Abstract. We used the recently developed commercially available Delta Ray isotope ratio infrared spectrometer (IRIS) to continuously measure the CO 2 concentration c and its isotopic composition δ 13 C and δ 18 O in a managed beech forest in central Germany. Our objectives are (a) to characterize the Delta Ray IRIS and evaluate its internal calibration procedure and (b) to quantify the seasonal variability of c, δ 13 C, δ 18 O and the isotopic composition of nighttime net ecosystem CO 2 exchange (respiration) R 13 eco C and R 18 eco O derived from Keeling plot intercepts. The analyzer's minimal Allan deviation (as a measure of precision) was below 0.01 ppm for the CO 2 concentration and below 0.03 ‰ for both δ values. The potential accuracy (defined as the 1σ deviation from the respective linear regression that was used for calibration) was approximately 0.45 ppm for c, 0.24 ‰ for 13 C and 0.3 ‰ for 18 O. For repeated measurements of a target gas in the field, the long-term standard deviation from the mean was 0.3 ppm for c and below 0.3 ‰ for both δ values. We used measurements of nine different inlet heights to evaluate the isotopic compositions of nighttime net ecosystem CO 2 exchange R 13 eco C and R 18 eco O in a 3-month measurement campaign in a beech forest in autumn 2015. During this period, an early snow and frost event occurred, coinciding with a change in the observed characteristics of both R 13 eco C and R 18 eco O. Before the first snow, R 13 eco C correlated significantly (p < 10 −4 ) with time-lagged net radiation R n , a driver of photosynthesis and photosynthetic discrimination against 13 C. This correlation became insignificant (p > 0.1) for the period after the first snow, indicating a decoupling of δ 13 C of respiration from recent assimilates. For 18 O, we measured a decrease of 30 ‰ within 10 days in R 18 eco O after the snow event, potentially reflecting the influence of 18 O depleted snow on soil moisture. This decrease was 10 times larger than the corresponding decrease in δ 18 O in ambient CO 2 (below 3 ‰) and took 3 times longer to recover (3 weeks vs. 1 week). In summary, we conclude that (1) the new Delta Ray IRIS with its internal calibration procedure provides an opportunity to precisely and accurately measure c, δ 13 C and δ 18 O at field sites and (2) even short snow or frost events might have strong effects on the isotopic composition (in particular 18 O) of CO 2 exchange on an ecosystem scale.

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Isotope ratio measurements and scale definitions are typically related to mass specectroscopy. This work discusses the challenges of optical isotope ratio spectroscopy and its prospects to significantly complement isotope ratio mass spectrometry.

&lt;p&gt;The emission of greenhouse gases and the resulting global warming is one of the most important and challenging issues of the 21&lt;sup&gt;st&lt;/sup&gt; century. Carbon dioxide is one of the major contributors to the greenhouse e&amp;#64256;ect and its atmospheric abundance has growing constantly since the beginning of the industrialization. The isotope ratios n(&lt;sup&gt;13&lt;/sup&gt;C)/n(&lt;sup&gt;12&lt;/sup&gt;C) and n(&lt;sup&gt;18&lt;/sup&gt;O)/n(&lt;sup&gt;16&lt;/sup&gt;O) are important tools for studying the impact of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt;. Usually, isotopic compositions of CO&lt;sub&gt;2&lt;/sub&gt; are reported as &amp;#948;-values, that express isotope ratios relative to an artifact based on a fossil calcite called VPDB. This relative VPDB scale was necessary, since absolute and SI-traceable isotope ratios of CO&lt;sub&gt;2&lt;/sub&gt; are currently not available, neither by isotope ratio mass spectrometry (IRMS) nor by optical isotope ratio spectroscopy (OIRS). In this study we present a potential way of deriving absolute carbon and oxygen isotope ratios of carbon dioxide via IRMS based on the gravimetric mixture approach. Besides practical improvements like an air buoyancy correction scheme for masses of gases, we show first results applying our method which demonstrate its feasibility, limitations, and achievable uncertainties. Also, we show the mathematics behind our approach and discuss further improvements and applications. Furthermore, we show how these absolute ratios can be used in field applications by OIRS methods including a new approach on OIRS uncertainty assessments according to the GUM. For this contribution we report on our recent results within in the European metrology research projects SIRS (16ENV06). and STELLAR (19ENV05).&lt;/p&gt;

