the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2026
Amine Ozonation Products Promote Sulfate Formation and Accelerate New Particle Growth
Abstract. New particle formation (NPF) significantly influences aerosol burden and cloud condensation nuclei in the atmosphere, but whether amine ozonation products retain nucleation activity remains unclear. This study combines quantum chemical calculations and cluster dynamics simulations to investigate how OH‑amine and CHO‑amine promote sulfate formation and subsequent cluster growth. Both type amines are found to dominantly reduce SO₂ hydrolysis barrier in the presence of water clusters, and rate-limiting step is changes to ozonation step. Further analysis of growth pathways reveals NH₂OH and CH₂NOH exhibit flexible multi‑core routes that accelerate progression toward critical sizes, whereas CH₃NHOH and CHO‑amines are largely confined to single‑ or double‑core pathways. Simulations of cluster formation rates show several ozonation products achieve rates comparable to their parent amines, indicating amine‑driven NPF can persist after ozonation rather than terminating with the consumption of the parent species. Humidity strongly activates all systems, while temperature effects diverge sharply. OH‑amine formation rates drop by 3 – 14 orders of magnitude from 280 to 320 K, whereas CHO‑amines decrease by only about one order over the same range. Together, these results demonstrate that OH‑ and CHO‑amines effectively accelerate sulfate‑based NPF through favorable kinetics in humid environments.
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RC1: 'Comment on egusphere-2026-4704', Anonymous Referee #2, 03 Sep 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-4704/egusphere-2026-4704-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-4704-RC1 -
AC2: 'Reply on RC1', Taicheng An, 17 Oct 2026
Manuscript ID: egusphere-2026-4704
Title: Amine Ozonation Products Promote Sulfate Formation and Accelerate New Particle Growth
The corresponding authors: Prof. Taicheng An
Dear Anonymous Referee #2,
Thank you for the helpful and valuable review and comment. We have made careful revisions on the original manuscript according to your kind and helpful comments. The changed sentences have been marked as red color in the revised manuscript. Below is our point-by-point response to your comments:
General Comments: This manuscript presents a comprehensive theoretical investigation into the nucleation activity of amine ozonation products, combining quantum chemical calculations with cluster dynamics simulations. The findings are novel and of significant interest to the atmospheric chemistry community, particularly regarding the sustained role of amine derivatives in sulfate-driven new particle formation (NPF) under humid and dark conditions.
Response: We are truly grateful to the reviewer for the thorough reading of our manuscript and for the highly positive and encouraging evaluation.
Question 1. The abstract and conclusion state that “humidity strongly activates all systems”, which is correct. However, the temperature effects (3–14 orders of magnitude drop for OH-amines, 1 order for CHO-amines) are equally striking and deserve more prominent emphasis in the graphical abstract.
Response: We have updated the graphical abstract and incorporated the effects of temperature and humidity into the graphic.
Question 2. The paper demonstrates that the pseudo-first-order hydrolysis rate constants kp of these amine derivatives are orders-of-magnitude higher than the ·OH-initiated SO2 oxidation rate constant from literature. It should be noted that kp and real-atmosphere effective rate, considering realistic low ambient concentrations of these OH-/CHO- amines. Readers need to better understand under what real-world concentration ranges this non-radical pathway can outcompete traditional ·OH oxidation.
Response: We agree with the reviewer’s point. kp here includes water concertation at variable humidity values. Actually, it is hard to directly calculate the reaction rate values of OH-/CHO-amine promoted reactions because we cannot find related atmospheric concentrations. Therefore, according to your advice, we reconsidered that we should delete direct comparisons with OH-initiated oxidation. Around Line 190, we have revised the discussions into “This suggests that even the less efficient CHO-amine pathway is a potential sulfate formation route besides conventional OH-initiated oxidation channel under favorable humidity conditions.” We deleted the description that “… can compete with the OH-initiated oxidation channel”. Meanwhile, we deleted “The apparent pseudo-first-order rate constants…under haze conditions.” in Conclusions.
