Atmospheric oxidation mechanism and kinetics of isoprene initiated by chlorine radicals: A computational study

Xirui Guo; Fangfang Ma; Cong Liu; Junfeng Niu; Ning He; Jingwen Chen; Hong-Bin Xie

doi:10.1016/j.scitotenv.2019.136330

Atmospheric oxidation mechanism and kinetics of isoprene initiated by chlorine radicals: A computational study

Sci Total Environ. 2020 Apr 10:712:136330. doi: 10.1016/j.scitotenv.2019.136330. Epub 2019 Dec 27.

Authors

Xirui Guo¹, Fangfang Ma¹, Cong Liu¹, Junfeng Niu², Ning He³, Jingwen Chen¹, Hong-Bin Xie⁴

Affiliations

¹ Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), Dalian University of Technology, Dalian 116024, China.
² Research Center for Eco-environmental Engineering, Dongguan University of Technology, Dongguan 523808, China.
³ State Key Laboratory of Fine Chemicals & School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China.
⁴ Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), Dalian University of Technology, Dalian 116024, China. Electronic address: hbxie@dlut.edu.cn.

PMID: 31931210
DOI: 10.1016/j.scitotenv.2019.136330

Abstract

The reaction with chlorine radicals (·Cl) has been considered to be one of indispensable sinks for isoprene. However, the mechanism of ·Cl initiated isoprene reaction was not fully understood. Herein, the reaction of isoprene with ·Cl, and ensuing reactions of the resulting isoprene relevant radicals were investigated by combined quantum chemistry calculations and kinetics modeling. The results indicate that ·Cl addition to two terminal C-atoms of two double bonds of isoprene, forming IM₁_-₁ and IM₁_-₄, are more favorable than H-abstractions from isoprene. Interestingly, the predicted reaction rate constant for the direct H-abstraction pathway is much lower than that of the indirect one, clarifying a direct H-abstraction mechanism for previously experimental observation. The IM₁_-₁ and IM₁_-₄ have distinct fate in their subsequent transformation. The reaction of IM₁_-₁ ends after the one-time O₂ addition. However, IM₁_-₄ can follow auto-oxidation mechanism with two times O₂ addition to finally form highly oxidized multi-functional molecules (HOMs), C₅H₇ClO₃ and ·OH. More importantly, the estimated contribution of ·Cl on HOMs (monomer only) formation from isoprene is lower than that of ·OH in addition pathway, implying overall HOMs yield from atmospheric isoprene oxidation could be overestimated if the role of ·Cl in transforming isoprene is ignored.

Keywords: HOMs; SOA formation; VOCs.