (Berndt et al, 2018;Shen et al, 2022;Xu et al, 2019). Despite the molecular similarities of HOM monomers or 85 dimers products from different monoterpenes (Jokinen et al, 2015;Ehn et al, 2012), HOM yields are quite different between different monoterpenes in a given oxidation reaction (e.g.…”

“…The kinetics of the chemistry following the ring breaking is expected to be fairly well described based on structure-activity relationships (SARs), and no explicit theoretical kinetic calculations were performed. We employ the same approach for deducing the mechanism as described in our recent work (Shen et al, 2022). Briefly, we only consider a limited oxidation 210 network by considering all possible reaction channels, predict their rate by SARs, and select few dominant channels.…”