From cylinder to city: How the recondensation-induced nucleation shapes urban aerosol number

Chen, Jen-Ping; Tsai, I-Chun; Kuo, Li-Wei; Hwang, Gong-Do

doi:10.5194/egusphere-2025-5487

Preprints

https://doi.org/10.5194/egusphere-2025-5487

Preprints

20 Nov 2025

| 20 Nov 2025

From cylinder to city: How the recondensation-induced nucleation shapes urban aerosol number

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

Abstract. Air-quality models frequently underestimate fine particle number concentration (PNC), particularly in the nucleation/Aitken range—while reproducing PM_2.5 mass more accurately, suggesting that key number-forming processes are missing from current frameworks. We propose and investigate a physically motivated pathway, Recondensation-Induced Nucleation (RIN), in which pre-existing ambient aerosols are vaporized during combustion and subsequently re-nucleate as the exhaust cools, selectively boosting particle number with negligible impact on mass.

Controlled four-stroke engine experiments demonstrate that a distinct nucleation mode (<30 nm) appears only when ambient aerosols are present in the intake air, providing direct laboratory evidence of RIN. Parcel-model simulations of H₂SO₄–H₂O systems were then used to probe particle evaporation under in-cylinder condition and the observed self-limiting behavior of nucleation. A parameterized RIN module was then implemented in the Community Multiscale Air Quality (CMAQ) model and tested over Taiwan. Without RIN, CMAQ underpredicted PNC by 75 % and overpredicted PM_2.5 by 21 % at the Xitun urban site; incorporating RIN reduced the PNC bias to 14 % with negligible change in PM_2.5. The RIN mechanism thus transfers accumulation-mode mass to Aitken-mode number, not only improve the low-PNC bias but also the low Aitken- to accumulation mode number ratio bias found at the Xitun site. Therefore, the inclusion of the RIN mechanism provides a mechanistic basis for improving health-relevant urban aerosol simulations.

Received: 05 Nov 2025 – Discussion started: 20 Nov 2025

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Journal article(s) based on this preprint

21 May 2026

From cylinder to city: how recondensation-induced nucleation in vehicle exhaust shapes urban aerosol number

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

Atmos. Chem. Phys., 26, 6909–6927, https://doi.org/10.5194/acp-26-6909-2026,https://doi.org/10.5194/acp-26-6909-2026, 2026

Short summary

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-5487', Anonymous Referee #1, 15 Dec 2025

This study conducted laboratory research on the nucleation mechanisms during the cooling process of motor vehicle exhaust, constructed mechanistic models, and empirically applied them in a chemical transport model. Although the authors have undertaken substantial work, the logical connections among the three parts of the manuscript remain somewhat unclear. Significant revisions are required before reconsideration for publication.

Major comments:

(1) In the introduction, the review of research progress on RIN is insufficient. It only briefly outlines various reasons that may lead to underestimation in particle size spectrum simulations, without introducing the mechanistic research advancements specifically related to RIN, which is the focus of this study.
(2)In section2, Is the use of SMPS alone sufficient to characterize the number concentration of nucleation-mode particles? Nucleated particles often exist in large quantities below 3 nm, while the detection limit of SMPS starts at approximately 11 nm. This likely leads to an underestimation of particle counts at 11 nm and below.
（3) In section3, the experiments in section2 were conducted using gasoline engines to quantify the nucleation process during exhaust cooling. Is this mechanism applicable to diesel vehicles? At the very least, this should be discussed. In real-world emissions, non-road mobile machinery, diesel vehicles, and even ships often use fuels with higher sulfur content. If the sulfuric acid-water binary nucleation mechanism can explain particle nucleation during cooling, these sources with higher fuel sulfur content might be more representative than gasoline vehicles. And what about the votile organic
(4) Does the current mechanistic model lack consideration of the role of volatile organic compounds and semi-volatile organic compounds in nucleation during the condensation process? What impact would this have on the established mechanistic module and application of air quality model?
(5) The validation of the numerical simulation is limited. Firstly, the validation of meteorological simulation is missing, making it difficult to confirm whether the underestimation of simulated concentrations is due to biases in the meteorological simulation.
(6) Validation for gaseous precursors of PM2.5 such as SO2, NO2 and O3 is absent. Additionally, validation for related components is lacking, making it challenging to quantify the bias in simulated PM2.5 mass. This should be supplemented.
(7) It is recommended to supplement the time-series simulation of particle number concentration and particle size distribution. Presenting only statistical average results lacks persuasiveness.

Specific editorial issues:

- Line 276: "can be" is repeated.

- Line 310: The abbreviation PNC and its full form appears redundantly; similar issues occur with PSD and RIN.

