Huangfu et al. present airborne measurements over Beijing and Baodong to investigate the vertical distributions of VOCs above these cities. They identified that VOC concentrations were typically higher near the surface, with some exceptions during some flights, informing the sources, removal, and transport of these VOCs. Higher VOC concentrations were observed above Baoding across the whole vertical profile, pointing toward different regional emissions and photochemical processing. They utilize VOC ratios to determine the emission sources contributing to elevated VOC concentrations at higher altitudes during some flights, suggesting elevated industrial and biomass burning VOCs. This manuscript provides information on the vertical and spatial distribution of VOCs over highly populated cities, informing sources, removal, vertical transport, and photochemical production. I recommend this manuscript for publication following edits in response to the following comments.

The manuscript can be improved by including more context and discussion throughout the text, as identified by several of the specific comments below.

 Specific Comments:

Line 16 – Include September 2017 and July 2019 so readers know the time of year for these observations.

Line 35-39 – This sentence is somewhat long and may be difficult to read for some people. I suggest splitting into two statements.

Line 73 – The authors should provide additional context regarding the previous airborne VOC measurements to further bolster the importance of their study. For the 3 listed studies, which cities were the VOCs collected above? What were the time/spatial/vertical resolutions Which VOCs? Relevant conclusions to your study?

Line 105 – Can you provide more information on the “sampling device”?

Line 134-135 – I suggest moving “The vertical profiles of meteorological factors are shown in Figure S1.” Up to ~line 118 along with measurements of meteorological parameters. Then revise the statement here (line 134-135) to “The vertical profiles of potential temperature…”

Section 2.2 – I believe more VOC quantification details are necessary.

• When / how often was the PTR-MS calibrated? Zeroed?
• Which standards were used to quantify the species listed in Table S1? For example, were monoterpenes calibrated based on the sensitivity of one isomer? Was MVK&MAC calibrated using an MVK standard, MACR standard, or both? This info could be included in Table S1 or elsewhere in the SI.
• Were any measurements near the limit of detection? If so, the LODs should also be listed in the SI. If not, a brief statement in the methods would be helpful.
• Typical propagated uncertainties for reported measurements should be included. Depending on the VOC, humidity would account for up to a 10% contribution (line 115) on top of the measurement error and error propagation through the calibration.
• Have you considered any interfering species in your measurements? For example, ethylbenzene impacting benzene signal, or aldehydes from cooking emissions impacting isoprene signal? See e.g., Coggan et al. (2024).

Line 141 – Include the distance between the air quality station and the airport.

Line 144 – Please include the full term for NAQS before the acronym. What are the Level II threshold values? It would be useful to add corresponding horizontal bars to Figs. 2 and S8 to show the reader when ground-level exceedances happened relative to the flights.

Line 152 – What do the errors represent? The standard deviation of all measurements across all September 2017 flights?

Line 154 – Provide more context regarding the other studies for the reader. Include the time of year when the previous measurements were made, that they were ground-based, and the measurement techniques. For the 2010 study, are you comparing against the PTR data or the GC data, since that could skew your comparison?

Line 156 – Provide more context to explain why the 2010 measurements were comparable to your measurements. Later you compare to the 2017 IAP measurements and explain the differences based on measurement location and traffic/industry influence. Please do the same for this comparison to 2010. Were the 2010 measurements also primarily influenced by industry?

Line 160 – Provide full term for IAP before acronym.

Line 160 – Add “and”: “… meteorology tower and represented…”

Line 163 – Regarding traffic vs industrial emissions when comparing to the 2017 IAP measurements: Fig. 3c suggests a greater enhancement in the C8 and C9 aromatics compared to benzene and toluene. You should discuss this observation in the context of the literature, comparing distributions of the aromatics you observed against typical traffic emissions and industrial solvents (as you do with MEK later, line 137).

Line 167 - Perhaps industrial tetrahydrofuran could also contribute to “MEK” (C4H8OH+)?

Line 174 – Somewhere, it would be useful to briefly discuss your vertical distributions in the context of the other airborne VOC studies you cited in the introduction (lines 73-76). Even though these studies are in different cities, do the vertical profiles generally agree? Why or why not?

Line 191 – For the composite profiles, the increase in styrene (and the OVOCs) with altitude appears to be driven by one flight (Fig. S2). Currently the discussion implies that styrene, acetonitrile, and the OVOCs were increasing with altitude for all flights. Add a clear explanation about the impact of the one flight on the composite profile.

Line 194 – Following from my previous comment, it would be useful to distinguish that the analysis of biomass burning and industrial sources in Section 3.3 refers to different profiles. Following my initial reading, I assumed the discussion would be in reference to a single profile.

Line 226 – I’m not sure what you mean by grey area in Fig. 4. Are you referring to the red shaded areas?

Lines 233-237 – Break this sentence into two parts. The “above the PBL” and “within the PBL” portions are two distinct points.

Line 268 – What was the predominant wind direction? Was Baoding downwind of Beijing or other cities, contributing to an accumulation of pollutants and the formation of photochemical products during transport?

Line 274 – Provide some discussion as to which VOCs had the higher and lower ratios between the cities and why. For example, C9 aromatics appear to be enhanced by a factor of ~3 while benzene is ~2 (Fig. 7). Differences in these aromatics suggest either (1) different emission sources dominate the aromatics between the cities, or (2) there is a greater degree of oxidation for the aromatic emissions in Beijing compared to Baodong.

