Water Isotopologues Research Articles

Stable Water Isotopologue Fractionation During Soil‐Water Evaporation: Analysis Using a Coupled Soil‐Atmosphere Model

AbstractThe atmosphere‐soil system forms a highly coupled system, which makes key processes such as evaporation complex to analyze as the mass, energy, and momentum transfer is influenced by both domains. To enhance the understanding of evaporation processes from soils, stable water isotopologues are suitable tools to trace water movement within these systems as heavier isotopologues enrich in the residual liquid phase. Due to the complex coupled processes involved in simulating soil‐water evaporation accurately, quantifying fractionation during flow and transport processes at the soil‐atmosphere interface remains an open research area. In this work, we present a multi‐phase multi‐component transport model that resolves flow through the near‐surface atmosphere and the soil, and models transport and fractionation of the stable water isotopologues using the numerical simulation environment DuMux. Using this coupled model, we simulate transport and fractionation processes of stable water isotopologues in soils and the atmosphere by solving compositional flow equations and by using suitable coupling conditions at the soil‐atmosphere interface instead of commonly used parameterization. In a series of examples of evaporation from bare soil, the transport and distribution of stable water isotopologues are evaluated numerically with varied conditions and assumptions, including different atmospheric conditions (turbulent/laminar flow, wind speed) and their impact on the spatial and temporal distribution of the isotopic composition. Building on these results, we observed how the enrichment of the isotopologues in soil is linked with the different stages of the evaporation process. A qualitative study is conducted to verify single fractionation processes in our approach.

Active Separation of Water Isotopologues by Local Molecular Motion in Microporous Framework Materials.

Around 10-15 % of the world's energy consumption is accounted for by the separation and purification of chemicals. Among them is the enrichment and separation of isotopologues which are an essential aspect of modern chemistry. In their recent work, Su et al. demonstrate the separation of water isotopologues by responsive dynamic pore windows in a microporous coordination polymer with unprecedented selectivity based on an elegant mechanism.

Active Separation of Water Isotopologues by Local Molecular Motion in Microporous Framework Materials

AbstractAround 10–15 % of the world's energy consumption is accounted for by the separation and purification of chemicals. Among them is the enrichment and separation of isotopologues which are an essential aspect of modern chemistry. In their recent work, Su et al. demonstrate the separation of water isotopologues by responsive dynamic pore windows in a microporous coordination polymer with unprecedented selectivity based on an elegant mechanism.

Frequency comb-referenced cavity ring-down spectroscopy of natural water between 8041 and 8633 cm−1

The 1.25 µm atmospheric transparency window is of importance for a number of atmospheric applications. As a continuation of our previous works on the improvement of water vapor line parameters in the near infrared, the room temperature absorption spectrum of water vapor in natural isotopic abundance is recorded with unprecedented sensitivity between 8041 and 8633 cm−1, using comb-referenced cavity ring-down spectroscopy. The line positions and intensities of more than 5400 lines were retrieved. Their intensities range between 3.6 × 10−30 and 1.5 × 10−22 cm/molecule. The high sensitivity and low noise level of the recordings (αmin≈ 10−11 cm−1) allow for measuring more than 1600 new lines and determine their positions with an accuracy of about 10−4 cm−1 in the case of isolated features. The rovibrational assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The final line list is assigned to more than 5400 transitions of the first six water isotopologues (H216O, H218O, H217O, HD16O, HD18O and HD17O). The measured line positions allow to determine the energy of 79 new levels of H216O, H218O, H217O, and HD16O, and to correct 139 previously reported term values. Although a good agreement is generally observed, the comparison to the HITRAN2020 spectroscopic database and to the W2020 transition frequencies reveals a number of discrepancies both for line positions and line intensities. The lack of traceability of some HITRAN line parameters and some biases in the derivation procedure of the W2020 energy levels are confirmed in the studied range. Validation tests of the theoretical values of the line intensities against measured values show both band-by-band variations of the deviations on the order of a few % and line-by-line fluctuations within a given band.

