the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jan 2024
A novel, balloon-borne UV/visible spectrometer for direct sun measurements of stratospheric bromine
Abstract. We report on a novel, medium weight (∼ 25 kg) optical spectrometer coupled to an automated sun tracker (∼ 12 kg) for direct sun observations from azimuth controlled balloon platforms. It is designed to measure a suite of UV/vis absorbing gases (O3, NO2, BrO, OClO, HONO and IO) relevant in the context of stratospheric ozone depletion using the DOAS method. Here, we describe the design and major features of the instrument. Further, the instrument’s performance during two stratospheric deployments from Esrange/Kiruna (Sweden) on 21 August 2021 and from Timmins (Ontario, Canada) on 23 August 2022 are discussed along with first results concerning inferred mixing ratios of BrO above balloon float altitude. Using a photochemical correction for the partitioning of stratospheric bromine ([BrO]/[Bry]) obtained by chemical transport simulations, the inferred total stratospheric bromine load [Bry] amounts to (17.5 ± 2.2) ppt (pure statistical error amounts to 1.5 ppt) in (5.5±1.0) yrs old air, resulting in a stratospheric entry early 2017±1 yr, the latter being inferred from simultaneous measurements of N2O by the GLORIA mid-IR instrument.
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RC1: 'Comment on egusphere-2023-2912', Anonymous Referee #1, 08 Feb 2024
This is a well-written paper that presents the design and first results from a new balloon spectrometer for solar occultation measurements of BrO. The instrumentation is described in detail and the co-authors present improvements to the standard retrieval approach together with a convincing handling of instrumental artifacts. The inferred bromine load and its derived uncertainty shows the usefulness of these measurements for monitoring stratospheric bromine trends. The paper is a good fit for AMT and should be published after addressing these minor revisions:
The case that the 1-2 ppt difference quoted here is actually of scientific significance should be made. On the surface it seems small in light of the total budget of 20 ppt.
The system level descriptions of the solar tracker and the spectrometer units are well done, but would be greatly helped with schematic level drawings of the designs (in addition to the picture, which is a bit hard to follow for one not familiar with the instrument).
How big is the non-linearity correction in the processing step, even when the detector saturation is kept to 30-60%?
Something is missing the explanation of how the dependence of etalon optical density on SCD is used for the retrieval process. How is the extrapolation to the larger SCDs performed throughout the retrieval range beyond the high sun range used for the characterization of the dependence?
The significant difference in [BrO] for different polynomial degrees is somewhat concerning. Is there some level of non-orthogonality between the higher degree polynomials and the BrO cross section? If the residuals are the same, would it not make sense to take the result for the lowest degree polynomial as the most robust?
How will new CCD detectors avoid the contamination problem? It seems the cause is not well understood by the co-authors, so might it not happen again? Maybe it is from internal reflection in the focal plan array itself (this is a known phenomenon)?
Prolific use of brackets for parenthetical phrases and around quantities make the text a bit difficult to read. Check with the editor for guidance on correct typesetting.
Line 78: “correct for them”
Line 88: rephrase “all electronics threatened to overheat”
Line 155: rephrase “were more threatened”
Citation: https://doi.org/10.5194/egusphere-2023-2912-RC1 -
RC2: 'Comment on egusphere-2023-2912', Anonymous Referee #2, 10 Apr 2024
In this manuscript, the authors describe a new balloon-borne DOAS instrument for measurements of stratospheric species. The instrument's performance during two deployments is discussed, and data from one measurement flight is used to estimate stratospheric Bry and compare the results to previous estimates.
The manuscript is well written and discusses an instrument and measurements relevant to atmospheric science and the study of the ozone layer. I understand that this article is intended as a reference for future work reporting on more measurement results achieved with this instrument. I recommend it for publication after the issues listed below have been addressed.
Major comments
As this is not the first stratospheric balloon-borne DOAS instrument, it would be good to highlight the actual progress of this instrument compared to the previous versions. This is not clear to the reader, and my impression is that while this instrument is new, it is not particularly innovative or better than past ones.
The description of the instrument is very detailed (I would say too detailed) in some parts, but it lacks drawings to understand the set-up of the telescope and the overall system. Figure 2 does not really help in this respect. Please add schematics providing more details on the set-up.
As discussed in the manuscript, there are two major problems with the current system: a) the sun-tracker is not fast enough to allow for measurements during ascent when the gondola is not stabilized, and b) there are artefacts in the spectra, which make BrO analysis difficult.
The first problem is only mentioned, and no indication is given regarding how it will be solved in future deployments. Not being able to measure profiles is a severe limitation of the instrument, so this needs to be discussed more.
The second problem is discussed in great detail, but only concerning the approach taken to correct it in the spectra. Very little is said about the possible origins of the artefacts beyond the fact that they are linked to etaloning on the CCDs. I'm not entirely convinced by this explanation, as this appears to be a rapid process, and the question is, what would be condensing on the CCDs at this rate in an evacuated housing? In the sense of "what can the reader learn from this manuscript", I would hope for a more detailed discussion of possible sources and the tests performed to investigate them:
- Has the effect been reproduced in the lab?
- Does it occur also if only the CCDs are cooled but not the rest of the optical system?
- Does it happen for both channels?
- Are the phases of the etalons on the two CCDs linked?
- Could it be etaloning due to condensation on another surface (mirrors, fibres, filters)?
Concerning the proposed correction approaches, I do not understand the rationale behind using SCD_air as the axis – I would have thought that time is the relevant parameter here. Please explain.
Clearly, the etaloning introduces substantial additional uncertainty even after correction, and this limits the value of the current measurements in addition to the limitation imposed by the lack of profiling capability.
Minor comments
Figure 1: What is the sharp gradient in the green shading at 80.5°? This looks like an artefact.
Figure 3: The FWHM of the UV instrument is relatively small – is a convolution of the literature cross-sections necessary (and even possible) at this high spectral resolution?
L264: Calling the vertical column density VD is inconsistent with the other notation. Please use VCD_air. Alternatively, if you meant density here (as suggested by your use of VD in equations 4 and 5), use another symbol such as \rho_air for air density.
L268: Why is it a good assumption that BrO is constant above floating altitude?
Equation 4: This is not correct. Either you remove the delta h_i, or you replace VD with the concentration of air
Equation 5: Same problem as for equation 4
L291: predominant
L461: I wouldn't say that NO2 SCDs are unaffected, but they are not much affected.
Citation: https://doi.org/10.5194/egusphere-2023-2912-RC2
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