the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jan 2025
Vertical distribution of halogenated trace gases in the summer Arctic stratosphere determined by two independent in situ methods
Abstract. Many halogenated trace gases are important greenhouse gases and/or contribute to stratospheric ozone depletion, yet their spatial distribution and temporal evolution in the stratosphere remain poorly constrained. We here present a new high-altitude dataset of a large range of these gases. The results are based on a large balloon flight in the Arctic in summer 2021. Air samples were collected using a passive (AirCore) as well as an active (cryogenic) technique; the former being the largest AirCore flown to date, thus enabling the quantification of an expanded variety of halogenated gases. The evaluation of the results demonstrates good comparability in most cases, but also revealed strengths and weaknesses for both sampler types. In addition, we show examples of the scientific value of this data, including the identification of air masses likely originating from the Asian Monsoon region, and the derivation of the average stratospheric transit times (i.e., the mean ages of air) from multiple tracers.
Competing interests: At least one of the authors is a member of the editorial board of Atmospheric Measurement Techniques
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2024-4034', Anonymous Referee #1, 12 Feb 2025
Review of “Vertical distribution of halogenated trace gases in the summer Arctic stratosphere determined by two independent in situ methods” by Laube et al.
General comment
This manuscript presents measurement results from two types of balloon-borne air samplers to study vertical distribution of halogenated trace gases in the stratosphere. The authors combined conventional and newly developed air-sampling and analytical techniques which required very careful and complex steps. Although there seem to be issues for some species, I would like to send congratulation to the authors for the reasonable success. The manuscript is well structured and written. I recommend publication of this manuscript after the minor comments below are considered.In section 2.3, the authors described possible diffusion effects that deteriorate vertical resolution of the measurements. Since the diffusion is molecular dependent, it means that altitudinal resolution is different from species to species, as the authors explains with Figures S1 and S2. I wonder if the curves in these figures could be interpreted as uncertainties of the altitude assignment of the measurement data and if this was considered in presented figures. The authors also infer that the diffusion would more affected CO2 and CH4. As the measurement data of these species have been presented in the companion paper (Shuck et al. 2025) (and more commonly in previous AirCore measurements), it might be worth presenting magnitudes of the diffusion effect in CO2 and CH4 as well.
The section also presents the interesting and decisive component of this study, subsampling. Because I have found information in the reference (Laube et al. 2020) is limited, I understand that this paragraph (P2 L118ff) will be reference for the authors’ future measurements. Although I likely followed their system and steps, it might have be much easier and clearer for readers and future studies if a schematic figure of the subsampling system is presented.
As written in the manuscript (Figure 3 caption), presented for the CRYO samples are from the averages of measurements at both FZJ and GUF. I would consider that presentation of the data from both labs in the figures might be another option. It would also allow profile comparison between the CRYO and MAC data from the single lab (FZJ), which could highlight difference from sampling techniques only. In addition, some general explanation about agreement between the data from both labs, at least for representative species in central discussion in this study, might be added. One option could be to move the supplement text (P1) to Section 2.4. Table S2 needs more reader-friendly sentences in its caption (see the comment below).
Specific comment
Title: I am not sure if these are called “in situ” methods. The methods are not certainly remote sensing, and air samples were collected in situ in the stratosphere, but the methods include subsequent lab analyses. I am confused at least with “determined by…in situ methods”.
P3 L68: “the low weight” it would be good to mention to the exact weight here
P3 L73: “withing” to “within”
P12 L337: For CFC-12, the figure shows the data deemed as contaminated in blue, while the valid data are in orange. How did the classification work? There are the data with clear excursions from an expected profile. On the other hand, there are also those apparently aligned near the CRYO data but considered as contaminated.
Table S2: needs more explanations in the caption. What e.g., y.y.y means should be explained in the caption in a full sentence (not as part of the table). What the colors indicate? What are number columns at right? Now readers have to read between the lines.Citation: https://doi.org/10.5194/egusphere-2024-4034-RC1 -
RC2: 'Comment on egusphere-2024-4034', Anonymous Referee #2, 25 Mar 2025
This study investigates two different methods for sampling the atmospheric chemical composition from the mid troposphere to the mid stratosphere up to 30 km. The high altitude balloon borne cryogenic whole air sampler for stratospheric sampling has been used multiple times in the past. The AirCore sampler has been used less frequently for stratospheric sampling and, as such, required additional evaluation. This manuscript provides a detailed evaluation of the sampling and analytical methods used for both samplers. The manuscript is very well written and is clear in the detailed evaluations of the sampler performances. Given the difficulty in sampling the stratosphere, this work is exceptionally important, especially with the higher vertical resolution of the AirCore and the high cost of cryogen for the cryogenic sampler. As a next step, comparison of the cryogenic sampler and the AirCore with an in situ instrument that measures a subset of the organic halogens would be extremely informative. I commend the authors for their excellent treatment of the comparison of the two samplers and I recommend publication of this manuscript after addressing the minor comments/questions below.
