BG2.2

The lower troposphere is where the surface evapotranspiration flux has a strong impact on the atmospheric water vapor isotopic composition, enabling the investigation of the hydro-ecological features of a specific study area. Even though several studies investigated in the last decade the spatial and temporal variability of tropospheric water vapor isotopic composition with ships, aircrafts, satellites and at fixed locations at ground level, vertical profiles and spatial observations acquired within the same time window in the lower troposphere (<3000 m) are still rare. As part of the ground validation of the EU H2020 LEMON project, we used an UltraLight Aircraft (ULA) equipped with a flight-enabled CRDS water vapor isotopes analyzer to probe the vertical and spatial structure of the lower troposphere in Ardèche, Southern France, between 17 and 23 September 2021. In total, 16 flights with different flight strategies were performed for a total flight time of ~20 hours. The flight patterns were mainly designed to obtain representative vertical profiles of the water vapor column below 3000 m for comparison with ground-based LIDAR and to obtain precise estimates of the humidity and water vapor isotopic composition at specific altitude levels, spanning an area of approximately 10 km x 10 km. Due to the flexibility of the ULA, it was also possible to fly several times throughout the day, allowing to study the daytime temporal evolution of the water vapor column within the boundary layer. In general, vertical profile measurements showed evidence of strong mixing process throughout the lower atmospheric column, with both input from free tropospheric layer and surface evapotranspiration. Water vapor stratification, characterized by a large vertical gradient of the isotopic composition, was observed during early morning flights with increased steepness of the vertical isotopic profile along the day. In some cases, flights focused on horizontal and spatial gridding of water vapor isotopic composition showed variation of more than 10‰ for dD in ~5 km² and in less than 0.12 hours. We hypothesize this large horizontal variability to be related to development of thermals within the boundary layer. Our next step will be to summarize the spatial and temporal variability of water vapor isotopic composition for allowing a fair comparison between high-resolution isotope-enabled general circulation models, remote sensing and water vapor observations in the boundary layer.

How to cite: Zannoni, D., Steen-Larsen, H. C., Sodemann, H., Durand, A., Monod, A., Clémençon, A., Dherbecourt, J.-B., Melkonian, J.-M., Hamperl, J., Totems, J., Geyskens, N., Geneau, P., Chazette, P., Nicolas, P., Ravier, S., Flamant, C., and Raybaut, M.: Temporal and spatial variability of water vapor isotopic composition in the lower troposphere: insights from ultralight aircraft measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6295, https://doi.org/10.5194/egusphere-egu22-6295, 2022.

Clumped isotope thermometry deals with the relative abundance of molecules that contain more than one of the rare isotopes. For methane, ¹³CH₃D and ¹²CH₂D₂ isotopologues have been recently proposed as promising tracers in geological, biogeochemical, and atmospheric studies. Their relative abundance denoted as Δ¹³CH₃D and Δ¹³CH₃D is a direct temperature proxy which may, however, also be influenced by kinetic isotope effects. Therefore, thermometry using two independent clumped isotopologues increases the reliability of temperature reconstruction, since departures from thermodynamic equilibrium can be interpreted with respect to kinetic processes or mixing of methane from various methane formation pathways [1,2].

We present an analytical technique based on direct absorption laser spectroscopy for precise, direct, and simultaneous detection of all isotopologues involved in the isotope exchange reactions ¹²CH₄ + ¹³CH₃D = ¹³CH₄ + ¹²CH₃D and ¹²CH₄ + ¹²CH₂D₂ = 2·¹²CH₃D. In contrast to HR-IRMS, which requires ultra-high mass-resolving power M/ΔM > 30000 to achieve a reasonable selectivity for M/z = 18 isotopologues, optical detection is intrinsically free from isobaric interferences and is capable to analyze comparable amounts of sample within a measurement time of tens of minutes. We achieved a precision of 0.02‰ and 0.2‰ for Δ¹³CH₃D and Δ¹²CH₂D₂, respectively, with an external reproducibility of better than 0.1‰ and 1‰ (1σ) for 10 reference-sample repetitions. The instrument employs two quantum cascade lasers (DFB QCL, Alpes Lasers) emitting around 8.6 μm and 9.3 μm spectral regions to simultaneously probe the transitions of all five above-mentioned isotopologues. An astygmatic Herriott-type optical multipass cell with 413 m optical path length (Aerodyne Research Inc.) allows for working with pure methane samples as little as 10 ml to enable the measurement of both Δ¹³CH₃D and Δ¹²CH₂D₂. Rare isotopologues line positions and intensities were surveyed using high-resolution FTIR spectroscopy and validated by laser spectroscopy. The instrument is coupled to a fully automated inlet system and a cryogen-free methane preconcentration unit [3]. Relevant aspects of instrument calibration using methane re-equilibrated in 50-300°C range over γ-Al₂O₃ catalyst and overview of future applications will also be discussed.

