SI-traceable balloon-borne measurements of water vapor in the upper atmosphere

Water vapor (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 is considered as the reference method for balloon-borne measurements of UTLS H₂O [1]. However, these devices must be fundamentally reconceived due their use of fluoroform (HFC-23) as cooling agent, which has to be phased out due to its high global warming potential. There is thus an urgent need for alternative, reliable technologies to monitor UTLS H₂O, e.g. in long-term global observing networks, such as the GCOS Reference Upper Air Network (GRUAN).

Here we present a new mid-IR quantum-cascade laser absorption spectrometer for balloon-borne measurements of UTLS H₂O (ALBATROSS). The spectrometer incorporates a specifically designed segmented circular multipass cell (SC-MPC) that allows for an optical path length of 6 m [2], and it fulfills stringent requirements in terms of mass (< 3.5 kg), size, and temperature resilience. Two successful test flights demonstrated the instrument's outstanding capabilities under real atmospheric conditions up to 28 km altitude [3].

During flights, the instrument experiences a harsh environment with up to 80 K temperature variations and two orders of magnitude change in pressure and H₂O amount fraction. To achieve reliable and SI-traceable, we determined the spectral performance and the accuracy of the retrieved amount fractions of ALBATROSS at UTLS-relevant conditions using a dynamic-gravimetric permeation method [4]. SI-traceable reference mixtures were generated with H₂O amount fractions as low as 2.5 µmol/mol (or parts per million, ppm) in synthetic air. The results show that ALBATROSS achieves an accuracy better than ±1.5 % at all investigated pressures (30–250 mbar) and H₂O amount fractions (2.5–35 ppm). The 1 s relative precision is better than 0.3 %, while 5 nmol/mol (i.e., parts per billion, ppb) is reached by averaging for 100 s. Furthermore, ALBATROSS reaches a linear response within ±2 ppm up to 180 ppm H₂O.

To achieve this level of performance, the Voigt profile was found to be inadequate. Therefore, we empirically determined all parameters needed to implement the quadratic speed-dependent Voigt profile (qSDVP), which accounts for speed-dependent collision broadening. The qSDVP more accurately captures the H₂O line shape at all pressure conditions, and thus significantly improves the accuracy of the retrieved water vapor amount fraction.

Overall, ALBATROSS achieves an unprecedented level of accuracy and precision for a balloon-borne hygrometer, and it demonstrates the exceptional potential of mid-IR laser absorption spectroscopy for in-situ measurements of UTLS H₂O. Further in-flight validation campaigns from Lindenberg (Germany) and Payerne (Switzerland) 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.