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As an important contributor to the atmospheric SOA formation, the oxidation of the sesquiterpenes including α-Cedrene are widely considered to play a significant role in atmospheric chemistry, especially for the HOMs formation. Comparable concentrations of the sesquiterpenes (SQTs) detected over the marine boundary layer as those of in terrestrial ecosystems in recent atmospheric campaign, which aroused extensive concern of the SQTs oxidation and its impact on SOA formation in the marine atmosphere. In this work, we comprehensively investigated the oxidation of a representative sesquiterpene (α-Cedrene) initiated by OH, Cl, ClO and BrO radicals and the resulting HOMs formation. For the initial reactions, OH/Cl/ClO/BrO addition reactions are favored over H-atom abstractions by these radicals from the thermodynamic aspect. Seven main first-generation products yielded from the sequential oxidation of the addition adducts by O 2 and NO/HO 2 radical under different atmospheric conditions, such as 2-acetyl-1,1,4-trimethyloctahydropentalene-3a-carbaldehyde. Furthermore, three pathways of the HOMs formation were proposed and compared, (1) autoxidation, (2) multiple H abstractions by OH, Cl, ClO and BrO radicals and the succeeding reactions with HO 2 , and (3) the sequential association reaction of peroxy radicals and OH radical to form ROOOH. For the HOMs formation from the oxidation of α-Cedrene by OH and BrO radicals, the pathway (2) and (3) are thermodynamically feasible under the atmospheric conditions. As most of the H abstractions by Cl and ClO radicals could be difficult to occur, the produced HOMs with more than four or five oxygen atoms could be restricted. To our knowledge, this is the first report for clarifying the oxidation processes of SQTs in the marine atmosphere, which would contribute to the evaluation of the impacts of biogenic volatile organic compounds on the Marine atmosphere. [Display omitted] • The atmospheric oxidation of α-Cedrene by OH, Cl, ClO and BrO radical was clarified. • The first-generation products of α-Cedrene oxidation were elaborated. • The oxidation processes of SQTs initiated by halogen radicals were first studied. • The HOMs formation mechanisms from α-Cedrene oxidation were proposed in marine air. [ABSTRACT FROM AUTHOR]

(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 HO 2. 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 O 2 and HO 2 , 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. • The atmospheric oxidation of (E)-β-farnesene by OH radical was comprehensively investigated. • The first- and second-generation oxidation products were clarified. • A new formation mechanism of HOMs from (E)-β-farnesene oxidation was proposed. [ABSTRACT FROM AUTHOR]

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