Citation: https://doi.org/10.5194/egusphere-2025-5487-RC1
- AC1: 'Reply on RC1', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC1
RC2:
'Comment on egusphere-2025-5487', Anonymous Referee #2, 27 Dec 2025

The authors propose a new mechanism affecting particle number concentration and conduct extensive work from laboratory experiments to modeling, which may help improve the simulation of particle number concentrations. However, several statements and assumptions lack sufficient support. Major revisions are required before the manuscript can be reconsidered for publication.

(1)Lines 47–50: the authors state that “In urban areas, the underestimation of PNC is unlikely to be caused by weak/missing nucleation pathways, because the high concentration of existing aerosols will efficiently deplete precursors (e.g., sulfuric acid), leaving the nucleation process inconsequential.” This statement is insufficiently supported. Existing studies have shown that nucleation processes, particularly previously underappreciated acid–base nucleation, are a major source of particle number concentration, and that sulfuric acid concentrations in urban environments can remain high and are still highly sensitive in controlling nucleation rates and particle number concentrations.

(2) Line 73: the authors should provide supporting data, such as the proportion of motorcycles in the total vehicle fleet or their contribution to emissions, to justify the representativeness of motorcycles in this study.

(3) Line 77: extrapolating experimental results from a single motorcycle to all motorcycles and passenger vehicles may introduce significant uncertainty. An uncertainty analysis is needed to discuss how differences in vehicle type, engine technology, and operating conditions may affect the results, and the applicability range of the proposed parameterization should be clearly stated.

(4) Line 162: the authors state that “new particle formation is dominated by H₂SO₄–H₂O nucleation,” which is inconsistent with the widely accepted understanding that acid–base nucleation is the dominant nucleation mechanism in urban boundary-layer environments. Moreover, the two references cited in support of this statement do not appear to provide direct evidence for such a conclusion. The authors should provide clearer evidence to support this claim and explicitly discuss the uncertainties introduced by adopting only the H₂SO₄–H₂O nucleation framework while neglecting acid–base nucleation, as well as how the presence of ammonia or semi-volatile organic compounds in urban environments may modulate the efficiency of the proposed RIN mechanism.

(5) Lines 334–335: Although the simulations cover the period from 5 to 25 November 2020, the validation results shown in Figures 8 and 9 are limited to only two specific days (20 and 25 November). This limited validation makes it difficult to demonstrate the robustness of the modeling framework over a broader time period. It is recommended to provide validation results over longer time scales and to supplement the evaluation with quantitative statistical metrics such as RMSE.

(6) The title of the manuscript should be further constrained to explicitly indicate that the proposed recondensation-induced nucleation is related to traffic or vehicle exhaust emissions. The current wording is overly generalized. Although “cylinder” may implicitly refer to engine combustion processes, this terminology is too implicit and not sufficiently clear for an atmospheric science audience.

Citation: https://doi.org/10.5194/egusphere-2025-5487-RC2
- AC2: 'Reply on RC2', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC2
- AC3: 'Reply on RC2', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-5487', Anonymous Referee #1, 15 Dec 2025

This study conducted laboratory research on the nucleation mechanisms during the cooling process of motor vehicle exhaust, constructed mechanistic models, and empirically applied them in a chemical transport model. Although the authors have undertaken substantial work, the logical connections among the three parts of the manuscript remain somewhat unclear. Significant revisions are required before reconsideration for publication.

Major comments:

(1) In the introduction, the review of research progress on RIN is insufficient. It only briefly outlines various reasons that may lead to underestimation in particle size spectrum simulations, without introducing the mechanistic research advancements specifically related to RIN, which is the focus of this study.
(2)In section2, Is the use of SMPS alone sufficient to characterize the number concentration of nucleation-mode particles? Nucleated particles often exist in large quantities below 3 nm, while the detection limit of SMPS starts at approximately 11 nm. This likely leads to an underestimation of particle counts at 11 nm and below.
（3) In section3, the experiments in section2 were conducted using gasoline engines to quantify the nucleation process during exhaust cooling. Is this mechanism applicable to diesel vehicles? At the very least, this should be discussed. In real-world emissions, non-road mobile machinery, diesel vehicles, and even ships often use fuels with higher sulfur content. If the sulfuric acid-water binary nucleation mechanism can explain particle nucleation during cooling, these sources with higher fuel sulfur content might be more representative than gasoline vehicles. And what about the votile organic
(4) Does the current mechanistic model lack consideration of the role of volatile organic compounds and semi-volatile organic compounds in nucleation during the condensation process? What impact would this have on the established mechanistic module and application of air quality model?
(5) The validation of the numerical simulation is limited. Firstly, the validation of meteorological simulation is missing, making it difficult to confirm whether the underestimation of simulated concentrations is due to biases in the meteorological simulation.
(6) Validation for gaseous precursors of PM2.5 such as SO2, NO2 and O3 is absent. Additionally, validation for related components is lacking, making it challenging to quantify the bias in simulated PM2.5 mass. This should be supplemented.
(7) It is recommended to supplement the time-series simulation of particle number concentration and particle size distribution. Presenting only statistical average results lacks persuasiveness.

