the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Sep 2025
Ethylamine-Driven Amination of Organic Particles: Mechanistic Insights via Key Intermediates Identification
Abstract. Atmospheric amines critically contribute to secondary aerosols formation via heterogeneous reactions, yet the molecular mechanisms governing heterogeneous amination chemistry of aerosols remain unclear. Here, we utilize an integrated tandem flow-tube system coupled with online ultrahigh-resolution mass spectrometry to elucidate the amination chemistry of ethylamine (EA) with representative organic aerosol components, including C20-C54 secondary ozonides (SOZs), C17-C27 carboxylic acids, and aldehydes. Our experiments provide evidence for the formation of four key intermediates: hydroxyl peroxyamines, amino hydroperoxides, peroxyamines, and amino ethers, which mediate SOZs conversion to hydroxyimines, amides, and imines. Furthermore, dihydroxylamines and hydroxylamines are identified as characteristic intermediates in carboxylic acids and aldehydes amination. Quantitative heterogeneous reactivity measurements (Δγeff) reveal that SOZs exhibit a pronounced inverse dependence on carbon chain length, e.g., C21 SOZ (Δγeff = 1.0 × 10-4) > C49 SOZ (Δγeff = 5.7 × 10-6), with consistently lower reactivity than acids and aldehydes, e.g., C17 acid (Δγeff = 2.3 × 10-4). The amination mechanism of SOZs is initiated by EA addition, followed by either hydroxyl peroxyamines-mediated dehydration yielding hydroxyimines and amides, or amino hydroperoxides-driven H2O2 elimination forming imines. For carboxylic acids and aldehydes, EA addition leads to dihydroxylamines and hydroxylamines formation, which subsequently dehydrate to produce amides and imines. These findings provide a mechanistic framework for understanding amine-driven aerosol aging processes that affects atmospheric chemistry, air quality, and climate systems.
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RC1: 'Comment on egusphere-2025-4260', Anonymous Referee #1, 10 Oct 2025
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AC1: 'Reply on RC1', Meirong Zeng, 07 Nov 2025
We sincerely thank the reviewer for the insightful comments, which helped to improve the clarity and presentation of our results. In the supplement, we reproduce the reviewer's comments (in italics), followed by our responses and the changes made to the revised manuscript and SI.
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AC1: 'Reply on RC1', Meirong Zeng, 07 Nov 2025
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RC2: 'Comment on egusphere-2025-4260', Anonymous Referee #2, 13 Oct 2025
This manuscript presents a mechanistic study of the heterogeneous reactions between ethylamine and SOZs (carboxylic acids and aldehydes) by using the flowtube reactor coupled with online APPI-HRMS. The direct identification of featured intermediates enables the proposal of a comprehensive amination mechanism. The quantitative measurement of effective uptake coefficients of the organic aerosols upon ethylamine exposure provides valuable data for understanding the evolution of atmospheric aerosols. I recommend publication of this work with minor revisions.
1. It is suggested to provide the material of the tandem flow-tube (e.g., Pyrex, stainless steel).
2. The range of O3 concentration is not mentioned in the paper. It is suggested that this parameter be clarified, as it is critical for the SOZ formation.
3. A supplementary table, listing values of γeff,2FT and γeff,1FT for representative SOZs, carboxylic acids, and aldehydes, can be useful to add.
4. The curves for different intermediates (e.g., dihydroxylamine vs. hydroxylamine) in Figure 6 are distinguished only by color. Adding distinct symbol types would improve the clarity of the figure.
5. It seems that the caption for Figure 6 mentions “Relative abundance”, which does not align with the y-axis label “Signal (arb. unit)”. Please update the caption to ensure the consistency between the figure and its description.
6. Line 30, the phrase “initiates through the nucleophilic attack” should be grammatically expressed as “is initiated by a nucleophilic attack”.
7. Line 38, “ethylamine and on a C15 SOZ” contains a superfluous conjunction. For clarity, this should be corrected to “ethylamine on a C15 SOZ”.
8. Line 116-117, the sentence “the net contribution of heterogeneous reactions… were quantitatively determined” has a subject-verb agreement error. Please correct it to “the net contribution... was quantitatively determined.”
9. Check the capitalization style of the article titles in the references. It seems that there is an inconsistency in the use of capitalization, specifically regarding sentence case vs. title case. It is recommended to standardize the format according to the journal's guidelines.
10. Both relative humidity and pH are critical factors influencing the heterogeneous reactions involving amines. Thus, if the reaction system has recorded humidity and pH data, it is advisable to explicitly state them in the manuscript.Citation: https://doi.org/10.5194/egusphere-2025-4260-RC2 -
AC2: 'Reply on RC2', Meirong Zeng, 07 Nov 2025
We sincerely thank the reviewer for the insightful comments, which helped to improve the clarity and presentation of our results. In the supplement, we reproduce the reviewer's comments (in italics), followed by our responses and the changes made to the revised manuscript and SI.
-
AC2: 'Reply on RC2', Meirong Zeng, 07 Nov 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-4260', Anonymous Referee #1, 10 Oct 2025
This paper investigated the heterogeneous amination of ethylamine with three representative organic aerosols (SOZs, carboxylic acids, and aldehydes). A tandem flowtube reactor coupled with online APPI-HRMS was employed to elucidate the ethylamine-driven amination chemistry. The identification of key intermediates and the proposal of novel reaction pathways provide valuable mechanistic insights. Quantification of differential effective uptake coefficients across a wide range of carbon chain lengths for SOZs and carbonyls is particularly valuable for advancing atmospheric chemistry models. The manuscript is well-written, and conclusions are well-supported by the experimental data. I recommend this work for publication, and some minor comments can be considered.
