the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Feb 2025
Formation of Chlorinated Organic Compounds from Cl Atom-Initiated Reactions of Aromatics and Their Detection in Suburban Shanghai
Abstract. Chlorine (Cl) atoms generated from the photolysis of atmospheric reactive chlorine species can rapidly react with various volatile organic compounds (VOCs), forming chlorine- and non-chlorine-containing low-volatile oxygenated organic molecules. Yet, the formation mechanisms of chlorine-containing oxygenated organic molecules (Cl-OOMs) from reactions of Cl atoms with aromatics in the presence and absence of NOx are not fully understood. Here, we investigated Cl-OOMs formation from Cl-initiated reactions of three typical aromatics (i.e., toluene, m-xylene, and 1,2,4-trimethylbenzene (1,2,4-TMB)) in the laboratory and searched for ambient gaseous Cl-OOMs in suburban Shanghai. From our laboratory experiments, 19 Cl-containing peroxyl radicals and a series of Cl-OOMs originating from the Cl-addition-initiated reaction were detected, which provides direct evidence that the Cl-addition-initiated reaction is a non-negligible pathway. In addition, a total of 51 gaseous Cl-OOMs were identified during the winter in suburban Shanghai, 38 of which were also observed in laboratory experiments, hinting that Cl-initiated oxidation of aromatics could serve as a source of Cl-OOMs in an anthropogenically influenced atmosphere. Toxicity evaluation of these Cl-OOMs shows potential adverse health effects. These findings demonstrate that Cl-OOMs can be efficiently formed via the Cl-addition pathway in the reactions between aromatics and Cl atoms and some of these Cl-OOMs could be toxic.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-607', Anonymous Referee #1, 31 Mar 2025
The authors Li et al describe a series of laboratory flow tube studies and ambient urban measurements focused on Cl-radical initiated chemistry and resulting product Cl-OOMs. The results of the flow tube studies are potentially interesting and the ambient measurements are novel. However, I have major questions about the chemistry proposed within this work and how it may lead to the observed products. I believe the manuscript will only be suitable for publication with the addition of direct evidence in support of the proposed mechanisms.
My major issue is with the proposed mechanism for Cl addition to the aromatic rings. To my knowledge, this chemistry has not been directly supported through laboratory measurements, and in support of this mechanism the authors cite a lone theoretical paper. While theory is undeniably useful in guiding mechanism development, I am hesitant to accept the Cl addition mechanism at the stated probability (14%) without direct measurements. Does the Cl-aromatic product analogous to the first-generation phenolic product from OH addition form? Are any other first-generation products observed that are analogous to those from established OH chemistry? Formation of smaller molecular fragmentation products should be common, given the high level of RO2-RO2 chemistry (Figure S10) and the expected fragmentation of ensuing RO radicals formed during some RO2-RO2 reactions. The Vocus-PTR should be well-suited to measuring at least some of these molecules. The authors do note indirect evidence for initial Cl addition in the lack of observation of potential second-generation radicals in the family C8H12ClOx (Line 345). However, the authors also note that the detection of intermediate radicals is difficult (Line 285), and on its own this indirect evidence is not sufficiently convincing. I would suggest substantially deeper analysis of the data from the present experiments and ideally further experiments more tailored specifically to detecting first-generation products of the Cl-aromatic reaction and constraining these reaction pathways.
Relatedly, I do not believe the mechanism illustrated in the upper half of scheme 1 describing OOM formation following H-abstraction by Cl is reasonable. H-abstraction is expected to occur on the methyl substituents, and the internal RO2-H migrations described following initial H-abstraction are not supported by prior literature. To my knowledge, internal H-migration of an aromatic H is not expected to be a reasonable pathway. Additionally, though internal H-migration across an aromatic ring hasn't been studied (to my knowledge), a 1,7-migration between primary carbons is predicted to be slow (Vereecken and Noziere, 2020). If this were to occur, the second H-migration to form the radical C8H9O4 would be expected to immediately collapse to form an aldehyde in the closed shell molecule C8H8O3 and an OH radical (Bianchi et al., 2019). This suggests other formation pathways for the observed non-Cl containing radicals and closed-shell products. The simplest explanation would be OH addition chemistry, with OH forming from HO2 through H-abstraction at the methyl groups (see, e.g., the already cited Bhattacharyya et al., 2023). These observations call into question the authors' statements on a lack of OH chemistry in NOx-free experiments (line 360-361), with further implications for the potential mechanisms by which Cl-OOMs may form.