Towards learned emulation of interannual water isotopologue variations in General Circulation Models

Abstract Simulating abundances of stable water isotopologues, that is, molecules differing in their isotopic composition, within climate models allows for comparisons with proxy data and, thus, for testing hypotheses about past climate and validating climate models under varying climatic conditions. However, many models are run without explicitly simulating water isotopologues. We investigate the possibility of replacing the explicit physics-based simulation of oxygen isotopic composition in precipitation using machine learning methods. These methods estimate isotopic composition at each time step for given fields of surface temperature and precipitation amount. We implement convolutional neural networks (CNNs) based on the successful UNet architecture and test whether a spherical network architecture outperforms the naive approach of treating Earth’s latitude-longitude grid as a flat image. Conducting a case study on a last millennium run with the iHadCM3 climate model, we find that roughly 40% of the temporal variance in the isotopic composition is explained by the emulations on interannual and monthly timescale, with spatially varying emulation quality. The tested CNNs outperform simple baseline models such as random forest and pixel-wise linear regression substantially. A modified version of the standard UNet architecture for flat images yields results that are as good as the predictions by the spherical CNN. Variations in the implementation of isotopes between climate models likely contribute to an observed deterioration of emulation results when testing on data obtained from different climate models than the one used for training. Future work toward stable water-isotope emulation might focus on achieving robust climate–oxygen isotope relationships or exploring the set of possible predictor variables.

Range-resolved detection of boundary layer stable water vapor isotopologues using a ground-based 1.98 µm differential absorption LIDAR.

This paper presents a first demonstration of range-resolved differential absorption LIDAR (DIAL) measurements of the water vapor main isotopologue H2 16O and the less abundant semi-heavy water isotopologue HD16O with the aim of determining the isotopic ratio. The presented Water Vapor and Isotope Lidar (WaVIL) instrument is based on a parametric laser source emitting nanosecond pulses at 1.98 µm and a direct-detection receiver utilizing a commercial InGaAs PIN photodiode. Vertical profiles of H2 16O and HD16O were acquired in the planetary boundary layer in the suburban Paris region up to a range of 1.5 km. For time averaging over 25 min, the achieved precision in the retrieved water vapor mixing ratio is 0.1 g kg-1 (2.5% relative error) at 0.4 km above ground level (a.g.l.) and 0.6 g kg-1 (20%) at 1 km a.g.l. for 150 m range bins along the LIDAR line of sight. For HD16O, weaker absorption has to be balanced with coarser vertical resolution (600 m range bins) in order to achieve similar relative precision. From the DIAL measurements of H2 16O and HD16O, the isotopic abundance δD was estimated as -51‰ at 0.4 km above the ground and -119‰ in the upper part of the boundary layer at 1.3 km a.g.l. Random and systematic errors are discussed in the form of an error budget, which shows that further instrumental improvements are required on the challenging path towards DIAL-profiling of the isotopic abundance with range resolution and precision suitable for water cycle studies.

Disentangling the Contribution of Moisture Source Change to Isotopic Proxy Signatures: Deuterium Tracing with WRF-Hydro-Iso-Tag and Application to Southern African Holocene Sediment Archives

