Unexpected high volatile organic compounds emission from vehicles on the Tibetan Plateau

Huang, Weichao; Wang, Sihang; Cheng, Peng; Chen, Bingna; Yuan, Bin; Yu, Pengfei; Wang, Haichao; Ma, Nan; Li, Mei; Lu, Keding

doi:https://doi.org/10.5194/egusphere-2025-1835

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https://doi.org/10.5194/egusphere-2025-1835

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28 May 2025

| 28 May 2025

Unexpected high volatile organic compounds emission from vehicles on the Tibetan Plateau

Weichao Huang, Sihang Wang, Peng Cheng, Bingna Chen, Bin Yuan, Pengfei Yu, Haichao Wang, Nan Ma, Mei Li, and Keding Lu

Abstract. Vehicle emissions significantly affect atmospheric composition, but their behavior in high-altitude environments is still poorly understood. In this paper, we conducted a comprehensive survey of vehicle emissions from ten tunnels in the Qinghai-Tibet plateau, covering an altitude change of nearly 3,000 meters. The results show that the total emission factor of volatile organic compounds (VOCs) increases with elevation, mainly due to a significant increase in evaporation emissions. Source analysis shows that evaporation emissions account for 67 % of VOC and are significantly higher than 24 % of exhaust emissions. This model differs from observed patterns at low elevations, where exhaust emissions are predominant. We believe that low pressure is a key factor in enhancing the release of evaporative VOCs at high altitudes. The study provides important new insights into vehicle emission mechanisms in the highlands and highlights the need to consider specific environmental conditions and fuel evaporation when developing emission control strategies at high altitudes. The promotion of electric vehicles (EVs) at high elevations such as Tibet is a win-win solution for reducing emissions and making use of the abundance of renewable energy sources in Tibet.

Received: 17 Apr 2025 – Discussion started: 28 May 2025

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Weichao Huang, Sihang Wang, Peng Cheng, Bingna Chen, Bin Yuan, Pengfei Yu, Haichao Wang, Nan Ma, Mei Li, and Keding Lu

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-1835', Anonymous Referee #1, 28 Jul 2025
The manuscript provides a unique and valuable dataset on vehicular VOC emissions from the Tibetan Plateau, highlighting the significant role of low atmospheric pressure in enhancing evaporative emissions at high altitudes. This addresses a critical knowledge gap, impacting emission inventories and mitigation strategies. The study's comprehensive methodology is commendable. Addressing the detailed comments, especially regarding the sampling strategy’s fit with EF calculations and the comprehensive validation of source apportionment, will significantly strengthen the manuscript and its impact.
Lines 74-77: This approach of sampling "accumulated air masses" seems to contradict the standard method for calculating fuel-based emission factors (EF) using simultaneously measured CO and CO₂ (Eq. 1), which typically assumes a well-mixed plume representing instantaneous emissions. Please provide a more detailed and rigorous explanation of how the sampling strategy (capturing accumulated air via piston effect) aligns with the EF calculation method. This might involve discussing the length of the tunnels, travel speed, and how "accumulation" truly translates to the average emission.

Please clarify what “59 species, including those common to this work and other research endeavors” (Line 126) precisely means. Is this a consistent subset used for comparison across studies?

The observation that PMF-resolved tailpipe exhaust (Factor 3) shows “relatively poor similarity” (38º) with chassis dynamometer-tested gasoline vehicle exhaust (Figure 3c, Table 1). The authors attributed this potentially to “the influence of diesel vehicles, as well as potential influences from other sources.” Can the authors quantify the likely contribution of diesel vehicles in these tunnels or explain why their influence leads to such a discrepancy? Were diesel vehicles included in the comparison dynamometer data?

The presented average EFs and ERs come with relatively high standard deviations (e.g., EF of 3.3 ± 3.1 ug·kgfuel⁻¹, ER of 87 ± 92 ppb/ppm). Does it reflect differences in vehicle types, driving conditions within tunnels, or other factors? How does this high variability impact the statistical significance of the observed altitude trends?

Comparisons to low-altitude tunnels in Hong Kong, Taiwan, Tianjin, Henan, and Haikou are valuable. However, fleet compositions, fuel standards, and driving conditions can vary significantly across these regions and study years. Briefly acknowledge these potential differences and how they might affect direct comparisons.

The absence of an altitude-specific distribution for the CO/CO2 ratio is interesting given theoretical expectations. While attributed to “other factors”, please elaborate on this, potentially with supporting evidence explaining why altitude isn't the dominant influence.

Please clarify what “Direct measurement” refers to in Table 1. Is it the average source profile from all tunnel measurements?

