Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO 2 in air

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1 and Physics ess doi: /amt Author(s) CC Attribution 3.0 License. Atmospheric Measurement Techniques Biogeosciences Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO 2 in air M. Wendeberg, J. M. Richter, M. Ro, and W. A. Brand Max Planck Institute for Biogeochemistry Jena, P.O. Box , Jena, Germany Climate of Past Correspondence to: W. A. Brand (wbrand@bgc-jena.mpg.de) Received: 30 July 2012 Published in Atmos. Meas. Tech. Discuss.: 13 September 2012 Revised: 7 March 2013 Accepted: 8 March 2013 Published: 26 March 2013 Earth System Dynamics Abstract. The need for a unifying scale anchor for isotopes of CO 2 in air was brought to light at 11th WMO/IAEA Meeting of Experts on Carbon Dioxide in Tokyo During discussions about persistent discrepancies in isotope measurements between worlds leading laboratories, it was concluded that a unifying scale anchor for Vienna Pee Dee Belemnite (VPDB) of CO 2 in air was desperately needed. Ten years later, at 2011 Meeting of Experts on Carbon Dioxide in Wellington, it was recommended that Jena Reference Air Set (JRAS) become official scale anchor for isotope measurements of CO 2 in air (Brailsford, 2012). The source of CO 2 used for JRAS is two calcites. After releasing CO 2 by reaction with phosphoric acid, gases are mixed into CO 2 -free air. This procedure ensures both isotopic stability and longevity of CO 2. That reference CO 2 is generated from calcites and supplied as an air mixture is unique to JRAS. This is made to ensure that any measurement bias arising from extraction procedure is eliminated. As every laboratory has its own procedure for extracting CO 2, this is of paramount importance if local scales are to be unified with a common anchor. For a period of four years, JRAS has been evaluated through IMECC 1 program, which made it possible to distribute sets of JRAS gases to 13 laboratories worldwide. A summary of data from six laboratories that have reported full set of results is given here along with a description of production and maintenance of JRAS scale anchors. 1 IMECC refers to EU project Infrastructure for Measurements of European Carbon Cycle ( 1 Introduction 1.1 State of affairs Geoscientific Instrumentation Methods and Data Systems Prior to 11th WMO/IAEA Meeting of Experts on Carbon Dioxide in Tokyo 2001, a project funded by International Atomic Energy Agency (IAEA) called CLAS- SIC (Circulation of Laboratory Geoscientific Air Standards for Stable Isotope inter-comparisons) had revealed significant discrepancies between four Model of world s Development leading laboratories (Allison et al., 2003). The results from CLASSIC project were based on five high-pressure cylinders filled with chemically modified air and two canisters with pure CO 2. These Hydrology and Earth System Sciences gases had been circulated during two campaigns ( , ) between CMAR/CSIRO 2 (Aspendale, Australia), INSTAAR/NOAA (Boulder, USA), Scripps Institution of Oceanography (La Jolla, USA), and Center for Atmospheric and Oceanic Studies at Tohoku University (TU), Sendai, Japan. The results reported from pure CO 2 canisters exposed differences between Ocean laboratories Science exceeding interlaboratory target precision of 0.01 for δ 13 C, leading authors of report to state: The mean differences in reported values are consistent with uncertainties in assignment of isotopic composition of in-house pure CO 2 working standards onto VPDB CO Solid 2 scale. Earth For analysis of air in high pressure cylinders, results were not much better. Although differences between two campaigns within laboratories met target precision, average differences relative to CMAR/CSIRO results for δ 13 C between laboratories ranged from to and for δ 18 O range 2 See Table 1. The Cryosphere M Published by Copernicus Publications on behalf of European Geosciences Union.