Abstract It is well accepted that global circulation models equipped with stable water isotopologues help us better understand the relationships between atmospheric circulation changes and isotope records in paleoclimate archives. Still, isotope-enabled models do not disentangle the different processes affecting precipitation isotopic compositions. Furthermore, the relevance of this model-oriented approach relies on the realism of the modeled isotope results, which would support the interpretation of the proxy records in terms of modeled climate changes. To alleviate these limitations, the newly developed WRF-Hydro-iso-tag, a version of the isotope-enabled regional coupled model WRF-Hydro-iso enhanced with an isotope-tracing procedure, is presented. Physics-based WRF-Hydro-iso-tag ensembles are used to regionally downscale the isotope-enabled Community Earth System Model for southern Africa, for two 10-yr slices of mid-Holocene and preindustrial times. The isotope-tracing procedure is tailored to assess the origin of the hydrogen isotope deuterium contained in southern African precipitation, between the Atlantic and Indian Oceans. In comparison to the global model, WRF-Hydro-iso-tag simulates lower precipitation amounts with more regional details, as well as mid-Holocene-to-preindustrial changes in precipitation isotopic compositions that better match plant-wax deuterium records from two marine sediment cores off the Orange and Limpopo River basins. Linear relationships between mid-Holocene-to-preindustrial changes in temperature, precipitation amount, moisture source, and precipitation deuterium compositions are derived from the ensemble results. A deuterium enrichment in the Orange River-related sediment core may not be related to an aridification but rather indicate a summer circulation change enabling a larger contribution of more isotopically enriched moisture from the Atlantic Ocean. Significance Statement The knowledge of past climates is crucial for understanding our Earth system and apprehending future climate change. Plant materials in sediment archives contain atoms of hydrogen from past precipitation that allow paleoclimate reconstructions, using compositions of the hydrogen isotope deuterium. However, in the tropics, deuterium-depleted plant remains can either denote a wetter climate phase or a change in atmospheric circulation patterns with longer distances between ocean evaporation and land precipitation. This work provides an innovative dynamical downscaling method of global paleoclimate models to disentangle the effects of precipitation amount change and moisture source change on deuterium records, and ultimately to improve paleoclimate reconstructions. The interpretation of a deuterium enrichment in a marine sediment core as a marker for aridification is revised. The enrichment caused by an atmospheric circulation change bringing a larger amount of more isotopically enriched moisture flow from the Atlantic Ocean to southern African precipitation would be a more physically sound explanation.

Separating water isotopologues using diffusion-regulatory porous materials.

The discovery of a method to separate isotopologues, molecular entities that differ in only isotopic composition1, is fundamentally and technologically essential but remains challenging2,3. Water isotopologues, which are very important in biological processes, industry, medical care, etc. are among the most difficult isotopologue pairs to separate because of their very similar physicochemical properties and chemical exchange equilibrium. Herein, we report efficient separation of water isotopologues at room temperature by constructing two porous coordination polymers (PCPs, or metal-organic frameworks) in which flip-flop molecular motions within the frameworks provide diffusion-regulatory functionality. Guest traffic is regulated by the local motions of dynamic gates on contracted pore apertures, thereby amplifying the slight differences in the diffusion rates of water isotopologues. Significant temperature-responsive adsorption occurs on both PCPs: H2O vapour is preferentially adsorbed into the PCPs, with substantially increased uptake compared to that of D2O vapour, facilitating kinetics-based vapour separation of H2O/HDO/D2O ternary mixtures with high H2O separation factors of around 210 at room temperature.

On the two-body dissociation of dications of water isotopologues

Multiple ionization of water isotopologues is studied using near-IR fs laser beams and asymmetric ω/2ω phase-locked fields, in order to elucidate the involved dissociation mechanics and isotopic effects. The double ionization mechanism of water is identified by the laser polarization dependence of the released, via two body fragmentation, ions. The angular distribution of the H+/D+ fragments, reveal the participation of different molecular orbitals in the ionization process, indicating excitation of both the ground and first few excited states of the dications. The rate of the dissociative processes of the dication is found to be different for the two isotopologues, highlighting an isotopic effect. Inspection of the H2+/D2+ fragments formed prior to dissociation, reveals that they may have been generated from parent cations and parent dications. In the case of dication fragmentation, we find evidence of autoionization. Finally, the formation of the H2+/D2+ ions, is completed within the duration of the pulse.

