Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests

Lee, Wen-Chien; Huang, Ming-Hao; Huang, Wei-Chieh; Chen, Jen-Ping; Lai, Yen-Jen; Ren, Haojia; Hung, Hui-Ming

doi:10.5194/egusphere-2025-3950

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https://doi.org/10.5194/egusphere-2025-3950

Preprints

19 Sep 2025

| 19 Sep 2025

Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

Abstract. Reactive nitrogen (Nr) species such as particulate ammonium (pNH₄⁺) and nitrate (pNO₃⁻) cause air pollution and affect ecosystems, yet their transformation processes in mountain forests are not well-characterized. Size-resolved isotope analysis of aerosols could reveal these processes, but is rarely performed due to low particle concentrations. We overcame this limitation by combining size-segregated aerosol sampling at Xitou, Taiwan, with sensitive isotopic techniques and Bayesian modeling. Functional groups were analyzed by Fourier-transform infrared spectroscopy (FTIR-ATR), and isotopes δ¹⁵N and δ¹⁸O were measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), enabling quantification of pNH₄⁺ source contributions and pNO₃⁻ formation pathways. Typical diurnal patterns, with higher daytime particle concentrations, were disrupted during a 26-hour fog caused by stagnant atmospheric conditions. During fog, the average δ¹⁵N-NH₄⁺ decreased from 11.75±2.42 ‰ (mean±1σ) during clear periods to 7.75±1.37 ‰, while δ¹⁵N-NO₃⁻ dropped from −2.57±1.80 ‰ to −4.51±1.79 ‰, indicating continued isotopic fractionation under reduced urban influence. Size-resolved isotope results revealed nitrate evolution during transport: urban plumes retained O₃-driven oxidation signatures with isotopic fractionation, whereas mountain-formed nitrate was produced via RO₂-involved processes with greater isotopic fractionation and enhanced biogenic contributions. Bayesian modeling indicated that 50−83 % of NH₃ emissions originated from combustion-related sources, while 42−95 % of pNO₃⁻ formed through RO₂-initiated oxidation during daytime and 6−84 % through heterogeneous reactions at night. These findings emphasize the importance of controlling urban NO_x and combustion-related NH₃ emissions to reduce downwind Nr pollution and demonstrate how size-resolved isotope analysis elucidates aerosol evolution along transport pathways.

Received: 13 Aug 2025 – Discussion started: 19 Sep 2025

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06 Aug 2026

Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests: A case study

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

Atmos. Chem. Phys., 26, 10947–10963, https://doi.org/10.5194/acp-26-10947-2026,https://doi.org/10.5194/acp-26-10947-2026, 2026

Short summary

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3950', Anonymous Referee #1, 23 Dec 2025

Please find attached the pdf file.

Citation: https://doi.org/10.5194/egusphere-2025-3950-RC1
- AC1: 'Reply on RC1', Wen-Chien Lee, 14 Apr 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3950/egusphere-2025-3950-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3950-AC1
RC2:
'Comment on egusphere-2025-3950', Anonymous Referee #2, 01 Mar 2026
In their manuscript “Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests”, authors conducted size-resolved isotope analysis of aerosol sampled at a mountain forest in Xitou, Taiwan to reveal sources and processes. The study is on a topic of relevance and general interest to the readers of ACP. Methodologically, studying reactive nitrogen species with isotopic techniques and source attribution modeling is not new. I am mostly concerned with the short period of sampling (8 days, 2 filter samples per day) and the lack of gas-phase nitrogen-containing species measurement, which makes it hard to form conclusive sources and formation pathways identification. Yet the data in the region of study during a prolonged fog is valuable, and the analyses were carried out comprehensively and rigorously. I am recommending a minor revisions with detailed comments listed below.

