Water Vapor Isotope Research Articles

What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large-Eddy Simulations.

The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the “amount effect.” Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large‐eddy simulations. Results from large‐eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large‐scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large‐scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently.

Correcting the impact of the isotope composition on the mixing ratio dependency of water vapour isotope measurements with cavity ring-down spectrometers

Abstract. Recent advances in laser spectroscopy enable high-frequency in situ measurements of the isotope composition of water vapour. At low water vapour mixing ratios, however, the measured stable water isotope composition can be substantially affected by a measurement artefact known as the mixing ratio dependency, which is commonly considered independent of the isotope composition. Here we systematically investigate how the mixing ratio dependency, in a range from 500 to 23 000 ppmv of three commercial cavity ring-down spectrometers, is affected by the isotope composition of water vapour. We find that the isotope composition of water vapour has a substantial and systematic impact on the mixing ratio dependency for all three analysers, particularly at mixing ratios below 4000 ppmv. This isotope composition dependency can create a deviation of ±0.5 ‰ and ±6.0 ‰ for δ18O and δD, respectively, at ∼2000 ppmv, resulting in about 2 ‰–3 ‰ deviation for the d-excess. An assessment of the robustness of our findings shows that the overall behaviour is reproducible over up to 2 years for different dry gas supplies, while being independent of the method for generating the water vapour and being the first order of the evaluation sequence. We propose replacing the univariate mixing ratio dependency corrections with a new, combined isotope composition–mixing ratio dependency correction. Using aircraft- and ship-based measurements in an Arctic environment, we illustrate a relevant application of the correction. Based on our findings, we suggest that the dependency on the isotope composition may be primarily related to spectroscopy. Repeatedly characterising the combined isotope composition–mixing ratio dependency of laser spectrometers when performing water vapour measurements at high elevations, on aircraft, or in polar regions appears critical to enable reliable data interpretation in dry environments.

Groundwater Origin in Qanats, Chemo-Isotopic, and Hydrogeological Evidence.

A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO3 -Na-Mg and Cl-Na water types. The Gonabad meteoric water line (Gn MWL) was drawn at the local scale as δ2 H = 6.32×δ18 O + 8.35 (with R2 = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ18 O = (-0.0031 × H(m.a.s.l) )-1.3). The δ2 H and δ18 O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat.

Relating Moisture Transport to Stable Water Vapor Isotopic Variations of Ambient Wintertime along the Western Coast of Korea

Atmospheric water vapor transfers energy, causes meteorological phenomena and can be modified by climate change in the western coast region of Korea. In Korea, previous studies have utilized precipitation isotopic compositions in the water cycle for correlations with climate variables, but there are few studies using water vapor isotopes. In this study, water vapor was directly collected by a cryogenic method, analyzed for its isotopic compositions, and used to trace the origin and history of water vapor in the western coastal region of Korea during the winter of 2015/2016. Our analysis of paired mixing ratios with water vapor isotopes can explain the mechanism of water vapor isotopic fractionation and the extent of the mixing of two different air masses. We confirm the correlation between water vapor isotopes and meteorological parameters such as temperature, relative humidity, and specific humidity. The main water vapor in winter was derived from the continental polar region of northern Asia and showed an enrichment of 10 per mil (δ18O) through the evaporation of the Yellow Sea. Our results demonstrate the utility of using ground-based isotope observations as a complementary resource for constraining isotope-enabled Global Circulation Model in future investigations of atmospheric water cycles. These measurements are expected to support climate studies (speleothem) in the west coast region of Korea.

