the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Jul 2024
Field evaluation of a novel charge transfer ionization TOF MS for ambient VOC measurements
Abstract. A newly developed charge transfer orthogonal Time-of-Flight Mass Spectrometer (oToF-MS), was deployed for the first time in the field in order to evaluate its ability to perform online real-time measurements of volatile organic compounds (VOCs) for a prolonged time period. The study focused on urban air sampling and targeted a specific range of VOCs, namely: acetone, isoprene, benzene, toluene, and xylene. The measurement campaign took place from May to August 2023 at the suburban area in Athens Greece. The measured VOC level were consistent with those reported in previous summer campaigns suggesting that the instrument did not face any unexpected problems moving from the laboratory to the field. The variability of the measurements of the various VOCs was used to gain insights about their sources. Transportation emissions were a dominant source of the BTX compounds (benzene, toluene and xylene). Acetone and isoprene were emitted by both anthropogenic and biogenic sources. During the summer biogenic sources were responsible for most of the isoprene in the site.
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RC1: 'Comment on egusphere-2024-2126', Anonymous Referee #1, 13 Aug 2024
The paper titled "Field evaluation of a novel charge transfer ionization TOF MS for ambient VOC measurements" discusses ambient measurements results from a newly designed and built TOF MS that was deployed in Athens. The instrument details are primarily discussed in a previous paper and here the authors focus on validating the instrument for field measurements. For this purpose, other traditional instruments were co-located at the measurement site with which the results are compared. The paper specifically focuses on 5 VOCs that generally have well-defined anthropogenic or biogenic source contributors.
It is straightforward to read, which is appreciable. However, there are several grammatical errors across the manuscript that suggests the writing has not been thoroughly vetted by the authors. I would highly recommend that the authors review their writing more closely to eliminate such errors. For e.g., "sear" in line 24, confusing use of "in" vs. "at" vs. "of" at several instances etc. Periods are missing at the end of some sentences.
I also felt that the introduction builds very limited ground for the need to develop a new mass spectrometer. There should be some more discussion of what already exists, what is it that is missing and why this mass spectrometer is important. Perhaps this is discussed in the previous instrument-focused paper but some discussion is necessary here as well in my perspective since measurements from a new instruments are discussed.
Line 187: It is not clear what an "external calibration" means. Please provide more detail. I understand that ambient temperature fluctuations can have an effect on the flight path length but how was the instrument externally calibrated?
Line 197: "electron ionization" can be easily confused with EI. I think the authors mean charge transfer by dropping an electron. Please revise for clarity.
Line 199-200: The authors state that fragmentation pattern may be condition specific. Do they mean instrument-conditions-specific or ambient conditions-specific such as the influence of relative humidity? If ambient, please explain how providing some citations.
I have several questions about the data shown in figure 5:
Figure 5b: Why does MO-OOA follow BCff trends in the second half of the day and BCff does not correlate with benzene? Also benzene does not show a trend that would be consistent with evening commute hours. How can this be explained?
Figures 5d/e: BBOA and eBCbb don't seem to line up. BBOA peaks close to mid-night but eBCbb peaks around 8 PM. Acetone appears to level out in the evening irrespective of variations in eBCbb and BBOA. Please explain.
Line 308: Benzene is stated to align with traffic-related markers. However, when Fig 5a/b are compared, HOA and benzene don't align after an initial morning period.
Line 309-310: I am not sure if I would call R = 0.31 a medium correlation. I'd suggest this to be a minor to poor correlation even considering field situations. This also raises the question of why benzene and eBCff won't align, especially since the authors attribute aromatic species to be mainly emitting from vehicular emissions in line 324.
Lines 318-319: Isoprene's morning peak is attributed to biogenic emissions. However, such enhancement can also be seen in BBOA. How do these things reconcile?
Table 2: I would recommend showing correlations of each target compound with all external tracers instead of a select few to get a better of sense of agreements vs contrasts.
Figure 6: I suggest clearly marking the city center on each plot for readers' convenience. The spatial scale is also not clear on these plots. As a reader, I am not sure how far am I looking at the terrain in these plots for source contributions.