Specific comments:
In Introduction, Line 66-81, the scientific significance of the study is not well explained. It is stated that “relatively little is known about the sources and atmospheric processing of Nr in East Asian mountain forests”, yet the authors later referred to Chen, T. Y. et al. (2022) that used the similar methodology (isotope signatures) at the same location. I’d love to see the authors further highlight the uniqueness of the location; for example, is Xitou a representative mountain forest area for East Asian, or at least for Taiwan (meteorologically wise, BVOC profile wise, urban-forest mixing wise)? Is Xitou known for abundant nitrogen-containing particles, mass concentration wise or fractional wise? Moreover, more information should be added to highlight the importance of aerosol chemistry under fog conditions (and summarize the current knowns and unknowns on aqueous phase processing etc.), followed by how your size-dependent isotopic approach could improve such understanding (see comment#5). This will enhance the scientific uniqueness of your study and can actually distinguish it from the previous one.

In Methods, Line 90, please justify your choice of the sampling period. Is April meteorologically favorable for foggy conditions or prone to higher ambient aerosol loadings or what? Is the week-long measurement representative enough for reaching your conclusions (e.g. those around Line 422)?

In Methods, section 2.3.2, the inclusion and exclusion of certain pathways during daytime and nighttime analysis reads arbitrary. It is recommended to provide results of sensitivity tests for the mentioned inclusion and exclusion to see if such decisions alter the results or not in the SI.

In Results, Line 270-284 and Figure 4 are too speculative to me. I am not convinced how useful it is to have three independent linear regression lines on such limited data points. Moreover, grouping the clear data into <~1 micron and >1 micron reads arbitrary and is too mechanical. The hypotheses and suggested explanations were not supported by any ancillary or referable data, and I am not sure if the trend or differences are statistically significant. For example, if the cutoff is not at 1 micron but at 1.5 or 2 micron, will the revealed increase go from 7.14 to 16.08 per mil? Will the δ15N-NH4+ values of this group really be lower then? When you say “coarse particles (1−10 µm) are likely produced from urban areas…that were subsequently transported to the sampling site”, does your wind data support such a hypothesis? I agree that a bell-shaped pattern is revealed in Figure 4 and it becomes less apparent during the fog, but the corresponding discussions shall be revised with more supporting evidence.

Figures corresponding to results discussed in section 3.4.2 and 3.4.3 are all in the SI. I’d suggest combining the two sections into one and bringing Figure S14 into the main manuscript for better interpretation. You may consider discussing the shifts in formation pathways (PXa, PXb) in a more descriptive, mechanistic language (Line 380 onward). I’d also suggest highlighting the importance of your pathway identifications (even though with uncertainties and assumptions) for improving the understanding of the particulate nitrate formation. Recent studies coupling the organonitrates formation during day and night with varied meteorological conditions, such as a few listed below, could be used for your reference and to put your findings into the broader picture:

Guo et al., 2024 (https://doi.org/10.1016/j.atmosenv.2024.120362)
Ward et al., 2025 (https://doi.org/10.1126/sciadv.adt8957)
Murphy et al., 2025 (https://doi.org/10.1021/acsestair.5c00206).

Technical corrections:
In Figure 6a, I am not sure if the regression line is needed given how scattered the data are.

Same as above for Figure S15.

What’s your take on larger particles being more neutral and smaller being more acidic from your Figure S6? Is it consistent with your hypothesis that more coarse particles were transported from the urban area?
Citation: https://doi.org/10.5194/egusphere-2025-3950-RC2
- AC2: 'Reply on RC2', Wen-Chien Lee, 14 Apr 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3950/egusphere-2025-3950-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3950-AC2

Peer review completion

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

AR by Wen-Chien Lee on behalf of the Authors (14 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 Apr 2026) by Alex Lee

RR by Anonymous Referee #2 (17 Apr 2026)

RR by Anonymous Referee #3 (14 May 2026)