2 818 M. Wendeberg et al.: Jena Reference Air Set (JRAS) Table 1. Laboratories that participated in JRAS project. Laboratoire des Sciences du Climat et de l Environnement (LSCE) Gif sur Yvette Cedex France Department of Environmental Physics AGH-University of Science Kraków Poland and Technology Institute of Environmental Physics University of Heidelberg (UH-IUP) Heidelberg Germany Scripps Institution of Oceanography University of La Jolla USA California-San Diego (SIO) CSIRO Center for Marine and Atmospheric Research Aspendale, Victoria Australia (CMAR/CSIRO) Environment Canada Stable Isotope Research Laboratory (EC) Downsview Ontario Canada National Institute for Environmental Studies (NIES) Tsukuba Japan Institute for Reference Materials and Measurements (IRMM) Geel Belgium Climate and Environmental Physics University of Bern (UB) Bern Switzerland Institute of Arctic and Alpine Research University of Colorado Boulder, Colorado USA (INSTAAR/NOAA) Center for Atmospheric and Oceanic Science Indian Institute of Bangalore India Science (IIS) Max Planck Institute for Biogeochemistry (MPI-BGC) Jena Germany National Institute of Water and Atmospheric Research (NIWA) Wellington New Zealand was to Quoting report: As with GS20B 3, mean difference between laboratories is consistent with problems in assignment onto VPDB-CO 2 reference scale. At root of this problem was and still is fact that both methods for scale anchoring and choice of anchor vary from laboratory to laboratory. A furr complicating factor was that all available scale anchors were pure CO 2, thus forcing laboratories to anchor ir scales by comparing CO 2 y extracted from air to pure reference CO 2 a comparison that adds any possible bias caused by extraction of CO 2 from air to local scale. As extraction method is unique to laboratory, resulting local scale will hence differ from laboratory to laboratory. In addition to this weakness in scale definition, results were hampered by cross-contamination or eta effect that has been plaguing isotope mass spectrometers over years (Meijer et al., 2000; Verkouteren et al., 2003a, b) and, thus, causing scale contraction. Furr inconsistencies were found in correction procedures for 17 O contribution used when translating ion currents into delta values (Brand et al., 2010) and in algorithms applied for removing inevitable presence of N 2 O in CO 2 gas extracted from air cryogenically (Sirignano et al., 2004; Ghosh and Brand, 2004). 1.2 The Jena Reference Air Set JRAS A JRAS set consists of two 5 L glass flasks containing CO 2 from two different calcites mixed into CO 2 -free air. Reference gases are problematic to store and keep unaltered regarding isotopic composition. For reliable longterm stability, solid (or liquid) materials are preferred. For 3 One of two pure CO 2 canisters. carbon and oxygen isotopes, internationally available and agreed upon primary reference materials are carbonates: NBS 19 with δ 13 C VPDB = and LSVEC with δ 13 C VPDB = 46.6 (Coplen et al., 2006). By also using solid calcites for anchoring air-co 2 scale, enough material can be prepared and stored without risk of isotopic alteration to ensure both longevity and stability of JRAS scale anchors. The first of two calcites, MAR-J1, with δ 13 C VPDB +2, has characteristics (isotopic composition, trace elements, grain size) that are almost identical to NBS 19, primary scale anchor for Vienna Pee Dee Belemnite (VPDB). The second calcite, OMC-J1, is also similar to NBS 19, but not as close as MAR-J1 (see Ghosh et al., 2005). It was chosen as a compromise after a long and unsuccessful search for an ideal candidate to serve as a second anchor point to VPDB isotope scale. Ideally, this point should be close to that of atmospheric CO 2 ; however, OMC- J1 provides an adequate range between two anchor points with a δ 13 C value of about 4 on VPDB scale (Ghosh et al., 2005). In order to keep variability arising from preparation of reference CO 2 at a minimum, a computer-controlled system has been designed and built at MPI-BGC stable isotope facility (Ghosh et al., 2005). The system prepares CO 2 by digestion of calcites in highly concentrated phosphoric acid and mixes it into CO 2 -free air in batches large enough to provide three 5 L flasks (p = 1.5 bar) at a time. The δ 13 C scale at BGC Isolab has been established by repeated analysis of preparations of primary calcites, NBS 19 and LSVEC, using automated preparation system (Ghosh et al., 2005; Brand et al., 2009). This scale, JRAS-06, which is firmly anchored at and 46.6 exactly and is continuously maintained, has been

