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A system for high‐quality CO 2 isotope analyses of air samples collected by the CARIBIC Airbus A340‐600

2009; Wiley; Volume: 23; Issue: 9 Linguagem: Inglês

10.1002/rcm.4008

1097-0231

Sergey Assonov, Philip Taylor, Carl A. M. Brenninkmeijer,

Atmospheric chemistry and aerosols

Abstract In 2007, JRC‐IRMM began a series of atmospheric CO 2 isotope measurements, with the focus on understanding instrumental effects, corrections as well as metrological aspects. The calibration approach at JRC‐IRMM is based on use of a plain CO 2 sample (working reference CO 2 ) as a calibration carrier and CO 2 ‐air mixtures (in high‐pressure cylinders) to determine the method‐related correction under actual analytical conditions (another calibration carrier, in the same form as the samples). Although this approach differs from that in other laboratories, it does give a direct link to the primary reference NBS‐19‐CO 2 . It also helps to investigate the magnitude and nature for each of the instrumental corrections and allows for the quantification of the uncertainty introduced. Critical tests were focused on the instrumental corrections. It was confirmed that the use of non‐symmetrical capillary crimping (an approach used here to deal with small samples) systematically modifies δ 13 C(CO 2 ) and δ 18 O(CO 2 ), with a clear dependence on the amount of extracted CO 2 . However, the calibration of CO 2 ‐air mixtures required the use of the symmetrical dual‐inlet mode. As a proof of our approach, we found that δ 13 C(CO 2 ) on extracts from mixtures agreed (within 0.010‰) with values obtained from the ‘mother’ CO 2 used for the mixtures. It was further found that very low levels of hydrocarbons in the pumping systems and the isotope ratio mass spectrometry (IRMS) instrument itself were critical. The m/z 46 values (consequently the calculated δ 18 O(CO 2 ) values) are affected by several other effects with traces of air co‐trapped with frozen CO 2 being the most critical. A careful cryo‐distillation of the extracted CO 2 is recommended. After extensive testing, optimisation, and routine automated use, the system was found to give precise data on air samples that can be traced with confidence to the primary standards. The typical total combined uncertainty in δ 13 C(CO 2 ) and δ 18 O(CO 2 ) on the VPDB‐CO 2 scale, estimated on runs of CO 2 ‐air mixtures, is ±0.040‰ and 0.060‰ (2‐ σ values). Inter‐comparison with MPI‐BGC resulted in a scale discrepancy of a similar magnitude. Although the reason(s) for this discrepancy still need to be understood, this basically confirms the approach of using specifically prepared CO 2 ‐air mixtures as a calibration carrier, in order to achieve scale unification among laboratories. As important practical application and as a critical test, JRC‐IRMM took part in the passenger aircraft‐based global monitoring project CARIBIC ( http://www.caribic‐atmospheric.com ). In this way, reliable CO 2 isotope data for the tropopause region and the free troposphere were obtained. From June 2007 to January 2009, approximately 500 CARIBIC air samples have been analysed. Some flights demonstrated a compact correlation of both δ 13 C(CO 2 ) and δ 18 O(CO 2 ) with respect to CO 2 concentration, demonstrating mixing of tropospheric and stratospheric air masses. These excellent correlations provide an independent, realistic data quality check. Copyright © 2009 John Wiley & Sons, Ltd.