Quantum-cascade laser absorption spectrometer (QCLAS) for balloon-borne measurements of UTLS water vapor

Water vapor (H₂O) is the strongest greenhouse gas in our atmosphere, and it plays a key role in multiple processes that affect weather and climate. Particularly, H₂O in the upper troposphere - lower stratosphere (UTLS) is of great importance to the Earth's radiative balance, yet accurate measurements of H₂O in this region are notoriously difficult, and significant discrepancies were found in the past between different techniques (both in-situ and remote sensing). Currently, cryogenic frostpoint hygrometry (CFH) 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 CFH (freon R23) urges the need of an alternative solution to maintain the monitoring of UTLS H₂O in long-term global observing networks, such as the GCOS Reference Upper Air Network (GRUAN).

As an alternative method, we developed a compact 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 within a small footprint [3], while meeting the stringent requirements in terms of mass, size, and temperature resilience, posed by the balloon platform and by the harsh environmental conditions of the UTLS. Two successful test flights performed in December 2019, in collaboration with the German Meteorological Service (DWD), demonstrated the instrument's outstanding capabilities under real atmospheric conditions up to 28 km altitude.

The accuracy and precision of QCLAS at UTLS-relevant conditions were validated by a dedicated laboratory campaign conducted at the Swiss Federal Institute of Metrology (METAS). Using a dynamic-gravimetric permeation method, we generated SI-traceable reference gas mixtures with H₂O amount fractions as low as 2.5 ppmv and 1.5 % uncertainty in synthetic air. All measurements by QCLAS were found within ± 1.5 % of the reference value, corresponding to a maximum absolute deviation of 210 ppbv, and with an absolute precision better than 30 ppbv at 1 s resolution. This represents an unprecedented level of accuracy and precision for a balloon-borne hygrometer. Further in-flight validation campaigns from Lindenberg (Germany) are currently in preparation.

[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.