Control of seasonal water vapor isotope variations at Lhasa, southern Tibetan Plateau

Water isotopes in precipitation provide new insight on the controlling factors for moisture source variations on the Tibetan Plateau. However, understanding the intra-annual water isotope cycle is difficult for a region lack of winter precipitation. Here, we reported water vapor δ18O and δ2H time series over two and half years at Lhasa on the southern Tibetan Plateau. We compare our values with concurrent daily precipitation isotopes to assess the seasonal controls of vapor isotopes and possible correlation to precipitation isotopes. Results show an intra-seasonal dependence of water vapor δ18O on large scale meteorological regime. During the winter months, vapor water δ18O shows positive dependence on regional-scale temperature, confirming a Rayleigh-distillation controlling moisture transport. During the summer months, lower vapor δ18O is weekly in association with the heavy precipitation over tropical Indian Ocean and on the Tibetan Plateau, and this relation is further enhanced by the evaporation of precipitation at the sampling site. We also found a weak seasonal variation of vapor d-excess, likely resulted from climate seasonality over the large moisture sources region. Moreover, the isotope dataset contains a high-frequency of fluctuations for both δ18O and d-excess lasting several days during the summer monsoon season. These high frequency fluctuations are in phase with precipitation events, confirming significant local evaporation effects on the short-term water cycle, which further complicated the control of near surface water vapor isotopes. These findings improve our regional scale understanding of hydrological cycle on the southern Tibetan Plateau, and will potentially improve our understanding of isotope variations in proxy archives in the Tibetan Plateau.

Coupled rainfall and water vapour stable isotope time series reveal tropical atmospheric processes on multiple timescales

AbstractHigh‐frequency stable isotope data are useful for validating atmospheric moisture circulation models and provide improved understanding of the mechanisms controlling isotopic compositions in tropical rainfall. Here, we present a near‐continuous 6‐month record of O‐ and H‐isotope compositions in both water vapour and daily rainfall from Northeast Australia measured by laser spectroscopy. The data set spans both wet and dry seasons to help address a significant data and knowledge gap in the southern hemisphere tropics. We interpret the isotopic records for water vapour and rainfall in the context of contemporaneous meteorological observations. Surface air moisture provided near‐continuous tracking of the links between isotopic variations and meteorological events on local to regional spatial scales. Power spectrum analysis of the isotopic variation showed a range of significant periodicities, from hourly to monthly scales, and cross‐wavelet analysis identified significant regions of common power for hourly averaged water vapour isotopic composition and relative humidity, wind direction, and solar radiation. Relative humidity had the greatest subdiurnal influence on isotopic composition. On longer timescales (weeks to months), isotope variability was strongly correlated with both wind direction and relative humidity. The high‐frequency records showed diurnal isotopic variations in O‐ and H‐isotope compositions due to local dew formation and, for deuterium excess, as a result of evapotranspiration. Several significant negative isotope anomalies on a daily scale were associated with the activity of regional mesoscale convective systems and the occurrence of two tropical cyclones. Calculated air parcel back trajectories identified the predominant moisture transport paths from the Southwest Pacific Ocean, whereas moisture transport from northerly directions occurred mainly during the wet season monsoonal airflow. Water vapour isotope compositions reflected the same meteorological events as recorded in rainfall isotopes but provided much more detailed and continuous information on atmospheric moisture cycling than the intermittent isotopic record provided by rainfall. Improved global coverage of stable isotope data for atmospheric water vapour is likely to improve simulations of future changes to climate drivers of the hydrological cycle.

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

AbstractStable water isotopes (δ18O and δ2H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ18O values of precipitation and the most positive δ18O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ2H and δ18O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.

Wintertime decoupling of urban valley and rural ridge hydrological processes revealed through stable water isotopes