Lines 285-286 – It appears that acetone was also slightly enhanced at altitude. Is there a reason it was excluded in the text?

Line 304 – The ratio analysis compares against fresh emissions. You should comment on the lifetime of styrene vs benzene in the context of transport times from the surface, and how that would affect your observed ratios.

Line 321 – Acetonitrile and benzene don’t appear to correlate well in Fig. 9d, suggesting different sources. Please include some discussion. Did acetonitrile correlate better with a different VOC which could better support the biomass burning (or other) source?

Table 1 – It looks like most columns were partially cut off (e.g., column 2 lists “Beijin”, the date column lists Sep. 09, “201”). It’s not clear to me if the error is on my end, but please double check.

Tables 1 and S2 – I assume the times are local? Please clarify.

Figure 8 – The caption and legend mention two types of industrial emissions and the corresponding shaded areas. I see three shaded areas (light orange, dark orange, purple). Does the dark orange region in the middle represent an overlap between the two types of industrial emissions? Please clarify in the caption.

Table S1 – Remove the extra “c” in “C8 acromatics”

Table S2 – I suggest adding to the caption “as discussed in Section 2.3” or similar. I also suggest reiterating the 10% uncertainty (mentioned on line 136).

Table S3 – Is there a purpose for the superscript “c” above MEK?

Tables S3 and S4 – You provide summary statistics for overall observations (Table S3) and below the PBL (Table S4). I suggest adding another SI table for above the PBL since you use those values in Fig. 5.

References

Coggon, M. M., Stockwell, C. E., Claflin, M. S., Pfannerstill, E. Y., Xu, L., Gilman, J. B., Marcantonio, J., Cao, C., Bates, K., Gkatzelis, G. I., Lamplugh, A., Katz, E. F., Arata, C., Apel, E. C., Hornbrook, R. S., Piel, F., Majluf, F., Blake, D. R., Wisthaler, A., Canagaratna, M., Lerner, B. M., Goldstein, A. H., Mak, J. E., and Warneke, C.: Identifying and correcting interferences to PTR-ToF-MS measurements of isoprene and other urban volatile organic compounds, Atmospheric Meas. Tech., 17, 801–825, https://doi.org/10.5194/amt-17-801-2024, 2024.

In this manuscript, the authors present aircraft-based vertical observations of volatile organic compounds (VOCs) measured using high-time resolution mass spectrometry. Despite the technical challenges associated with such data collection, the authors provide valuable insights into the complex evolution of VOC species under the combined effects of surface emissions, chemical removal, and regional transport. The topic is certainly of interest, but the analysis does not yet sufficiently support the proposed conclusions. The existing data requires further exploration, and additional information is needed to strengthen the findings. There are also several aspects that require clarification. Below are my specific comments and suggestions.

Specific Comments:

1.My primary concern is that the current analysis does not robustly support the statement about the “complex VOC species evolution under the joint impacts of surface emission, chemical removal, and regional transport.”

1.1 Features of VOC species profiles:

The VOC species selected for the study are appropriate, but a more thorough classification and combination would enhance the analysis. For instance, methanol and acetonitrile are largely chemically inactive and primarily derived from primary emissions. In contrast, acetaldehyde is active and predominantly the result of secondary chemical transformations.

To better address the relationship between VOC species profiles, emissions, and chemical removal, I recommend replotting Figures 4, 6, 8, and 9 to incorporate the ratios of VOC species based on their chemical reactivity (see Zhu et al., 2025) or the ratios of primary vs. secondary species (see Yang et al., 2024). Including CO and NOx profiles would also provide useful tracers for distinguishing between inactive and active species, provided the data are available.

1.2 Regional Transport of VOCs:

For the analysis of VOC regional transport, I suggest incorporating synoptic charts and diagnosing vertical velocity (including vertical transport and advection). In lines 192 and 222, the CO concentrations at ~3500 m and 2000 m could serve as effective tracers for regional transport.

Overall, the manuscript would benefit from better organization and additional data analysis to substantiate the points raised by the authors.

2. In lines 129-132, the criteria for determining the HPBL are necessary to include, but the equation for potential temperature seems unnecessary in this context.

3.The analysis presented in lines 230-243 is intriguing. The highest ratio of ethanol between below and above the boundary layer (BL), compared to acetaldehyde, acetone, and MEK, may reflect the more uniform vertical distribution of secondary VOCs (OVOCs) from various chemical sources. However, if the ratios of active species (e.g., styrene and monoterpenes) to inactive species (e.g., toluene and benzene) were compared, a different ratio feature of OVOCs/ethanol might emerge. These features could provide further meaningful insights that are directly related to the “complex VOC species evolution under the joint impacts of surface emission, chemical removal, and regional transport.”

References:

Zhu et. al., Revealing Distinct Photochemical Ages within the Vertical Boundary Layer and Seasons by Observed VOC Species Ratios, ACS ES&T Air 2(7), DOI:10.1021/acsestair.4c00319

Yang, S., et. al., Vertical Features of Volatile Organic Compounds and Their Potential Photochemical Reactivities in Boundary Layer Revealed by In-Situ Observations and Satellite Retrieval. Remote Sens. 2024, 16, 1403. https://doi.org/10.3390/rs16081403