- In the introduction, the authors justify the use of squalene as a model system. However, a brief statement for selecting ethylamine as the model amine, compared to more commonly studied amines (e.g., ammonia or dimethylamine), can be helpful. Is it due to its atmospheric relevance, reactivity, or as a simpler model for primary amines?
- The use of APPI-HRMS is critical to the identification of intermediates and products. To facilitate a better assessment of the methodology, it is suggested to provide key instrumental parameters in the supplementary information. These should include, but not be limited to: the ionization mode used (positive/negative), mass resolution, mass range scanned, and vaporizer temperature.
- The mass spectrum in Figure 3b would be more clearly distinguishable if the different product classes (adducts, -H2O, -H2O2) were labeled with different colors or symbols directly on the spectrum.
- The identification of the four key intermediates is a core finding of this work. For clarity, it is suggested to add a simplified schematic flowchart, either in Figure 4 or as a supplementary figure, to summarize the competing pathways and subsequent consumption pathways. This would facilitate visualization of the complex reaction network.
- For the newly proposed intermediates (peroxyamines and amino ethers), it is suggested to provide their exact mass and compare them with the experimentally observed m/z values in a supplementary table. Listing representative intermediates and products observed for each SOZ in this table can provide additional evidence for their identification.
Technical corrections:
- There are inconsistencies in the terminology used for differential effective uptake coefficients, which are sometimes wrriten as “∆γeff” and sometimes abbreviated as “∆γ”.
- Line 203, “see Sect. 3.2 for mechanistic details,” but the mechanistic details are in Sect. 3.3. Please correct it.
- Line 254, “has be demonstrated”, this should be correct to “has been demonstrated”
- Line 268, “C32O55O2N (peroxyamine)” should be corrected to C32H55O2
- Line 269, “C32O57O3N” should be corrected to C32H57O3
Citation: https://doi.org/10.5194/egusphere-2025-4260-RC1 -
AC1: 'Reply on RC1', Meirong Zeng, 07 Nov 2025
We sincerely thank the reviewer for the insightful comments, which helped to improve the clarity and presentation of our results. In the supplement, we reproduce the reviewer's comments (in italics), followed by our responses and the changes made to the revised manuscript and SI.
-
RC2: 'Comment on egusphere-2025-4260', Anonymous Referee #2, 13 Oct 2025
This manuscript presents a mechanistic study of the heterogeneous reactions between ethylamine and SOZs (carboxylic acids and aldehydes) by using the flowtube reactor coupled with online APPI-HRMS. The direct identification of featured intermediates enables the proposal of a comprehensive amination mechanism. The quantitative measurement of effective uptake coefficients of the organic aerosols upon ethylamine exposure provides valuable data for understanding the evolution of atmospheric aerosols. I recommend publication of this work with minor revisions.
1. It is suggested to provide the material of the tandem flow-tube (e.g., Pyrex, stainless steel).
2. The range of O3 concentration is not mentioned in the paper. It is suggested that this parameter be clarified, as it is critical for the SOZ formation.
3. A supplementary table, listing values of γeff,2FT and γeff,1FT for representative SOZs, carboxylic acids, and aldehydes, can be useful to add.
4. The curves for different intermediates (e.g., dihydroxylamine vs. hydroxylamine) in Figure 6 are distinguished only by color. Adding distinct symbol types would improve the clarity of the figure.
5. It seems that the caption for Figure 6 mentions “Relative abundance”, which does not align with the y-axis label “Signal (arb. unit)”. Please update the caption to ensure the consistency between the figure and its description.
6. Line 30, the phrase “initiates through the nucleophilic attack” should be grammatically expressed as “is initiated by a nucleophilic attack”.
7. Line 38, “ethylamine and on a C15 SOZ” contains a superfluous conjunction. For clarity, this should be corrected to “ethylamine on a C15 SOZ”.
8. Line 116-117, the sentence “the net contribution of heterogeneous reactions… were quantitatively determined” has a subject-verb agreement error. Please correct it to “the net contribution... was quantitatively determined.”
9. Check the capitalization style of the article titles in the references. It seems that there is an inconsistency in the use of capitalization, specifically regarding sentence case vs. title case. It is recommended to standardize the format according to the journal's guidelines.
10. Both relative humidity and pH are critical factors influencing the heterogeneous reactions involving amines. Thus, if the reaction system has recorded humidity and pH data, it is advisable to explicitly state them in the manuscript.Citation: https://doi.org/10.5194/egusphere-2025-4260-RC2 -
AC2: 'Reply on RC2', Meirong Zeng, 07 Nov 2025
We sincerely thank the reviewer for the insightful comments, which helped to improve the clarity and presentation of our results. In the supplement, we reproduce the reviewer's comments (in italics), followed by our responses and the changes made to the revised manuscript and SI.
-
AC2: 'Reply on RC2', Meirong Zeng, 07 Nov 2025
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This paper investigated the heterogeneous amination of ethylamine with three representative organic aerosols (SOZs, carboxylic acids, and aldehydes). A tandem flowtube reactor coupled with online APPI-HRMS was employed to elucidate the ethylamine-driven amination chemistry. The identification of key intermediates and the proposal of novel reaction pathways provide valuable mechanistic insights. Quantification of differential effective uptake coefficients across a wide range of carbon chain lengths for SOZs and carbonyls is particularly valuable for advancing atmospheric chemistry models. The manuscript is well-written, and conclusions are well-supported by the experimental data. I recommend this work for publication, and some minor comments can be considered.
Technical corrections: