Cities, responsible for 67 to 72% of global fossil fuel CO2 emissions, play an important role in climate change mitigation. However, uncertainties in city-level inventories can affect assessments of urban emission reduction measures. Monitoring CO2 mole fractions in urban boundary layers can help constrain total emission estimates, while additional measurements of stable carbon isotope composition of CO2 (δ13C-CO2) can infer respective contributions from different anthropogenic and natural sources. This study reports observations from June 2022 to August 2023 using a Picarro G2131-i analyzer set up for automated, high-precision measurements of atmospheric CO2 mole fractions and δ13C-CO2 on a tall-tower 144 m above Vienna, Austria. The elevated inlet height enables representative sampling of air masses influenced by both regional and local urban sources. To separate regional and urban signals, a city baseline was modelled from the diel-seasonal cycle and regional background CO2. Discrete short-lived enhancements were identified and used to estimate isotopic signature of the local urban contribution (δ13C-LUC) with two-component mixing models. The δ13C-LUC values fall between endmembers for liquid-fuel and natural-gas combustion, consistent with Vienna's emission inventory. Monthly δ13C-LUC values were increased during summer and decreased during winter, significantly correlating with δ13C values modelled from “Emissions Database for Global Atmospheric Research” data for Vienna. This pattern reflects the shifts from low natural-gas use in summer toward high natural-gas consumption for heating in winter. This study highlights the potential use of these tall-tower observations as basis for detecting temporal changes in Vienna's fuel mixture.

Like other cities across the globe, Vienna has announced an ambitious climate target of reaching net zero emissions of greenhouse gases (GHG) by 2040. Developing and implementing appropriate urban polices and measures to reach this goal requires robust understanding and quantification of the emissions of all GHGs, including Methane (CH4). According to the Austrian provincial emission inventory (Bundesländer Luftschadstoff-Inventur) that sets the current baseline for Vienna’s planned emission reductions, CH4 emissions contribute only ca. 1% of the city’s total GHG budget (87 of 8387 kt CO2eq in 2021; 100-year global warming potential). However, urban sources of methane are associated with large uncertainties (e.g. leaks from gas distribution networks, post-meter fugitive emissions) with inventories often producing substantial underestimations of these emissions. The Vienna Urban Carbon Laboratory is currently investigating the CH4 budget of Austria’s capital city with local atmospheric observations of turbulent fluxes and total column mole fractions. Since May 2022, CH4 fluxes are being measured at the Arsenal radio tower using an eddy covariance system installed 144 m above the city. Furthermore, between May and July 2022, a parallel measurement campaign with four ground-based, sun-viewing FTIR spectrometers (EM27/SUN) was conducted to measure horizontal gradients in total column CH4 mole fractions. This conference contribution will discuss the temporal and spatial patterns in the CH4 fluxes derived directly from eddy covariance observations so far, as well as the inverse estimates of summer CH4 emissions from the EM27/SUN observations. The two independent, observation-based methods will provide robust, and policy-relevant indications on the potential accuracy (or potential bias) in Vienna’s CH4 inventory.

The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO2) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gas by far. For this reason, we are presenting a measuring system here that enables a wide range of users to carry out stable isotope analysis of atmospheric CO2 using off the bench hard- and software. The fully automated system uses cryogenic and gas chromatographic separation to analyse CO2 from 12 mL whole air samples and consists of an autosampler, a Gasbench II, a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/Gasbench II/Cold Trap/Conflo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO2 sources (four CO2 reference gases and four artificial air tanks). The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO2 source) in the determination of the carbon and oxygen isotope ratio was 0.04 ‰ and 0.09 ‰ (n=24). Furthermore, we were able to show that the measurement data also allowed for the quantification of the CO2 mole fraction, with a precision of 1.2 µmol mol-1 in the analysis range of 400 to 500 µmol mol-1. The method to be presented was summarized and published in the form of a protocol (DOI: 10.1002/rcm.9647) providing a detailed description of the measurement setup and the analysis procedure, how raw data should be evaluated and gives recommendations for sample preparation and sampling to enable a fully automated whole air sample analysis. We look forward to further discussion with interested users to elaborate on potential improvements/extensions/application options.

