Elucidating the sources of upper tropospheric and lower stratospheric humidity through the in situ measurement of hydrogen-deuterium-oxygen and water

Water in the upper troposphere (UT) and lower stratosphere (LS) plays a central role in the coupled chemistry-climate system through the radiative effects of water vapor and clouds and through stratospheric ozone chemistry. As the atmosphere changes in response to increased concentrations of greenhouse gasses, UT and LS humidity will change in accordance with the climate sensitivity of water transport mechanisms. Identifying and quantifying the transport mechanisms is the first step toward predicting how humidity will be affected by climate change. Water isotopes hold promise as a tracer capable of discerning between water transport mechanisms. Measurements of H2O and H DO that are accurate and precise enough to be used for the quantitative study of water transport in the UT and LS pose an exceptional challenge due to the low concentration of water at the tropopause and the propensity of water to contaminate instrument surfaces.;We have built and successfully flown an H2O-deltaD instrument with the sensitivity to operate in the upper troposphere and lower stratosphere by employing a photolysis laser-induced fluorescence technique to measure H2O and H DO concentrations. The instrument (Hoxotope) exhibits the requisite sensitivity, accuracy, time response, and lack of systematic biases necessary to make a credible deltaD measurement and has demonstrated those same attributes both in the laboratory and in flight aboard the NASA WB-57. Data from the Aura Validation Experiment Water Isotope Intercomparison Flights (AVE-WIIF) in Summer 2005 allowed for the intercomparison between Hoxotope water and the established Harvard Water Vapor instrument, as well as between Hoxotope H DO and ICOS H DO. The data show a close agreement between the calibrations of the instruments and they demonstrate the quick time response of Hoxotope. The successful intercomparison validated Hoxotope as a credible source of deltaD data in the upper troposphere and lower stratosphere. Hoxotope AVE-WIIF data provided anecdotal evidence of convective ice lofting into the summertime mid-latitude overworld, but more geographic and seasonal spread in measurements is necessary before a quantitative determination of water transport mechanisms will be possible.