3 factor JRAS-06 are negligible scale. factor are BGC negligible results validate BGC results ability validate of Isolab to ability maintain of Isolab to maintain JRAS-06 scale. JRAS-06 scale. M. Wendeberg et al.: Jena Reference Air Set (JRAS) 819 Fig. 1. Production variability of JRAS gases over time. Each data point represents an average of nine measurements of prepared CO 2 air mix, three per flask within a batch. Thus overall Figure 1. Production variability of JRAS gases over time. Each data point represents an average of nine measurements of prepared CO2-air mix, three per flask within a batch. Thus overall standard deviation (1-σ) reflects combined variability between prepared CO2 as well as Figure 2. Regression Figure and residuals 2. Regression plots of and MPI-BGC residuals measurements plots of MPI-BGC (n=15) measurements of JRAS gases (n=15) treated of as JRAS gases treated as standard uncertainty related deviation to extraction (1 σ and ) mass reflects spectrometric measurements combined performed variability in MPI between independent samples BGC laboratory. prepared CO 2 as well as uncertainty related to extraction independent (n to = 15) originally samples of JRAS assigned to gases values originally treated on assigned JRAS-06 as independent values scale. on Left panels JRAS-06 samples show scale. toleft 13 C panels orig-shoinally 13 C and mass spectrometric measurements performed in MPI-BGC data and right panels data and corresponding assigned right panels values 18 O corresponding data. on JRAS-06 scale. Left panels show 18 O data. 2. laboratory. Results Figure 2. Regression and δ 13 residuals C data plots and of right MPI-BGC panels measurements corresponding (n=15) of JRAS δ 18 Ogases data. treated as Figure 2. Regression and residuals plots of MPI-BGC measurements (n=15) of JRAS gases treated as Test and Evaluation independent samples to originally assigned values on JRAS-06 scale. Left panels show 13 C independent samples to originally assigned values on JRAS-06 scale. Left panels show 13 C The JRAS concept has been evaluated from 2007 to 2011 as a part of European Table 2. δ 13 data and right panels corresponding C results. Linear regression resulting from comparing 18 O data. data Table and right 3. panels δ 18 O results. corresponding 7Linear 18 regression O data. resulting from comparing Commission funded program - Infrastructure for Measurements of European 7 local scales to JRAS-06. Carbon Cycle, IMECC. Thanks to IMECC isotope laboratory at MPI-BGC has been able to produce and distribute 50 JRAS sets. Thirteen laboratories from all over y = a + bx world volunteered to participate in IMECC-JRAS project (Table 1). Each of a b R 2 INSTAAR, University of Colorado, 5 USA NIES, Japan Scripps Institution of Oceanography, USA University of Heidelberg, Germany NIWA, New Zealand MPI-BGC, Germany Fig. 2. Regression and residuals plots of MPI-BGC measurements local scales to JRAS y = a + bx a b R 2 INSTAAR, University of Colorado, USA NIES, Japan Scripps Institution of Oceanography, USA University of Heidelberg, Germany NIWA, New Zealand MPI-BGC, Germany used to assign initial values of JRAS sets before distribution. Figure 1 shows inter-batch δ 13 C variability of produced CO 2 between 2007 and These data provide information regarding maintenance and repeatability of JRAS-06 scale over time. The preparations should not vary by a large amount in order to maintain anchor point fixed in time. It is important to note that because isotopic composition of flasks is measured and assigned during this procedure preparation variability is not propagated into an uncertainty of scale anchor. 2 Results Test and evaluation The JRAS concept has been evaluated from 2007 to 2011 as a part of European Commission funded program Infrastructure for Measurements of European Carbon Cycle, IMECC. Thanks to IMECC isotope laboratory at MPI-BGC has been able to produce and distribute 50 JRAS sets. Thirteen laboratories from all over world volunteered to participate in IMECC-JRAS project (Table 1). Each of laboratories received a JRAS set which, when exhausted, was replaced with a new set. The number of analyses performed varied between participating laboratories due to differences in workload and instrument availability. The project as a whole totaled over 250 analyses of JRAS scale anchors.

