Water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is of great importance to the Earth's radiative balance. However, 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, these devices are currently undergoing a fundamental reconception due their use of fluoroform (HFC-23) as cooling agent, which must be phased out due of its high global warming potential. Here, we present a new mid-IR quantum-cascade laser absorption spectrometer for balloon-borne measurements of UTLS H₂O (ALBATROSS). The spectrometer incorporates a specially designed segmented circular multipass cell that allows an optical path length of 6 m [3], while meeting stringent requirements in terms of mass (< 3.5 kg), 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 ALBATROSS at UTLS-relevant conditions were validated by a laboratory campaign at the Swiss Federal Institute of Metrology (METAS). Using a dynamic-gravimetric permeation method [4], we generated SI-traceable reference mixtures with H₂O amount fractions as low as 2.5 ppmv in synthetic air. The results show that ALBATROSS achieves an accuracy better than ± 1.5 % at all investigated pressures (30-250 mbar) and H₂O amount fractions (2.5-35 ppmv), and a precision of 0.3 % at 2.5 ppmv H₂O and 1 s resolution. Precision can be further improved until 2 ppbv by integrating the measurements in time over approximately 100 s. ALBATROSS also achieves a linear response within ± 2 ppmv up to 180 ppmv H₂O. Overall, this represents an unprecedented level of accuracy and precision for a balloon-borne hygrometer. ALBATROSS recently participated to the AquaVIT-4 international intercomparison of atmospheric hygrometers (Karlsruhe, Germany) [5], and further in-flight validation campaigns are planned in 2022.

[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] Guillevic et al., Atmos. Meas. Tech., 2018, 11, 3351–3372.
[5] https://www.hemera-h2020.eu/aquavit-4/