Review of “Dominant Influence of Biomass Combustion and Cross-Border Transport on Nitrogen-Containing Organic Compound Levels in the Southeastern Tibetan Plateau” by Wang et al.

The authors reported measurement results of nitrogen-containing organic compounds (NOCs) in PM2.5 at a regional background site in South Tibetan Plateau. Careful speciation and quantification on NOCs with 60+ samples over 2 months of time span. Concentration levels and variations of main compound classes, including free amino acids, amines and urea, as well as relatively minor ones (in terms of mass concentrations) such as alkyl amides, nitriles, cyclic NOCs, and isocyanates, were discussed. In addition, concentrations of these NOCs and those of other particulate pollutants (e.g, metals and EC/OC) were used to apportion the sources of NOCs at this site. Results suggested that biomass burning and secondary formation were main contributors of NOCs at this site. Furthermore, regional model was used to evaluate the contribution of cross-boundary transport to particulate matter at this site, hinting that such contribution is also important for the NOCs measured. The study is of importance to understand the climate-related pollutants in the less-explored region of Tibetan Plateau. The chemical analysis and data interpretation in this work in rigorous. The manuscript is also clear to follow. I have a few comments as below.

1. Can the authors briefly justify why nitro-aromatic compounds were not included in this study? They are also light-absorbing and can affect climate. The authors hinted on secondary formation of this class of compounds in P11/L394. Is it that the NOx and aromatic VOC level at the regional background site are not high enough to make them important at this site?
2. I have some reservation on using regional model to estimate PM2.5, and then infer that NOCs are also dominantly transported from nearby regions. High PM2.5 might not necessarily mean high NOCs. It is better to build a stronger linkage between the model results of PM2.5 and the PSCF results of NOCs, such that the conclusion of regional transport of NOCs would be more convincing.
3. I am also a bit confused about how free amino acids are formed secondarily. Do you mean the processes of breaking down proteins into free amino acids, or converting, say amines, into amino acids by introducing the COOH group via oxidation? Please clarify.
4. P4/L120: define FAAs here.
5. P5/L135: why not using nmol/m^-3 that is consistent with those in the previous paragraphs?
6. P6/168: add “solution” after “7-factor”.
7. P7/L210: “EC” or “EP”?
8. P8/L276-278: citation needed.
9. P9/L287: subscript for 6 and 20 to be consistent with the notation earlier in the sentence.
10. P9/L297: provide mean +/- standard deviation as in the previous paragraph?
11. P9/L311: remove “firmly”.
12. P9/L319: replace “which” with “and they”.
13. P10/L357: how much is “a portion”?

The manuscript provides a comprehensive analysis of nitrogen-containing organic compounds (NOCs) in fine particulate matter (PM2.5) in the southeastern Tibetan Plateau (TP). The authors have conducted a detailed field study and employed robust analytical methods to identify the sources and concentrations of NOCs, emphasizing the significant impact of biomass combustion and cross-border transport. The study is well-structured, and the results are important for understanding the atmospheric chemistry and climate implications in this sensitive high-altitude region. I recommend the manuscript for publication after addressing these minor points.

1. Abstract: the abstract would benefit from a brief mention of the specific analytical techniques used to quantify NOCs, which would provide readers with a better understanding of the study's methodological robustness.
2. Introduction: The introduction provides a good background on the significance of NOCs and the TP region. It would be helpful to include a brief discussion on the potential implications of NOCs on local human health and ecosystems, in addition to their climatic impact.
3. line 232: provide a more detailed explanation of the criteria used to segment the campaign into EC1-EC5 periods. For instance, specifying the exact meteorological parameters and concentration thresholds that define each period would enhance clarity
4. line 254-261: The mean concentrations of protein-type and non-protein-type FAAs are provided, but it would be useful to discuss the potential reasons for the observed differences in their concentrations. For example, what environmental or biological processes might account for the higher prevalence of protein-type FAAs.
5. line 298: add references for “…Urea can be released into the atmosphere through agricultural activities and biomass burning, and it can also be formed secondarily in the atmosphere through chemical reactions…”
6. line 329: While Simoneit et al. (2003) is cited for the formation mechanisms, it would be useful to reference additional studies that have observed similar formations of alkyl amides and nitriles in biomass burning contexts. This would help to further validate the findings and place them within a broader research context.
7. line 383: While other sources accounted for less than 10%, it would be beneficial to briefly mention what these sources are and their potential impact. Even minor contributors can provide important context for a comprehensive understanding of NOC sources.
8. Conclusion: Suggest areas for future research that could build on this study. For instance, further investigation into the specific chemical pathways of NOC formation during transport, or more detailed source apportionment studies in different regions, could be valuable.
9. Figure 1: There is a minor typographical error in the description of the clean period: "5/6 to -5/11" should be corrected to "5/6 to 5/11". Consistency in date formats will prevent confusion.