Characterization of a New Laser Isotopologue Hygrometer for Measurements of Isotopes in Cloud Water

Observations of water isotopologues in clouds can play an important role in studies of cloud microphysical and dynamical processes that are important in the hydrological cycle, including elucidation of various aspects of the aerosol indirect effect. Among the challenges for obtaining observations in clouds are representative in-situ measurements of HD¹⁶O, H₂¹⁸O, and H₂¹⁶O in clouds with precision and accuracy that can both (1) track important changes, and (2) remain stable throughout the observation period over a wide range of atmospheric conditions. To address some of these observational challenges, we are developing a new laser isotopologue hygrometer suitable for in-situ aircraft measurements with provisions to reduce measurement drift, including during warm-up periods and cloud encounters. We have also developed new algorithms for fitting spectra obtained by lasers that measure pairs of water isotopologues simultaneously, thus improving measurement precision when water concentrations are changing rapidly during short-duration cloud penetrations. The instrument will be small, lightweight, and autonomous to allow for broad deployment on a variety of platforms. This presentation will focus on characterization of the precision of laboratory measurements of HD¹⁶O/H₂¹⁶O and H₂¹⁸O/H₂¹⁶O ratios at water mixing ratios expected in clouds, stability of the optics under realistic environmental conditions, and intercomparisons with a flight-ready Picarro water isotope analyzer. We will also discuss plans for future tests of an instrument on research aircraft.