Figure S6 shows very interesting distributions for isoprene around Athens during May-August. Isoprene is very high near the city center (from what I understand where the city center is from these plots) in June but low in the mountainous regions in the south-east. The city center is somewhat unchanged in July but a high intensity area appears in the north-east in July with emissions upto 2.5 ppb. This exceeds the 1 ppb max scale when the mountainous sources dominate isoprene contributions in August. I am curious why the sources of isoprene are changing so dramatically over a short span of a couple of months. Are there multiple biogenic sources of isoprene all around Athens other than the mountains in the south-east and north-west? Or the wind direction is flipping around? The authors discuss this briefly in lines 336-342. Perhaps it might be useful to include a windrose plot in the SI.
Minor comment:
I suggest adding ionization reaction equations to section 2.1 to help make the details more comprehensive.
Citation: https://doi.org/10.5194/egusphere-2024-2126-RC1 - AC1: 'Reply on RC1', Olga Zografou, 12 Oct 2024
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RC2: 'Comment on egusphere-2024-2126', Anonymous Referee #2, 20 Aug 2024
This manuscript reports on the long-term deployment of a newly developed charge transfer orthogonal ToF-MS (oToF-MS) in Athens, Greece, for online VOC measurements. My major concern is with the scope of the manuscript. The introduction gives the impression that the primary focus of this study is to evaluate the long-term deployment of this new instrument. To achieve this goal, it is essential to compare the measurements to an established method, such as GC-MS or GC-FID. Unfortunately, this comparison is missing in the manuscript. The "Results and Discussion" section extensively discusses the sources and geographical origins of VOCs, which deviates from the main focus. If source apportionment of VOCs is indeed the primary focus, the analysis presented here is superficial, and several conclusions are not well supported. Therefore, my overall recommendation is to better define the scope, include the necessary measurements to support it, and remove the distracting analysis. I do not want to discourage the authors. The development of a highly sensitive instrument with ~20,000 mass resolution is very impressive. I hope to see more detailed characterization, evaluation, and results from this instrument in the future.
Other comments:
- Line 39: It is true that proton transfer causes less fragmentation than electron impact ionization. However, the strong electric field in the PTR still causes significant fragmentation, which introduces challenges in product identification. A recent study by Coggon et al.1 showed that fragmentation from higher-carbon aldehydes and cycloalkanes substantially contributes to m/z 69 and interferes with isoprene measurements.
- Line 71: Heated SS tubing may cause measurement interference. For example, hydroperoxides may convert to carbonyls on metal surfaces2. Again, this is why validation of VOC measurements by an independent instrument should be included in this study.
- Table 1: Are the relative abundances reported in this study from calibration or ambient measurements? As mentioned above, ambient measurements may have interferences.
- Figure 5: Why is benzene shown in the same plot as MO-OOA, as they do not have a strong correlation?
- Lines 346-349: The logic here is problematic. Comparing VOCs and OA factors is useful for understanding their sources. However, given that both have complex sources in the atmosphere, it is challenging to use one measurement to evaluate the reliability of the other. In other words, it is difficult to use "the relationship between VOCs and OA factors" to demonstrate "the successful implementation of the new oToF-MS."
Reference
- Coggon, M. M.; Stockwell, C. E., et al., Identifying and correcting interferences to PTR-ToF-MS measurements of isoprene and other urban volatile organic compounds. Atmos. Meas. Tech. 2024, 17 (2), 801-825.
- Rivera-Rios, J. C.; Nguyen, T. B., et al., Conversion of hydroperoxides to carbonyls in field and laboratory instrumentation: Observational bias in diagnosing pristine versus anthropogenically controlled atmospheric chemistry. Geophysical Research Letters 2014, 41 (23), 8645-8651.
Citation: https://doi.org/10.5194/egusphere-2024-2126-RC2 -
AC2: 'Reply on RC2', Olga Zografou, 12 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2126/egusphere-2024-2126-AC2-supplement.pdf
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