The substantial urban contribution to global anthropogenic greenhouse gas (GHG) budgets underlines the importance of improved GHG emissions monitoring in cities. Reducing urban emissions of carbon dioxide (CO2) and methane (CH4) will be critical to mitigating climate change; yet, GHG budgets of individual cities as quantified by emission inventories can be very uncertain. This is due to a lack of appropriate activity data and emission factors for compiling city-scale inventories or uncertainties in spatial downscaling of regional/national emissions.The Vienna Urban Carbon Laboratory is currently investigating how monitoring of CO2 and CH4 emissions in Austria’s capital city can be supported by a range of atmospheric measurement methods, including a tall-tower application of eddy covariance flux measurements. Cities can represent non-ideal conditions for eddy covariance due to the aerodynamically rough surface conditions and spatial heterogeneity in GHG sources (and sinks). Nonetheless, if biases/errors caused by these factors are acceptable, the method provides a potentially significant advantage in that net urban emissions can be directly inferred from the measured vertical turbulent fluxes. Since December 2017, CO2 fluxes at 144 m above the surface have been measured using an eddy covariance system deployed at the A1 Arsenal radio tower in Vienna’s city centre. The original rationale for the tall tower approach was to partially mitigate the aforementioned challenges of urban eddy covariance (e.g. to get above the deep urban roughness layer and measure in the surface layer) and to increase the area of the city sampled by the flux footprint. In May 2022, the observations at the tower were expanded to measure CH4 fluxes, as well as atmospheric mixing ratios of CO2 and its stable carbon isotope composition. Furthermore, between May and July 2022, a parallel measurement campaign with four ground-based, sun-viewing FTIR spectrometers (EM27/SUN) was conducted to measure horizontal gradients in total column CO2 and CH4 concentrations.This conference contribution will present an analysis of the tall-tower eddy covariance measurements of CO2 and CH4 fluxes and discuss potential applications within the scope of operational emissions monitoring. In addition to discussing the encouraging agreement between eddy covariance measurements and local CO2 emission inventories for the years 2018 to 2020, the initial eddy flux-inventory comparison for CH4 will be presented. Moreover, planned analyses (and initial results, where available) on several relevant fronts will be briefly discussed: comparison of the eddy fluxes with inverse modelled CO2 and CH4 fluxes using differential column concentration measurements; comparison of partitioned CO2 fluxes with source-sector emission estimates derived from local inventories and measurements of stable carbon isotope composition of atmopsheric CO2. Finally, trends in CO2 fluxes between 2018 and 2022 will be presented to highlight the potential early indicator function and immediate societal benefits that urban eddy covariance can provide.

<p>Cities contribute significantly to global carbon dioxide (CO<sub>2</sub>) emissions, and it is important to understand and accurately measure these emissions in order to effectively mitigate climate change. Current methods for estimating emissions, such as emission inventories, can be very uncertain at the scale of individual cities. Measurement methods that involve analyzing local atmospheric CO<sub>2</sub> levels and the respective stable carbon isotopic composition of CO<sub>2</sub> can provide additional, independent information on local emissions, particularly in terms of source contributions from combustion of different fossil fuels and natural respiration. As part of the Vienna Urban Carbon Laboratory (VUCL), a cavity-ring-down laser isotope spectrometer (G2201-<em>i</em>, Picarro Inc., USA) has been operating on a radio tower in Vienna’s city centre since May 2022 to measure atmospheric mixing ratios of CO<sub>2</sub> and stable carbon isotopic composition of CO<sub>2</sub> (δ<sup>13</sup>C) 144 m above the surface.</p> <p>The overall objective here is to establish an analysis framework to best utilize these measurements in combination with tall-tower eddy covariance measurements for the identification and quantification of local CO<sub>2</sub> emission emitters in Vienna. Initial analysis of the half-hourly CO<sub>2</sub> concentrations and fluxes between May and Dec 2022 show that a night-time increase of measured CO<sub>2</sub> concentrations are followed by an early morning peak, due to a nocturnal build-up of surface-level CO<sub>2</sub> that is followed by an upward flush of CO<sub>2</sub> in the morning. The δ<sup>13</sup>C of CO<sub>2</sub> (based on keeling plot analysis) suggests that fluxes from natural respiration are dominant over the night. In the afternoon, the δ<sup>13</sup>C of CO<sub>2</sub> sources decreases, which may be due to an increased contribution from sources with isotopically depleted CO<sub>2</sub>, such as traffic emissions and small-scale stationary methane combustion. We also observed higher concentrations CO<sub>2</sub> that are isotopically depleted, during the summer when winds came from the area southeast of the tower, which has more industrial and refinery activity. In addition to these initial results from keeling plot analysis, our presentation will also include results from the ongoing winter measurements, where we expect to see indications of enhanced methane combustion for space heating. Furthermore, results from ongoing tests of other analysis methods for identifying emitting sources (e.g., application of the miller trans model method, analysis of the data at higher temporal resolutions) will be presented.</p>

