Characterization of a customized calibration unit for continuous measurements of the isotopic composition of water vapor

The objective of this work is the development, standardization and creation of a method to carry out continuous measurement of oxygen and hydrogen isotopic composition of the atmospheric water vapor using a wavelengthscanned cavity ring down spectroscopy (WS-CRDS) instrument produced by Picarro, L1102-i model. Some technical improvements of the standard instrument configuration have been made to create three different inlet gas lines: a “standard” line, a calibration line and a line connected with the external sampler. The calibration line is composed of a syringe-pump that continuously injects standard water into a steel tee heated at the temperature of 170◦C and flushed with dry nitrogen gas. In this way, instantaneous and complete vaporization of the standard water takes place. The resulting steam is characterized by a well-defined composition in δD e δ18O values. To allow comparison with other international data, we have characterized the individual instrumental response to variation of the isotopic composition of the water vapor. Several humidity-isotope response functions (6000-26000 ppmv) have been estimated with three different internal standards (0.35h -8.75h -29.11h and -40.28h for δ18O; 2.31h -58.91h -222.19h and -317.78h for δD). Moreover, we have measured the instrumental drift at regular time intervals to apply the opportune corrections to instrument data. The setup has been tested using a 3.5 day continuous measurements carried out with the Picarro sampling the water vapor outside our campus in Venice and parallel sampling using the classical cryogenic trapping procedure, obtaining excellent results. Furthermore, our analysis technique has given good results for the standards with values which are similar to those obtained with the isotope ratio mass spectrometry (IRMS) technique

Stable isotopes in water vapor and precipitation for a coastal lagoon at mid latitudes

The stable oxygen and hydrogen isotope composition in precipitation can be used in hydrology to describe the signature of local meteoric water. The isotopic composition of water vapor is usually obtained indirectly from measurements of δD and δ18O in precipitation, assuming the isotopic equilibrium between rain and water vapor. Only few studies report isotopic data in both phases for the same area, thus providing a complete Local Meteoric Water Line (LMWL). The goal of this study is to build a complete LMWL for the lagoon of Venice (northern Italy) with observations of both water vapor and precipitation. The sampling campaign has started in March 2015 and will be carried out until the end of 2016. Water vapor is collected once a week with cold traps at low temperatures (−77◦C). Precipitation is collected on event and monthly basis with a custom automatic rain sampler and a rain gauge, respectively. Liquid samples are analyzed with a Picarro L1102-i and results are reported vs VSMOW. The main meteorological parameters are continuously recorded in the same area by the campus automatic weather station. Preliminary data show an LMWL close to the Global Meteoric Water Line (GMWL) with lower slope and intercept. An evaporation line is clearly recognizable, considering samples that evaporated between the cloud base and the ground. The deviation from the GMWL parameters, especially intercept, can be attributed to evaporated rain or to the humidity conditions of the water vapor source. Water vapor collected during rainfall shows that rain and vapor are near the isotopic equilibrium, just considering air temperature measured at ground level. Temperature is one of the main factor that controls the isotopic composition of the atmospheric water vapor. Nevertheless, the circulation of air masses is a crucial parameter which has to be considered. Water vapor samples collected in different days but with the same meteorological conditions (air temperature and relative humidity) show differences in terms of δ18O up to 3h. Isotopic ratios in rain events and water vapor are in fact dominated by a seasonal component but outliers are clearly linked to air parcel origin. The monthly measurements of δD and δ18O in precipitation of August 2015, for instance, are lower than in colder months, considering monthly average temperatures. Single rain events show a small sequence of precipitation, that leads to 40% of total precipitation of August, which lowers δ−values considerably. The sampling on event basis during occasional and discontinuous rain also allows to identify the rainout effect, which leads to lightening water during a rainfall. Statistics based on back trajectories (48 hours) show that the major part of air parcels travels across central Europe and derives from sources located in the north Atlantic, whereas, a smaller fraction of the water vapor can be attributed to editerranean sources

