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Original Research Papers

On the long-term stability of reference gases for atmospheric O2/N2 and CO2 measurements

Authors:

Ralph F. Keeling ,

Scripps Institution of Oceanography, University of California, US
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Andrew C. Manning,

Formerly at Scripps Institution of Oceanography, currently at the School of Environmental Sciences, University of East Anglia, GB
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William J . Paplawsky,

Scripps Institution of Oceanography, University of California, US
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Adam C. Cox

Scripps Institution of Oceanography, University of California, US
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Abstract

Measurements of changes in the atmospheric O2/N2 ratio have typically relied on compressed air derived from highpressure tanks as the reference material against which atmospheric changes are assessed. The validity of this procedure is examined here in the context of the history of 18 O2/N2 reference tanks compared over a 12-yr time-frame. By considering differences in tank sizes, material types, and by performing additional tests, the long-term stability of the delivered gas is evaluated with respect to surface reactions, leakage, regulator effects, and thermal diffusion and gravimetric fractionation. Results are also reported for the stability of CO2 in these tanks. The results emphasize the importance of orienting tanks horizontally within a thermally insulated enclosure to reduce thermal and gravimetric fractionation of both O2/N2 and CO2 concentrations, and they emphasize the importance of avoiding elastomeric Orings at the head-valve base. With the procedures documented here, the long-term drift in O2/N2 appears to be zero to within approximately ±0.4 per meg yr-1, which projects to an uncertainty of ±0.16 Pg C yr-1 (1ó) in O2-based global carbon budgets.

How to Cite: Keeling, R.F., Manning, A.C., Paplawsky, W.J. and Cox, A.C., 2007. On the long-term stability of reference gases for atmospheric O2/N2 and CO2 measurements. Tellus B: Chemical and Physical Meteorology, 59(1), pp.3–14. DOI: http://doi.org/10.1111/j.1600-0889.2006.00196.x
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  Published on 01 Jan 2007
 Accepted on 27 Jul 2006            Submitted on 29 Mar 2006

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