Chemistry and the Environment, Contributed Talk (15min)

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

S. Brunamonti1, M. Graf1, T. Bühlmann2, C. Pascale2, H. Looser1, L. Emmenegger1, B. Tuzson1
1Empa, Laboratory for Air Pollution/Environmental Technology, Dübendorf, Switzerland, 2METAS, Laboratoy Gas Analysis, Bern, Switzerland

Water vapor (H2O) is the strongest greenhouse gas in our atmosphere, and it plays a key role in multiple processes that affect weather and climate. Particularly, H2O in the upper troposphere - lower stratosphere (UTLS) is of great importance to the Earth's radiative balance, and has a significant impact on the rate of global warming. Currently, the reference method used for in-situ measurements of UTLS H2O aboard meteorological balloons is cryogenic frostpoint hygrometry (CFH) [1]. However, the cooling agent required for this technique (trifluoromethane) is phasing out as of 2020, due to its strong global warming potential. This represents a major challenge for the continuity of long-term UTLS H2O monitoring programs worldwide. As an alternative to CFH, we developed a compact instrument based on mid-IR quantum-cascade laser absorption spectroscopy (QCLAS) [2]. The spectrometer incorporates a specifically developed segmented circular multipass cell to extend the laser path length to 6 m [3], while meeting the stringent requirements, in mass, size and temperature resilience, posed by the balloon platform and by the harsh environmental conditions of the UTLS. Two successful test flights performed in December 2019 demonstrate the instrument's outstanding capabilities up to 28 km altitude [2].

The spectrometer relies on a calibration-free retrieval approach, i.e. the H2O amount fractions are determined from first principles from the acquired spectra, hence it is necessary to validate its absolute accuracy with respect to a high-accuracy reference. To this aim, a dedicated laboratory campaign was conducted at the Swiss Federal Institute of Metrology (METAS) in April-May 2021. Using a dynamic-gravimetric permeation method combined with dynamic dilution [4], we generated SI-traceable reference gas mixtures of H2O in a synthetic air matrix, with amount fractions between 2.5-35 ppmv and uncertainty < 1.5 %, which were measured by QCLAS at pressures between 30-250 mbar. This dataset provides the basis for the absolute validation in terms of accuracy and linearity of the spectrometer at UTLS-relevant conditions. Furthermore, it allows to investigate secondary effects originating from the assumption of a simplified shape model (the Voigt profile) for the absorption lines in the fitting algorithm. In particular, their contribution to the overall uncertainty of the instrument will be investigated and compared to more sophisticated parameterizations, such as the Hartman-Tran profile (HTP), for a large set of pressure and amount fraction conditions. The ultimate goal is to demonstrate the potential of QCLAS as a highly valuable technique for quantitative balloon-borne measurements of UTLS H2O, which are directly traceable to the SI units.

[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.
[4] Guillevic et al., Atmos. Meas. Tech., 2018, 11, 3351–3372.