Water from fossil fuel combustion can represent >10% of urban specific humidity, but this fraction is difficult to constrain from meteorological measurements alone. Stable water vapor isotopes can be used to estimate the fraction of combustion-derived vapor (CDV) and characterize the contrast between anthropogenically-altered and natural systems due to CDV's distinctive isotope composition. However, accurate estimates of the CDV fraction of urban humidity requires information on vapor isotope ratios in the absence of anthropogenic emissions. We present data from an urban site in Salt Lake City, UT and from a high-elevation site in the adjacent Wasatch Mountains. Urban vapor d-excess values (d-excess = δ2H–8δ18O) closely track CO2 concentrations on diurnal to weekly timescales, but high-elevation vapor values do not. Instead, the high-elevation site captures large-scale atmospheric variability, with d-excess changes largely following changes in δ18O and humidity. Isotope ratios at the two sites remain distinct throughout most of the winter, indicating that these sites are regularly decoupled from each other and that high-elevation winter measurements may rarely be representative of valley conditions in the absence of urban emissions. Furthermore, high-elevation d-excess may be higher than emissions-free values at the same elevation as the urban area, as vapor d-excess changes non-linearly during condensation at low specific humidity, and would result in an overestimate of urban humidity amounts. Therefore, the high-elevation site may not help place additional constraints on the amount of CDV in urban systems, yet the paired sites show changes in vapor isotope ratios, particularly in d-excess, that capture differences in urban and natural impacts on water vapor cycling.

Resolving the controls of water vapour isotopes in the Atlantic sector

Stable water isotopes are employed as hydrological tracers to quantify the diverse implications of atmospheric moisture for climate. They are widely used as proxies for studying past climate changes, e.g., in isotope records from ice cores and speleothems. Here, we present a new isotopic dataset of both near-surface vapour and ocean surface water from the North Pole to Antarctica, continuously measured from a research vessel throughout the Atlantic and Arctic Oceans during a period of two years. Our observations contribute to a better understanding and modelling of water isotopic composition. The observations reveal that the vapour deuterium excess within the atmospheric boundary layer is not modulated by wind speed, contrary to the commonly used theory, but controlled by relative humidity and sea surface temperature only. In sea ice covered regions, the sublimation of deposited snow on sea ice is a key process controlling the local water vapour isotopic composition.

Miniaturized ring-down spectrometer for CubeSat-based planetary science.

A robust, miniaturized cavity ring-down spectrometer has been developed as a laboratory demonstration model for future CubeSat deployments of near- and mid-infrared spectrometers for in situ planetary science. The spectrometer is compact enough to ensure compatibility with standard CubeSat spacecraft buses, with a probed gas volume of less than 2.5 cc to ease mass, volume, and power requirements of sample gas handling subsystems. When operated at 1.39μm for water vapor isotope measurements, a noise-equivalent absorption coefficient of 3.7×10-9 cm-1 Hz-1/2 is obtained. Oxygen isotope measurements were performed to demonstrate scanning performance. The spectrometer has been designed to use only components with functional equivalents throughout the 1-5μm range to maintain flexibility across a wide array of planetary science targets. Preliminary results from a 3.27μm implementation intended for methane measurements are also presented.

Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor

Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales.

Stable isotope variations of daily precipitation from 2014\u20132018 in the central United States

Stable isotopes of hydrogen and oxygen (δ2H, δ18O and δ17O) serve as powerful tracers in hydrological investigations. To our knowledge, daily precipitation isotope record especially 17O-excess is rare in the mid-latitudes. To fill such knowledge gap, daily precipitation samples (n=446) were collected from June 2014 to May 2018 in Indianapolis, Indiana, U.S. A Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique was used to concurrently measure precipitation isotopic variations (δ2H, δ18O and δ17O). Meanwhile, 17O-excess and d-excess as second-order isotopic variables were calculated to provide additional information on precipitation formation and transport mechanisms. This study presents a four-year daily precipitation isotope dataset for mid-latitudes, and makes it available to researchers around the world who may use it as a reference for site comparisons and for assessing global hydrological models.

