Preprints
https://doi.org/10.5194/egusphere-2024-3137
https://doi.org/10.5194/egusphere-2024-3137
21 Oct 2024
 | 21 Oct 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Mechanistic insights into chloroacetic acid production from atmospheric multiphase VOC-chlorine chemistry

Mingxue Li, Men Xia, Chunshui Lin, Yifan Jiang, Weihang Sun, Yurun Wang, Yingnan Zhang, Maoxia He, and Tao Wang

Abstract. Chlorine-containing oxygenated volatile organic compounds (Cl-OVOCs) are indicators of atmospheric chlorine chemistry involving volatile organic compounds (VOCs). However, their formation mechanisms are insufficiently understood. Herein, a strong diel pattern of chloroacetic acid (C2H3O2Cl) was observed with daytime peaks at 19 and 13 ppt (1-hour averages) in 2020 and 2021, respectively, at a coastal site in southern China. Ethene was previously proposed as the primary precursor responsible for daytime C2H3O2Cl levels, but a photochemical box model based on Master Chemical Mechanism (MCM) simulations indicates that ethene accounts for less than 1 %. Quantum chemical calculations suggest that other alkenes also can act as chloroacetic acid precursors. Using an updated gas-phase VOC-Cl chemistry model, we find that isoprene, the most abundant VOC at the sampling site, along with its oxidation products, accounts for 7 % of the observed C2H3O2Cl levels. Moreover, the simulation with the updated MCM produces appreciable levels of other Cl-OVOCs, especially chloro-acetaldehyde, a precursor of C2H3O2Cl. We proposed the multiphase reaction of Cl-OVOCs to reconcile the overestimation of Cl-OVOCs and the underestimation of C2H3O2Cl in our gas-phase model. The estimated reactive uptake coefficients for various Cl-OVOCs range from 3.63 × 10-5 to 2.34 × 10-2, according to quantum chemical calculations and linear relationship modeling. Box model simulation with multiphase chemistry reveals that the heterogeneous conversion of chloro-acetaldehyde to C2H3O2Cl is a more important source of C2H3O2Cl than gas-phase reactions. Our study thus proposes a formation mechanism of gaseous C2H3O2Cl and highlights the potential importance of multiphase processes in VOC-Cl chemistry.

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Mingxue Li, Men Xia, Chunshui Lin, Yifan Jiang, Weihang Sun, Yurun Wang, Yingnan Zhang, Maoxia He, and Tao Wang

Status: open (until 03 Dec 2024)

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Mingxue Li, Men Xia, Chunshui Lin, Yifan Jiang, Weihang Sun, Yurun Wang, Yingnan Zhang, Maoxia He, and Tao Wang
Mingxue Li, Men Xia, Chunshui Lin, Yifan Jiang, Weihang Sun, Yurun Wang, Yingnan Zhang, Maoxia He, and Tao Wang

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Short summary
Our field campaigns observed a strong diel pattern of chloroacetic acid as well as a strong correlation between its level and those of reactive chlorine species at a coastal site. Using quantum chemical calculations and box model simulation with updated MCM, we found that the formation pathway of chloroacetic acid involved multiphase processes. Our study deepens the understanding of atmospheric VOC-Cl chemistry and highlights the crucial role of multiphase reactions in atmospheric chemistry.