The Principal Investigator (PI) plans to: (1) To refine and apply a new experimental technique to measurements of the kinetic isotopic fractionation of water associated with evaporation and condensation; and (2) to calibrate a said instrument with traditional mass spectrometry techniques focusing on relatively low water vapor concentrations (<6000 ppmv) and on the deuterium excess of water vapor. It is expected that both of these goals will be beneficial to a wide range of scientists with interest in hydrology and atmospheric climate dynamics. The PI has developed a novel experimental technique for measuring the fractionation associated with evaporation and condensation that takes advantage of the rapid analytical capabilities of newly commercially available laser-based cavity ring-down spectrometers (CRDS). The new technique has several advantages over traditional mass spectrometry as simultaneous, real-time measurements of water vapor concentration and water vapor hydrogen and oxygen isotopic ratios may be made. Preliminary evaporation data corroborate the published equilibrium fractionations (100% relative humidity), suggesting that this approach may provide a new way to advance understanding of isotopic fractionation during disequilibrium evaporation and condensation. This work is exploratory in nature and the approach largely untested and thus qualifies for an EAGER.
This work will provide a foundation for future research in a wide range of hydrologic and atmospheric research. The research methodologies developed in this project will be published in peer-reviewed journals and presented at national conferences, and the project will partially fund the Ph.D. research of a female Ph.D. student.
In this project, we determined the change in the stable oxygen and hydrogen isotope composition of water vapor during evaporation into undersaturated air. The stable isotope composition of waters can be used to trace moisture sources and evaporation and transpiration fluxes in the environment, however the critical fractionation parameter associated with kinetic-based evaporation has not been well constrained. We are able to use our experimental system, combining an evaporation chamber with our new cavity ringdown laser spectrometer to construct an empirical equation for evaporation which can be incorporated into climate/hydrology models in future research. This work respresents the major portion of a Ph.D. research student’s work. Already we have one publication on the use of cavity ringdown lasers for oxygen and hydrogen isotope variations in water vapor in the upper troposphere, and have used our system to measure isotope fractionation into air as a function of temperature and relative humidity. Our work has added greatly to the total dataset for this critical parameter in the global meteoritic water system and will ultimately allow us to construct a global circulation model based on the stable isotope composition of river water combined with the calculated fractionation over the ocean and lakes. We are using these concepts to determine the relative contributions of evaporation and transpiration over the continents on the basis of the stable hydrogen and oxygen isotope composition of lakes and rivers worldwide. Our initial work suggests that transpiration (water loss through plant leaves) is much larger than previously thought.
