Review of “Measurement report: Greenhouse gas profiles and age of air from the 2021 HEMERA-TWIN balloon launch” by Schuck et al.

General comment

This manuscript presents vertical profiles of trace gases from the stratosphere over Scandinavia in summer 2021. The successful payload of a variety of instruments onboard the balloon and subsequent analysis data were fruits of great efforts, for which I would like to send congratulations to the authors. In particular, AirCores and the laser spectrometer (Pico) seem to be the new efforts from the recent decade. The team’s long and dedicated efforts deserve the new exciting data. The manuscript is well organized and written. Below I provide some minor comments that might be considered.

I believe that this work has been achieved on the basis of the long history of experimental advances in balloon observations by the European scientific community. I suppose that earlier works such as Schmidt et al. and Fabian et al. have paved the way. It might be interesting if the authors provide readers opportunity to follow the origins of the active techniques and historical development. One additional paragraph in the introduction section could help. Technical advantages over the balloon observation systems from the US and Japan teams might be also interesting.

Age of air calculation should be elaborated so that readers can understand principles of the method better. Although I understand that this study stands on the past technical efforts, needed is more exact explanations about the method, not the software. The package should assume an age spectra formulation to find the best-match mean age of air. It could be more clearly explained why the surface reference time series of CH₄ is used for calculation of the mean ages from CO₂ and SF₆ (were they or stratospheric data used for the CO₂ correction?). What kind of assumptions brought the bias correction applied to SF₆-derived ages of air might be also more explained with additional sentences, in addition to reference to Garny et al. paper. Although they are complex, the authors should provide essential parts. Since age of air is not uniquely determined from the data, I think the manuscript should provide the method in a way independently understandable.

Figures 2, 3, 4, 5 and 7 might be merged into a multi-panel graph. I am interested in comparing vertical profiles of different species particularly for correspondences of gradients among species. The tropopause could be marked, otherwise readers wonder at places like “in the troposphere.” Profiles down to the troposphere are great advantage of this study.

Specific comment

Throughout the manuscript: I’ve been wondering what HEMERA and TWIN stand for.

P2 L51: Though with some difference in methods, use of halocarbons for estimating mean age of air can be further back in time e.g., Daniel et al. (1996) and Harnisch et al. (1999). It might be worth mentioning that the idea existed much earlier than the recent extensive studies like Ray et al. (2017) and Leedham Elvidge et al. (2018).

P3 L63: I could not find the companion paper (Laube et al. 2024) available. It must be made available before considering the decision of this manuscript. Otherwise this manuscript should provide more information without mentioning the reference.

P4 L84: “position data” do they include the GPS altitude? Regarding this, the altitude coordination in this study is unclear. Are the altitudes in Figure 2 and the following figures the GPS altitude recorded by the Pico-SDLA instrument? Or was pressure recorded and converted to altitude?

P4 Section 2.2: It is regrettable that the Pico-STRAT Bi Gaz instrument for CO₂ and H2O did not work well. Looking at Figure 2, I hope successful measurement at next opportunity as the present profiles were all from “reconstruction” from subsequent analyses.

P5 Table 1: I think that this table could be somehow merged with Table 3, because I have sometimes confused with which instrument for which species. A merged table might provide a quick look for such correspondences.

P8 L192: Is there a reference that described usefulness of the cotton filter for COS measurements? Does co-existence of ozone and COS deteriorate their stability in the stainless steel flask? Such an explanation is valuable and desirably presented in this manuscript or the companion one.

P8 Section 2.5: I think that it would be valuable to describe how much sample amount (volume) was used for each analysis. This information should be critical when the campaign was planned.

P9 L219: How long does it take to have the instrument output stabilized with the flow rate ~50 sccm. I imagine that those stable data points were taken to calculate the measurement value and the time for stabilization determined the amount of sample used.

P10 Section 2.6: As in my earlier comment, I hope this section be reformulated so that readers can correctly follow principles, assumptions and processes to calculate the age of air.

P10 L265: “in the troposphere” the tropopause should be presented. Otherwise readers cannot be clear about which altitude range is mentioned.

P14 L303: It is interesting to see the ups and downs in CH₄ above 20 km observed by the Pico instrument. Is there any possible mechanism that could shape such layers?

Technical Comments

P4 L70: “known” to “considered”

P11 L274: “caused”

P18 L350: “Aircore” to “AirCore”

P18 L356: “altitde" to “altitude”

P18 L358: “Age of Air” to “age of air”