The ALBATROSS spectrometer for balloon-borne measurements of UTLS water vapor: Laboratory and in-flight validation

The amount of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) plays a critical role for the Earth's radiative balance. However, due to its low abundance, accurate measurements of H₂O in this region (~8‒25 km altitude) are still very challenging, and large discrepancies were often found between different techniques.

Here, we present the validation of a laser absorption spectrometer, ALBATROSS, specifically developed for balloon-borne measurements of UTLS H₂O [1]. ALBATROSS is a compact (< 3.5 kg)  instrument using a continuous-wave (cw) distributed feedback quantum cascade laser (DFB-QCL) emitting at 6.014 μm, and a monolithic segmented circular multipass cell [2] with an optical path length of 6 m within a cell diameter of 10.8 cm. The multipass cell is highly resistant to thermal and mechanical stress, and can be operated both in a closed-path (laboratory) and an open-path (flight) configuration.

The performance of the spectrometer was assessed at UTLS-relevant conditions using SI-traceable reference gases generated by a dynamic-gravimetric permeation method [3]. The results show that ALBATROSS achieves an accuracy better than ±1.5 % with respect to the SI-traceable reference at all investigated pressures (30‒250 mbar) and H₂O amount fractions (2.5‒35 ppm), and a precision better than 0.3 % at 1 s resolution. The quadratic speed dependent Voigt profile (qSDVP) line shape model was implemented to assure this level of accuracy using first principles.

Further laboratory-based validation activities included the AquaVIT4 intercomparison of atmospheric hygrometers, held at the AIDA cloud simulation chamber in Karlsruhe, Germany. Here, the performance of four airborne hygromenters, including ALBATROSS, was evaluated under a wide range of challenging environmental conditions (pressure 20‒600 mbar, temperature 190‒245 K, H₂O amount fraction 0.5‒530 ppm).

Recently, ALBATROSS was deployed in a series of atmospheric test flights conducted from the Meteoswiss Payerne Observatory (Switzerland), within the framework of the Swiss H₂O-Hub project. In tandem with ALBATROSS, a cryogenic frospoint hygrometer (CFH) was also deployed as a reference. Good agreement within ±10 % was found between ALBATROSS and CFH up to about 24 km altitude (~30 mbar pressure).

Altogether, our results demonstrate the exceptional potential of mid-IR laser spectroscopy for in-situ measurements of UTLS H₂O. This is particularly relevant considering the ongoing reconception of the CFH method, currently used in long-term climate monitoring networks (e.g., the GCOS reference upper air network, GRUAN), due to its use of fluoroform (HFC-23) as cooling agent, which must be phased out due to its high global warming potential.

[1] Graf et al., Atmos. Meas. Tech., 14, 1365–1378, 2021.

[2] Graf et al., Opt. Lett., 43, 2434-2437, 2018.

[3] Brunamonti et al., Atmos. Meas. Tech., 16, 4391–4407, 2023.