4 a b R² measurements, deviations become more prominent (Table 2, Figure 4). 820 INSTAAR, University of Colorado, USA NIES, Japan et al.: Jena Reference 1 M. Wendeberg Air Set (JRAS) Scripps Institution of Oceanography, USA University of Heidelberg, Germany NIWA, New Zealand MPI-BGC, Germany Table 2. 13C results. Linear regression resulting from comparing local scales to JRAS-06. y = a + bx a b R² INSTAAR, University of Colorado, USA Scripps Institution of Oceanography, USA University of Heidelberg, Germany NIWA, New Zealand MPI-BGC, Germany Figureof 3. Extrapolation of presented MPI-BGCinresults in Figure to isotopic Fig. 3. Extrapolation MPI-BGC results Fig. 2 presented to isotopic range 2spanned by NBSrange 19 and LSVEC. NIES, Japan spanned by NBS 19 and LSVEC. In Figure 4 results from different laboratories are extrapolated in same fashion as shown in Figure 3 for MPI-BGC to demonstrate relationship between local scales and VPDBgas. The offsets between local scales and VPDB-CO2 (term a in Table 2) determined by JRAS project are not much different from those 18 Table 3. O results. Linear regression resulting from comparing local scales to JRAS-06. reported in CLASSIC project. Hence, situation has remained largely unchanged over last decade. With JRAS project scale comparison is going Figure based 4.than Extrapolation based on linear ofc JRAS C dataanchor, reby allowing to furr during CLASSIC using multi point scale Fig. 4. Extrapolation on linear regression ofby regression JRAS aδ 13 data. 13 test local scales for scale contraction issues. Scale contraction (i.e. scale offset 13 The resultsdependent for oxygenon isotopes canvalue) be evaluated in same way as those is linearly delta can be caused by differences in for C. Here offsets methods/materials are expected to be larger, reflecting commonly observed standardization or cross contamination in mass spectrometer. uncertainties in oxygen (Table 3, Figure 5).and Factors The results from JRAS isotope clearly measurements suggest that se issues are real significant (slope uncertainty correctionfor for N analyzed 2O present bcontributing in Table 2),toalthough not forare all laboratories. example; infactor b valuesgas of ( correction m/z 46 current isand larger thanare thatsofor m/z that 45; see regressions for for University of ion Heidelberg NIWA small Sirignano slope is 18 et al., 2004, Ghosh and Brand 2004), value used fororwise digesting calcite insignificant, thus explaining sign of Oslopes, which would indicate a during5.standardization (Wendeberg et al., ) and O correction (Brand et al., Figure Extrapolation based on linear regression of JRAS δ O data virtual scale expansion. Fig. 5. Extrapolation based on linear regression of JRAS δ O data. 2010). The latter have been focus of WMO recommendations for several In spring of where 2010 project was use augmented square) linear regression years, JRAS need for a unified of 8 Assonov correction has been represents agreement between 3. Conclusions with 5 L glass flasks containing calibrated (dry ambient) air; round-robin measurements and initial assignment reemphasized (Assonov et al., 2003). this was a result requests made during at on 2 JRAS-06 scale. The additive term in equaten of years after need for adiscussions unifying scale anchorsults for CO in air was formulated, 15th WMO/IAEA Meeting of Experts on Carbon Dioxide tion signifies an offset between two sets of data, and participants of 16th WMO CO2 Experts-Meeting in Wellington 2011 have decided in Jena 2009 (Brand, 2010). While se air samples do not factor/slope describes differences in standardization straterecommend JRAS(this gases for use as officialgies scale anchorlaboratories for measurement of as a quantification of any constitute an to immediate scale anchor is still exclusively between or serves 9 made via calcite materials MAR-J1 and OMC-J1), y scale contraction occurring. As both term and factor atmospheric CO2 on VPDB scale (Brailsford, 2012). This along with a unified use add an important data point right at isotope composition are negligible, BGC results validate ability of Isolab of CO 217, O correction contribute to harmonize various local of atmospheric thus allowingwill for ahopefully three-point compar- greatly to maintain JRAS-06 scale. ison of local scale implementations. In Fig. 3 BGC Isolab data presented in Fig. 2 are exscales. We obtained complete data sets including ambient trapolated to a wider delta scale, now covering whole isotheonly MPI-BGC lab will continue to aid inthese this process, now as aofwmo-endorsed