[1] Douglas, P., et al. Methane clumped isotopes: Progress and potential for a new isotopic tracer, Organic Geochemistry, 113, 262-282, (2017) https://doi.org/10.1016/j.orggeochem.2017.07.016

[2] Chung, E., & Arnold, T. Potential of clumped isotopes in constraining the global atmospheric methane budget. Global Biogeochemical Cycles, 35, e2020GB006883, (2021) https://doi.org/10.1029/2020GB006883

[3] Prokhorov, I. and Mohn, J.: Cryogen-free fully automated preconcentration unit to enable Δ¹³CH₃D and Δ¹²CH₂D₂ analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-132, (2021) https://doi.org/10.5194/egusphere-egu21-132

How to cite: Prokhorov, I., Tuzson, B., Kueter, N., Rosskopf, R., Li, G., Ebert, V., Emmenegger, L., Bernasconi, S. M., and Mohn, J.: Analysis of methane clumped isotopologues with laser absorption spectroscopy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1335, https://doi.org/10.5194/egusphere-egu22-1335, 2022.

The concentration of atmospheric carbon dioxide (CO₂) has increased since the pre-industrial era (1750) due to human activity leading to a warming of the global land and ocean surface of 1.0 ± 0.2 ºC over the last 30 years could reach 1.5 ºC between 2030 and 2052. A better understanding of the fossil fuel CO₂ emission sources is essential to develop strategies to reduce these emissions, and thus trying to stop the global warming produced by the accumulation of CO₂ in the atmosphere. Policies to achieve these reductions require accurate and robust estimates of these emissions by a monitoring system based on independent atmospheric observations. This system must be able to separate the impact of anthropogenic CO₂ emissions from the effect of the complex natural carbon cycle, which both affect atmospheric CO₂ concentrations.

Radiocarbon (¹⁴CO₂) measurements have been used in conjunction with total CO₂ measurements on both local (e.g. Indianapolis and Heidelberg) and regional scales (e.g. North America and Europe) to separate fossil fuel CO₂ fluxes from biogenic CO₂. The estimation of fossil fuel emissions from atmospheric observations can, in principle, be done by inverse modeling. In this work we will use the LUMIA (Lund University Modular Inversion Algorithm) for performing a series of observation system simulation experiments (OSSEs) inverting simultaneously terrestrial CO₂ and ¹⁴CO₂ observations from the Integrated Carbon Observation System (ICOS) station network to solve for both the natural fluxes (mainly terrestrial) and the anthropogenic fossil fuel emissions, accounting also for the ocean and terrestrial ¹⁴C disequilibrium fluxes. The OSSEs will be performed on a spatial domain over Europe, with a spatial resolution of 0.1° for fossil fuel CO₂ sources and 0.5° for natural CO₂ fluxes and a weekly temporal resolution for natural and anthropogenic emissions and monthly for ocean and terrestrial disequilibrium fluxes 2009 to 2011. We will assess the suitability of the current ICOS ¹⁴CO₂ observation network as well as potential extensions to estimate anthropogenic fossil fuel emissions.

How to cite: Gómez-Ortiz, C., Monteil, G., Karstens, U., Basu, S., Hammer, S., and Scholze, M.: Simultaneous CO2 and 14CO2 atmospheric inversions over Europe to quantify fossil fuel CO2 emissions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12406, https://doi.org/10.5194/egusphere-egu22-12406, 2022.