Acyl chain selection couples the consumption and synthesis of phosphoinositides

Phosphoinositides (PIPn) in mammalian tissues are enriched in the stearoyl/arachidonoyl acyl chain species (“C38:4”), but its functional significance is unclear. We have used metabolic tracers (isotopologues of inositol, glucose and water) to study PIPn synthesis in cell lines in which this enrichment is preserved to differing relative extents. We show that PIs synthesised from glucose are initially enriched in shorter/more saturated acyl chains, but then rapidly remodelled towards the C38:4 species. PIs are also synthesised by a distinct ‘re‐cycling pathway’, which utilises existing precursors and exhibits substantial selectivity for the synthesis of C38:4‐PA and ‐PI. This re‐cycling pathway is rapidly stimulated during receptor activation of phospholipase‐C, both allowing the retention of the C38:4 backbone and the close coupling of PIPn consumption to its resynthesis, thus maintaining pool sizes. These results suggest that one property of the specific acyl chain composition of PIPn is that of a molecular code, to facilitate ‘metabolic channelling’ from PIP2 to PI via pools of intermediates (DG, PA and CDP‐DG) common to other lipid metabolic pathways.

2D-TMDC-enhanced condensed phase cavity ring-down spectroscopy coupled with surface plasmon resonance for investigation on water isotopologues

We report a novel scheme for sensitivity-enhanced liquid phase cavity ring-down spectroscopy (CRDS) exploiting the two-dimensional transition metal dichalcogenides (2D-TMDCs) based surface plasmon resonance (SPR). We investigated the performance of the proposed sensing scheme by monitoring water isotopologues i.e. D2O (heavy water) and H2O on the SPR chip utilizing a simulation-based characteristics transfer matrix method. The cavity-enhanced SPR coupled with 2D-TMDCs is employed to detect D2O from distilled (DI) water with good time response. This proposed approach exhibited minimum sensitivity up to 448 ns/RIU with 4-layers of molybdenum disulfide (MoS2) and maximum sensitivity up to​ 604 ns/RIU with 4-layers of tungsten diselenide (WSe2). Moreover, dependence of cavity-finesse and chip-sensitivity on the 2D-nanomaterials suggests that SPR-CRDS is a new-strategy for the applications of plasmonic chemical sensing in the condensed phase with absorbance of the order of 10−11.

Redox controls on H and N speciation and intermolecular isotopic fractionations in aqueous fluids at high pressure and high temperature: Insights from in-situ experiments

Aqueous magmatic fluids are essential to the transport of hydrogen (H), carbon (C), and nitrogen (N) from the mantle to the surface, during which changes in pressure, temperature, and redox conditions affect the chemical speciation and intermolecular isotopic fractionations of H, C, and N. Here, we performed a series of hydrothermal diamond-anvil cell experiments to evaluate the role of pressure, temperature, and redox conditions on the speciation and intermolecular fractionations of H and N during the decompression and cooling of aqueous fluids from 780 MPa to 800°C to 150 MPa and 200°C. We used Raman spectroscopy to investigate the distribution and exchange reactions of H and N isotopologues between water, methane, ammonia, and di-nitrogen molecules under changing physicochemical conditions. Our experiments show that upon decompression, a C- and N-bearing fluid will preferentially degas D-rich methane and 15N-rich N2, depleting the residual aqueous fluid in those isotopes. If this fluid precipitates N-rich (i.e., NH4+-bearing) minerals, the observed N isotopic fractionation is opposite to that during N2 degassing, enriching the aqueous fluid in 15N. Because these fractionations result from changes in H, C, and N speciation in the aqueous fluid, their magnitudes depend on redox conditions as well as pressure and temperature. Our new in-situ experimental results are consistent with the large H and N isotopic fractionations observed between water, methane, and ammonia species in aqueous fluids at high pressures and temperatures, although the magnitude of the fractionations in our experiments cannot be quantified. Nonetheless, our results suggest that statistical thermodynamic models likely underestimate isotopic fractionation effects for isotopic molecules under these conditions, and should account for solubility and isotopic effects of the solvent associated with the solvation of water, methane, and ammonia isotopologues in aqueous fluids.This work has significant implications for interpreting isotopic measurements of natural samples from hydrothermal systems because it offers insights into isotopic fractionations in multicomponent and multiphase systems under hydrothermal temperatures and pressures.