Suggestions for revision or reasons for rejection

The manuscript has been improved after the first-round review. I have several comments after reading the revised version and the authors’ responses to the previous comments.
1. Manuscript
The measurement period is relatively short, as already questioned by both reviewers during the first round. Only one fog event was observed although it last more than one day. In my view, this should be treated more as a case study, and it may not be sufficient to represent typical fog conditions in the studied forest. This limitation weakens the robustness of the conclusions.
One suggestion is to combine previous measurements at the same site.
Regarding the abundance of biogenic VOCs, I agree that forested areas are generally expected to exhibit elevated levels. However, this measurement site is located near a mountain top, where air masses may be substantially diluted. Additional observational evidence or supporting measurements are needed to support this statement.
L32–33: Please provide references to support this statement.
L110, L127, L169, and elsewhere throughout the manuscript: Please carefully check the reference formatting.
L405–408: Was there any direct measurement or observational evidence from this site to support this argument?
Figure S7: A height of 10 m AGL was selected for the back-trajectory analysis, which is not commonly used. At such low altitude, surface turbulence may substantially affect the trajectories, and the results may not represent regional transport well. However, I am not an expert in meteorology, so I suggest the authors further justify this setup and discuss the associated uncertainties.
Figure S8: Please provide more information on how CMAQ simulates gaseous NH₃.
In the conclusion section, please more clearly highlight the uncertainties associated with the short measurement period, especially because most conclusions regarding fog impacts rely on a single fog event.
2. Response to Reviewers’ Comments
R1C1: Similar observations and conclusions have been reported in previous studies, which is acceptable since this is what the data show. However, the authors still need to better justify the scientific significance and/or policy relevance of the study in order to satisfy ACP standards.
R1C4: The authors proposed multiple isotope-depleting mechanisms during the fog event. However, further evidence is needed to demonstrate that these lower isotopic values do not normally occur simultaneously under non-fog conditions. This would help establish the uniqueness of the fog event and strengthen the scientific significance of the study.
R1C6: I suggest the authors tone down the claims regarding “novel” observational findings, since only one fog event was observed during the measurement period.
R1C8: I suggest briefly introducing the relevant mechanisms when defining the terminology, since many new terms are introduced in this section. Otherwise, the discussion remains difficult to follow.

Hide

ED: Reconsider after major revisions (20 May 2026) by Alex Lee

AR by Wen-Chien Lee on behalf of the Authors (22 Jun 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Jun 2026) by Alex Lee

RR by Anonymous Referee #3 (19 Jul 2026)

ED: Publish as is (21 Jul 2026) by Alex Lee

AR by Wen-Chien Lee on behalf of the Authors (23 Jul 2026) Manuscript

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3950', Anonymous Referee #1, 23 Dec 2025

Please find attached the pdf file.

Citation: https://doi.org/10.5194/egusphere-2025-3950-RC1
- AC1: 'Reply on RC1', Wen-Chien Lee, 14 Apr 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3950/egusphere-2025-3950-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3950-AC1
RC2:
'Comment on egusphere-2025-3950', Anonymous Referee #2, 01 Mar 2026
In their manuscript “Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests”, authors conducted size-resolved isotope analysis of aerosol sampled at a mountain forest in Xitou, Taiwan to reveal sources and processes. The study is on a topic of relevance and general interest to the readers of ACP. Methodologically, studying reactive nitrogen species with isotopic techniques and source attribution modeling is not new. I am mostly concerned with the short period of sampling (8 days, 2 filter samples per day) and the lack of gas-phase nitrogen-containing species measurement, which makes it hard to form conclusive sources and formation pathways identification. Yet the data in the region of study during a prolonged fog is valuable, and the analyses were carried out comprehensively and rigorously. I am recommending a minor revisions with detailed comments listed below.

Specific comments:
In Introduction, Line 66-81, the scientific significance of the study is not well explained. It is stated that “relatively little is known about the sources and atmospheric processing of Nr in East Asian mountain forests”, yet the authors later referred to Chen, T. Y. et al. (2022) that used the similar methodology (isotope signatures) at the same location. I’d love to see the authors further highlight the uniqueness of the location; for example, is Xitou a representative mountain forest area for East Asian, or at least for Taiwan (meteorologically wise, BVOC profile wise, urban-forest mixing wise)? Is Xitou known for abundant nitrogen-containing particles, mass concentration wise or fractional wise? Moreover, more information should be added to highlight the importance of aerosol chemistry under fog conditions (and summarize the current knowns and unknowns on aqueous phase processing etc.), followed by how your size-dependent isotopic approach could improve such understanding (see comment#5). This will enhance the scientific uniqueness of your study and can actually distinguish it from the previous one.

In Methods, Line 90, please justify your choice of the sampling period. Is April meteorologically favorable for foggy conditions or prone to higher ambient aerosol loadings or what? Is the week-long measurement representative enough for reaching your conclusions (e.g. those around Line 422)?