<p>More than two thirds of global anthropogenic greenhouse gas (GHG) emissions originate from cities. Urban mitigation policies need a reliable emission data basis to effectively reduce emissions and given inventory uncertainties at the level of single cities, there is growing interest in measurement-based methods to support urban GHG emissions monitoring. Inverse modelling is a measurement-based approach that integrates atmospheric observations with emission inventories, whereby the inventories serve as prior estimates that are subsequently constrained against the observations. While such inverse systems rely on modelling frameworks that typically utilise in situ and/or remote measurements of atmospheric GHG mixing ratios, there is scope for city-scale inverse frameworks to utilise other types of GHG observations, such as flux measurements.</p> <p>In this study, we investigate such an approach based on a two month field campaign between 15th of May and 20th of July in 2022 in Vienna, Austria. In particular, for the prior information, we use tall-tower eddy covariance observations to constrain the CH<sub>4</sub> emissions within the tower's flux footprint and combine the measurements with 1km x 1km inventory data of the larger city area of Vienna. This refined and measurement-supported inventory serves as a-priori information for both, a Bayesian- and a Phillips-Tikhonov based inversion approach. The observational input for the inversion methods is delivered by MUCCnet (Munich Urban Carbon Column network) instruments consisting of four ground-based, sun-viewing FTIR spectrometers (EM27/SUN), with three of these instruments located on the outskirts of Vienna and one instrument located at the bottom of the tall-tower close to the city center.<span class="Apple-converted-space"> </span></p> <p>This study investigates the synergetic aspects of two different measurement systems: the eddy-covariance system is particularly sensitive to near field emissions with a range of hundreds of meters upwind of the tower, whereas the ground-based remote sensing instruments observe the differential total column concentration and are therefore sensitive to emissions originating several Kilometers upwind. Applying both measurement systems within a city inversion framework may indeed represent a viable option for further constraining city emissions and improving urban GHG emissions monitoring.</p>

RATIONALE:The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO2 ) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gases by far. For this reason, we are presenting a measuring system here that enables a wide range of users to carry out stable isotope analysis of atmospheric CO2 using off-the-bench hardware and software.METHODS:The fully automated system uses cryogenic and gas chromatographic separation to analyse CO2 from 12-mL whole air samples and consists of an autosampler, a Gasbench II (GB), a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/GB/Cold Trap/ConFlo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO2 sources (four CO2 reference gases and four artificial air tanks).RESULTS:The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO2 source) in the determination of the carbon and oxygen isotope ratio was 0.04‰ and 0.09‰ (n = 24). Furthermore, we were able to show that the measurement data also allowed for the quantification of the CO2 mole fraction, with a precision of 1.2 μmol mol-1 in the analysis range of 400-500 μmol mol-1 .CONCLUSIONS:Our protocol provides a detailed description of the measurement set-up and the analysis procedure, how raw data should be evaluated and gives recommendations for sample preparation and sampling to enable a fully automated whole air sample analysis. The quantification limit of CO2 mole fractions and measurement precision for carbon and oxygen isotope ratios of CO2 should meet the requirements of a wide range of users.

