Water as a Window: Application-Driven Design of Infrared Laser Spectrometers

We present two infrared laser spectroscopic instrument developments that illustrate how optimized measurements of H₂O enable advanced atmospheric sensing across very different operating regimes. In both cases, instrument performance is driven by careful selection of absorption lines, infrared optical design, and measurement strategies tailored to the specific application.

The first instrument is an airborne water vapor probe designed for the low-H₂O regime relevant to persistent aviation contrail cirrus. The second application employs a more sensitive mid-infrared measurement scheme, coupled to a novel sampling manifold, to measure molecular hydrogen - after catalytic conversion to H₂O - with high speed and sensitivity. Both instruments are based on tunable infrared laser direct absorption spectroscopy (TILDAS), scanning isolated H₂O absorption lines near 7205 cm⁻¹ (1.39 µm) and 1558 cm⁻¹ (6.4 µm), respectively.

This presentation will discuss the design details of these two very different instruments, as well as the application-driven requirements that informed their hardware architectures. A comprehensive set of comparison and validation results will be presented for the system developed for contrail avoidance. For the second application, hydrogen spectrometer performance and a range of deployment scenarios will be discussed, including the novel CLAIR-H₂ system for localization and quantification of hydrogen emissions using tracer release and atmospheric inversion.

Together, these examples demonstrate the flexibility of infrared laser spectroscopic instrumentation for enabling both aviation-relevant water vapor measurements and emerging atmospheric hydrogen observations.