Question 3. The conclusion that CHO-amines require RH > 80% to be effective (Section 3.4) is valuable, yet the manuscript does not quantify how often such high humidity co-occurs with elevated ozone and amine emissions in typical haze episodes. A brief discussion using representative field data would contextualize the practical relevance of this pathway.
Response: Thank you for your careful reading of our manuscript and for raising these critical structural concerns.
We have added in revise manuscript: “These conditions occur in marine environments, where ozone concentrations and amine emissions are both high. Under high humidity, amines dissolve directly in aerosols. (Liu et al., 2018;Yang et al., 2026).” around Line 300.
Question 4. In the ACDC simulations, the concentrations of OH-/CHO- amines is fixed at 1 ppt. However, the actual atmospheric concentrations of these amine ozonation products may be significantly lower than their parent amines, and would dynamically change with the ozonation process. Although the paper notes that the J values for certain products are comparable to their parent methylamine, this is based on the assumption that the product concentration is identical to the parent amine. If product concentrations were significantly lower (e.g., 10-100 times lower), would the J values still remain comparable? I suggest the authors include additional scenario simulations at different amine concentrations (e.g., 0.1 ppt, 0.01 ppt) to assess the impact of concentration changes on the conclusion of “functional equivalence” and to more realistically reflect the NPF potential under atmospheric conditions.
Response: We thank the reviewer for this constructive comment. We agree that fixing all amine species at 1 ppt may overestimate the NPF potential of the ozonation products if their steady-state concentrations are substantially lower than those of the parent amines. In the revised manuscript, we have performed additional ACDC simulations at 0.1 and 0.01 ppt for all five OH-/CHO-amines (Figure R1), and compared them with the original 1 ppt baseline.
Accordingly, we added discussions around Lines 270 - 280 “It should be noted that the above comparison assumes equal concentrations for the ozonation products and their parent amines. In reality, the steady-state concentrations of OH-/CHO-amines are determined by the balance between production from parent amine ozonation and loss via further oxidation, deposition, and cluster growth, and may be lower than those of the parent amines. Therefore, their initial concentrations should be lower than their parent amines. Accordingly, we performed additional ACDC simulations at 0.1 and 0.01 ppt of OH-/CHO-amines. As shown in Fig. S21, J decreases with decreasing OH-/CHO-amines concentration, approximately following a power-law dependence. When the concentrations of the five OH-/CHO-amines are reduced from 1 ppt to 0.1 ppt, their J decrease by about 1 order of magnitude; at 0.01 ppt, their decreases are about 2 orders of magnitude. Moreover, it can be observed that even when the concentration of the amine oxidation products drops to 0.01 ppt, the J value of CH2NOH’s cluster remains 12.8 orders at 0.01 ppt and 13.8 orders at 0.1 ppt, which are comparable to that of parent MA (14 orders). Likewise, the J of CH3NHCHO’s cluster is 12.0 orders at 0.01 ppt and 13.0 orders at 0.1 ppt, which is also comparable to that of parent DMA (13.6 orders). Therefore, CH2NOH and CH3NHCHO can sustain their ability of promoting NPF like their parent amines.”
Figure R1 Formation rates (J) of critical clusters from ACDC simulations. (a) J as a function of [OH-amine] at [H2O] = 1017 molecules cm-3 and 300 K; (b) J as a function of [CHO-amine] at [H2O] = 1017 molecules cm-3 and 300 K.
Question 5. The protonated form, such as NH3OH+·HSO3- in Figure 1b should be properly noted and described in the manuscript text.
Response: We thank the reviewer for this valuable question. NH2OH is not protonated during hydrolysis reaction, but CH3NHOH occurs. For the latter, this occurs because during the promotion of SO2 hydrolysis, CH3NHOH accepts a proton from H2O to form CH3NH2OH+. The reaction process and the molecular composition during the reaction are illustrated in Fig. S2 - S4. Therefore, in the subsequent ozonation reaction for sulfate formation, CH3NH2OH+ participate in their molecular forms.