As an alternative to activated carbon, biochar is a promising, environmentally friendly sorbent that can be used to remove organic groundwater pollutants, such as chlorinated ethenes (CEs). Stable isotope fractionation in biofilters is used to quantify pollutant degradation and to distinguish degradation from pollutant sorption on e.g. biochar. However, the sorption of CEs on biochar, and the potential abiotic fractionation processes remain to be tested. The sorption process of CEs and ethene on activated carbon and biochar was investigated with regard to the isotope effects for the differentiation from microbial degradation processes. Results from physical and chemical characterization of biochar indicated that biochar feedstock and pyrolysis conditions determined sorption performance depending on the surface chemistry and the pore size distribution of the coarse sorbent particles. The sorption capacity of the activated carbon was significantly higher with highly chlorinated ethenes, but similar to the biochars with low chlorination. Apparent carbon isotope fractionation factors (epsilon) of +0.1 to -4.4 parts per thousand were found above measurement uncertainties of GC/IRMS. The extent of isotope enrichment of the C-13 bearing isotopologues in the residual aqueous phase (epsilon < 0) was characteristic for individual pairs of pollutant and sorbent material and could be related to pore-filling processes limited by the micropore size distribution of sorbent materials and the chemical properties of sorbed pollutants. Especially the large isotope fractionation during the sorption of ethene led to the assumption that diffusion processes within the pore matrix of the sorbent particles contributed to the observed isotope effects, but should still be considered a property of sorption. Concluding on the results indicated that sorption processes can have a significant contribution to carbon isotope fractionation in CEs and ethene. These should not be neglected in the evaluation of biofilters for groundwater purification, in which CEs are simultaneously degraded by microbes.

The study herein reports on the development and testing of sampling systems (and subsequent analytical setups) that were deployed on an unmanned aerial vehicle (UAV) for the purpose of analysing greenhouse gases (GHGs) and volatile organic compounds (VOCs) in the lower atmospheric boundary layer. Two sampling devices, both of which can be mounted to an UAV with a payload capability greater than 1 kg, were tested for respective sampling and analysis of specific GHGs (carbon dioxide, CO2, and methane, CH4) and VOCs (chlorinated ethenes, CEs). The gas analyses included measurements of the molar amounts and the respective stable carbon isotope ratios. In addition to compound calibration in the laboratory, the functionality of the samplers and the UAV-based sampling was tested in the field. Atmospheric air was either flushed through sorbent tubes for VOC sampling or collected and sampled in glass vials for GHG analysis. The measurement setup for the sorbent tubes achieved analyte mass recovery rates of 63 %–100 % (more favourable for lower chlorinated ethenes), when prepared from gaseous or liquid calibration standards, and reached a precision (2σ) better than 0.7 ‰ for δ13C values in the range of 0.35–4.45 nmol. The UAV-equipped samplers were tested over two field sampling campaigns designed to (1) compare manual and UAV-collected samples taken up a vertical profile at a forest site and (2) identify potential emissions of CO2, CH4 or VOC from a former domestic waste dump. The precision of CO2 measurements from whole air samples was ≤7.3 µmol mol−1 and ≤0.3 ‰ for δ13C values and ≤0.03 µmol mol−1 and ≤0.2 ‰ for CH4 working gas standards. The results of the whole air sample analyses for CO2 and CH4 were sufficiently accurate to detect and localise potential landfill gas emissions from a secured former domestic waste dump using level flight. Vertical CO2 profiles from a forest location showed a causally comprehensive pattern in the molar ratios and stable carbon isotope ratios but also the potential falsification of the positional accuracy of a UAV-assisted air sample due to the influence of the rotor downwash. The results demonstrate that the UAV sampling systems presented here represent a viable tool for atmospheric background monitoring, as well as for evaluating and identifying emission sources. By expanding the part of the lower atmosphere that can be practicably sampled over horizontal and vertical axes, the presented UAV-capable sampling systems, which also allow for compound-specific stable isotope analysis (CSIA), may facilitate an improved understanding of surface–atmosphere fluxes of trace gas.

Elevated atmospheric reactive nitrogen (NR) deposition is considered one of the key components of human induced global change, threatening biodiversity and possibly altering carbon sequestration, one of the forest’s key ecosystem services. Carbon sequestration is the net result of plant production and of soil organic matter (SOM) decomposition. Ignoring the impact of N deposition on plant growth, decomposition or any major physical, biological or anthropogenic process that alters the rate of conversion of soil organic matter to atmospheric CO2 (decomposition) will have profound implications for the global C budget and consequently climate change. Soil nitrogen cycling is predicted to change as a result of increased atmospheric N deposition and mineralization due to temperature increases. However, experimental results on the effects of increased N input on SOM decomposition in the field are inconsistent, reporting positive, negative and neutral responses of SOM to N input. We set out to test the impacts of elevated reactive nitrogen NR addition, specifically on the soil processes in the field, independently of forest production effects. Using a suite of conventional, natural abundance and isotope pool dilution methods in situ, we traced C and N transformations of soil microbial and gaseous pools and monitored concomitant changes in gross mineralization and nitrification rates, as well as enzymatic activity. Over a number of growing seasons in a spruce dominated Austrian forest we found evidence to suggest N addition significantly reduces gross N mineralization rates and enzyme activity, in-line with an emerging consensus that N deposition reduces soil fungal abundance and activity, ultimately resulting in greater stocks of soil organic carbon. Simulated elevated nitrogen deposition decelerated SOM decomposition and consequently increased soil carbon storage, an N input effect on soil processes independent of the effect of N on tree growth and forest production.

