Global 3-D Simulations of the Triple Oxygen Isotope Signature Δ17O in Atmospheric CO2.

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 Δ 17 O = ln ( δ 17 O + 1 ) - λ RL · ln ( δ 18 O + 1 ) with λ RL = 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.

Determination of the triple oxygen and carbon isotopic composition of CO2 from atomic ion fragments formed in the ion source of the 253 Ultra high-resolution isotope ratio mass spectrometer.

RATIONALE: Determination of δ17 O values directly from CO2 with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of 13 C16 O16 O on 12 C17 O16 O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ17 O value of CO2 . In addition, δ13 C measurements require correction for the interference from 12 C17 O16 O on 13 C16 O16 O since it is not possible to resolve the two isotopologues. METHODS: We present a technique to determine the δ17 O, δ18 O and δ13 C 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 17 O-correction algorithm for δ17 O and δ13 C values, respectively. RESULTS: The scale contractions for δ13 C and δ18 O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ17 O and Δ17 O values of CO2 can be measured on the 17 O+ 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 17 O+ 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 Δ17 O measurements with the O-fragment method agree with the CO2 -O2 exchange method over a range of Δ17 O 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.

Measurement of 18 O18 O and 17 O18 O in atmospheric O2 using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples.

RATIONALE: The doubly substituted isotopologues (e.g., 18 O18 O, 17 O18 O) in atmospheric O2 are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high-resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences. METHODS: O2 from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to -78°C. The δ17 O, δ18 O, Δ17 O, Δ35 and Δ36 values are measured on the extracted O2 with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues. RESULTS: Interferences from isobars, mainly 35 Cl for 17 O18 O and H35 Cl and 36 Ar for 18 O18 O, are sufficiently resolved to enable high-precision determination of Δ35 and Δ36 . The Δ35 and Δ36 values of O2 after photochemical isotope equilibration at -63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ35 and Δ36 values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O2 differ from those expected at equilibrium. CONCLUSIONS: The 253 Ultra allows interference-free clumped isotope measurements of O2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.