High precision measurement of multiply substituted ("clumped") isotopologues of CO₂ is a topic of significant interest in the fields of isotope geochemistry and paleoclimate research [1, 2]. The temperature-dependent behavior of ¹³C and ¹⁸O isotopes in gaseous carbon dioxide is widely used as a temperature proxy for paleoclimate reconstruction. The basis for it lies in the temperature dependence of the equilibrium constant, K(T), of the isotope exchange reactions ¹²C¹⁸O₂ + ¹²C¹⁶O₂ ↔ 2٠¹²C¹⁶O¹⁸O and ¹³C¹⁶O¹⁸O + ¹²C¹⁶O₂ ↔ ¹³C¹⁶O₂+¹²C¹⁶O¹⁸O [3, 4] as these reactions have a slight tendency to move towards the right at higher temperatures. Currently, the established method to perform clumped isotope thermometry is Isotope Ratio Mass Spectrometry (IRMS) [5]. However, IRMS measurements, in particular for rare isotopologues, typically require several hours of analysis time and extensive sample preparation to properly separate isobaric interferences. In contrast to IRMS, optical absorption spectroscopic techniques allow the realisation of isotopologue specific, non-destructive, and compact spectrometers with short analysis time and high-precision capabilities. Recently, Wang et al. [6], Prokhorov et al. [3], and Nataraj et al. [4] have demonstrated the great promise of laser absorption spectroscopy for measurements of clumped isotopes of carbon dioxide.

The major challenge for clumped isotope thermometry using ¹²C¹⁸O₂ resides in its very low natural relative abundance (4.1 ppm) and the spectral interference from the major (¹²C¹⁶O₂) and singly substituted isotopologues. These factors seriously limit the achievable analytical performance of spectroscopic measurements and thus the applicability of this technique. However, the interference caused by the hot-band transitions of the abundant species can be suppressed by reducing the gas temperature. Moreover, working at low pressure (5 mbar) narrows the absorption lines and reduces the overlap between neighbouring transitions.

Here, we present a novel quantum cascade laser absorption spectrometer (QCLAS) employing a low-volume segmented circular multipass cell (SC-MPC) [7] operated at cryogenic temperatures (153 K) and low pressure (5 mbar). For the first time, we optically measure the abundances of all three isotopologues involved in the reaction ¹²C¹⁸O₂ + ¹²C¹⁶O₂ ↔ 2٠¹²C¹⁶O¹⁸O simultaneously. We report a precision of 0.05 ‰ in the isotope ratios [¹²C¹⁸O₂/¹²C¹⁶O₂] and [¹²C¹⁶O¹⁸O/¹²C¹⁶O₂] with 25 s integration time. In addition, we determine and resolve the tiny variation in the equilibrium constant, K(T), of the above exchange reaction for carbon-dioxide samples equilibrated at 300 K and 1273 K, respectively. This versatile system can be extended to other chemical species where spectroscopic measurements are impacted by the hot-band transitions of abundant isotopologues — (e.g., methane and its deuterated isotopologues, CH₃D and CH₂D₂, or propane and the two isotopomers, ¹²CH₃¹³CH₂¹²CH₃ and ¹³CH₃¹²CH₂¹²CH₃) — thereby opening up new perspectives in environmental sciences and fundamental research.

[1] J. M. Eiler, Quaternary Science Reviews,doi: 10.1016/j.quascirev.2011.09.001.

[2] S. M. Bernasconi et al., Applied Geochemistry doi: 10.1016/j.apgeochem.2011.03.080.

[3] I. Prokhorov et a.l, Sci Rep, doi: 10.1038/s41598-019-40750-z.

[4] A. Nataraj et al., Optics Express accepted, doi:10.1364/OE.447172

[5] Fiebig et al, Chemical Geology doi: 10.1016/j.chemgeo.2019.05.019

[6] Z. Wang et al., Anal. Chem., doi: 10.1021/acs.analchem.9b04466.