The Vienna Urban Carbon Laboratory (VUCL) has begun testing in situ measurement-based options for monitoring local carbon dioxide (CO2) and methane (CH4) emissions in Austria’s capital city. Building upon the groundwork of the CarboWien project, VUCL extends and expands the current tall-tower eddy covariance flux system and will furthermore conduct campaigns to measure carbon isotopes and isofluxes, as well as upwind-downwind gradients in total column CO2 and CH4 mixing ratios. The project, which runs between 2021 and 2024 and is funded by the Vienna Science and Technology Fund (WWTF), will be implemented by a collaboration between the University of Natural Resources and Life Sciences Vienna (BOKU), the Technical University of Munich (TUM), the Environment Agency Austria (EAA) and A1 Telekom Austria AG (A1). In addition to contributing to international research into measurement-based greenhouse gas emissions monitoring, the multi-method approach provides an opportunity to demonstrate measurement-based emissions monitoring options directly to Vienna’s civil servants responsible for climate change mitigation action in the city. Continuous local stakeholder engagement over the project duration is therefore planned. This conference contribution to the WMO-IG3IS session at vEGU21 will allow VUCL to be introduced to relevant scientists and stakeholders in the international community. Given the recent project start (01 Feb 2021), the foreseen discussions on the project’s planned implementation will provide an important and timely input into VUCL. Finally, initial VUCL results will be presented together with data from the preceding CarboWien project (2018-2020) to show how the measured CO2 fluxes in Vienna have been impacted by the lockdown restrictions due to the COVID-19 pandemic.

Rationale Measurement of greenhouse gas (GHG) concentrations and isotopic compositions in the atmosphere is a valuable tool for predicting their sources and sinks, and ultimately how they affect Earth\u0027s climate. Easy access to unmanned aerial vehicles (UAVs) has opened up new opportunities for remote gas sampling and provides logistical and economic opportunities to improve GHG measurements. Methods This study presents synchronized gas chromatography/isotope ratio mass spectrometry (GC/IRMS) methods for the analysis of atmospheric gas samples (20-mL glass vessels) to determine the stable isotope ratios and concentrations of CO2 , CH4 and N2 O. To our knowledge there is no comprehensive GC/IRMS setup for successive measurement of CO2 , CH4 and N2 O analysis meshed with a UAV-based sampling system. The systems were built using off-the-shelf instruments augmented with minor modifications. Results The precision of working gas standards achieved for δ13 C and δ18 O values of CO2 was 0.2‰ and 0.3‰, respectively. The mid-term precision for δ13 C and δ15 N values of CH4 and N2 O working gas standards was 0.4‰ and 0.3‰, respectively. Injection quantities of working gas standards indicated a relative standard deviation of 1%, 5% and 5% for CO2 , CH4 and N2 O, respectively. Measurements of atmospheric air samples demonstrated a standard deviation of 0.3‰ and 0.4‰ for the δ13 C and δ18 O values, respectively, of CO2 , 0.5‰ for the δ13 C value of CH4 and 0.3‰ for the δ15 N value of N2 O. Conclusions Results from internal calibration and field sample analysis, as well as comparisons with similar measurement techniques, suggest that the method is applicable for the stable isotope analysis of these three important GHGs. In contrast to previously reported findings, the presented method enables successive analysis of all three GHGs from a single ambient atmospheric gas sample.

