On polar ice sheets, water vapor interacts with surface snow, and through the exchange of water molecules, imprints an isotopic climate signal into the ice sheet. This exchange is not well understood due to sparse observations in the atmosphere. There are currently no published vertical profiles of water isotopes above ice sheets that span the planetary boundary layer and portions of the free troposphere. Here, we present a novel dataset of water-vapor isotopes ( δ18O , δD, dxs) and meteorological variables taken by fixed-wing uncrewed aircraft on the northeast Greenland Ice Sheet (GIS). During June-July (2022), we collected 105 profiles of water-vapor isotopes and meteorological variables up to 1500 m above ground level. Concurrently, surface snow samples were collected at 12-hour intervals, allowing connection to surface-snow processes. We pair observations with modeling output from a regional climate model as well as an atmospheric transport and water-isotope distillation model. Climate model output of mean temperature and specific humidity agrees well with observations, with a mean difference of +0.095 °C and -0.043 g/kg (-2.91 %), respectively. We find evidence that along an air parcel pathway, the distillation model is not removing enough water prior to onsite arrival. Below the mean temperature inversion (~200m), water-isotope observations indicate a kinetic fractionating process, likely the result of mixing sublimated vapor from the ice sheet surface along with an unknown fraction of katabatic wind vapor. Modeled dxs does not agree well with observations, a result that requires substantial future analysis of kinetic fractionation processes along the entire moisture pathway.

The isotopic effects of sublimating ice is poorly understood and disagreement from diverging results from studies spans decades. The core question is whether sublimation occurs layer-by-layer with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. Here, small ice spheres were suspended in an unsaturated atmosphere and a Rayleigh distillation model was used to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, fractionation (103lna18O of ~ -0.6‰ (ɑ = 0.999) and 103lna2H -3 to -6‰ (ɑ = 0.994 to 0.997 ) was observed, smaller than predicted for equilibrium fractionation at this temperature and humidity. Assuming a modest porosity of 0.0005%, porosity could sufficiently increase diffusivity to explain the observed fractionation. The results help reconcile how sublimation varies between experimental and observational studies where uncontrolled porosity varies substantially across a continuum from porous firn layers to low porosity ice deep in glaciers.

Isotopic evaporation models, such as the Craig-Gordon model, rely on the description of non-equilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient-diffusion type measurements have been performed in ocean settings to test the validity of the commonly used non-equilibrium fractionation factor parametrizations for ocean evaporation. In this work we present six months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating the best non-equilibrium fractionation factors (k, ‰) for ocean evaporation and their dependency on wind speed. Gradient-diffusion measurements are sensitive enough to resolve non-equilibrium fractionation factors during evaporation and provide mean values of k18= 5.2±0.6 ‰ and k2= 4.3±3.4 ‰. In this study, we furthermore evaluate the relationship between k and 10-m wind speed over the ocean. Such relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (i) sensitivity of k to wind speed is small, in the order of -0.16 to 0.20 ‰ s/m for k18, and (ii) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. Instead, k18 monotonically decreases within our observed wind speed range [0 – 10 m/s]. Implications for using such k values in modelling ocean vapor d-excess are briefly discussed.