Isotopic Equilibration in Convective Downdrafts

AbstractWhile it is becoming increasingly clear that stable water isotopologues are important tools in the study of atmospheric convection, many questions remain on how different processes affect them. This work is focused on water vapor isotopes in precipitation‐driven downdrafts. Using idealized simulations, the contributions of rain evaporation and liquid‐vapor equilibration processes are analyzed. It is shown that rain evaporation tends to deplete water vapor isotopes throughout the downdraft's descent, whereas equilibration has a neutral or depleting effect in the first half of the downdraft's life cycle and an enriching one in the second half. The total contribution of the two processes is then discussed, and it is argued that equilibration has an overall small effect compared to rain evaporation. Finally, using two sensitivity experiments, it is shown that the role played by equilibration varies significantly in regimes with different concentrations of cloud condensation nuclei.

Water vapor absorption spectroscopy and validation tests of databases in the far-infrared (50–720 cm-1). Part 1: Natural water

The rotational spectrum of water vapor in natural isotopic abundance has been recorded by high resolution (≈ 0.001 cm-1) Fourier transform spectroscopy at the AILES beam line of the SOLEIL synchrotron. The room temperature absorption spectrum has been recorded between 50 and 720 cm-1 using five pressure values up to 7 mbar and an absorption pathlength of 151.75 m. Line parameters were retrieved for the five recorded spectra and then combined in a global list of 2867 water lines with line intensity ranging between a few 10–26 and 10–19 cm/molecule. 454 of the measured lines are newly observed by absorption spectroscopy. The spectral calibration based on a statistical matching with about 700 accurate reference line positions allows for line center determinations with an accuracy of 5 × 10–5 cm-1 for well isolated lines of intermediate intensity.The large spectral coverage, the achieved position accuracy and sensitivity of the constructed line list make it valuable for validation tests of the current spectroscopic databases. Six water isotopologues (H218O, H216O, H217O, HD18O, HD16O, and HD17O) were found to contribute to the spectrum. The line position comparison to the recent HITRAN2020 spectroscopic database and to the W2020 line lists of H216O, H217O and H218O, [Furtenbacher et al. J. Phys. Chem. Ref. Data 49 (2020) 043103; https://doi.org/10.1063/5.0030680] shows an overall very good agreement. Nevertheless, a number of significant deviations are observed. Part of them has an amplitude largely exceeding the W2020 claimed error bars. On the basis of the experimental data at disposal for the main isotopologue (1310 transitions), the best agreement is achieved with the positions calculated using the effective Bending–Rotation Hamiltonian [Coudert et al. J Mol Spectrosc 2014;303:36–41. https://doi.org/10.1016/j.jms.2014.07.003].

Investigating stable oxygen and carbon isotopic variability in speleothem records over the last millennium using multiple isotope-enabled climate models

Abstract. The incorporation of water isotopologues into the hydrology of general circulation models (GCMs) facilitates the comparison between modeled and measured proxy data in paleoclimate archives. However, the variability and drivers of measured and modeled water isotopologues, as well as the diversity of their representation in different models, are not well constrained. Improving our understanding of this variability in past and present climates will help to better constrain future climate change projections and decrease their range of uncertainty. Speleothems are a precisely datable terrestrial paleoclimate archives and provide well-preserved (semi-)continuous multivariate isotope time series in the lower latitudes and mid-latitudes and are therefore well suited to assess climate and isotope variability on decadal and longer timescales. However, the relationships of speleothem oxygen and carbon isotopes to climate variables are influenced by site-specific parameters, and their comparison to GCMs is not always straightforward. Here we compare speleothem oxygen and carbon isotopic signatures from the Speleothem Isotopes Synthesis and Analysis database version 2 (SISALv2) to the output of five different water-isotope-enabled GCMs (ECHAM5-wiso, GISS-E2-R, iCESM, iHadCM3, and isoGSM) over the last millennium (850–1850 CE). We systematically evaluate differences and commonalities between the standardized model simulation outputs. The goal is to distinguish climatic drivers of variability for modeled isotopes and compare them to those of measured isotopes. We find strong regional differences in the oxygen isotope signatures between models that can partly be attributed to differences in modeled surface temperature. At low latitudes, precipitation amount is the dominant driver for stable water isotope variability; however, at cave locations the agreement between modeled temperature variability is higher than for precipitation variability. While modeled isotopic signatures at cave locations exhibited extreme events coinciding with changes in volcanic and solar forcing, such fingerprints are not apparent in the speleothem isotopes. This may be attributed to the lower temporal resolution of speleothem records compared to the events that are to be detected. Using spectral analysis, we can show that all models underestimate decadal and longer variability compared to speleothems (albeit to varying extents). We found that no model excels in all analyzed comparisons, although some perform better than the others in either mean or variability. Therefore, we advise a multi-model approach whenever comparing proxy data to modeled data. Considering karst and cave internal processes, e.g., through isotope-enabled karst models, may alter the variability in speleothem isotopes and play an important role in determining the most appropriate model. By exploring new ways of analyzing the relationship between the oxygen and carbon isotopes, their variability, and co-variability across timescales, we provide methods that may serve as a baseline for future studies with different models using, e.g., different isotopes, different climate archives, or different time periods.

