the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jul 2025
The TropoPause Composition TOwed Sensor Shuttle (TPC-TOSS): A new airborne dual platform approach for atmospheric composition measurements at the tropopause
Abstract. In this paper we introduce the new TropoPause Composition TOwed Sensor Shuttle (TPC-TOSS), which constitutes an advanced development of the AIRcraft TOwed Sensor Shuttle (AIRTOSS), introduced by Frey et al. (2009). As part of a tandem measurement platform with a Learjet 35A, both platforms were equipped with redundant instruments for collocated measurements of aerosol size distribution (Ultra-High Sensitivity Aerosol Spectrometer, UHSAS), ozone (2BTech model 205), cloud particles (Back-Scatter Cloud Probe, BCP), as well as relative humidity, temperature and pressure. To measure the exact position of the two platforms as well as the relative distance of the TPC-TOSS to the Learjet a Global Positioning System (GPS) is installed on both platforms. Two identical Inertial Navigation Systems (INS) further allow to monitor attitude angles (roll, pitch, and heading) and accelerations.
Laboratory tests before and ground tests as well as inflight tests during the intensive operation period show a good agreement of the ozone and temperature measurements of better than 4.2 ppbv + 1.1 % (ozone) and 0.5 °C (temperature) at a noise level of ± (2 ppbv + 0.5 %) for 2 s data (ozone) and 0.1 K for 1 Hz data (temperature). Stability of the ozone monitor mounted in the TPC-TOSS has been tested and is estimated to be 2.2 ppbv (offset, 1 σ) and 0.7 % (gain, 1 σ), respectively, based on the drift of offset and gain during regular calibrations between measurement flights in the two weeks operation period.
The new TPC-TOSS was successfully flown during the TPEx I (TropoPause composition gradients and mixing Experiment) mission in June 2024 and performed four flights covering the altitude range between 6 and 12 km. The tropopause was crossed several times as evident from different temperature and ozone gradients as well as gradients of the aerosol number density. With the setup we are able to resolve transient stability and composition gradients ranging from almost zero or even negative to strong positive gradients of up to 25 K km−1 for potential temperature and from inverted to strong positive vertical gradients of ozone of up to 800 ppbv km−1, respectively. These gradients are caused by transport and mixing due to convection or shear induced turbulence at the tropopause.
Competing interests: At least one of the (co-)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 paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-3175', Charles Brock, 10 Jul 2025
This is a nice manuscript describing the use of a novel sensor package that is towed behind an aircraft on a cable. Case studies that demonstrate the use of this technique in the tropopause region where there can be very significant but transient structures that produce strong vertical gradients are presented. The manuscript is well written and generally clear, and the subject matter is appropriate for publication in AMT. Minor revisions are needed to address a few questions, correct technical issues, and provide additional information.
1) Table 2: Please provide a column for instrument uncertainties at the stated sampling frequency. I believe the SkyPOC particle size range is misstated; Bundke et al report a lower detection limit of 0.25 µm.
2) Section 3. At some point here I would like a brief discussion of how the cable system works and how far the TPC-TOSS module can be lowered. What is the total cable length and the typical vertical separation? This is evident only in graphs. What is the range of deltaZ (or cable length) that could be used safely? Alternatively this could go in Section 5.1.
3) Line 214. Change to, "the addition of insulation to protect the instrument by maintaining temperatures above 0 degrees C." Is this an arbitrary temperature limit or would the ozone instrument still function at colder temperatures? I ask because tropical missions near the tropopause might see much colder temperatures than this (if ~13-14 km altitude could be reached).
4) Figure 6b. What is the standard deviation of the Gaussian fit? This would inform as to total instrument variance.
5) Line 343. Surprising use of imperial length units. I thought this was strictly a problem in the U.S.!
6) Line 354 "peeks" -> "peaks" and line 302 "week" -> "weak".
7) Line 382 "Atomizer" -> "atomizer"
8) Line 395. Reference to Fig. 17 before Fig. 12. Generally figures need to be cited in order. You could place the bin diameters as vertical lines in Fig. 11b instead, if you prefer. This might help see how they span the range of compositions. Except, see the next comment below.
9) Fig. 11. There is a substantial (~20%) shift in diameter from the manufacturer's calibration, consistently for both instruments. It's not clear if this correction has been applied when the new wider bins were created. I'm not sure of the reason for creating the wider bins, other than some way to represent the range of possible sizes. It may be better to calculate a low-refractive index calibration and a high-refractive index calibration (by calibration, I mean relationship between channel number and calibration diameter for each calibrant), then a medium-refractive index calibration as the default value. Uncertainty bars would then span across the low- and high- refractive index cases and you could still use the full 99-channel resolution of the UHSAS. I'm not sure what the wider bins gains you since using that method a central bin diameter is assumed and only one size distribution, with no uncertainty range, comes out. Uncertainty ranges might be more useful than grouped wider bins.
10) Fig. 13. There are some surprising size-dependent counting efficiency differences between units here, which pass without much comment. ~30% is a big counting difference (i.e., 350 nm). What is going on? Any ideas?
