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Laboratory Validation of a Laser Absorption Spectrometer for Balloon-borne Measurements of UTLS Water Vapor

S. Brunamonti1, M. Graf1, L. Emmenegger2 and B. Tuzson1

1Swiss Federal Laboratories for Materials Science and Technology (Empa) Laboratory for Air Pollution and Environmental Technology, Dübendorf, Switzerland; +41 58 76 56 559, E-mail: simone.brunamonti@empa.ch
2Swiss Federal Laboratories for Materials Science and Technology (Empa), Dübendorf, Switzerland

Water vapor (H2O) is the strongest greenhouse gas in our atmosphere, and its abundance in the upper troposphere - lower stratosphere (UTLS) is of great importance to the Earth's radiative balance. Nevertheless, accurate measurements of H2O in this region are notoriously difficult, and significant discrepancies were found in the past between different techniques. Currently, cryogenic frostpoint hygrometry is considered as the reference method for balloon-borne measurements of UTLS H2O [1]. However, the ongoing phasing-out of the cooling agent required by this technique (freon R23) urges the need of an alternative solution to maintain the monitoring of UTLS H2O in long-term global observing networks.

As an alternative method, we developed a compact (< 3.5 kg) instrument based on mid-IR quantum-cascade laser absorption spectroscopy (QCLAS) [2]. The spectrometer incorporates a specially designed segmented circular multipass cell to extend the optical path length to 6 m [3], while meeting stringent requirements in terms of mass, size, and temperature resilience. Two successful test flights demonstrated the instrument's outstanding capabilities under real atmospheric conditions up to 28 km altitude [2].

The accuracy and precision of the QCLAS at UTLS-relevant conditions were assessed by a dedicated laboratory campaign at the Swiss Federal Institute of Metrology (METAS). Using a dynamic-gravimetric permeation method, SI-traceable reference gas mixtures (synthetic air) at various pressures (30-250 mbar) were generated with H2O amount fractions in the range of 2.5–35 ppmv and 1.5 % uncertainty. The results of the spectral retrieval show that QCLAS achieves an accuracy better than ± 1.5 % at all investigated experimental conditions. In addition, the precision was determined to be better than 30 ppbv (0.3 %) at 1 s resolution, while the best achievable precision was obtained after 50 s integration time, resulting in 2 ppbv H2O. This represents an outstanding level of accuracy and precision for a balloon-borne hygrometer.

Currently, the instrument participates in the AquaVIT-4 international intercomparison of atmospheric hygrometers at the AIDA chamber (KIT, Germany) [4]. Further in-flight validation campaigns are foreseen during this year.

References

[1] Brunamonti et al., J. Geophys. Res. Atmos., 2019, 124, 13, 7053-7068.

[2] Graf et al., Atmos. Meas. Tech., 2021, 14, 1365-1378.

[3] Graf, Emmenegger and Tuzson, Opt. Lett., 2018, 43, 2434-2437.

[4] https://www.hemera-h2020.eu/aquavit-4/

Figure 1

Figure 1. Left: fully assembled spectrometer as deployed for the balloon flights (adapted from [2]). Right: representative spectra measured during the laboratory validation campaign at METAS, corresponding to 35 ppmv H2O and pressure levels 30, 60, 100 and 200 mbar.