Quantifying the Stable Water Isotopologue Exchange Between the Snow Surface and Lower Atmosphere by Direct Flux Measurements

Surface processes in high latitudes play an important role in global climate and thus understanding the physics of these systems is critical for improving climate projections. The characterization of the stable water isotopologue flux between the surface and the atmosphere offers the potential to constrain parameterizations of these physical surface exchange processes in numerical models. In addition, observations of isotopologue surface fluxes allow the evaluation of surface fluxes as a process influencing the formation of the climate signal retrieved from ice core isotopologue records. Here, we present a record of stable water isotopologue surface fluxes measured in-situ in the accumulation zone of the Greenland Ice Sheet at the East Greenland Ice Core Project site. We measured isotopologue fluxes above the snow surface directly by combining high-frequency eddy covariance measurements with low-frequency isotopologue measurements from a cavity ring-down spectrometer (CRDS). We developed a method to correct for the high-frequency loss of the CRDS by combining humidity measurements from both the CRDS and eddy covariance instruments. Using this approach our measurements provide the first direct observations of water isotopologue fluxes in polar areas. We observed a clear diurnal cycle in the fluxes of the different water isotopologues. The isotopic composition of the sublimation and deposition flux showed to be dependent on the snow and vapor isotopic composition, respectively. To a first order, the isotopic composition of the sublimation flux could be derived assuming equilibrium fractionation during sublimation. Plain Language Summary Heavy water molecules behave differently than light water molecules during phase change processes. By measuring both, the heavy and light water concentration throughout the atmospheric water cycle, we gain a better understanding of the processes involved. Processes that are not well understood and therefore difficult to represent in climate models, are surface exchange processes in the polar areas. Including heavy water molecules as an additional parameter in climate models can improve our understanding of the water exchange between snow and atmosphere. In order to test model results, we need observations of the exchange of heavy and light water molecules between snow and air. We have therefore developed a method that allows us to measure the flux of heavy water molecules directly. To do so, we combined high-frequency wind and humidity measurements from a flux system, with slower laser spectroscopy heavy water measurements to obtain the flux. We correct for the losses in the high-frequency range of the slower instrument, by combining the humidity measurements of both the fast and the slow instruments. With this method, we observed that the heavy water flux follows the diurnal cycle, and that, on average, the snow surface defines the heavy water content in the sublimation flux. Key Points Direct measurements of the stable water isotopologue fluxes between the snow surface and atmosphere Combination of eddy covariance method with cavity ring-down spectroscopy isotopologue measurements Isotopic composition of sublimation flux shows isotopic fractionation during sublimation

Automated summary

Direct measurements of stable water isotopologue fluxes in polar areas revealed diurnal variations in different isotopologue fluxes. The isotopic composition of sublimation and deposition fluxes showed dependency on snow and vapor isotopic composition, respectively.