Accordingly, we have revised Fig. 1b in the revised manuscript and added the following clarification in Section 3.1 around Line 125: “During the promotion of SO2 hydrolysis, CH3NHOH accepts a proton from H2O to form CH3NH2OH+, which then participates in the subsequent ozonation reaction in its protonated form. Both NH2OH and CH2NOH participate in subsequent reactions in their molecular forms.”
Question 6. In the graphical abstract, the term “bisulfate” should be corrected to “bisulfite”, which means HSO3-. Such errors should be carefully checked and corrected throughout the manuscript.
Response: We appreciate this careful correction. We have carried out full-text search and proof-reading to eliminate similar terminology mistakes throughout manuscript, figures and captions. The graphical abstract is as follows.
References
Liu, F., Bi, X., Zhang, G., Lian, X., Fu, Y., Yang, Y., Lin, Q., Jiang, F., Wang, X., Peng, P. a., and Sheng, G.: Gas-to-particle partitioning of atmospheric amines observed at a mountain site in southern China, Atmospheric Environment, 195, 1-11, https://doi.org/10.1016/j.atmosenv.2018.09.038, 2018.
Yang, X. Y., Cao, F., Wu, C. L., Zhang, Y. X., Song, W. H., Lin, Y. C., and Zhang, Y. L.: Secondary formation dominated low molecular weight amines origins in aerosols over the marginal seas of China, Atmos. Chem. Phys., 26, 7031-7046, 10.5194/acp-26-7031-2026, 2026.
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AC2: 'Reply on RC1', Taicheng An, 17 Oct 2026
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RC2: 'Comment on egusphere-2026-4704', Anonymous Referee #1, 14 Sep 2026
This manuscript presents a computational investigation of how amine ozonation products may promote SO2 hydrolysis and subsequent sulfate-containing cluster growth. The topic is relevant to atmospheric chemistry and new particle formation, and the combination of quantum chemical calculations with ACDC simulations is potentially useful. Overall, the manuscript is interesting and the main conclusions appear to be broadly supported by the calculations, although several methodological details and definitions should be clarified to improve the transparency, reproducibility, and interpretation of the results. I therefore recommend minor revision before publication. Specific comments
- The manuscript should more clearly distinguish the different energetic and thermodynamic quantities used in the calculations. In Section 2.1, the reaction barriers and reaction energies are defined from single-point electronic energies, whereas Section 2.2 subsequently uses these quantities to derive equilibrium and rate constants. Please clarify whether zero-point energy and entropy corrections are included in the quantities used for Keq, reaction rates, and cluster evaporation rates. If electronic energies and Gibbs free energies are used for different purposes, this distinction should be stated explicitly.
- Please define the Core units explicitly at their first occurrence and reconcile their compositions with the cluster notation used in Section 2.4. Consistent notation should be used throughout the text, Table 1, and Fig. 4.
- The nomenclature and units of the kinetic quantities should also be checked carefully. For example, Tables S1–S3 refer to “unimolecular rate constants” while reporting units of cm3 molecule-1 s-1.
- Section 2.3 states that 500 initial configurations were generated for each cluster composition and that the lowest-energy structure was selected for further optimization. For reproducibility, please provide the Cartesian coordinates and corresponding energies of the optimized structures used in the subsequent cluster analyses, preferably in the Supplementary Information. Please also clarify whether the electronic and/or Gibbs energies used as inputs to the growth-pathway and ACDC calculations.
- Please provide a clearer description of the ACDC boundary conditions, including the concentrations of all fixed species.
- Please describe how the collision and evaporation coefficients are calculated in the ACDC simulations, or provide appropriate references for these calculations.
Technical comments
- Abstract: “Both type amines are found to dominantly reduce SO2 hydrolysis barrier …” should be corrected to “Both types of amines are found to substantially reduce the SO2 hydrolysis barrier …”.
