Laboratory validation of a compact laser spectrometer for trace-level water vapor measurements

Accurate measurements of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS, ~8-25 km altitude) are still very challenging, due to the low abundance of H₂O in this region (~5 ppm). The standard method for balloon-borne measurements of UTLS H₂O in global monitoring networks (e.g., GRUAN, GCOS reference upper air network) is chilled-mirror hygrometry. However, this technique is currently undergoing a major reconception, with the introduction of Peltier-based instruments as well as alternative cooling agents to the phasing-out fluoroform (HFC-23). Therefore, alternative, high-accuracy methods for in-situ measurements of UTLS H₂O are required.

To this aim, we developed ALBATROSS, a lightweight (< 3.5 kg) laser absorption spectrometer for balloon-borne measurements of UTLS H₂O [1]. ALBATROSS is based on a continuous-wave (cw) distributed feedback quantum cascade laser (DFB-QCL) emitting at 6.014 μm, and a monolithic segmented circular multipass cell with an optical path length of 6 m and a cell diameter of 10.8 cm. The multipass cell is highly resistant to thermal and mechanical stress, and can be operated either in a closed-path (laboratory) or an open-path (flight) configuration.

The performance of the spectrometer at UTLS-relevant conditions was assessed by a series of laboratory-based validation experiements. These measurements require the generation of reference gases with H₂O amount fractions in the low-ppm range (< 5 ppm), and their quantification at low pressures (< 100 mbar). At such conditions, artifacts due to the strong surface adsorption/desorption properties of H₂O become critical. These "memory" effects must be minimized by a careful design of the gas handling system and of the experimental procedure. At the same time, to achieve the required accuracy of ~1-2 % (i.e., 50-100 ppb), high-order line shape models, beyond the standard Voigt profile, must be considered for the spectroscopic retrieval.

In this presentation, we focus on the technical challenges and the results achieved in two distinct activities performed with ALBATROSS: an SI-traceable validation, using reference gases generated by a dynamic-gravimetric permeation method [2], and the AquaVIT-4 intercomparison of atmospheric hygrometers, held at the AIDA cloud simulation chamber in Karlsruhe, Germany [3]. Particularly, we highlight the best practices to address surface effects and other artifacts related to the gas handling system, as well as the importance of using an advanced line shape model (namely, the quadratic speed-dependent Voigt profile, qSDVP), and how to empirically obtain the necessary parameters not contained in the HITRAN database. This provides a general blueprint for the validation of a laser spectrometer dealing with a highly adsorbing gas at very low concentrations and pressures, such as H₂O at UTLS-relevant conditions, in a laboratory setting.

The instrument is currently deployed in a series of atmospheric test flights within the Swiss H₂O-Hub GCOS-project. Overall, the results demonstrate the exceptional potential of mid-IR laser absorption spectroscopy as a new reference method for in situ measurements of UTLS H₂O.

 

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

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

[3] Brunamonti et al., Atmos. Meas. Tech., 18, 5321–5348, 2025.