The Dominant Role of Brewer‐Dobson Circulation on 17O‐Excess Variations in Snow Pits at Dome A, Antarctica

AbstractRecent studies have suggested that water isotopologues in snow pits from remote East Antarctica can be influenced by the input of stratospheric water, which has anomalously high 17O‐excess values. However, it remains unclear whether the 17O‐excess records preserved in snow and ice from this region can be used to reconstruct stratosphere‐troposphere exchange (STE). In this study, we present high‐resolution 17O‐excess records from two snow pits at Dome A, the highest point of the Antarctic ice sheet. The 17O‐excess records show a significant positive correlation with the strength of the Brewer‐Dobson circulation (BDC), the hemispheric‐scale troposphere‐stratosphere overturn circulation. Stronger BDC leads to more stratospheric water input over Antarctica and higher 17O‐excess, and vice versa. In addition, the 17O‐excess records also have a significant positive correlation with the Southern Annular Mode (SAM) index, because SAM modulates Antarctic precipitation, which has a dilution effect on the stratospheric water input. The 17O‐excess records do not show significant correlations with local temperature and relative humidity in the moisture source region. These results suggest the dominant effect of BDC on 17O‐excess and indicate the potential for using 17O‐excess records in ice cores from remote sites in East Antarctica for reconstructing long‐term variations of STE, and understanding their mechanisms and climate effects.

Assessment of the changes in contributions from water sources to streamflow induced by urbanization in a small-sized catchment in Southeastern Brazil using the dual stable isotopes of water (18O and 2H).

Urban growth often results in changes in the urban hydrological cycle, causing impacts on water availability in densely populated regions. The water isotopologues can provide relevant information about the origin of water under different hydrogeological scenarios, aiding to implement better strategies for water conservation in coupled natural-urbanized environments. In this study, the isotopic compositions of multiple water sources were assessed in a pristine (Ipanema National Forest, FLONA) and an urbanized (Lavapés catchment, SOR) watershed located in the Sorocaba River basin (State of São Paulo, Southeastern Brazil), seeking to understand the causes of isotopic variability and to determine the relative contribution from different sources to streamflow, using the Bayesian mixing model approach. Differences in isotopic composition were observed, as FLONA yielded the most depleted water (ca. -7.5 ‰ [Formula: see text]18O for surface and groundwater and ca. + 11.0 ‰ d-excess), while SOR yielded the most enriched water (ca. -5.5‰ [Formula: see text]18O for surface and groundwater and -3.8‰ [Formula: see text]18O for the water supply system), with evidence of evaporation (ca. + 8.2 ‰ d-excess). The differences observed in isotopic compositions are related to a combination of different factors, such as geological framework, groundwater recharge, and evaporation associated with the Itupararanga water reservoir. Both in FLONA and SOR, groundwater discharge is the most important factor that regulates streamflow. However, in SOR, losses from the water supply system were almost constant along the year, representing an important contribution. The results presented here highlight the use of isotope hydrology techniques to solve problems related to urban hydrology.