&lt;p&gt;Ecosystem assimilation and respiration result in anti-correlated fluxes of oxygen (O&lt;sub&gt;2&lt;/sub&gt;) and carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;). While the ecosystem O&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; molar exchange ratio is usually assumed constant at &amp;#8776;1.1 on longer timescales, variations for individual ecosystem compartments or shorter timescales have been reported in the past. We hypothesize that these exchange ratio variations can reveal information about underlying biotic and abiotic processes in plants or soil that cannot be inferred from traditional net ecosystem exchange measurements. To date, oxygen measurements have not been widely implemented in ecosystem research due to the technical challenge of detecting very small variations (ppm-level) against an atmospheric background of &amp;#8776;21% (&amp;#8776;210,000 ppm).&lt;/p&gt; &lt;p&gt;We evaluate the performance and applicability of two commercial oxygen analyzers Integrated into custom-built gas handling and calibration systems, and report first results from measurements of O&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; exchange ratios in a managed European beech forest in central Germany.&lt;/p&gt; &lt;p&gt;System 1, consisting of a relatively slow response differential fuel cell O&lt;sub&gt;2&lt;/sub&gt; analyzer (Oxzilla FC-2, Sable Systems Inc., USA) together with a non-dispersive infrared CO&lt;sub&gt;2&lt;/sub&gt; analyzer (LI-840, LI-COR Biosciences, USA), was used to simultaneously measure O&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2 &lt;/sub&gt;mole fractions in air sampled from soil, stem, and branch chambers. Chambers were operated in an open flow-through steady-state design aimed at equilibrium mole fractions within a few hundred ppm of atmospheric background. Using a multiplexer valve design, we measured chambers sequentially by directing chamber air at a controlled flow rate to the gas analyzing system.&lt;/p&gt; &lt;p&gt;Preliminary analysis of August to December 2018 data show that chamber-based flux estimates for O&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt; were anti-correlated at all times, and that the O&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; molar exchange ratios (defined as &amp;#8209;&amp;#916;[O&lt;sub&gt;2&lt;/sub&gt;]/&amp;#916;[CO&lt;sub&gt;2&lt;/sub&gt;]) varied considerably over time and between the different ecosystem compartments (soil, stems, and branches) with a median (interquartile range) of 0.94 (0.75 to 1.09).&lt;/p&gt; &lt;p&gt;In system 2, CO&lt;sub&gt;2&lt;/sub&gt;, O&lt;sub&gt;2&lt;/sub&gt; and water vapor (H&lt;sub&gt;2&lt;/sub&gt;O) measurements were performed with a fast response (5 Hz) gas analyzer using tunable infrared laser direct absorption spectroscopy (TILDAS, Aerodyne Research Inc., USA). We measured fluctuations in O&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; exchange ratios in air sampled at 1.5 times the canopy height, i.e. a typical eddy covariance set-up.&lt;/p&gt; &lt;p&gt;Analysis of the high-frequency data revealed instrumental noise levels of &amp;#8776;&amp;#177;12 ppm O&lt;sub&gt;2&lt;/sub&gt;. Fourier transformation of high-frequency data obtained during well-mixed boundary layer conditions indicate that turbulent fluctuations of the O&lt;sub&gt;2&lt;/sub&gt; signal were insufficiently resolved when compared to the CO&lt;sub&gt;2&lt;/sub&gt; power spectra. When averaging high-frequency data to 2-min aggregates, instrumental noise was reduced to &amp;#8776;&amp;#177;1 ppm, similar to the precision of system 1. At this timescale, contemporaneous measurements of above-canopy air revealed agreement between the fuel cell and the laser systems, both in O&lt;sub&gt;2&lt;/sub&gt; mole fraction (R&lt;sup&gt;2&lt;/sup&gt; = 0.6 slope = 0.7, MAE = 1.6 ppm) and in estimated O&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; exchange ratios of 1.01 and 0.97 for system 1 and 2, respectively.&lt;/p&gt; &lt;p&gt;Our presentation will expand on the applicability of both O&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2 &lt;/sub&gt;measurement systems with regard to micrometeorological flux techniques. Specifically, we elucidate on the potential of using O&lt;sub&gt;2 &lt;/sub&gt;flux measurements as a constraint for estimating ecosystem-scale gross primary production.&lt;/p&gt;

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Abstract. Measurements of the isotopic composition of water vapor, δv, as well as measurements of the isotopic composition of evaporation and transpiration provide valuable insights in the hydrological cycle. Here we present measurements of δv in the surface boundary layer (SBL) in combination with eddy covariance (EC) measurements of the isotopic composition of evapotranspiration δET for both δD as well as δ18O over a full growing season above a managed beech forest in central Germany. Based on direct measurements of isoforcing IF and the height h of the planetary boundary layer (PBL), we provide an estimate of isoforcing-related changes in δv, revealing the influence of local evapotranspiration (ET) on δv. At seasonal time scales we find no evidence for a dominant control of δv by local ET. Rayleigh distillation could at most explain 35 % of the observed variability and we did not find indications for the influence of entrainment at seasonal time scales. Instead, we obtain a strong significant correlation (R2 ≈ 0.52; p

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