Eddy covariance measurements of the dual-isotope composition of evapotranspiration

Measurements of the isotopic composition of water vapor (H2Ov) provide valuable insights into the hydrological cycle, whereas eddy covariance (EC) measurements are widely used to quantify biosphere-atmosphere exchange processes. Yet, the direct combination of water isotope approaches and the EC method remains technically challenging. Here, we present the first EC measurements of δ18O and δD of evapotranspiration (ET) over the full growing season of a beech forest in central Germany. This EC implementation is based on 2 Hz measurements of the mole fraction and isotopic composition of water vapor (CH2O,v, δD and δ18O) with a customized version of a commercially available water vapor isotope analyzer. The isotopic composition of ET showed a seasonal variability from −19 to 0‰ for δ18OET and from −140 to −25‰ for δDET. The setup-specific limitations of our measurements yield a mean bias of 0.03 mmol m−2 s−1 of the measured net water flux. Spectral and cospectral analysis showed that high-frequency dampening was less pronounced for our EC setup (which was equipped with heated tubing) than for the standard EC setup at this site. Thus, we conclude that direct EC measurements of the isotopic composition of ET are feasible for both, δ18OET and δDET. We propose that EC-based measurements of the isotopic composition of ET are feasible to improve our understanding of the hydrological cycle, especially where flux gradient methods show limited applicability. Our simultaneous measurements of δDET and δ18OET reveal the difference between transpiration-dominated and evaporation-dominated periods. This study highlights the potential of simultaneous measurements of δ18OET and δDET.

Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals

Abstract. Retrievals of vertical profiles of key atmospheric gases provide a critical long-term record from ground-based Fourier transform infrared (FTIR) solar absorption measurements. However, the characterization of the retrieved vertical profile structure can be difficult to validate, especially for gases with large vertical gradients and spatial–temporal variability such as water vapor. In this work, we evaluate the accuracy of the most common water vapor isotope (H216O, hereafter WV) FTIR retrievals in the lower and upper troposphere–lower stratosphere. Coincident high-quality vertically resolved WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA frost point hygrometers (FPHs) are used as reference to evaluate the performance of the retrieved profiles at two sites: Boulder (BLD), Colorado, and at the mountaintop observatory of Mauna Loa (MLO), Hawaii. For a meaningful comparison, the spatial–temporal variability has been investigated. We present results of comparisons among FTIR retrievals with unsmoothed and smoothed FPH profiles to assess WV vertical gradients. Additionally, we evaluate the quantitative impact of different a priori profiles in the retrieval of WV. An orthogonal linear regression analysis shows the best correlation among tropospheric layers using ERA-Interim (ERA-I) a priori profiles and biases are lower for unsmoothed comparisons. In Boulder, we found a negative bias of 0.02±1.9 % (r=0.95) for the 1.5–3 km layer. A larger negative bias of 11.1±3.5 % (r=0.97) was found in the lower free troposphere layer of 3–5 km attributed to rapid vertical change of WV, which is not always captured by the retrievals. The bias improves in the 5–7.5 km layer (1.0±5.3 %, r=0.94). The bias remains at about 13 % for layers above 7.5 km but below 13.5 km. At MLO the spatial mismatch is significantly larger due to the launch of the sonde being farther from the FTIR location. Nevertheless, we estimate a negative bias of 5.9±4.6 % (r=0.93) for the 3.5–5.5 km layer and 9.9±3.7 % (r=0.93) for the 5.5–7.5 km layer, and we measure positive biases of 6.2±3.6 % (r=0.95) for the 7.5–10 km layer and 12.6 % and greater values above 10 km. The agreement for the first layer is significantly better at BLD because the air masses are similar for both FTIR and FPH. Furthermore, for the first time we study the influence of different WV a priori profiles in the retrieval of selected gas profiles. Using NDACC standard retrievals we present results for hydrogen cyanide (HCN), carbon monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the ground truth and evaluating the impact of other WV profiles. We show that the effect is minor for C2H6 (bias <0.5 % for all WV sources) among all vertical layers. However, for HCN we found significant biases between 6 % for layers close to the surface and 2 % for the upper troposphere depending on the WV profile source. The best results (reduced bias and precision and r values closer to unity) are always found for pre-retrieved WV. Therefore, we recommend first retrieving WV to use in subsequent retrieval of gases.