- Abstract: “… and rate-limiting step is changes to ozonation step” should be corrected to “… and the rate-limiting step shifts to the ozonation step”.
- Introduction: “… initiating the followed ozonation step” should be corrected to “… initiating the subsequent ozonation step”.
Citation: https://doi.org/10.5194/egusphere-2026-4704-RC2 -
AC1: 'Reply on RC2', Taicheng An, 17 Oct 2026
Manuscript ID: egusphere-2026-4704
Title: Amine Ozonation Products Promote Sulfate Formation and Accelerate New Particle Growth
The corresponding authors: Prof. Taicheng An
Dear Anonymous Referee #1,
Thank you for the helpful and valuable review and comment. We have made careful revisions on the original manuscript according to your kind and helpful comments. The changed sentences have been marked as red color in the revised manuscript. Below is our point-by-point response to your comments:
General Comments: This manuscript presents a computational investigation of how amine ozonation products may promote SO2 hydrolysis and subsequent sulfate-containing cluster growth. The topic is relevant to atmospheric chemistry and new particle formation, and the combination of quantum chemical calculations with ACDC simulations is potentially useful. Overall, the manuscript is interesting and the main conclusions appear to be broadly supported by the calculations, although several methodological details and definitions should be clarified to improve the transparency, reproducibility, and interpretation of the results. I therefore recommend minor revision before publication.
Response: We sincerely thank you for your critical and constructive comments regarding the experimental transparency of our manuscript.
Question 1. The manuscript should more clearly distinguish the different energetic and thermodynamic quantities used in the calculations. In Section 2.1, the reaction barriers and reaction energies are defined from single-point electronic energies, whereas Section 2.2 subsequently uses these quantities to derive equilibrium and rate constants. Please clarify whether zero-point energy and entropy corrections are included in the quantities used for Keq, reaction rates, and cluster evaporation rates. If electronic energies and Gibbs free energies are used for different purposes, this distinction should be stated explicitly.
Response: We thank the reviewer for this important clarification. We apologize for the unclear description in the original manuscript. We have now explicitly clarified the definition of all energy quantities used in this work.
In our calculations, all reported energy barriers, reaction energies, and binding energies are defined as
ΔE = ΔG + ΔCCSD(T)
where ΔCCSD(T) is the high-level electronic energy difference obtained from single-point calculations at the M06-2X/6-311++G(3df,3pd) level, and ΔG is the thermal Gibbs free energy correction (including zero-point energy, enthalpy, and entropy) obtained from frequency calculations at the M06-2X/6-311G(d,p) level. Therefore, all ΔE values reported in this work correspond to Gibbs free energy changes, not purely electronic energies.
Consequently, the equilibrium constant Keq, the rate constants, and the apparent pseudo-first-order rate constant kp were all derived from these Gibbs free energy quantities. Similarly, the cluster evaporation rates in the ACDC simulations were calculated from the same Gibbs free binding energies. No purely electronic energies were used in the kinetic or ACDC calculations. We have revised around lines 65-70 to state this explicitly.
Question 2. Please define the Core units explicitly at their first occurrence and reconcile their compositions with the cluster notation used in Section 2.4. Consistent notation should be used throughout the text, Table 1, and Fig. 4.
Response: We thank the reviewer for this important comment.
At the first occurrence around Line 200, we previously stated that “To further evaluate the nucleation potential of the amines investigated above, we simulated the hygroscopic growth of clusters containing an OH-/CHO-amine-sulfate complex as the core…”
According to your suggestion, Consistent notation of “OH-/CHO-amine-sulfate complex is set as one Core” is added in Table 1, and Fig. 4 in the revised manuscript.
Question 3. The nomenclature and units of the kinetic quantities should also be checked carefully. For example, Tables S1–S3 refer to “unimolecular rate constants” while reporting units of cm3 molecule-1 s-1.