Specific editorial issues:

- Line 276: "can be" is repeated.

- Line 310: The abbreviation PNC and its full form appears redundantly; similar issues occur with PSD and RIN.

Citation: https://doi.org/10.5194/egusphere-2025-5487-RC1
- AC1: 'Reply on RC1', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC1
RC2:
'Comment on egusphere-2025-5487', Anonymous Referee #2, 27 Dec 2025

The authors propose a new mechanism affecting particle number concentration and conduct extensive work from laboratory experiments to modeling, which may help improve the simulation of particle number concentrations. However, several statements and assumptions lack sufficient support. Major revisions are required before the manuscript can be reconsidered for publication.

(1)Lines 47–50: the authors state that “In urban areas, the underestimation of PNC is unlikely to be caused by weak/missing nucleation pathways, because the high concentration of existing aerosols will efficiently deplete precursors (e.g., sulfuric acid), leaving the nucleation process inconsequential.” This statement is insufficiently supported. Existing studies have shown that nucleation processes, particularly previously underappreciated acid–base nucleation, are a major source of particle number concentration, and that sulfuric acid concentrations in urban environments can remain high and are still highly sensitive in controlling nucleation rates and particle number concentrations.

(2) Line 73: the authors should provide supporting data, such as the proportion of motorcycles in the total vehicle fleet or their contribution to emissions, to justify the representativeness of motorcycles in this study.

(3) Line 77: extrapolating experimental results from a single motorcycle to all motorcycles and passenger vehicles may introduce significant uncertainty. An uncertainty analysis is needed to discuss how differences in vehicle type, engine technology, and operating conditions may affect the results, and the applicability range of the proposed parameterization should be clearly stated.

(4) Line 162: the authors state that “new particle formation is dominated by H₂SO₄–H₂O nucleation,” which is inconsistent with the widely accepted understanding that acid–base nucleation is the dominant nucleation mechanism in urban boundary-layer environments. Moreover, the two references cited in support of this statement do not appear to provide direct evidence for such a conclusion. The authors should provide clearer evidence to support this claim and explicitly discuss the uncertainties introduced by adopting only the H₂SO₄–H₂O nucleation framework while neglecting acid–base nucleation, as well as how the presence of ammonia or semi-volatile organic compounds in urban environments may modulate the efficiency of the proposed RIN mechanism.

(5) Lines 334–335: Although the simulations cover the period from 5 to 25 November 2020, the validation results shown in Figures 8 and 9 are limited to only two specific days (20 and 25 November). This limited validation makes it difficult to demonstrate the robustness of the modeling framework over a broader time period. It is recommended to provide validation results over longer time scales and to supplement the evaluation with quantitative statistical metrics such as RMSE.

(6) The title of the manuscript should be further constrained to explicitly indicate that the proposed recondensation-induced nucleation is related to traffic or vehicle exhaust emissions. The current wording is overly generalized. Although “cylinder” may implicitly refer to engine combustion processes, this terminology is too implicit and not sufficiently clear for an atmospheric science audience.

Citation: https://doi.org/10.5194/egusphere-2025-5487-RC2
- AC2: 'Reply on RC2', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC2
- AC3: 'Reply on RC2', Jen-Ping Chen, 10 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5487/egusphere-2025-5487-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-5487-AC3

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Jen-Ping Chen on behalf of the Authors (11 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (12 Jan 2026) by Fangqun Yu

RR by Anonymous Referee #1 (28 Jan 2026)

Suggestions for revision or reasons for rejection

Although the authors have made certain revisions to the article, further modifications are still required prior to publication consideration. The specific issues are as follows:
1. Validation through numerical simulations is a crucial component for demonstrating the scientific validity of the RIN mechanism in this study. Although the particle number size distribution (PNSD) timeseries (Figure S6) was provided in the previous revision, the three subplots are arranged side by side, resulting in compressed long-term time series data that obscures the daily evolution of the particle size distribution. Since the hourly evolution of the size spectrum is essential for determining the occurrence of new particle formation events, it is recommended to optimize and adjust the presentation. Additionally, further improvement in color scheme is advised. Furthermore, it is suggested to include a discussion on the overall PNSD simulation results in the main text. Otherwise, validating the nearly month-long simulation by selecting only two days for analysis lacks persuasiveness.