In Methods, section 2.3.2, the inclusion and exclusion of certain pathways during daytime and nighttime analysis reads arbitrary. It is recommended to provide results of sensitivity tests for the mentioned inclusion and exclusion to see if such decisions alter the results or not in the SI.

In Results, Line 270-284 and Figure 4 are too speculative to me. I am not convinced how useful it is to have three independent linear regression lines on such limited data points. Moreover, grouping the clear data into <~1 micron and >1 micron reads arbitrary and is too mechanical. The hypotheses and suggested explanations were not supported by any ancillary or referable data, and I am not sure if the trend or differences are statistically significant. For example, if the cutoff is not at 1 micron but at 1.5 or 2 micron, will the revealed increase go from 7.14 to 16.08 per mil? Will the δ15N-NH4+ values of this group really be lower then? When you say “coarse particles (1−10 µm) are likely produced from urban areas…that were subsequently transported to the sampling site”, does your wind data support such a hypothesis? I agree that a bell-shaped pattern is revealed in Figure 4 and it becomes less apparent during the fog, but the corresponding discussions shall be revised with more supporting evidence.

Figures corresponding to results discussed in section 3.4.2 and 3.4.3 are all in the SI. I’d suggest combining the two sections into one and bringing Figure S14 into the main manuscript for better interpretation. You may consider discussing the shifts in formation pathways (PXa, PXb) in a more descriptive, mechanistic language (Line 380 onward). I’d also suggest highlighting the importance of your pathway identifications (even though with uncertainties and assumptions) for improving the understanding of the particulate nitrate formation. Recent studies coupling the organonitrates formation during day and night with varied meteorological conditions, such as a few listed below, could be used for your reference and to put your findings into the broader picture:

Guo et al., 2024 (https://doi.org/10.1016/j.atmosenv.2024.120362)
Ward et al., 2025 (https://doi.org/10.1126/sciadv.adt8957)
Murphy et al., 2025 (https://doi.org/10.1021/acsestair.5c00206).

Technical corrections:
In Figure 6a, I am not sure if the regression line is needed given how scattered the data are.

Same as above for Figure S15.

What’s your take on larger particles being more neutral and smaller being more acidic from your Figure S6? Is it consistent with your hypothesis that more coarse particles were transported from the urban area?
Citation: https://doi.org/10.5194/egusphere-2025-3950-RC2
- AC2: 'Reply on RC2', Wen-Chien Lee, 14 Apr 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3950/egusphere-2025-3950-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-3950-AC2

Peer review completion

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

AR by Wen-Chien Lee on behalf of the Authors (14 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 Apr 2026) by Alex Lee

RR by Anonymous Referee #2 (17 Apr 2026)

RR by Anonymous Referee #3 (14 May 2026)

Suggestions for revision or reasons for rejection

Hide

ED: Reconsider after major revisions (20 May 2026) by Alex Lee

AR by Wen-Chien Lee on behalf of the Authors (22 Jun 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Jun 2026) by Alex Lee

RR by Anonymous Referee #3 (19 Jul 2026)

ED: Publish as is (21 Jul 2026) by Alex Lee

AR by Wen-Chien Lee on behalf of the Authors (23 Jul 2026) Manuscript

Journal article(s) based on this preprint

06 Aug 2026

Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests: A case study

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

Atmos. Chem. Phys., 26, 10947–10963, https://doi.org/10.5194/acp-26-10947-2026,https://doi.org/10.5194/acp-26-10947-2026, 2026

Short summary

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

Supplement

https://doi.org/10.5194/egusphere-2025-3950-supplement

Wen-Chien Lee, Ming-Hao Huang, Wei-Chieh Huang, Jen-Ping Chen, Yen-Jen Lai, Haojia Ren, and Hui-Ming Hung

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We studied nitrogen pollution in Taiwan's mountain forests to track how urban emissions reach and transform in remote areas. Isotope analysis and statistical modeling revealed that combustion sources contributed 50–83 % of ammonia, while nitrate forms continuously from urban to rural sampling sites. The findings show that persistent urban pollution strongly impacts mountain ecosystems, offering key insights for air quality management.


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