Response: We thank the reviewer for catching this error. We have carefully checked the nomenclature and units of all kinetic quantities. In Tables S1 - S3, the quantities with units of cm3 molecule-1 s-1 are bimolecular rate constants, not unimolecular rate constants. We have already made the necessary revisions in this regard. We have also checked all other kinetic quantities in the manuscript and Supplementary Information to ensure that the terms “unimolecular”, “bimolecular”, and “pseudo-first-order” are used consistently with their units.
Question 4. Section 2.3 states that 500 initial configurations were generated for each cluster composition and that the lowest-energy structure was selected for further optimization. For reproducibility, please provide the Cartesian coordinates and corresponding energies of the optimized structures used in the subsequent cluster analyses, preferably in the Supplementary Information. Please also clarify whether the electronic and/or Gibbs energies used as inputs to the growth-pathway and ACDC calculations.
Response: We thank the reviewer for this helpful suggestion. For reproducibility, we have included the optimized structures of each cluster in the file of “the lowest-energy structure”, and have provided the corresponding ΔG and ΔCCSD(T) values for each cluster in Table S4 - S5. Additionally, the cluster energies used in the ACDC calculations were derived from the ΔE values listed in Table 1.
Question 5. Please provide a clearer description of the ACDC boundary conditions, including the concentrations of all fixed species.
Response: We thank the reviewer for this request.
The boundary is described around Line 105 “The simulated system comprises clusters of the form (Core)m·(H2O)n, where Core denotes a strongly bound complex of an OH-/CHO-amine with H2SO4 and two H2O molecules, with m ranging from 0 to 3 and n from 0 to 4.”
About concentrations, you can find them in the following sentences “In these simulations, the concentration of Core is fixed at 1 ppt, while the water concentration ([H2O]) is varied from 1015 to 1017 molecules cm-3 to assess its influence on nucleation. The primary simulations are run at 300 K, with additional sets at 280 K and 320 K to evaluate temperature effects.”
Question 6. Please describe how the collision and evaporation coefficients are calculated in the ACDC simulations, or provide appropriate references for these calculations.
Response: We thank the reviewer for this request.
In the revised Section 2.4, we added descriptions around Line 115: “The collision coefficients are calculated from hard-sphere collision theory using cluster masses, densities, and effective diameters, while the evaporation coefficients are derived from the Gibbs free binding energies via detailed balance. The detailed methodology is described in McGrath et al. (2012) and Liu et al. (2022)” to calculate the collision and evaporation coefficients.
The detailed information of the two references is as follows:
McGrath, M. J., Olenius, T., Ortega, I. K., Loukonen, V., Paasonen, P., Kurtén, T., Kulmala, M., and Vehkamäki, H.: Atmospheric Cluster Dynamics Code: a flexible method for solution of the birth-death equations, Atmos. Chem. Phys., 12, 2345-2355, 10.5194/acp-12-2345-2012, 2012.
Liu, Y., Xie, H. B., Ma, F., Chen, J., and Elm, J.: Amine-Enhanced Methanesulfonic Acid-Driven Nucleation: Predictive Model and Cluster Formation Mechanism, Environ Sci Technol, 56, 7751-7760, 10.1021/acs.est.2c01639, 2022.
Question 7. Abstract: “Both type amines are found to dominantly reduce SO2 hydrolysis barrier …” should be corrected to “Both types of amines are found to substantially reduce the SO2 hydrolysis barrier …”. “… and rate-limiting step is changes to ozonation step” should be corrected to “… and the rate-limiting step shifts to the ozonation step”.
Response: We thank the reviewer for these careful corrections. We have corrected all sentences in the revised manuscript. The change has been highlighted for your easy reference.
Question 8. Introduction: “… initiating the followed ozonation step” should be corrected to “… initiating the subsequent ozonation step”.
Response: We thank the reviewer for catching this grammatical error. In the revised manuscript Line 36, we have corrected the sentence in the section of Introduction. The change has been highlighted for your easy reference.
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