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

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.