Changes in climate will bring along changes in precipitation patterns, and as such, it will determine the availability of water in agricultural systems. We aimed to investigate the impact of climate-induced altered precipitation regimes on crop performance and soil processes such as organic matter mineralisation and nutrient release. The experiment took place at the lysimeter station located in Hirschstetten, Vienna, Austria (48° 15' 22" N, 16° 289 3" E, 160 m a.s.l.) where a future precipitation scenario was compared with current precipitation patterns on two different soil types – a sandy calcaric Phaeozem and a calcic Chernozem, both being representative for the Marchfeld region in Lower Austria. The future precipitation regime was calculated from four regionalised scenarios from Euro-Cordex out of the ÖKS 15 ensemble following the GHG emission scenarios RCP 4.5 and RCP 8.5. Stable isotope analysis has become a useful tool for sensitively tracing biogeochemical processes in soils. In this study, plant residues of white mustard (Sinapis alba), isotopically labelled with carbon 13C and nitrogen 15N in a controlled laboratory environment were applied as organic fertiliser (green manure) on the lysimeter soils in April 2018. Soil, plant, gas and groundwater samples were collected from the lysimeters throughout the growing season of 2018 and 2019 and analysed using cavity ring-down spectrometry (CRDS) for 15N-N2O in the field and by isotope ratio mass spectrometry. Crop results showed an increase in the shoot 13C signatures, indicative of drought stress, which resulted in diminished plant production by -20 to -50% under the decreased precipitation. Isotope analysis showed lower decomposition and mineralisation rates of labelled green manure only during the first few days under the future precipitation treatment, followed by an increase in 15N enrichment of soil solution NO3- during summer, emphasising the importance of plant biomass production on root NO3- uptake from the soil. N2O emissions were higher after the application of synthetic fertiliser during the first year, highlighting the importance of available NO3- in agricultural systems for nitrification and denitrification processes. However, lower N2O emissions were observed during the second year, indicating possible N stress. Overall we found that N losses through NO3- leaching and N2O emissions were most sensitive to reduced precipitation when NO3- is available, which can cause aggravating environmental problems in the future. The stable isotope labelling technique proved to be successful for tracing and identifying drought stress effects on plant and soil processes in agricultural systems, allowing for a better understanding of soil-plant processes under changing climate conditions.

Currently sampling of the atmosphere for gas emission measurements involves building towers or hiring airplanes - capital-intensive methods. Easy access to unmanned aerial vehicles (UAV) has opened-up new opportunities for remote gas sampling. The project Iso-2-Drone aims to develop and produce a modular UAV-based gas monitoring system for emission measurements to substitute current technologies. A key feature of the UAV-attached gas sampler design was the ready-to-use nature of the system. This meant that the system was designed to mesh with commonly available equipment, using collection vessels which can be easily and immediately measured by common continuous flow - isotope ratio mass spectrometer (CF-IRMS) instrumentation. The target compounds comprise the three major natural greenhouse gases CH4, CO2 and N2O to be measured at natural isotopic abundance and ambient levels. We use 20 mL headspace vials for CH4 and CO2 sampling. Vials can be conditioned on-sight with our sample preparation prototype using repeatedly evacuating and synthetic air refilling cycles to prevent ambient air contamination. On the UAV-attached sampler atmospheric air is sampled passively by pressure compensation of the vacuum. N2O is sampled actively via adsorption tubes, filled with Molecular Sieve 5Å and conditioned in the lab. Both a prototype device and two UAV-attached samplers have been designed, built and are currently tested. The measurement setup in the lab comprises of two autosamplers, a purge & trap system (VSP 4000, IMT Innovative Maschinentechnik GmbH) and a headspace sampler (CTC CombiPal, Chromtech GmbH) in order to switch from ppb range necessary for CH4 and N2O to a ppm range for CO2. For CO2 measurements the CTC injects 600 µl of sampled air to a Restek Micropacked Column (Shin Carbon ST 100/120, 2m x 1mm ID and 1/16” OD) within a Thermo Scientific Trace GC Ultra heated up from 40°C to 110°C, maintained for 5 min, before heating up to 180°C by 12°C per minute. Thereby CO2 is properly separated from the potentially interfering N2O. For CH4 the residual air sample is cryo-focused at -140°C in a HayeSep D filled trap, transferred to the GC and targeted with a Poraplot Q (30m x 0.32mm) held at 35°C. Using the similar GC method and autosampler N2O is desorbed after switching the autosampler to thermal desorption mode. All three analytes pass an oxidation/reduction reactor (1030°C) before they are introduced into the IRMS (Thermo Scientific DeltaV Advantage) via a universal gas interface (Thermo Scientific Conflo IV). The IRMS continuously scans the intensity of the mass-to-charge ratios of mass 44, 45, 46 for CH4 and CO2 and 28, 29 for N20 converted to N2. δ13C and δ15N are referenced against calibrated laboratory reference gases. We are currently tuning the methods and testing the prototypes and will present the lasted results and open questions at the conference.