Minor comments
Line 365: more reduced molecules could also form through multi-generation chemistry when H-abstraction occurs from a carbon with an OH or OOH substituent, leading to the formation of a carbonyl.
Lin 443, Figure 4: I would like to see more discussion on some of the higher-concentration compounds, specifically C2O2, C6O3/4, C8O5, and C9O1. Even if these compounds were not observed in the flow tube studies, the observation is both novel and useful and can provide further insight on ambient Cl chemistry and/or primary Cl-OVOC/Cl-OOM emissions.
Line 459: fix citation.
Line 474: label concluding section
References not already within text
Vereecken, L., & Nozière, B. (2020). H migration in peroxy radicals under atmospheric conditions. Atmospheric Chemistry and Physics, 20(12), 7429–7458. https://doi.org/10.5194/acp-20-7429-2020Citation: https://doi.org/10.5194/egusphere-2025-607-RC1 -
RC2: 'Comment on egusphere-2025-607', Anonymous Referee #2, 30 Apr 2025
This manuscript presents a detailed and well-structured investigation into the formation mechanisms of chlorine-containing oxygenated organic molecules (Cl-OOMs) from Cl-initiated reactions with aromatics in both laboratory experiments and ambient measurements. The study provides direct evidence of Cl-addition reactions and explores the role of NOx in modulating Cl-OOM formation. The identification of 51 Cl-OOMs in suburban Shanghai, with 38 also detected in the laboratory, strengthens the claim that Cl-initiated oxidation of aromatics is an important atmospheric process. Furthermore, the toxicity evaluation adds valuable insights into the potential health risks associated with these compounds. Overall, the study is comprehensive and provides significant contributions to the field. However, several points require further clarification or additional discussion to strengthen the manuscript.
- Line 121 (experimental setup): The mixing of a few sccm of gas (2.5 sccm) with a 10 lpm flow may lead to incomplete mixing or loss of reactants. Have the authors verified that the minor flow contributions are fully entrained and do not experience losses? Additionally, is nitrate-CI-APi-LToF sampling from the center of the flow tube reactor while other instruments sample from different radial positions? If so, have the authors considered the potential radial inhomogeneity in reactions, especially in cases where reactants are not fully mixed before entering the flow tube?
- Line 127: How was the VOC introduced into the system specifically? Was it through a permeation tube or another method? A clearer description of the VOC introduction process would be beneficial.
- Lines 135-140: The consistency of Cl atom concentrations across experiments with different VOCs needs further support. Have the authors confirmed that Cl atom concentrations remain consistent for the three VOCs under the same flow conditions? Providing a graphical representation (e.g., VOC concentration versus time) would strengthen this claim.
- Lines 234-242: When NOx was introduced, both non-Cl-OOMs and Cl-OOMs increased in molar yield, but the explanation focuses mainly on OH chemistry. Given that Cl-OOMs also increased, further discussion is needed on why this occurs.
- Lines 423-424: The manuscript mentions measurements of Cl-OOMs with one oxygen atom using nitrate-Cl-APi-LTOF. How confident are the authors in detecting these low-oxygenated species using nitrate-Cl-APi-LTOF? Additional justification for measurement accuracy would be helpful.
- Lines 434-438: While 38 species were observed in both laboratory and field settings, only three were selected for diurnal variation analysis. What was the rationale for this selection? Do the remaining 35 species exhibit similar daily trends? A discussion or additional data supporting the selection of these three species would enhance clarity.
- Lines 436-441: Cl Radical Concentrations at Noon: The manuscript suggests that Cl radicals peak at noon, but many studies report higher morning concentrations due to ClNO2 photolysis. What evidence supports a midday peak at this measurement site in this study? If none exists, could these Cl-containing compounds be formed from Cl-VOC reactions involving OH rather than Cl oxidation? Providing further discussion or alternative hypotheses would improve the robustness of this conclusion.
Citation: https://doi.org/10.5194/egusphere-2025-607-RC2
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