Please provide a clearer “good consistency” threshold from the literature (e.g., <20° or <25°) when discussing the 38°for tailpipe exhaust, to better contextualize the PMF factor.
Citation: https://doi.org/10.5194/egusphere-2025-1835-RC1
RC2:
'Comment on egusphere-2025-1835', Anonymous Referee #2, 30 Jul 2025
This pioneering study reveals unexpectedly high VOC emissions from vehicles on the Tibetan Plateau, with evaporative sources dominating due to low-pressure enhancement—novel and rigorously validated findings. The experimental design (multi-tunnel mobile measurements) and analytical rigor (PMF/NNLS source apportionment) are exceptional. While the proposal for electric vehicle (EV) adoption in Tibet offers a promising pathway for emission reduction, it overlooks severe battery efficiency decay in low-temperature high-altitude environments, weakening policy relevance. I recommend softening the emphasis on this aspect.

Specific comments:
Line 38-39: This study emphasizes the importance of non-tailpipe emissions such as evaporative emissions. Therefore, it is necessary to add some descriptions about non-tailpipe emissions. Do non-tailpipe emissions only include the evaporation of fuel and solvents? Furthermore, what does "solvents" refer to? Is it windshield wiper fluid, or automotive surface/interior coatings? How are they emitted? Are they emitted intentionally by humans, or are they continuously emitted like fuel? The literature support provided here may not be sufficient.

Line 50: Do the terms Qinghai-Tibet plateau and Tibetan Plateau used in this article convey the same meaning or are there any differences between them?

Line 53-56 This part seems more like a statement of conclusion, and it is not recommended to place it in section 1 Introduction. It is suggested to revise it.

Line 60 and many others：Either use Table x / Figure x or Tab. x / Fig. x. Unify throughout the manuscript according the journal's format requirements.

Line 73: This statement states that analysis will be conducted a week after sampling. Considering that the entire mobile observation spans a long distance, how many days did the sampling last, and are all samples taken at the same time in the same tunnel?

Line 74-75: The piston effect is generally aimed at vehicles traveling in the same direction within the same tunnel. The author mentioned sampling at the rear of the tunnel. Has this study measured tunnels with vehicles traveling in both directions simultaneously? If so, how are these types of tunnels sampled? In addition, please also check the correctness of the references cited in the literature (Chung and Chung, 2007).

Line 119-122：When comparing plateau vs. plain EF/ER, the authors selected only two tunnel examples (Hong Kong 50m and Taiwan 330m). Please include additional examples to enhance representativeness.

Line 137：Inconsistent spacing around “±” symbols.

Line 137：Replacing "contributing to" with "accounting for".
Citation: https://doi.org/10.5194/egusphere-2025-1835-RC2
RC3: 'Comment on egusphere-2025-1835', Anonymous Referee #3, 11 Aug 2025

Hereby I offer only one comment to complement other reviewer's comments on a rather important aspect of the paper. It is not true as the authors stated that "However, as far as we know, the influence of pressure on evaporative emissions has not been documented, posing a challenge to our comprehension of vehicular emissions in high-altitude regions.".
In fact, this effect of evaporative emissions on altitude has been well documented, e.g., in MOVES model by the US EPA (US EPA, 2024, p20, Equation 3-6). Therein, the effect is clearly considered, i.e., "Tank vapor generated depends on the rise in fuel tank temperature (F), ethanol content (vol. percent), Reid vapor pressure (RVP, psi) and altitude". And there is also a table comparing model parameters appropriate for Denver, a city that is ~1700 meters above sea level versus those for at sea level.
Therefore, it is crucial for the authors to put their study in the context of what is already known, by changing the statement above to reflect the state of the science, and, more importantly to reconcile the measurement inferred altitude effect with those documented in the literature.
References
USEPA, 2024, Evaporative Emissions from Onroad Vehicles in MOVES5, November 2024, EPA-420-R-24-014, https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P101CTZI.pdf (last accessed 8/10/2025).

Citation: https://doi.org/10.5194/egusphere-2025-1835-RC3

Weichao Huang, Sihang Wang, Peng Cheng, Bingna Chen, Bin Yuan, Pengfei Yu, Haichao Wang, Nan Ma, Mei Li, and Keding Lu

Supplement

https://doi.org/10.5194/egusphere-2025-1835-supplement

Weichao Huang, Sihang Wang, Peng Cheng, Bingna Chen, Bin Yuan, Pengfei Yu, Haichao Wang, Nan Ma, Mei Li, and Keding Lu

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Short summary

We studied vehicle emissions from 10 3000-metre tunnels in the Qinghai-Tibet plateau. Since low pressure causes fuel evaporation, emissions of volatile organic compounds rise with elevation, unlike in low-altitude areas where exhaust gas is predominant. This indicates the need for specific emission control measures. Electric vehicles can use renewable energy in Tibet to reduce emissions. The study aims to understand emissions at high altitudes and guide cleaner transport.


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