The spatial variability in isotopic composition of surface snow and snowpits on the East Antarctic Ice Sheet

The water isotope composition of snow precipitations, archived in the Antarctic ice sheet every year, is an important proxy of climatic conditions. This signal depends on several parameters such as local temperature, altitude, moisture source areas and air mass pathways. However, especially in areas where snow accumulation is very low (as on the East Antarctic Plateau), the isotopic composition is affected by additional spatial variability induced by the interactions between the atmosphere and snow surface, and the pristine signal may be modified through isotopic exchanges, sublimation processes and mechanical mixing originated from wind action. Here, we present the isotopic composition (D and 18O) and the second-order parameter d-excess of surface snow and snowpit samples collected during the Italian-French campaign in Antarctica (2019-2020). The sampling sites cover the area from Dumont D'Urville to Concordia Station and from Concordia Station towards the South Pole (EAIIST – East Antarctic International Ice Sheet Traverse). These data, compared with a previous dataset of Antarctic surface snow isotopic composition (Masson-Delmotte et al. 2008), are analyzed to determine the variability of the spatial relationship between precipitation isotopic composition and local temperature in relation to geographical parameters (latitude, distance from the coast and elevation). The interpretation of these factors determining the isotope signature is the base to better define the amount of the effects caused by subsequent interaction between atmosphere and surface snow, and by the wind action. Understanding the spatial variability of this proxy, which strongly decreases the signal-to-noise ratio, could permit to improve the use of the “isotopic thermometer” to quantify past changes in temperature based on the stable isotopic record of deep ice cores

Investigating two possible schemes of Laser Ablation – Cavity Ring Down Spectrometry for water isotope measurements on ice cores

Thinning of the deep ice core layers must be considered when the water isotopic composition of the Oldest Ice Core is to be analyzed. From an experimental point of view, a novel instrument combining a micro-destructive cold femtosecond - Laser Ablation (LA) sampling system, that provides high spatial resolution together with minimal usage of ice sample, and a Cavity Ring Down Spectrometer is being built for high-quality water isotope measurements. Laser ablation results in crater formation and its morphology depends on the laser parameters used. Optical images that show crater morphology under different experimental conditions allow crater characterization towards an efficient cold LA sampling. An ablation chamber and a transfer line are both the connecting parts between the LA system and the CRDS instrument. They are to be designed and constructed in the optimal size and shape to collect the ablated mass and guarantee its smooth delivery to the CRDS analyzer with minimum disturbance. Coupling a Laser Ablation system with a CRDS analyzer has already been achieved using a laser operating at the nanosecond regime and a cryo-cell as the ablation chamber. Comparison of the two Laser Ablation systems, by the means of ice sampling and collection of the ablated material, will be of great importance to understand the ablation mechanism and post-ablation processes on ice and further develop a system dedicated to water isotope measurements

An upgraded CFA - FLC - MS/MS system for the semi-continuous detection of levoglucosan in ice cores

A new Continuous Flow Analysis (CFA) system coupled with Fast Liquid Chromatography – tandem Mass Spectrometry (FLC-MS/MS) has been recently developed for determining organic markers in ice cores. In this work we present an upgrade of this innovative technique, optimized for the detection of levoglucosan in ice cores, a crucial tracer for reconstructing past fires. The upgrade involved a specific optimization of the chromatographic and mass spectrometric parameters, allowing for a higher sampling resolution (down to 1 cm) and the simultaneous collection of discrete samples, for off-line analysis of water stable isotopes and additional chemical markers. The robustness and repeatability of the method has been tested by the analysis of multiple sticks of ice cut from the same shallow alpine ice core, and running the system for several hours on different days. The results show similar and comparable trends between the ice sticks. With this upgraded system, a higher sensitivity and a lower limit of detection (LOD) was achieved compared to discrete analysis of alpine samples for levoglucosan measurements. The new LOD was as low as 66 ng L−1, a net improvement over the previous LOD of 600 ng L−1