A compact mid-IR laser absorption spectrometer for water vapor isotopologue measurements in the upper air

Water vapor is the main natural greenhouse gas controlling the Earth's energy balance. It significantly influences atmospheric chemistry and climate dynamics, particularly in the upper troposphere and lower stratosphere (UTLS), which makes it an essential climate variable. Therefore, systematic monitoring of the variability of water vapor in the UTLS is crucial for the understanding of the climate system. However, measurements of its concentration lack important information about the history and origin of the water. This issue can be addressed by considering stable isotopologues of water (e.g. H₂¹⁶O, H₂¹⁸O, and HDO) and quantifying tiny variations in their distribution, which is driven by environmental conditions and chemical/physical processes, making stable water isotopologues an ideal proxy for process studies of the hydrological cycle. Despite the large potential of such measurements, the availability of in-situ data is still very limited due to a lack of adequate analytical tools.

This project focuses on the development of a mobile laser absorption spectrometer (LAS) for airborne in-situ measurements of water vapor isotopologues up to the UTLS region, leveraging on our recent advances in the development of compact instruments [1, 2]. We target the strong absorption features of H₂¹⁶O, H₂¹⁸O, and HDO simultaneously using a single cw-DFB quantum cascade laser (QCL) at around 1359 cm^-1. This range was selected after a detailed spectral survey covering the whole infrared domain. We are currently evaluating a laboratory setup using an astigmatic Herriott multipass cell with an optical path length (OPL) of 76 m to assess the performance of the approach.

The mobile design will rely on our succesful concept [1, 2] including a segmented circular multipass cell [3] in open path configuration, which allows for an effective mitigation of memory effects and exhibits the fastest response time. However, the low abundance of the rare water vapor isotopologues needs to be compensated by substantially extending the OPL (30-fold) to enhance the signal-to-noise ratio. Simulations and laboratory tests have shown that this can be achieved while keeping the rigidity, low weight, and tolerance needed for harsh environmental conditions.

Ultimately, our concept should provide an easy-to-deploy tool for isotope-resolved water vapor profiles at high spatio-temporal resolution.

 

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

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

[3] Graf et al., Opt. Lett., 43, 2434-2437, 2018.