RationaleMeasurement of greenhouse gas (GHG) concentrations and isotopic compositions in the atmosphere is a valuable tool for predicting their sources and sinks, and ultimately how they affect Earth's climate. Easy access to unmanned aerial vehicles (UAVs) has opened up new opportunities for remote gas sampling and provides logistical and economic opportunities to improve GHG measurements.MethodsThis study presents synchronized gas chromatography/isotope ratio mass spectrometry (GC/IRMS) methods for the analysis of atmospheric gas samples (20‐mL glass vessels) to determine the stable isotope ratios and concentrations of CO2, CH4 and N2O. To our knowledge there is no comprehensive GC/IRMS setup for successive measurement of CO2, CH4 and N2O analysis meshed with a UAV‐based sampling system. The systems were built using off‐the‐shelf instruments augmented with minor modifications.ResultsThe precision of working gas standards achieved for δ13C and δ18O values of CO2 was 0.2‰ and 0.3‰, respectively. The mid‐term precision for δ13C and δ15N values of CH4 and N2O working gas standards was 0.4‰ and 0.3‰, respectively. Injection quantities of working gas standards indicated a relative standard deviation of 1%, 5% and 5% for CO2, CH4 and N2O, respectively. Measurements of atmospheric air samples demonstrated a standard deviation of 0.3‰ and 0.4‰ for the δ13C and δ18O values, respectively, of CO2, 0.5‰ for the δ13C value of CH4 and 0.3‰ for the δ15N value of N2O.ConclusionsResults from internal calibration and field sample analysis, as well as comparisons with similar measurement techniques, suggest that the method is applicable for the stable isotope analysis of these three important GHGs. In contrast to previously reported findings, the presented method enables successive analysis of all three GHGs from a single ambient atmospheric gas sample.

(1) Institute of Soil Research, University of Natural Resources and Life Sciences, Vienna, Austria (simon.leitner@boku.ac.at), (2) Institute of Terrestrial Ecosystem, University of Vienna, Vienna, Austria , (3) International Atomic Energy Agency, Soil and Water Management & Crop Nutrition Laboratory, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Vienna, Austria, (4) Department for Soil Health and Plant Nutrition, AGES Austrian Agency for Health and Food Safety, Vienna, Austria, (5) Institute for Land and Water Management Research, Federal Agency for Water Management, Petzenkirchen, Austria, (6) Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna, Austria

The evaluation of groundwater contaminant e.g. tetrachloroethene (PCE) degradation processes requires complete quantification of and pathway analysis of the groundwater contaminant under investigation. For example the reduction of PCE concentrations in the groundwater by unknown dissolution and/or sorption processes will impede interpretation of the fate and behaviour of such contaminants. In the present study PCE dissolution and sorption processes during anaerobic microbial degradation of chlorinated ethenes were investigated. For this purpose, microcosms were prepared using sediment samples from a PCE-contaminated aquifer, which in previous studies had demonstrated anaerobic organohalide respiration of PCE. Solid/water distribution coefficients (kd) of PCE were determined and validated by loss-on-ignition (LOI) and PCE sorption experiments. The determined kd magnitudes indicated methodological congruency, yielding values for sediment samples within a range of 1.15±0.02 to 5.93±0.34L·kg−1. The microcosm experiment showed lower PCE concentrations than expected, based on spiked PCE and observed anaerobic microbial degradation processes. Nevertheless the amount of PCE spike added was completely recovered albeit in the form of lower chlorinated metabolites. A delay due to dissolution processes was not responsible for this phenomenon. Sorption to sediments could only partially explain the reduction of PCE in the water phase. Accordingly, the results point to reversible sorption processes of PCE, possibly onto bacterial cell compartments and/or exopolymeric substances.