Rotranslational dynamics of confined water. II. Spectroscopic evidence of confinement effects on the far-infrared spectra of water isotopologues in argon and krypton matrices.

Water molecules trapped in rare gas matrices exhibit conspicuous shifts in their far-infrared (FIR), rotranslational spectral features compared with the corresponding transitions observed in the gas phase. These confinement-induced perturbations have been related not only to the quantization of translational motion but also to the coupling between the orientational and positional degrees of freedom: the rotation-translation coupling (RTC). As the propensity displayed by the nuclear spin isomers (NSI) of water to undergo interconversion in confinement is intimately related to how its nuclear spin degrees of freedom are coupled with those for intra- and intermolecular motions, confinement-induced RTC should also strongly impact the NSI interconversion mechanisms and rates. Insight into the rotranslational dynamics for H2 16O, H2 17O, and H2 18O, confined in argon and krypton matrices, is provided here based on the evolution of rotranslational spectra induced by NSI interconversion while a definitive assignment is provided from the transition energies and intensities calculated using the confined rotor model [Paper I, Wespiser et al., J. Chem. Phys. 156, 074304 (2021)]. In order to build a complete rotranslational energy diagram of confined water, which is fundamental to understand the NSI interconversion rates, the energy difference between the ground ortho and para rotranslational states is derived from the temperature dependence of the intensity ratio of mid-infrared lines emerging from these states. These investigations should provide deeper insight of the factors that control NSI interconversion of water isotopologues under extreme confinement.

Charge Transfer Reactions between Water Isotopologues and Kr+ ions.

Astrochemical models often adopt capture theories to predict the behavior of experimentally unmeasured ion–molecule reactions. Here, reaction rate coefficients are reported for the charge transfer reactions of H2O and D2O molecules with cold, trapped Kr+ ions. Classical capture theory predictions are found to be in excellent agreement with the experimental findings. A crossing point identified between the reactant and product potential energy surfaces, constructed from high-level ab initio calculations, further supports a capture-driven mechanism of charge transfer. However, ion–molecule reactions do not always agree with predictions from capture theory models. The appropriateness of using capture theory-based models in the absence of detailed experimental or theoretical studies is discussed, alongside an analysis of why capture theory is appropriate for describing the likelihood of charge transfer between Kr+ and the two water isotopologues.

Quantification the diffusion-induced fractionation of 1H217O isotopologue in air accompanying the process of water evaporation

Advances in laser technology enabling simultaneous analyses of three heavy water isotopologues (1H2H16O, 1H218O and 1H217O) triggered renewed interest in studies of the hydrological cycle using full isotopic composition of water. Such studies require adequate knowledge of equilibrium and kinetic isotope effects accompanying phase transitions of water. Here we report the results of laboratory experiments aimed at quantification of diffusion-induced fractionation of 1H217O isotopologue in air, accompanying the process of water evaporation. Three evaporation experiments have been conducted: (i) evaporation under constant temperature and relative humidity levels, (ii) evaporation under constant temperature and at three different relative humidity levels, and (iii) evaporation under constant relative humidity level and at two different temperatures. Experiment (i) yield the value of diffusive fractionation coefficient of 1H217O isotopologue in air equal 14.64 ± 0.24‰ at 20.4 °C. This value is indistinguishable within the quoted uncertainties from the value of 14.71 ± 0.05‰ derived experimentally for the first time by Barkan and Luz (2007). We confronted these experimental values with the results of state-of-the art quantum–mechanical calculations published recently by Hellmann and Harvey (2020). Experiments (ii) and (iii) yield for the first time the experimental evidence that, contrary to the d-excess, the 17O-excess parameter is largely insensitive to changes in temperature but is sensitive to changes of the relative humidity and this sensitivity increases with the degree of isotope enrichment of the evaporating water body. The linear dependence of kinetic fractionation coefficients of heavy water isotopologues in air on the relative humidity deficit, postulated by the Craig-Gordon isotope model of evaporation process, was confirmed by the results of experiment (ii) for all three isotope systems studied, pointing to correctness of the conceptual framework underlying this widely used model.