the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Oct 2024
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3137', Anonymous Referee #1, 21 Nov 2024
Li et al. present observations of chloroacetic acid at a rural site in Hong Kong and showed that its formation cannot be fully explained by existing mechanisms. They propose two potential pathways for its production: (1) isoprene and its oxidation products, and (2) multiphase reactions. The draft is well-structured, and the results are clearly presented. However, I suggest the authors address the following comments before the manuscript is accepted for publication in ACP:
- The estimation of the reactive uptake coefficient relies on linear relationship models based on several OVOCs. I recommend evaluating the accuracy of this linear model and discussing the uncertainties associated with these uptake coefficients.
- The updated gas-phase chlorine chemistry and VOC-Cl model predict Cl-OVOCs concentrations up to 1 ppb, which significantly exceeds the observed values. The authors should provide an explanation for this large discrepancy.
- The inclusion of both updated gas-phase chemistry and heterogeneous reactions increases the simulated levels of chloroacetic acid by 32–56%. I suggest adding a discussion on the potential role of other missing mechanisms to account for the remaining gap.
- Line 193: Considering the slow rate of hydrolysis reactions, how do QC calculations support the plausibility of chloroacetic acid formation via multiphase processes? This result appears contradictory.
- Lines 131–134 and Figure S1: The correlation coefficient between Sa and C2H3O2Cl is not strong enough to indicate a robust correlation. Please address this limitation.
- Figure 1: Why are the observed C2H3O2Cl levels higher in 2020 compared to 2021? Can this discrepancy be explained by the proposed mechanisms?
- Figure 4: The model-simulated diurnal cycle of Cl-OVOCs does not match the observed diurnal pattern of C2H3O2Cl shown in Figure 1. Please explain this inconsistency.
- Figure 4: The gray shading representing C2H3O2Cl is not visible in the plot. Please clarify or correct this issue.
Citation: https://doi.org/10.5194/egusphere-2024-3137-RC1 -
AC1: 'Reply on RC1', Tao Wang, 13 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3137/egusphere-2024-3137-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-3137', Anonymous Referee #2, 26 Nov 2024
The manuscript “Mechanistic insights into chloroacetic acid production from atmospheric multiphase VOC-chlorine chemistry” by Li et al. describes the potential heterogeneous processes of chloroacetic acid production in observations at Hong Kong by performing quantum chemical calculations and chemical box model simulations. The formation mechanism of chloroacetic acid in the atmosphere is poorly understood, as such, I feel this study, which provides a comprehensive assessment of this chemistry, is of interest to the community and falls within the scope of ACP. The manuscript is also well written and organized. I recommend publication after addressing the following points.
- Page 3, line 93-: I am not very sure whether a linear relationship analysis between hydrolysis of OVOCs and their reported γ values could be used to predict γ of Cl-VOCs. For N2O5 in liquid water maybe it is fine as its bulk hydrolysis rate is quite fast, and there are no other significant processes in pure liquid water. But here, the hydrolysis rates of Cl-VOCs are slow, oxidation in aqueous phase also plays a role, and the composition in aerosol water is really complicated. Please discuss whether these factors would have an impact.
- It seems that the method for deriving γ also assumes that gas-phase diffusion limitations were negligible, is this true? Please clarify.
- Page 3, line 70, please also report measurement accuracy.
- Page 5, first paragraph: it seems that the correlations could be mostly attributed to diurnal variation, is this true? Considering the diurnal variation of oxidants, VOCs and other emissions, it is not surprised to have chloroacetic acid, aerosol mass, photolysis rate, etc. all peak in daytime. Similarly, correlation with RH could also be due to the diurnal variation of humidity.
- Page 5, line 150-152: the energy barrier (~8 kcal mol-1) between IM1 and IM2 is unclearly shown in the 3-D relaxed scan (Fig. 2). Texts on the axes need to be clearer.
- According to Figure 1, 4, 6, and 8, it seems that the diurnal variation of Cl-OVOCs/chloroacetic acid can only be represented by adding a reactive uptake, and cannot be resolved by adding another gas formation pathway related to oxidants levels. Is this correct? If so, this could be evidence to support the importance of multiphase process. Also, does adding reactive uptake change the diurnal pattern of precursors?
- Figure 8 has not been mentioned in the main text, please add it to the corresponding description.
- Figure 8: Why does adding reactive uptake (scenario VI) decrease chloroacetic acid level at night (compare to scenario IV).
- I might suggest adding description for simulated contribution from heterogeneous reactions of chloro-acetaldehyde to the observed chloroacetic acid in abstract and conclusion.
- Abstract: “multiphase processes in VOC-Cl chemistry” in the last sentence may lead to confusion. It sounds like a multiphase process involve gas VOC with oxidation by Cl in aqueous phase. Please consider rephase.