A global database of water vapor isotopes measured with high temporal resolution infrared laser spectroscopy

The isotopic composition of water vapour provides integrated perspectives on the hydrological histories of air masses and has been widely used for tracing physical processes in hydrological and climatic studies. Over the last two decades, the infrared laser spectroscopy technique has been used to measure the isotopic composition of water vapour near the Earth’s surface. Here, we have assembled a global database of high temporal resolution stable water vapour isotope ratios (δ18O and δD) observed using this measurement technique. As of March 2018, the database includes data collected at 35 sites in 15 Köppen climate zones from the years 2004 to 2017. The key variables in each dataset are hourly values of δ18O and δD in atmospheric water vapour. To support interpretation of the isotopologue data, synchronized time series of standard meteorological variables from in situ observations and ERA5 reanalyses are also provided. This database is intended to serve as a centralized platform allowing researchers to share their vapour isotope datasets, thus facilitating investigations that transcend disciplinary and geographic boundaries.

Changes in the Multilayer Dielectric Coating Reflection Coefficient under Variation in the Medium Humidity

It is found that the reflection coefficient of multilayer dielectric mirrors strongly depends on the medium (gas) humidity. This effect can result in both an increase and decrease in the reflection coefficient, which is determined by a change in the refractive indices of the dielectric layers (when filling with water vapor). The mirror reflection coefficient can increase up to 0.9% in a gas with the humidity close to the dew point. Changes in the reflection coefficient of a mirror in gaseous media which contain different water vapor isotopes (H216O, H218O, and D2O) are studied. Mirrors of a CRDS spectrometer with the reflection coefficient R = 0.9999 are studied and the upper bound of the variation in the reflection coefficient versus air humidity is estimated.

Tracking the Strength of the Walker Circulation with Stable Isotopes in Water Vapor.

General circulation models (GCMs) predict that the global hydrological cycle will change in response to anthropogenic warming. However, these predictions remain uncertain, in particular for precipitation [IPCC, 2013]. Held and Soden [2006] suggest that as lower-tropospheric water vapor concentration increases in a warming climate, the atmospheric circulation and convective mass fluxes will weaken. Unfortunately, this process is difficult to constrain, as convective mass fluxes are poorly observed and incompletely simulated in GCMs. Here, we demonstrate that stable hydrogen isotope ratios in tropical atmospheric water vapor can trace changes in temperature, atmospheric circulation and convective mass flux in a warming world. We evaluate changes in temperature, the distribution of water vapor, vertical velocity (ω) and advection, and water isotopes in vapor (δD V ) in water isotopeenabled GCM experiments for modern vs. high CO 2 atmospheres to identify spatial patterns of circulation change over the tropical Pacific. We find that slowing circulation in the tropical Pacific moistens the lower troposphere and weakens convective mass flux, both of which impact the δD of water vapor in the mid-troposphere. Our findings constitute a critical demonstration of how water isotope ratios in the tropical Pacific respond to changes in radiative forcing and atmospheric warming. Moreover, as changes in δD V can be observed by satellites, our results develop new metrics for the detection of global warming impacts to the hydrological cycle and, specifically, the strength of the Walker Circulation.

Seasonal Patterns of Water Cycling in a Deep, Continental Mountain Valley Inferred From Stable Water Vapor Isotopes

AbstractThe origin of atmospheric moisture is difficult to determine from meteorological measurements alone. Stable water vapor isotopes can be used to trace the atmospheric history of water vapor, and therefore are a powerful tool for understanding water cycle processes across a range of spatial and temporal scales. We present subhourly measurements of summer and winter vapor isotope ratios from a deep mountain valley in northwestern Wyoming. Vapor isotopes are paired with local meteorological measurements, and relationships between local vapor isotopic compositions and large‐scale air transport are explored using atmospheric back trajectories and a Lagrangian moisture source diagnostic. Isotopic compositions vary on diurnal to seasonal timescales, and to first order, track changes in local specific humidity. Local vapor isotopic composition varies with air transport pathway in both seasons, but cannot be uniquely mapped to a source region due to continental recycling of vapor. Diurnal variability in d‐excess is substantial in summer, but not in winter. This seasonal contrast is likely driven by local surface and near‐surface processes including evapotranspiration and boundary layer mixing, which partially overprint the source isotope ratios of the regional source during the summer and casts doubt on the use of d‐excess over continental interiors as a tracer of evaporative conditions over the ocean. Continued monitoring of the isotopic composition of near‐surface vapor in continental settings may help detect changes in regional moisture convergence and the local evapotranspiration flux, as well as assess meso‐to‐regional scale responses to hydrologic variability.

Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions

An attractive tracer of water-surface interactions in humidity exposure experiments are the stable hydrogen and oxygen isotopes in water vapor, though the conventional method of using salt saturated aqueous solutions to control humidity is likely to exert strong effects on the H and O stable isotope composition of the vapor. The magnitude of these effects are virtually unknown for hyper-saline solutions near salt saturation and beyond. Here we explore the hydrogen and oxygen stable isotope effects of salt-water mixtures from dilute to the salt saturation point and beyond for BaCl2, CaCl2, KCl, K2SO4, MgCl2, NaBr, NaCl, and NaI salts, of which the O isotope results on BaCl2, K2SO4, and NaBr are the first to be reported, and the first for H with BaCl2. We find that vapor humidity levels, and hydrogen and oxygen isotope effects in the vapor generated from dilute solutions to salt-saturated mixtures increase in magnitude by predictable linear trends. In the case of hydrogen isotope salt effects, the cation identity matters, as the effect becomes stronger with decreasing size and increasing charge (increasing ionic potential) in the sequence of: K < Na < Ba < Ca < Mg. For oxygen, the cation effect becomes stronger with increasing ionic potential, in the order of Na < Ba < K < Ca < Mg (irrespective of sign). Hydrogen and oxygen isotope effects in vapor from salt concentrations greater than the salt saturation point do not behave as systematically, with some of the salts displaying no increase in isotope effects beyond saturation, while others show an increase in isotope effects beyond salt saturation, especially in the case of hydrogen. We make specific recommendations on the salts that exert hydrogen and oxygen isotope effects with the least magnitude and most predictability for humidity levels from 26% to 92%. The use of salt solutions to create vapor at less than 100% and with specific δ2H and δ18O values that differ from that defined by the global meteoric water relationship also seems possible. Hyper-saline surface waters can exist at and above saturation, and thus these isotope effects may be present in them, as well as in minerals that form from these waters.

Climatic and environmental controls on stable isotopes in atmospheric water vapor near the surface observed in Changsha, China

Continuous measurements were made of isotopic compositions in atmospheric water vapor (δv) and in atmospheric precipitation (δp) near the surface in the subtropical monsoon region (Changsha, China) from October 2014 to March 2017 to investigate the change characteristics of vapor isotopes and their interactions with precipitation isotopes. On a diurnal time scale, the δ18Ov values were more negative in the daytime and more positive at night, and the change in dv was reversed during non-rainy days; however, the δ18Ov values were more positive in the daytime and more negative at night, and the variation in dv was not significant during rainy days. On a seasonal time scale, affected by distinctive moisture sources, the vapor isotopes had obvious seasonal variations, with higher δ18Ov and dv values occurring in cold seasons than in warm seasons. There were generally consistent and significant negative linear relationships of dv with temperature and absolute humidity, but the linear relationships between δ18Ov and meteorological factors were variable on the different time scales, which was closely associated with the local weather conditions on the short time scales from diurnal to intra-seasonal and movements of air masses with distinct thermodynamic properties on seasonal and sometimes intra-seasonal time scales. Although the change trend of daily δ18Ov closely approximated that of δ18Op, δ18Op experienced significant relative enrichment compared to δ18Ov, with an average enrichment of 9.18‰. Overall, the isotopic compositions of atmospheric water vapor were in equilibrium with those of precipitation while rainfall events occurred, except for those in the cold season in 2014, and when tracing short-duration weather events, δ18Ov provided more details and information related to weather processes than δ18Op.