air sample from six of thirteen laboratories. topic range two primary scale anchors, NBS 19 and JRAS resultscentral were evaluated by comparing analytical LSVEC. This plot provides visual characterizacalibration Lab, by providing calibration for high-pressure tanks basedanonenhanced results on local scales with values assigned on tion of scale discrepancies caused by different standardizajras procedure. JRAS-06 scale. MPI-BGC also participated in evaluation tion schemes or possibly scale contraction issues. For MPIby intermittently measuring JRAS gases as independent sambgc, deviation of scale is insignificant, amounting to ples. These BGC results are shown in Fig. 2, where (least 0.04 only over a scale distance of almost 50. However, 10

5 M. Wendeberg et al.: Jena Reference Air Set (JRAS) 821 when we plot results from all participating laboratories that included air samples in ir measurements, deviations become more prominent (Table 2, Fig. 4). In Fig. 4 results from different laboratories are extrapolated in same fashion as shown in Fig. 3 for MPI- BGC to demonstrate relationship between local scales and VPDB gas. The offsets between local scales and VPDB- CO 2 (term a in Table 2) determined by JRAS project are not much different from those reported in CLASSIC project. Hence, situation has remained largely unchanged over last decade. With JRAS project scale comparison is going furr than during CLASSIC by using a multi-point scale anchor, reby allowing to test local scales for scale contraction issues. Scale contraction (i.e., scale offset is linearly dependent on delta value) can be caused by differences in standardization methods/materials or cross contamination in mass spectrometer. The results from JRAS clearly suggest that se issues are real and significant (slope b in Table 2), although not for all laboratories. For example, factor b values of regressions for University of Heidelberg and NIWA are so small that slope is insignificant, thus explaining sign of slopes, which orwise would indicate a virtual scale expansion. The results for oxygen isotopes can be evaluated in same way as those for δ 13 C. Here offsets are expected to be larger, reflecting commonly observed uncertainties in oxygen isotope measurements (Table 3, Fig. 5). Factors contributing to uncertainty are correction for N 2 O present in analyzed gas ( correction for m/z 46 ion current is larger than that for m/z 45; see Sirignano et al., 2004; Ghosh and Brand, 2004), δ 18 O value used for digesting calcite during standardization (Wendeberg et al., 2011) and 17 O correction (Brand et al., 2010). The latter have been focus of WMO recommendations for several years, where need for a unified use of Assonov correction has been emphasized (Assonov et al., 2003). 3 Conclusions Ten years after need for a unifying scale anchor for CO 2 in air was formulated, participants of 16th WMO/IAEA Meeting of Experts on Carbon Dioxide in Wellington 2011 have decided to recommend JRAS gases for use as official scale anchor for measurement of atmospheric CO 2 on VPDB scale (Brailsford, 2012). This along with a unified use of 17 O correction will hopefully contribute greatly to harmonize various local scales. The MPI-BGC lab will continue to aid in this process, now as a WMO-endorsed Central Calibration Lab, by providing calibration for high-pressure tanks based on JRAS procedure. Acknowledgements. We would like to thank all participants in various laboratories for extending ir best efforts into this (rar tedious) endeavor. In particular we would like to acknowledge ir openness in communicating ir results to general public. We are indebted to three reviewers, whose suggestions helped to clarify several issues and improved readability of manuscript. For financial support, we are indebted to EU-IMECC project. Without this support for funding flasks, tanks and providing financial support for M.W., this study would not have been possible. The service charges for this open access publication have been covered by Max Planck Society. Edited by: A. C. Manning References Allison, C. E., Francey, R. J., White, J. W. C., Vaughn, B., Wahlen, M., Bollenbacher, A., and Nakazawa, T.: What have we learnt about stable isotope measurements from IAEA CLASSIC, in: Proceedings of 11th WMO/IAEA Meeting of CO 2 Experts, Tokyo 2001, WMO/GAW publication series, 148, 17 30, Assonov, S. S. and Brenninkmeijer, C. A. M.: On O-17 correction for CO 2 mass spectrometric isotopic analysis, Rapid Commun. Mass Sp., 17, , Brailsford, G. 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