SI-traceable validation of a balloon-borne spectrometer for water vapor measurements in the upper atmosphere

Abstract. Despite its crucial role in the Earth's radiative balance, upper air water vapor (H₂O) is still lacking accurate, in-situ, and continuous monitoring. Especially in the upper troposphere-lower stratosphere (UTLS), these measurements are notoriously difficult, and significant discrepancies were reported in the past between different measuring techniques. Here, we present a laboratory validation of a recently developed mid-IR quantum-cascade laser absorption spectrometer for balloon-borne measurements of H₂O in the UTLS (ALBATROSS). The validation was performed using SI-traceable reference gas mixtures generated based on the permeation method and dynamic dilution. The accuracy and precision of ALBATROSS were evaluated at a wide range of pressures (30‒250 mbar) and H₂O amount fractions (2.5‒35 ppm), representative of the atmospheric variability of H₂O in the UTLS. Best agreement was achieved by implementing a quadratic Speed-Dependent Voigt Profile (qSDVP) line-shape model in the spectroscopic retrieval algorithm. The molecular parameters required by this parameterization were determined empirically using a multi-spectrum fitting approach over different pressure conditions. ALBATROSS achieves an accuracy better than ±1.5 % with respect to the SI-traceable reference at all investigated pressures and H₂O amount fractions. The measurement precision was found to be better than 30 ppb (i.e., 0.1 % at 35 ppm H₂O) at 1 s resolution for all conditions. This performance, unprecedented for a balloon-borne hygrometer, demonstrates the exceptional potential of mid-IR laser absorption spectroscopy as a new reference method for in-situ measurements of H₂O in the UTLS.