Why were cotton filters used to remove ozone, e.g. have they been used in the past or evaluated relative to other potential methods? Were they tested in the lab to determine their ozone removal efficiency?
Were the interior of the cryogenic sampling canisters coated with anything to improve trace gas inertness?
It's not clear to me how molecular diffusion is taken into account when evaluating mixing in the tubes and how mixing is taken into account when determining sample altitude. Perhaps an equation in the supplemental material would be helpful.
Table S2, what do the colors mean and what do the n’s and y’s mean?
An interesting feature in the altitude profiles is the near constant mixing ratios between about 11 and 14 km rather than decreasing rapidly just above the cpt. I realize the focus of the manuscript is on the comparisons, but it might be worth a mention of what you think is responsible for this part of the profiles.
Regarding the use of a constant value of 0.7 for the squared width of the age spectrum with mean age, it has, as the authors point out, been used in the past. However, according to Ray, et al., 2024 “Most studies of age of air have used values of 𝑅 ranging from 0.7-1.25 years based on model estimates (e.g., Hall and Plumb, 1994; Volk et al., 1997; Engel et al., 2008). However, with a better understanding of the effect of the exponential tail of 𝐺 for 𝑡’ > 10 years, the model estimates of 𝑅 have increased to values of 1.5 years or more with considerable variability in the stratosphere (e.g., Diallo et al., 2012; Ploeger and Birner, 2016; Fritsch et al., 2020).” I’m not suggesting the authors change the value they use, but they might consider qualifying their use beyond it’s been used in the past.
Citation: https://doi.org/10.5194/egusphere-2024-4034-RC2 -
RC3: 'Comment on egusphere-2024-4034', Anonymous Referee #3, 29 Mar 2025
This paper describes the results of a large stratospheric balloon flight in August 2021, including a large air-core sampler for halogenated compounds and a cryogenic whole air sampler, along with other instruments. Results from the two methods for halogenated compounds are compared and the relative strengths of each are described; results from the other instruments are described in a companion paper.
This is an important advance in stratospheric sampling and well suited for publication in Atmospheric Measurement Techniques. It is to my knowledge the first time that AirCores have been flown on a large balloon payload together with established instruments for halogenated compounds that sample air at reasonably well-defined altitudes. The large number of compounds measured and quantified is also quite impressive. The results of the flight were generally very good, and some important lessons were learned and discussed concerning both techniques and how to carry out this type of experiment. Many of the details of the balloon launch itself are in the companion paper by Schuck et al.; this is fine but hopefully the two papers will appear close in time (or even simultaneously, though in this day of electronic publishing the idea of “back-to-back” publications is perhaps a thing of the past).
The techniques used in this balloon flight are generally described well, the figures are clear, and the results are novel. In particular, the use of the “MegaAirCore” with extensive subsampling is an important step forward and resulted in a large number of samples in the stratosphere, at altitudes up to 30 km with excellent vertical resolution. My only comment here is that previous work is barely mentioned. Laube et al. 2020 used AirCore results together with aircraft measurements and models to probe changes in the stratospheric distributions of halogenated compounds, and Li et al., 2023 described a technique to measure some of the same molecules directly from an AirCore into a gas chromatograph. Are there any other relevant publications on halogenated molecules in AirCores? How does this new publication build on previous work (much of it by the same author)? This may only need a few sentences or a short paragraph to address in the Introduction, or Section 2.2; no need to make this manuscript much longer.
I also found Figure 3 (one of the most important figures in the manuscript) and the text surrounding it a little confusing. First, the legend and caption refer to the CRYO samples with the O3 scrubber as open squares, but the squares on the right panel are not completely open, and seem to have something inside (in contrast to the legend). Or are there regular (non-scrubbed) data points inside as well? This seems to be the case in Figures 4 and 5 as well. This is also relevant to the statement about agreement of CFC-11 on line 302. However, the CFC-11 CRYO data points at similar altitudes are very close to each other, and the explanation on lines 306-307 seems entirely reasonable. The low mixing ratios of SF6 near 24 km in both instruments are very interesting. I don’t see how the (relatively small at this altitude) correction for residual fill gas could allow the recovery of this structure; perhaps the mixing did not occur completely or is less efficient than expected. This also calls into question the explanation for the disagreement between the two methods near 20 km, which would be caused by mixing of the air collected at 24 km with very low SF6 (lower than the cryo-sample) and air with higher SF6 collected at lower altitudes. The points i and iii starting at line 270 about the larger diameter of the AirCore and the larger sample volume are well-taken, however. The gradient in CFC-11 from 19-27 km is more readily explained in terms of mixing, since the MAC gradient is less steep than for the CRYO data. But for SF6, the gradient in the MAC data from 19-24 km is actually steeper than for CRYO data (if you believe the minimum at ~24 km, which is observed by both techniques). This may not be a terribly important point; both data sets look very interesting and the comparison and combination of the two have led/will lead to additional insights into both techniques as well as possible changes in how to plan for and conduct balloon flights.
