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The degradation mechanism of benzo[ a ]pyrene (BaP) initiated by • OH and 1 O 2 in aqueous solution is investigated by density functional theory calculations. The main degradation products are BaP-1,6-quinone, BaP-3,6-quinone, BaP-4,6-quinone, and BaP-6,12-quinone. • OH and HO 2 are the main intermediate radical species. At a low initial concentration of • OH, 1 O 2 could be a primary driver for BaP degradation. The degradation mechanism includes six consecutive elementary reactions: (1) 1 O 2 initiation forming BaP-6-OO. (2) 1,3 H-shift (H atom shifts to the OO group) that is promoted by H 2 O, forming BaP-6-OOH. (3) BaP-6-OOH decomposes into the • OH radical and BaP-6-O. (4) • OH addition to BaP-6-O forming BaP-6- O -1(3,4,12)-OH. (5) Extracting the H atom from the carbon with the OH group by 1 O 2 . (6) Extracting the H atom from the OH group by HO 2 . At a high initial concentration of • OH, the • OH-initiated and 1 O 2 -initiated degradation reactions of BaP are both feasible. The degradation mechanism includes six consecutive elementary reactions: (1) • OH initiation forming BaP-6-OH or 1 O 2 initiation forming BaP-6-OO. (2) 1 O 2 addition to BaP-6-OH forming BaP-6-OH-12(1,3,4)-OO or • OH addition to BaP-6-OO forming BaP-6-OO-12(1,3,4)-OH. (3) Extracting the H atom from the carbon with the OH group by 1 O 2 , forming HO 2 . (4) 1,3 H-shift (H-shift from the carbon to the OO group), promoted by H 2 O. (5) The loss of the OH radical. (6) Abstracting the H atom from the OH group by HO 2 . In this paper, the formation of BaP-4,6-quinone via the BaP degradation is first reported. Water participates in the elementary reaction in which the H atom attached on the aromatic ring shifts to the OO group, serving as a bridge that stabilizes the transition state and transports the proton. A comprehensive investigation explains the degradation mechanism of BaP initiated by • OH and 1 O 2 in aqueous solution.

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Degradation Mechanism of Benzo[a]pyrene Initiated by the OH Radical and 1O2: An Insight from Density Functional Theory Calculations