Mechanistic insights into chloroacetic acid production from atmospheric multiphase volatile organic compound–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 h 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. Moreover, the simulation with the updated MCM produces appreciable levels of other Cl-OVOCs, especially chloroacetaldehyde, 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, based on quantum chemical calculations and linear relationship modelling. The box model simulation with multiphase chemistry shows that the heterogeneous conversion of chloroacetaldehyde to C2H3O2Cl can contribute 24 %–48 % of the observed levels. Our study thus proposes a formation mechanism of gaseous C2H3O2Cl and highlights the potential importance of multiphase processes in atmospheric organic acid formation.