Abstract
Abstract. This paper presents an analytical system for analysis of all single substituted isotopologues (12C16O17O, 12C16O18O, 13C16O16O) in nanomolar quantities of CO2 extracted from stratospheric air samples. CO2 is separated from bulk air by gas chromatography and CO2 isotope ratio measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio mass spectrometry (IRMS). The 17O excess (Δ17O) is derived from isotope measurements on two different CO2 aliquots: unmodified CO2 and CO2 after complete oxygen isotope exchange with cerium oxide (CeO2) at 700 °C. Thus, a single measurement of Δ17O requires two injections of 1 mL of air with a CO2 mole fraction of 390 μmol mol−1 at 293 K and 1 bar pressure (corresponding to 16 nmol CO2 each). The required sample size (including flushing) is 2.7 mL of air. A single analysis (one pair of injections) takes 15 minutes. The analytical system is fully automated for unattended measurements over several days. The standard deviation of the 17O excess analysis is 1.7‰. Multiple measurements on an air sample reduce the measurement uncertainty, as expected for the statistical standard error. Thus, the uncertainty for a group of 10 measurements is 0.58‰ for Δ 17O in 2.5 h of analysis. 100 repeat analyses of one air sample decrease the standard error to 0.20‰. The instrument performance was demonstrated by measuring CO2 on stratospheric air samples obtained during the EU project RECONCILE with the high-altitude aircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for δ13C, 0.07‰ (1σ) for δ18O and 0.55‰ (1σ) for δ17O for a sample of 10 measurements. This is sufficient to examine stratospheric enrichments, which at altitude 33 km go up to 12‰ for δ17O and up to 8‰ for δ18O with respect to tropospheric CO2 : δ17O ~ 21‰ Vienna Standard Mean Ocean Water (VSMOW), δ18O ~ 41‰ VSMOW (Lämmerzahl et al., 2002). The samples measured with our analytical technique agree with available data for stratospheric CO2.
Highlights
Isotopic studies of carbon dioxide (CO2) play an important role in understanding the global carbon cycle (Ciais et al, 1997; Farquhar et al, 1993; Trolier et al, 1996)
valve number 1 (V1) is in position LOAD, valve number 2 (V2) is in position INJECT, and valve number 3 (V3) is in position LOAD
We have established an online measurement system for measurement of 17O in CO2 based on complete oxygen isotope exchange with CeO2 at 650 ◦C (Assonov and Brenninkmeijer, 2001) and similar to the online system using a copper oxide exchange reagent by Kawagucci et al (2005)
Summary
Isotopic studies of carbon dioxide (CO2) play an important role in understanding the global carbon cycle (Ciais et al, 1997; Farquhar et al, 1993; Trolier et al, 1996). 33 / 32 ion current ratio of O2 originated from CO2 Following this approach several methods have been developed: Bhattacharya and Thiemens (1989) converted CO2 to O2 by reacting it with BrF5; Brenninkmeijer and Röckmann (1998) used conversion of CO2 into CH4 and H2O by reaction with H2; in a second step they fluorinated the H2O with F2 to produce O2 and HF; Barkan and Luz (2012) equilibrated CO2 with H2O and used water fluorination to produce O2. To investigate the complete oxygen isotopic composition of CO2, the isotopic composition of CO2 is measured before and after isotopic exchange with oxygen from a solid oxide (CeO2) of known isotopic composition (Assonov and Brenninkmeijer, 2003; Kaiser, 2008) This technique was developed as an offline analytical technique where the isotope ratios are measured with dual-inlet IRMS. We measured the triple oxygen isotope composition of three stratospheric CO2 samples obtained during the EU project RECONCILE
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