2. In the atmospheric environment, organic compounds play a significant role in particle growth. Based on the comparison of PNSD simulations between the No_RIN and RIN scenarios in this study, the RIN scenario appears to overestimate the particle number concentration within the smaller size range (diameter < 100 nm). Could this be attributed to a lack of consideration for mechanism of particle growth processes in the simulation? Please provide further clarification.

3. Figure S4 does not include validation for PM2.5, which is inconsistent with the response to the review comments. Given that this study focuses on particulate matter simulation, please supplement the figure with PM2.5 timeseries validation.

Hide

RR by Anonymous Referee #2 (02 Feb 2026)

Suggestions for revision or reasons for rejection

The authors have made efforts to address the previous round of comments, improving the manuscript's clarity and adding supporting information in several areas. However, two critical scientific issues remain inadequately resolved and require further attention before publication can be recommended.
The first major concern relates to the nucleation mechanism framework. While the authors now acknowledge that acid-base and organic nucleation are plausible in Taichung's atmosphere, the model continues to employ only H₂SO₄–H₂O binary nucleation. The justification provided is that binary nucleation is "sufficient" to reproduce observed particle number concentrations and that NH₃ or organics would "not fundamentally alter the qualitative behavior of the system." This assertion, however, lacks empirical support. Without sensitivity tests comparing other nucleation pathways, it remains unclear whether the current framework captures the dominant physics or merely achieves agreement through parameter adjustment. If NH₃ or organics significantly enhance nucleation efficiency—as recent urban nucleation studies increasingly suggest—the CMAQ parameterization developed in Section 4 (which assumes binary-only chemistry) could systematically misestimate PNC under varying NH₃/VOC conditions. The authors' statement that the parcel model serves "qualitative interpretation only" appears inconsistent with subsequently using its mechanistic insights to construct a quantitative CMAQ parameterization. At minimum, sensitivity simulations with at least one alternative nucleation scheme are needed to quantify the uncertainty introduced by this framework choice and to substantiate the claim that mechanism selection does not fundamentally affect the conclusions.
The second major issue concerns the scope and generalizability of the findings. The revised Introduction and Method appropriately acknowledges that the RIN mechanism operates under specific regional conditions similar to those in Taiwan—heavy pollution, high pre-existing aerosol loading, suppressed NPF events, and a high proportion of motorcycles. However, the title, abstract, and conclusions continue to imply general applicability across urban environments. This creates a misleading impression, as most global cities exhibit substantially different characteristics, and this mechanism may be regionally important in motorcycle-dominated Asian cities but not necessarily representative of urban environments globally. The title, abstract, and conclusions should be revised to explicitly reflect the specific region where this mechanism has been validated.

Hide

ED: Reconsider after major revisions (03 Feb 2026) by Fangqun Yu

AR by Jen-Ping Chen on behalf of the Authors (05 Mar 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (08 Mar 2026) by Fangqun Yu

RR by Anonymous Referee #1 (24 Mar 2026)

RR by Anonymous Referee #2 (04 Apr 2026)

ED: Reconsider after major revisions (08 Apr 2026) by Fangqun Yu

AR by Jen-Ping Chen on behalf of the Authors (09 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (10 Apr 2026) by Fangqun Yu

RR by Anonymous Referee #1 (27 Apr 2026)

ED: Publish subject to technical corrections (28 Apr 2026) by Fangqun Yu

AR by Jen-Ping Chen on behalf of the Authors (03 May 2026) Author's response Manuscript

Journal article(s) based on this preprint

21 May 2026

From cylinder to city: how recondensation-induced nucleation in vehicle exhaust shapes urban aerosol number

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

Atmos. Chem. Phys., 26, 6909–6927, https://doi.org/10.5194/acp-26-6909-2026,https://doi.org/10.5194/acp-26-6909-2026, 2026

Short summary

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

Supplement

https://doi.org/10.5194/egusphere-2025-5487-supplement

Jen-Ping Chen, I-Chun Tsai, Li-Wei Kuo, and Gong-Do Hwang

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This study identifies a previously unrepresented process, Recondensation-Induced Nucleation (RIN), linking combustion microphysics to ambient ultrafine particle formation. Laboratory engine experiments, parcel modeling, and CMAQ simulations show that including RIN can significantly reduce number concentration underestimation from 75 % to 14 % while maintaining PM_2.5 accuracy, providing a mechanistic basis for improving urban aerosol modeling.


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