A water isotope (2H, 17O and 18O) spectrometer based on optical-feedback cavity enhanced absorption for in-situ airborne applications.

Measurements of the isotopic composition of water are thought to help explain stratospheric aridity and related issues in atmospheric sciences. Simultaneous in situ measurements of 2H/1H, 17O/16O, and 18O/16O at high spatial resolution are required for this purpose. We present the design and laboratory performance of a device that will be used on high-altitude research aircraft. It is based on optical feedback cavity-enhanced spectroscopy (OF-CEAS), with better sensitivity than traditional multi-pass arrangements. It utilizes a near-infrared laser source, avoiding the need for cryogens. We demonstrate an airborne precision during tropospheric flight conditions of 1 ‰, 3 ‰, and 9 ‰ for δ18O, δ17O, and δ2H, respectively, for 30-s averaged data and a water concentration of about 200 ppm. With recent improvements we expect to remain within a factor of about three of these values under true stratospheric conditions (water mixing ratio ∼10 ppmv).