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 H-2/H-1, O-17/O-16, and O-18/O-16 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 parts per thousand, 3 parts per thousand, and 9 parts per thousand for delta O-18, delta O-17, and delta H-2, 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 similar to 10 ppmv).