[7] M. Graf et al.,Optics Letters doi: 10.1364/OL.43.002434.

How to cite: Nataraj, A., Gianella, M., Prokhorov, I., Tuzon, B., Bertrand, M., Mohn, J., Faist, J., and Emmenegger, L.: Quantum cascade laser absorption spectrometer with a low temperature multipass cell for precision clumped 12C18O2 measurement, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11659, https://doi.org/10.5194/egusphere-egu22-11659, 2022.

Ocean paleotemperatures have been reconstructed for almost the entirety of the Phanerozoic using the oxygen isotope compositions of calcium carbonates formed by marine organisms and preserved in ocean sediments. However, the isotopic composition of these calcitic tests and shells can be substantially altered through diagenetic processes. Here, we used ¹⁸O as an isotopic tracer in controlled experiments designed to simulate early diagenesis of modern benthic foraminifera tests to investigate how fluids penetrate into and exchange oxygen isotopes with these biogenic calcites. Initially pristine tests of Ammonia sp., Haynesina germanica, and Amphistegina lessonii were immersed in an ¹⁸O-enriched artificial seawater at 90 °C for hours to days. High-resolution SEM images of the tests before and after the experiments were indistinguishable yet the bulk oxygen isotope compositions of reacted tests revealed rapid and species-dependent isotopic exchange with the water. Correlated SEM, TEM and NanoSIMS imaging of ¹⁸O intra-test distributions showed that fluid penetration and exchange is ubiquitous yet heterogenous, and is intimately tied to test ultrastructure and associated organic matter. Species level differences in ultrastructure, quantified through image analysis, explained the observed species-dependent rates of isotopic exchange. Consequently, even calcitic skeletons considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange and hence a critical re-examination of the paleo-temperature record is warranted.

How to cite: Cisneros-Lazaro, D., Adams, A., Guo, J., Bernard, S., Baumgartner, L. P., Daval, D., Baronnet, A., Grauby, O., Vennemann, T., Stolarski, J., Escrig, S., and Meibom, A.: 18O tracer shows diagenetic isotope exchange in biocalcites to be fast, pervasive and species-dependent, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6996, https://doi.org/10.5194/egusphere-egu22-6996, 2022.

Partitioning the measured net ecosystem carbon dioxide (CO₂) exchange into gross primary productivity (GPP) and ecosystem respiration remains a challenge, which is usually tackled by disentangling the net ecosystem CO₂ exchange using various methods. A relatively new approach uses the trace gas carbonyl sulfide (COS) to estimate GPP. This is possible because of the very similar pathways CO₂ and COS take into and within leaves, allowing researchers to use COS uptake as a proxy for the CO₂ uptake in plants. In order to assess the viability of COS as a GPP proxy, COS sources and sinks in ecosystems have to be quantified. One of the biggest unknowns in this regard is the contribution of the soil.

In our study we looked at the effects of live roots on the soil COS-exchange, a topic that has not yet been explored in the literature. While in the last couple of years different working groups measured soil samples in the lab, no study to date looked at the impact of live roots on the soil COS flux. We hypothesized that live roots will change the COS flux by changing microbial community composition and activity via root exudates. In order to investigate the root contribution of a live plant we had to build an experimental setup that would allow us to only measure the belowground plant parts and the soil, while at the same time keep the whole plant alive. The plants used in this study were young beech trees (~2 years) and the soil was commonly used potting soil, in order to ensure a mostly homogeneous substrate for the trees. The measurements were spread out over one year to cover the different phenological stages of the trees, from no leaves in winter to new and mature leaves in spring and summer, respectively, to senescent leaves in autumn. Growth lamps were used to supply the aboveground parts of the plants with light during the day.

Most pots, with and without plants, emitted COS during the course of the experiment. COS and CO₂ emissions increased in pots with roots compared to the control pots, but the increase in CO₂ emissions was much stronger compared to the increase in the COS flux, which lead to consistently higher COS/CO₂ emission ratios in the control pots, which contained potting soil only. A diurnal pattern was visible in all the measurements with the largest emissions for COS and CO₂ occurring in the afternoon, when soil temperatures were the highest. Comparing the measurements over the whole experiment a clear difference in the COS/CO₂ ratio could be observed between the measurements without leaves in February compared to the measurements with leaves in summer and autumn, indicating a dynamic effect of live roots on the soil COS exchange.

