the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
SI-traceable validation of a balloon-borne spectrometer for water vapor measurements in the upper atmosphere
Simone Brunamonti
Manuel Graf
Tobias Bühlmann
Céline Pascale
Ivan Ilak
Lukas Emmenegger
Abstract. Despite its crucial role in the Earth's radiative balance, upper air water vapor (H2O) is still lacking accurate, in-situ, and continuous monitoring. Especially in the upper troposphere-lower stratosphere (UTLS), these measurements are notoriously difficult, and significant discrepancies were reported in the past between different measuring techniques. Here, we present a laboratory validation of a recently developed mid-IR quantum-cascade laser absorption spectrometer for balloon-borne measurements of H2O in the UTLS (ALBATROSS). The validation was performed using SI-traceable reference gas mixtures generated based on the permeation method and dynamic dilution. The accuracy and precision of ALBATROSS were evaluated at a wide range of pressures (30‒250 mbar) and H2O amount fractions (2.5‒35 ppm), representative of the atmospheric variability of H2O in the UTLS. Best agreement was achieved by implementing a quadratic Speed-Dependent Voigt Profile (qSDVP) line-shape model in the spectroscopic retrieval algorithm. The molecular parameters required by this parameterization were determined empirically using a multi-spectrum fitting approach over different pressure conditions. ALBATROSS achieves an accuracy better than ±1.5 % with respect to the SI-traceable reference at all investigated pressures and H2O amount fractions. The measurement precision was found to be better than 30 ppb (i.e., 0.1 % at 35 ppm H2O) at 1 s resolution for all conditions. This performance, unprecedented for a balloon-borne hygrometer, demonstrates the exceptional potential of mid-IR laser absorption spectroscopy as a new reference method for in-situ measurements of H2O in the UTLS.
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Simone Brunamonti et al.
Status: open (until 16 Jun 2023)
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RC1: 'Comment on amt-2023-83', Alan Fried, 20 May 2023
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The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-83/amt-2023-83-RC1-supplement.pdf
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RC2: 'Comment on amt-2023-83', Markus Miltner, 01 Jun 2023
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Review of the AMT manuscript by S. Brunamonti et al. with title:
SI-traceable validation of a balloon-borne spectrometer for water vapor measurements in the upper atmosphere
General comments
This paper is of interest for the AMT audience for several reasons: 1) It demonstrates that laser absorption spectrometers are a promising alternative to cryogenic frost-point hygrometers under the particularly difficult measurement conditions encountered in the upper atmosphere (variable pressure, low and variable water concentration). 2) It gives a blueprint of a laboratory validation for such measurement devices. 3) It shows the importance of choosing a more advanced (compared to the Voigt profile) line shape model (here the qSDVP) and demonstrates how to obtain the necessary parameters, which are not contained in the HITRAN data base.
The experiment is well-designed and the underlying measurements and data analysis are presented in detail, allowing fellow scientists to reproduce the measurements if they wish.
The paper is well written: It is easy to follow the authors through the chapters, thanks to the clear structure, the precise language and the supporting figures.
Specific comments
1 Introduction, p2, l25; State that only pressure broadening parameters are assessed and the line strength is not (since only pressure was varied, while temperature was held constant). This was not immediately clear to me when first reading the paper.
2.3 Gas handling system, p4, l13; Flow rates not consistent with what is stated in Figure 1 (0.05 to 3 slpm vs 0.05 to 4.5 slpm)
Figure 1; Should there not be a vent somewhere between the mixing solenoid valve and MFC 3? If not, where does the excessive flow go? Please clarify this.
p4, l19; how was the temperature controlled?
P4, l25; it would be interesting for fellow scientists trying to reproduce the experiments to know how the gas cylinder used for the secondary reference gas mixture was “conditioned” and what kind of synthetic air (upper boundary of water content?) was used to prefill and pressurize the bottle.
P4, l28; it would also be good to specify the type of the SilcoNert®2000-coated stainless-steel cylinder into which the reference gas mixture was expanded. Despite the coating, surface effects might be different for different bottle sizes.
3.2 Pre-processing, P9, l2; I imagine that the spectral range was about 0.845 cm-1, so you obtain a spectral-point resolution of 1.69 10^(-4) for your stated 5000 datapoints? Please mention the spectral range to make clear how you got to the spectral-point resolution.
3.4. Quadratic speed-dependent Voigt profile (qSDVP), p13, l14; I do not agree with the interpretation of the residuals obtained with the qSDVP as being free of any structure exceeding the normal noise level, although admittedly the features visible in Figure 6c and d are very small. Have you tried to see if an additional fit parameter (D2 != 0) or a higher order line shape model would be able to suppress this feature? If so, it would be nice to mention this. In any case, in my opinion it would be preferable to state that there is still some small structure observable, but that it is largely reduced compared to the VP. Stressing the argument that the QF obtained with the improved fit equals the SNR, one could nonetheless justify the choice of the qSDVP as line profile?
4.3. Linearity (extended-range validation), p18, l14; What about the repeatability of this measurement? Have you determined the ~180ppm concentration several times? From Figure 9b it looks like all measurements (except for the 180ppm one) are slightly too high, as if the undiluted gas actually had a slightly higher water concentration…
- Conclusion, p21, l10; “without systematic residuals”, I do not agree, as discussed above.
Technical corrections
P9, l15, replace “while secondary reference gas mixtures (panel d) at 3 pressure levels (60−200 mbar)” by “while secondary reference gas mixtures (panel d) were measured at 3 pressure levels (60−200 mbar)”
P 14, l4, delete double “the” in “while the the qSDVP fitting uses”
The color-code used in the figures for different pressures (Figure 3, 4, 6, 7) should be unified. A different color scheme should be used, since the different blue lines are hardly distinguishable.
P 25, l15-26; references are not in correct order (K after L)
Citation: https://doi.org/10.5194/amt-2023-83-RC2
Simone Brunamonti et al.
Simone Brunamonti et al.
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