Articles | Volume 25, issue 19
https://doi.org/10.5194/acp-25-12451-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.The critical role of volatile organic compound emissions in nitrate formation in Lhasa, Tibetan Plateau: insights from oxygen isotope anomaly measurements
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- Final revised paper (published on 09 Oct 2025)
- Supplement to the final revised paper
- Preprint (discussion started on 07 Mar 2025)
- Supplement to the preprint
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Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on egusphere-2025-164', Anonymous Referee #2, 26 Mar 2025
- AC2: 'Reply on RC1', Junwen Liu, 13 Jun 2025
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RC2: 'Comment on egusphere-2025-164', Anonymous Referee #1, 03 Apr 2025
- AC1: 'Reply on RC2', Junwen Liu, 13 Jun 2025
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AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Junwen Liu on behalf of the Authors (13 Jun 2025)
Author's response
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ED: Referee Nomination & Report Request started (15 Jun 2025) by Lisa Whalley
RR by Anonymous Referee #1 (08 Jul 2025)

ED: Publish as is (11 Jul 2025) by Lisa Whalley
AR by Junwen Liu on behalf of the Authors (18 Jul 2025)
Author's response
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Zheng et al., showed that volatile organic compounds play a critical role in the nitrate production on a plateau city, as inferred from the oxygen isotope anomaly of nitrate (Δ17O-NO3-). The Δ17O-NO3- hold a wealth information about the atmospheric oxidation environment, which can be used to complement the model work as an observational constraint for NOx chemistry. I believe this study is of significant importance to the community as there are very sparse measurements of oxygen isotope anomaly of nitrate in high-elevation plateau environments. While I agree with most of the interpretation, some of the results, i.e., the day-night difference in Δ17O-NO3- require further deliberation. In addition, considerable improvements could be made in the presentation of the results, refining the methodology, the layout of the figures, as well as enhancing the overall clarity of the writing. Overall, the manuscript should be subjected to major revisions listed below.
Specific comments:
1: The author highlights that VOCs+NO3 is of particular important for nitrate formation in Lhasa in spring based on the Δ17O measurements and a simple mass-balance model calculation (i.e., Bayesian). The author did a lot of statistically analysis based on the Bayesian model outputs. It is well known that the Bayesian models of this nature was mathematically underdetermined and there was no unique solution with only one constraint but for three solutions (see Phillips et al., 2014), therefore model results will be associated with significant uncertainty. The comparison, statistically analysis and any conclusions draw from these results should be approached with great caution. For example, the contribution of OH+NO2 likely fluctuates around 50% throughout the year.
2: Regarding the source of VOCs, the authors suggest that high ambient VOCs in spring may originate from South Asia via long-range transport. There are growing evidence that long-range transport of atmospheric pollutants from South Asia regulating the aerosol loadings in south of Tibetan Plateau in spring. Does nitrate aerosol in Lhasa also be impacted by the long-range transport, especially in spring? It is likely that the author assumed that long-range transported VOCs involve in the local nitrate production in Lhasa through NO3+VOC pathways. This should be explicitly addressed in the main text.
3: The methodology for the determination of specific pathway contribution to nitrate based on Δ17O should be clearly presented in the Method section. One of the most important of part is the A value (i.e, the relative importance of O3 versus RO2 in NO2 formation), when using Δ17O to distinguish nitrate formation pathways. First, I noticed that the author derived the RO2 concentrations based on a empirical relationship about O3 mixing ratio. This relationship between RO2 and O3 indeed has been widely used in relevant study as concurrent RO2 measurement is unavailable. This method is feasible at present. However, the relationship between RO2 and O3 the author used in this study is referred to Kanaya et al., 2007, which was conducted in urban site in central Tokyo. I believe that the atmospheric condition in Lhasa is completely different from that in Tokyo, i.e., the dominant RO2 source. RO2 production is najorly determined by the solar radiations, which is also different between the two sites, as noticed in the Introduction. I recommend the calculation of RO2 concentrations using MCM model and recent field observations of VOCs at Lhasa (see Chunxiang Ye et al., 2023).
Second, the author also suggests that nighttime RO2 may play a role in the NOx oxidations. Similarly, the derivation of nighttime RO2 is valid only when O3 oxidation VOC dominates the RO2 production (Kanaya et al., 2007). Nighttime RO2 production mechanisms in Lhasa maybe unknown, however, in other urban cities such as Beijing in China, NO3 radical + VOC is the dominant channel for nighttime RO2 production. In this case, nighttime RO2 will be roughly correlated with the NO3 radical production rate, kO3+NO2[O3][NO2]. Although, given the high nighttime O3 concentration in Lhasa, it maybe reasonable to assume O3 dominant nighttime NO oxidation. To improve the robustness of the pathway differentiation, I recommend that this part could be done according to the approach of Alexander et al., 2020, and compare the field Δ17O-NO3- measurements with the model results in Alexander et al., 2020.
3: I DONOT agree with the interpretation of the observed day-night differences in Δ17O-NO3- during winter and summer (Lines: 307-333). Remember that daytime NO3 and N2O5 chemistry should be negligible in nitrate chemistry, and no supporting evidence for this claim could be found in reference in Brown et al., 2011. Note high NO3 production rate not means high mixing ratio of NO3, NO3 and N2O5 will be rapidly decomposed under sunlight. Although there are increasing studies showing the potential impact of daytime NO3 radical chemistry, the importance of daytime NO3/N2O5 chemistry should be investigated with concurrent field observations or model experiments. The atmospheric residence time of nitrate should be considered for the comparison of day-night difference in Δ17O-NO3-, see Vicars et al., 2013.
General comment
The description of nitrate formation pathways (Text S1) and the associated Δ17O signatures should be presented in the main text.
Line 62-63 Numerous field experiments have demonstrated that the N2O5 uptake probability on aerosol varied significantly, depending on the aerosol composition, meteorological parameters.
Line 237 I think the highlight of the text is the comparison of nitrate chemistry in high-elevation city with that in plain region. More discussion is needed to explore the mechanisms regulating the nitrate oxidation pathways, rather than a simple comparison of relative importance.
Line 345-347 Recent field radical measurements in urban sites in China found that OH and HO2 radical during haze period is comparable to clean days, see Slater et al., 2020, Lu et al., 2019.
Line 373 The implication sounds impotent. It is well known that aerosol liquid water content (ALWC) and Ox (oxidation capacity) regulate nitrate concentrations—ALWC impacts gas-to-particle partitioning, while Ox affects oxidation efficiency. The authors should focus on the specific or unique environmental conditions in the Tibetan Plateau that could be reflected by the measurements of Δ17O-NO3-.
Additionally, many sentences throughout the manuscript require careful revision for clarity and grammar (e.g., Lines 31–33)