How to cite: Kitz, F., Wachter, H., Spielmann, F. M., Hammerle, A., and Wohlfahrt, G.: Exploring the impact of live roots on the soil COS flux, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6139, https://doi.org/10.5194/egusphere-egu22-6139, 2022.

Biogenic gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are regularly analysed in many environments to understand elemental cycling and processes through the ecosphere. They are also of interest to atmospheric chemists for their role in climate change. The Elementar Isoprime TraceGas, coupled with Elementar isotope ratio mass spectrometer (IRMS), has been a key to a significant number of studies providing data on the isotopes of these key dynamic molecules. We shall review some of the notable publications and modifications in the field of atmospheric gas monitoring.

The development of the recently launched isoprime precisION IRMS has permitted a new generation of control and automation of the mass spectrometer and integrated peripherals. This has greatly improved the accessibility and versatility of the instruments as a whole. Taking advantage of the inherent benefits of the isoprime precisION, the iso FLOW GHG has been developed for high performance analysis of CO₂, N₂O and CH₄ as a successor to the isoprime TraceGas, and has the capacity to be rapidly customised for specific needs with options for N₂ and N₂O analysis, analysis of hydrogen isotopes in CH₄, and high precision and sensitivity measurement of nitrate-derived N₂O as generated from denitrifier techniques. We present an outline of the latest generation hardware available to the gas researcher and explain how it’s standard modes and configurations take biogenic gas analysis further than before.

How to cite: Barker, S., Taylor, K. W. R., Hackett, P., and Price, W.: Analysis and Monitoring Atmospheric Gases in a High-Performing and Versatile Isotope Ratio Instrument, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1668, https://doi.org/10.5194/egusphere-egu22-1668, 2022.

Understanding the enzymes responsible for biological nitrogen fixation in the natural environment is crucial for understanding the global nitrogen cycle. The isotopic acetylene reduction assay (ISARA) is currently one of the only ways to distinguish between nitrogenase enzymes and it involves measuring the δ¹³C of ethylene generated via the reduction of acetylene. However, the classical method can only be applied to samples with ethylene concentrations >1,000 ppm which is limiting for environmental samples, where N₂ fixation activity is generally low resulting in a low headspace ethylene concentration (<300 ppm).

Here we describe an improved analytical method for analyzing δ¹³C of ethylene using a homemade gas pre-concentration system and reproducible in-house standards developed from commercially available ethylene tanks. We also present a simple methodology using mutants of Azotobacter vinelandii (Mo-only and V-only nitrogenase) and the removal of headspace acetylene by chemical precipitation to easily scale the ISARA experiment from δ¹³C to complementary nitrogenase contribution without the uncertainty and tediousness surrounding measurement of the source acetylene.

The new Low activity - ISARA (LISARA) method can now estimate contribution of complementary nitrogenase from environmental samples with as little as 10 ppm of ethylene. Updated limit of quantification for δ¹³C of ethylene is < 2 ppm. Finally, we demonstrate the applicability of the method using samples with characteristically low N₂ fixation activity (termites, wood, leaf litter, soil, moss), with substantial contribution of complementary nitrogenase across multiple sites in the northeastern United States.

Our results expand our knowledge of the contribution of complementary nitrogenase to temperate ecosystems. The new methodology will allow broader access to the classical ISARA method for pure culture experiments and high activity samples through the outsourcing of δ¹³C ethylene measurements, facilitating the study of complementary nitrogenases.

How to cite: Haynes, S., Darnajoux, R., Han, E., and Zhang, X.: Methodological and analytical improvement of the ISotopic Acetylene Reduction Assay for the assessment of complementary biological nitrogen fixation in low activity samples, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10896, https://doi.org/10.5194/egusphere-egu22-10896, 2022.