Specific comments:
P.2, line 41 – Are there six or seven gases measured in AirCores in Laube et al., 2020? And there are really only five halogenated gases measured in Li et al., 2023 (six gases in all, but one of them is N2O).
l.49 Schuck et al., 2024 also contains a few more details about the balloon flight, etc.; the authors could add something like “, along with additional details about the balloon flight” at the end, or something like that. (For example, I was curious how long the balloon stayed at altitude, and that is found in Schuck et al. No need to repeat it here.)
P.3, l. 78-95, The MegaAirCore is very interesting. Can it be flown by itself on smaller balloons?
P.6, l. 179 I thought the addition of the perfluoro amine compound was an interesting (and seemingly very helpful) innovation, along with figuring out how to account for its possible changing detector response (on the following page). As long as it is not “sticky” in any of the tubing or valves, etc. it should work fine. It certainly was useful in pointing out the possible leakage in the last loop of the first subsampler (P.7, l. 211-215), and then (I think) correcting for it.
P.8, l. 218-221 – Was there a pressure gradient between the two adjacent loops in this test? If not, it seems different than conditions that may have led to the outlier sample described on the previous page. Were these results used to correct the trace gas concentrations in that sample? And in Figure 1, P. 8, does the apparent fill gas fraction change abruptly on the logarithmic scale from sampler 1 to sampler 2? In any case, the explanations all seem reasonable.
P.9, l. 245-252 It seems that it might be worthwhile to calculate the altitudes in the MAC using the Tans method, if only for comparison.
P.11, l. 303 “agreement within two standard deviations”, compared to the Figure 3 caption, lines 292-293 “horizontal error bars are equivalent to 1 standard deviation”. These may both be true, but that doesn’t seem to be the clearest way of communicating this information. I agree with the point on l. 306 about “slightly different altitude ranges”; for a compound like CFC-11 with a large gradient and excellent measurement precision small (real) atmospheric variations are quite possibly the cause of some apparent disagreement.
P.12, l. 330-331 – I really don’t understand what “1/3 loops” means. Is it that in one third of the loops, the mixing ratio increased by 15%. And why “only”? And in the next line “+10 and +16%”?
l. 337-338 The contamination for CFC-12 does not look random at all. It only appears in the middle section of the data (~10km-23km). Or do you mean that within one (or two) subsamplers the contamination is random, and some of the cyan points are actually not contaminated?
P.15, l. 386-387 Can the distribution and mixing ratios of the surface origin tracer be diagnosed in the CLaMS model on a faster than daily timescale? If model output is once a day (at 12 UTC), how can one know that it changes “significantly within a few hours”? Perhaps I am mixing up model and measurements (or knowledge from past aircraft campaigns); but if so then the text could be made clearer.
P.16, l. 414 Shouldn’t the Ray et al., 2017 paper on the lifetime of SF6 go here? I am not sure if this is the correct spot for Andrews et al., 2001 either (I did not go back and look at that one), though it should certainly be included on line 416 (as it is).
l. 419-420, For this methods paper, using a constant value of 0.7 is fine, though it surely varies, at least somewhat. And the sentence “Following previous studies…” seems redundant and unnecessary.
P.17, l. 433 Uncertainties in the SF6 tropospheric trend do not add a year of uncertainty; perhaps for other trace gases used they can. I would say that the parameterization of the age spectrum could add at least a month; if it were only a month that would be great.
P.19, l. 472 - Is the vertical stability really particularly high? Is it different than other similar high latitude profiles in summer, or is this time/region typically very stable? If temperature actually increased with altitude, the atmospheric stability would be even stronger.
Technical and proofreading comments:
P.3, l. 66 This sentence could just start with “AirCores were invented [or developed] at the NOAA…”
P.6, l. 177-178 – You might consider putting the word “exceptions” in the earlier sentence to avoid somewhat contradicting yourself. Something like “…trace gas-free N2, with two exceptions. These were: i) small …”
P.9, l. 233 – It should probably be “Cumulative” instead of “cumulated”, but more importantly, the fill gas fraction is not cumulated. Only the subsampled volume is cumulative. At least that is how the figure appears to me.
l. 254 “Figure 3 shows the vertical profiles…” is good enough.
P.10, l. 279-280 “Similar differences were observed for CH4 when comparing CRYO samples and smaller AirCores (Schuck et al., 2024).”
P.19, l. 476 “as due to”
Citation: https://doi.org/10.5194/egusphere-2024-4034-RC3
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