Exploring the OH-initiated reactions of styrene in the atmosphere and the role of van der Waals complex. (November 2021) - Wu, Xiaoqing
Abstract: Reacting with OH provides a major sink for styrene in the atmosphere, with three possible pathways including OH-addition, H-abstraction and addition-dissociation reactions. However, the total rate coefficients of styrene + OH were measured as 1.2–6.2 × 10 −11 cm 3 molecule −1 s −1 under atmospheric conditions, varying by a maximum factor of 5. On the other hand, only one theoretical work reported this rate coefficient as 19.1 × 10 −11 cm 3 molecule −1 s −1, which exhibits up to 16 times that measured in laboratory studies. In the present study, the reaction kinetics of styrene + OH was extensively studied with high-level quantum chemical methods combined with RRKM/master equation simulations. In particular, we carried out theoretical treatments for the formation of pre-reaction Van der Waals complexes of styrene + OH, and examined their influence on the reaction kinetics. The total rate coefficient for styrene + OH is calculated to be 1.7 × 10 −11 cm 3 molecule −1 s −1 at 300 K, 1 atm. The main products are addβ (88.2%), add5 (6.9%), addα (1.9%) and add3 (1.7%). Using our computed rate coefficient and the global atmospheric hydroxyl radical concentration (2 × 10 6 radicals per cm 3 ), the lifetime of styrene in the atmosphere is estimated at 8.0 h. The degradation of styrene might be negligible for the formation of ozone in the atmosphere based upon the photochemical ozone creation potentials calculation. The computed product yields indicate that addβ via subsequent reactions could significantly produce formaldehyde and benzaldehyde that were observed in previous experimental studies on styrene oxidation, and contribute to the formation of secondary organic aerosols. Graphical abstract: Image 1 Highlights: Three oxidation pathways of styrene + OH including the major addition are proposed. The total rate coefficient is 1.7 × 10 −11 cm 3 molecule −1 s −1 at 300 K, 1 atm. The addβ is the predominant product with a yield of 88.2%. Tunneling and variational effects impact rate coefficients of pathways with pre-reaction complex. The lifetime of styrene determined by OH radical is 8.0 h.