Since the discovery of mass-independent isotope effects in stratospheric and tropospheric gases, the analysis of triple oxygen isotope abundance in carbon dioxide gained in importance. However, precise triple oxygen isotope determination in carbon dioxide is a challenging task due to mass-interference of (17)O and (13)C variations. Here, we present a novel analytical technique that allows us to determine slight deviations of CO(2) from the terrestrial fractionation line [TFL]. Our approach is based on isotopic equilibration between CO(2) gas and CeO(2) powder at 685 degrees C and subsequent mass spectrometric analysis of ceria powder by infrared-laser fluorination. We found that beta(CO2-CeO2), the exponent in the relation alpha(17/16) = (alpha(18/16))(beta), amounts to 0.5240 +/- 0.0011 at 685 degrees C. The oxygen isotope anomaly of CO(2) (Delta(17)O) can be determined for a single analysis of CeO(2) with a precision of +/-0.05 per thousand (1sigma). Our CO(2)-CeO(2) equilibration procedure is performed with an excess of CO(2) so that one analysis of Delta(17)O on CO(2) requires at least 3.5 mmol of CO(2) gas. Our new technique allows accurate and precise determination of Delta(17)O in CO(2) and opens up a new field for investigating triple oxygen isotope abundance in various types of natural CO(2).

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The triple oxygen isotope signature Δ17O in atmospheric CO2, also known as its “17O excess,” has been proposed as a tracer for gross primary production (the gross uptake of CO2 by vegetation through photosynthesis). We present the first global 3‐D model simulations for Δ17O in atmospheric CO2 together with a detailed model description and sensitivity analyses. In our 3‐D model framework we include the stratospheric source of Δ17O in CO2 and the surface sinks from vegetation, soils, ocean, biomass burning, and fossil fuel combustion. The effect of oxidation of atmospheric CO on Δ17O in CO2 is also included in our model. We estimate that the global mean Δ17O (defined as Δ17normalO=lnfalse(δ17normalO+1false)−λRL·lnfalse(δ18normalO+1false) with λRL = 0.5229) of CO2 in the lowest 500 m of the atmosphere is 39.6 per meg, which is ∼20 per meg lower than estimates from existing box models. We compare our model results with a measured stratospheric Δ17O in CO2 profile from Sodankylä (Finland), which shows good agreement. In addition, we compare our model results with tropospheric measurements of Δ17O in CO2 from Göttingen (Germany) and Taipei (Taiwan), which shows some agreement but we also find substantial discrepancies that are subsequently discussed. Finally, we show model results for Zotino (Russia), Mauna Loa (United States), Manaus (Brazil), and South Pole, which we propose as possible locations for future measurements of Δ17O in tropospheric CO2 that can help to further increase our understanding of the global budget of Δ17O in atmospheric CO2.

Rationale Determination of δ17O values directly from CO2 with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of 13C16O16O on 12C17O16O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ17O value of CO2. In addition, δ13C measurements require correction for the interference from 12C17O16O on 13C16O16O since it is not possible to resolve the two isotopologues. Methods We present a technique to determine the δ17O, δ18O and δ13C values of CO2 from the fragment ions that are formed upon electron ionization in the ion source of the Thermo Scientific 253 Ultra high‐resolution isotope ratio mass spectrometer (hereafter 253 Ultra). The new technique is compared with the CO2‐O2 exchange method and the 17O‐correction algorithm for δ17O and δ13C values, respectively. Results The scale contractions for δ13C and δ18O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ17O and Δ17O values of CO2 can be measured on the 17O+ fragment with an internal error that is a factor 1–2 above the counting statistics limit. The ultimate precision depends on the signal intensity and on the total time that the 17O+ beam is monitored; a precision of 14 ppm (parts per million) (standard error of the mean) was achieved in 20 hours at the University of Göttingen. The Δ17O measurements with the O‐fragment method agree with the CO2‐O2 exchange method over a range of Δ17O values of −0.3 to +0.7‰. Conclusions Isotope measurements on atom fragment ions of CO2 can be used as an alternative method to determine the carbon and oxygen isotopic composition of CO2 without chemical processing or corrections for mass interferences.