Citation: https://doi.org/10.5194/egusphere-2024-3137-RC2 -
AC2: 'Reply on RC2', Tao Wang, 13 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3137/egusphere-2024-3137-AC2-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3137', Anonymous Referee #1, 21 Nov 2024
Li et al. present observations of chloroacetic acid at a rural site in Hong Kong and showed that its formation cannot be fully explained by existing mechanisms. They propose two potential pathways for its production: (1) isoprene and its oxidation products, and (2) multiphase reactions. The draft is well-structured, and the results are clearly presented. However, I suggest the authors address the following comments before the manuscript is accepted for publication in ACP:
- The estimation of the reactive uptake coefficient relies on linear relationship models based on several OVOCs. I recommend evaluating the accuracy of this linear model and discussing the uncertainties associated with these uptake coefficients.
- The updated gas-phase chlorine chemistry and VOC-Cl model predict Cl-OVOCs concentrations up to 1 ppb, which significantly exceeds the observed values. The authors should provide an explanation for this large discrepancy.
- The inclusion of both updated gas-phase chemistry and heterogeneous reactions increases the simulated levels of chloroacetic acid by 32–56%. I suggest adding a discussion on the potential role of other missing mechanisms to account for the remaining gap.
- Line 193: Considering the slow rate of hydrolysis reactions, how do QC calculations support the plausibility of chloroacetic acid formation via multiphase processes? This result appears contradictory.
- Lines 131–134 and Figure S1: The correlation coefficient between Sa and C2H3O2Cl is not strong enough to indicate a robust correlation. Please address this limitation.
- Figure 1: Why are the observed C2H3O2Cl levels higher in 2020 compared to 2021? Can this discrepancy be explained by the proposed mechanisms?
- Figure 4: The model-simulated diurnal cycle of Cl-OVOCs does not match the observed diurnal pattern of C2H3O2Cl shown in Figure 1. Please explain this inconsistency.
- Figure 4: The gray shading representing C2H3O2Cl is not visible in the plot. Please clarify or correct this issue.
Citation: https://doi.org/10.5194/egusphere-2024-3137-RC1 -
AC1: 'Reply on RC1', Tao Wang, 13 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3137/egusphere-2024-3137-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2024-3137', Anonymous Referee #2, 26 Nov 2024
The manuscript “Mechanistic insights into chloroacetic acid production from atmospheric multiphase VOC-chlorine chemistry” by Li et al. describes the potential heterogeneous processes of chloroacetic acid production in observations at Hong Kong by performing quantum chemical calculations and chemical box model simulations. The formation mechanism of chloroacetic acid in the atmosphere is poorly understood, as such, I feel this study, which provides a comprehensive assessment of this chemistry, is of interest to the community and falls within the scope of ACP. The manuscript is also well written and organized. I recommend publication after addressing the following points.
- Page 3, line 93-: I am not very sure whether a linear relationship analysis between hydrolysis of OVOCs and their reported γ values could be used to predict γ of Cl-VOCs. For N2O5 in liquid water maybe it is fine as its bulk hydrolysis rate is quite fast, and there are no other significant processes in pure liquid water. But here, the hydrolysis rates of Cl-VOCs are slow, oxidation in aqueous phase also plays a role, and the composition in aerosol water is really complicated. Please discuss whether these factors would have an impact.
- It seems that the method for deriving γ also assumes that gas-phase diffusion limitations were negligible, is this true? Please clarify.
- Page 3, line 70, please also report measurement accuracy.
- Page 5, first paragraph: it seems that the correlations could be mostly attributed to diurnal variation, is this true? Considering the diurnal variation of oxidants, VOCs and other emissions, it is not surprised to have chloroacetic acid, aerosol mass, photolysis rate, etc. all peak in daytime. Similarly, correlation with RH could also be due to the diurnal variation of humidity.
- Page 5, line 150-152: the energy barrier (~8 kcal mol-1) between IM1 and IM2 is unclearly shown in the 3-D relaxed scan (Fig. 2). Texts on the axes need to be clearer.
- According to Figure 1, 4, 6, and 8, it seems that the diurnal variation of Cl-OVOCs/chloroacetic acid can only be represented by adding a reactive uptake, and cannot be resolved by adding another gas formation pathway related to oxidants levels. Is this correct? If so, this could be evidence to support the importance of multiphase process. Also, does adding reactive uptake change the diurnal pattern of precursors?
- Figure 8 has not been mentioned in the main text, please add it to the corresponding description.
- Figure 8: Why does adding reactive uptake (scenario VI) decrease chloroacetic acid level at night (compare to scenario IV).
- I might suggest adding description for simulated contribution from heterogeneous reactions of chloro-acetaldehyde to the observed chloroacetic acid in abstract and conclusion.
- Abstract: “multiphase processes in VOC-Cl chemistry” in the last sentence may lead to confusion. It sounds like a multiphase process involve gas VOC with oxidation by Cl in aqueous phase. Please consider rephase.
Citation: https://doi.org/10.5194/egusphere-2024-3137-RC2 -
AC2: 'Reply on RC2', Tao Wang, 13 Jan 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3137/egusphere-2024-3137-AC2-supplement.pdf
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Mingxue Li
Chunshui Lin
Yifan Jiang
Weihang Sun
Yurun Wang
Yingnan Zhang
Maoxia He
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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(1231 KB) - Metadata XML
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Supplement
(1526 KB) - BibTeX
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- Final revised paper