A first comprehensive insight into the sesquiterpene oxidation and sequential HOMs formation in the marine atmosphere: A case study of α-Cedrene

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 O2 and NO/HO2 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 HO2, 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.