Atmospheric oxidation of (E)-β-farnesene initiated by hydroxyl radical: New formation mechanism of HOMs

(E)-β-farnesene is one of the acyclic sesquiterpenes, whose atmospheric oxidation has been determined as a significant contributor to SOA formation due to the formed highly oxidized multifunctional molecules (HOMs). In this work, the atmospheric oxidation of (E)-β-farnesene initiated by OH radical was comprehensively investigated using quantum chemical calculation. For the initial reactions, OH addition reactions are dominant over H atom abstraction reactions because of lower free energy barriers. The first-generation products involving acetone, (E)− 4-methyl-8-methylenedeca-4,9-dienal, 4-methylenehex-5-enal (P3), 6-methylhept-5-en-2-one (P4), formaldehyde, (E)− 7,11-dimethyldodeca-1,6,10-trien-3-one and (E)− 6,10-dimethyl-2-methyleneundeca-5,9-dienal are produced from the subsequent reactions of the OH-(E)-β-farnesene adducts with NO and HO2. The second-generation products, such as 4-oxopentanal, 4-methylenehex-5-enal, (E)− 4-methyl-8-oxodeca-4,9-dienal, formaldehyde and (E)− 4-methyl-8-methylenenon-4-enedial, are also determined from the successive oxidations of P2 and P3 under high and low NOx conditions. For the formation of HOMs, the H atom shift is the bottleneck for the autooxidation mechanism while our newly proposed mechanism, i.e. the multiple H abstractions by OH radical and reactions with O2 and HO2, is a more thermodynamically favorable formation pathway of the HOMs. Due to the intricate oxidation mechanisms and products, it is difficult to comprise the oxidation mechanisms of sesquiterpenes (including (E)-β-farnesene) in the atmospheric chemical models to thoroughly evaluate their contributions to SOA formation. We expect that this comprehensive oxidation mechanism of (E)-β-farnesene from a theoretical perspective would be conducive to clarifying the atmospheric fate of (E)-β-farnesene and even sesquiterpenes.