the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Mar 2025
The Critical Role of Volatile Organic Compounds Emission in Nitrate Formation in Lhasa, Tibetan Plateau: Insights from Oxygen Isotope Anomaly Measurements
Abstract. Atmospheric particulate nitrate aerosol (NO3-), produced via the oxidation of nitrogen oxides (NOx = NO + NO2), plays an important role in atmospheric chemistry and air quality, yet its formation mechanism still poorly constrained the plateau region. In this study, we first reported the yearly variation of the signatures for the stable oxygen isotope anomaly (∆17O = δ17O - 0.52 × δ18O) in NO3- collected in the urban region of Lhasa city (3650 m a.s.l), Tibetan Plateau, China. Our results show that NO2 + OH is the largest contributor to NO3- formation (46 %), followed by NO3 + VOC (26 %), and N2O5 + H2O (28 %) using the Bayesian Isotope Mixture Model. Notably, there are significant differences in the NO2 + OH, NO3 + VOC, and N2O5 + H2O pathways between spring and other three seasons (p < 0.05). Our results highlight the influence of VOC emissions from regions such as Afghanistan and northern India, which enhance NO3- concentrations in Lhasa during spring. Furthermore, the diurnal distribution of NO3- oxidation pathways varied distinctly across seasons, suggesting that these difference in NO3- pathways are attributed to ALWC, VOC concentration, and pollution levels.
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RC1: 'Comment on egusphere-2025-164', Anonymous Referee #2, 26 Mar 2025
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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)
Citation: https://doi.org/10.5194/egusphere-2025-164-RC1 -
RC2: 'Comment on egusphere-2025-164', Anonymous Referee #1, 03 Apr 2025
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Summary: This manuscript presents an interesting study of aerosol nitrate formation in an elevated urban environment (Lhasa, Tibetan Plateau, China), with a focus on stable triple oxygen isotope measurements of aerosol nitrate. The authors collected high-volume aerosol samples for offline chemical and isotopic analyses and used the oxygen isotopic composition of nitrate (Δ17O) to infer the seasonality, and to a limited extent, diurnal variation of NOₓ oxidation and nitrate formation pathways. They conclude that oxidation by NO₃ + VOC contributes significantly to aerosol nitrate formation (~26%) based on a Bayesian isotope mixing model. While this study is timely and potentially impactful, particularly due to the high-elevation urban setting and the application of oxygen isotopes, it suffers from major methodological limitations and interpretive leaps. For example, the mixing model appears to be significantly underconstrained, and key assumptions (e.g., endmember Δ17O values) are not adequately justified or tested. The manuscript would benefit from deeper contextualization, more rigorous uncertainty analysis, and supplemental modeling to support the stated conclusions. I believe the study has potential but requires substantial revision before it can be considered for publication in ACP.
Comments:
Text S2: This section is never referenced in the main manuscript but contains a critical assumption about the fraction of NO2 oxidation. Since the Δ17O(NO3-) signature is largely derived from NO2, this section should be moved to the main text.
TOC Figure: The figure may mislead readers by implying significant seasonal differences in Δ17O(NO3-) that are not statistically supported in the results. Differences were observed only in spring. Uncertainties must be included for the pathway contributions, and once added, the seasonal distinctions may not hold.
Lines 35–37: The uncertainty in the calculated pathway contributions should be provided.
Lines 37–39: Explain how this difference between seasons was determined
Lines 39–40: Add context to how these statements were concluded.
Line 42: Acronyms such as ALWC, NO₃⁻, and VOC are not defined in the abstract and should be introduced.
Lines 87–89: It is unusual to target three "major" NO₃⁻ formation pathways in a continental setting, especially including NO3 + VOC, which is generally minor. Consider referencing Alexander et al., 2019 and revisiting the classification of major pathways.
Lines 93–94: Since the site is at high elevation, provide information on altitude-related meteorology and its influence on boundary layer mixing and transport.
Lines 94–95: Include more detail on the urban characteristics and land use of Lhasa to contextualize emissions.
Lines 125–126: The link did not work. Ensure that all supplemental data used in the manuscript is archived and accessible via a reliable digital repository.
Lines 131–132: The isotope mixing model assumes known Δ17O endmembers. How were these determined, particularly for Δ17O(NO2)? Please explain the derivation or source of these values.
Lines 150–153: The MDL for NO3- was given earlier (Line 114), but MDLs for other ions are missing here. Please include them.
Lines 154–157: The provided URL for the model is not a proper citation. Please cite the model formally and ensure access.
Lines 157–159: Add a supporting reference for the assumptions or parameterizations described here.
Lines 159–162: Why was 3,650 meters chosen for the model?
Lines 173–175: The claim of an "opposite" trend is unclear; visually, it seems the trends are actually consistent. Please clarify.
Lines 179–200: Nitrate concentrations depend strongly on gas-particle partitioning, which is influenced by chemical composition (e.g., NH4+) and meteorology. Discuss the observed spring peak in NO3- alongside NH4+ trends and partitioning behavior of HNO3.
Lines 186–188: Provide a possible explanation for the observed concentration increase. Is there a local emission or meteorological reason?
Lines 190–191: The COVID-19 shutdown period seems to have ended before sampling. If not, describe local shutdown policies in the methods section.
Lines 191–195: This statement implies minimal local influence. Consider emphasizing regional transport instead.
Lines 193–195: If COVID-19 restrictions impacted emissions, explain why no corresponding impact is evident in your data.
Lines 213–215: The data do not clearly show seasonal differences. Spring appears elevated, but other seasons are similar. Please clarify the interpretation.
Lines 216–218: Why was NO2 formation not included in the discussion? It's central to Δ17O(NO3-).
Lines 220–223: Consider including a plot of this data.
Lines 240–241: A more detailed description of how alpha was determined is needed. A supplementary figure showing alpha and estimated Δ17O(NO2) over time would strengthen this section.
Lines 244–246: The pathway model lacks independent validation. Aside from the mixing model (which appears underconstrained), consider constructing a simple box model to test the plausibility of the proposed NO3- formation routes.
Lines 252–253: In most continental urban settings, NO3 + VOC is a minor contributor to aerosol nitrate. Reassess this conclusion in light of existing literature.
Lines 254–256: If VOC data are available, use them to estimate the contribution of the NO3 + VOC pathway. Also, at this elevation, stratospheric intrusions may occur. Could this be a source of high Δ17O nitrate?
Lines 283–284: The logic in this sentence doesn’t follow clearly from the preceding text. Please revise.
Lines 286–287: High O3 levels increase Δ17O(NO2), which strongly influences Δ17O(NO3-). This should be acknowledged explicitly.
Lines 295–297: Why is VOC assumed to be the only contribution from the biomass burning plume? Could oxidized nitrogen compounds also be transported?
Lines 315–320: The diurnal nitrate interpretation doesn’t account for the atmospheric lifetime of NO3-. Some residual NO3- from nighttime may persist into the daytime. Please consider this in the discussion.
Lines 319–320: NO3 and N2O5 chemistry is unlikely to significantly contribute to daytime NO3- formation due to their short lifetimes in sunlight. Please calculate and discuss the expected lifetime.
Figure 5: Uncertainty/error bars are needed for all pathway contributions. These are model-derived estimates with inherent uncertainties and should not be presented as precise values.
Lines 355–364: Much of the mechanistic discussion here is speculative. Consider using a simple model framework (e.g., kinetic or box model) to evaluate the chemical feasibility of the proposed pathways.
Citation: https://doi.org/10.5194/egusphere-2025-164-RC2
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