Accurate and high-resolution measurements of UTLS water vapor by balloon-borne laser absorption spectroscopy

Abstract. Accurate and sustained monitoring of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H₂O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H₂O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H₂O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H₂O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H₂O.