Please find attached the pdf file. 

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:

1. 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. 
2. 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)? 
3. 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. 
4. 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. 
5. 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:

1. In Figure 6a, I am not sure if the regression line is needed given how scattered the data are.
2. Same as above for Figure S15. 
3. 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?