11) With PSL, when you are comparing numbers do you just integrate the PSL peak, or are you counting additional surfactant/contamination particles in the smallest bins (assuming no DMA is used for the PSL calibrations to remove the smaller contaminant particles)?
12) Line 417. The yaw angle (alignment with respect to the local wind vector) of -147 degrees must be an error. I might believe -1.47 degrees.
13) Fig. 16. What is the shading on this plot?
14) Line 483. Two periods after "cabin".
15) Figure 17. I don't find log-log size distributions very useful. Of more interest (at least to me) would be how well the integrated number, surface, volume, and effective radius agree. These are the parameters governing CCN activity, heterogeneous chemistry, extinction and mass transport, and remote sensing retrieval, respectively.
16) If data need to be plotted on a log axis, it implies that the parameter is not normally (Gaussianly) distributed. Thus standard deviation, which assumes Gaussian statistics, is not valid and is meaningless in describing the statistics. A geometric standard deviation might be better here. (But I would prefer linear plots of N, S, and V vs log diameter instead.)
17) The lateral and fore-aft spacing of the TPC-TOSS is mentioned in Section 6, but of more importance is the vertical spacing, which is not mentioned.
18) Please make sure that all figures are plotted using colors and/or symbols that would allow a person with a color vision impairment to distinguish the different parameters. There are two such scientists in my close acquaintance and it can be a struggle for them.
Citation: https://doi.org/10.5194/egusphere-2025-3175-RC1 -
RC2: 'Comment on egusphere-2025-3175', Anonymous Referee #2, 27 Jul 2025
Review of the manuscript „The TropoPause Composition TOwed Sensor Shuttle (TPC-TOSS): A new airborne dual platform approach for atmospheric composition measurements at the tropoopause“
by Bozem et al.
The article presents a very interesting new platform for airborne measurements at the tropopause altitude range. It is well-written, and fully fits in the range of the journal AMT. In my opinion it can be published after some minor revisions. Suggestions for improvement and small typos are specified below.
Suggestions for improvement:
- I would suggest to introduce the method earlier, e.g. the first figure should be a sketch of how it works, with the Lear Jet, the rope and the payload. In my opinion it takes too long for the reader to get a first impression in Fig. 3/ on page 8. This should also include a clear statement if there is only a mechanical connection, or also power supply
- Please state on the swinging behaviour of the system, e.g. show statistics on pitch/roll7yaw angles during one flight, mention critical situations, describe more in detail how the tethered system is handled, e.g. with a winch. There is some information in the summary (900 m behind, 200 m lateral) – how constant is this?
- There are different informations about altitude, e.g. in the intor it says 6-12 km. This is a contradiction to l. 69, studying vertical transport form the PBL into the UTLS. Then in l 84 it states that the maximum altitude with the TPC-TOSS was only 9700 m.
- Different informations about aerosol sizes: 95 nm-1 µm in l. 77
- Please motivate more in detail the 200 m rope length. Was this a choice based on technical constraints or scientific scales? In both cases please explain more in depth. L. 343 states that the rope was only 200 ft – is this flexible? Can it be chosen for each flight?
- If relevant, please explain quickly the Mission Support System, or omit.
- Please include technical details on temperature management. The aerosol sensors are for sure temperature stabilized? How cold does it get in the TPC-TOSS without heating, how much heating power is applied? Is it actively controlled depending on measured inside temperatures? L. 231 only mentions that the system is thermally isolated
- 150/151: the uncertainty is 1.25 and 2 m. Is this good enough? Please comment.
- 167/168: what is the temporal resolution of the humicap in the UTLS? A few minutes would be too much for the scientific questions, I suppose? Why not complement with an optical hygrometer?
- In general, how do you address the issue of response time? What corrections are applied? Maybe compare to the correction methods applied in Bärfuss et al., 2023 (https://amt.copernicus.org/articles/16/3739/2023/amt-16-3739-2023.html), who performed temperature and humidity measurements up to 10 km altitude based on a drone
- Explain coluors of figures only in captions, do not use in text, e.g. l. 297-299, 330, 346, 413
- 7: were temperature corrections applied, similar to Bärfuss et al.? If yes, please explain method in text. If not, why? What error does this imply?
- 16: include vertical lines for better overview, e.g. for begin of climb?
Minor details:
- 24: according to THE World Meteorological Organization
- aircraft is also aircraft in the plural form, please adapt throughout the manuscript, e.g. l.40, 54
- put references in chronological order, e.g. l. 51/52, 406
- 75: deploy IT during…
- 125: modificationS
- caption of Fig. 3: bracket missing
- 185: ThereforeE
- 231 thermalLy isolated
- 245: in Section 4.4
- 246: instrument output frequency of the ozone instrument
- explain all acronyms, e.g. l. 263 NIST
- use „laboratory“ instead of „lab“ throughtout the text, e.g. l. 328
- 344 AT a distance
- 373: brackets
- change order of Fig. 15 and 16, as mentioned in text?
- 482 dot missing
- 483 2 dots
- 500: AT two altitudes
- 504/505: rephrase
- 524: all authors
Citation: https://doi.org/10.5194/egusphere-2025-3175-RC2
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