As CO2 concentrations in the atmosphere increase due to fossil fuel burning, the concentration of atmospheric O2 drops. For every carbon atom that is burnt one molecule of O2 is consumed to produce CO2. The oxygen concentration decrease is tracked by measuring the ratio of oxygen to nitrogen, the idea being, that the nitrogen content in the atmosphere is constant, so that a change in the O2/N2 ratio indicates changing atmospheric O2 concentrations. Atmospheric O2 has sources and sinks that differ from those of CO2. Thus, a great deal can be learned about the global carbon cycle including the partitioning of oceanic and land biosphere sinks for CO2 by studying the O2/N2 ratio.
Our inlet system for measuring O2/N2 (and Ar/N2) ratios comprises a multiport valve, a drying tube and an open split from where air directly enters the IRMS called Luftikus. The system is unique in terms of sample consumption as reliable measurements can be made from gas flows as low as 0.2 ml/min. Measurements are made against in-house reference air that is housed in thermally insulated boxes. Since sample measurements are also susceptible to temperature induced errors the samples are kept under a polystyrene hub during analysis to protect against temperature fluctuations. The in-house standards are periodically verified by analysing “Scripps Tanks” supplied by Ralph Keeling (Scripps Oceanic Institute, La Jolla). Thus measurements are currently on the local realisation of the Scripps Scale.
Due to the very different diffusion rates of O2, N2 and Ar micro leaks in the sampling flasks lead to drastic shifts in the O2/N2 and Ar/N2 ratios. Thus the 1 L sampling flasks for atmospheric sampling and storage were specifically designed to provide the best possible long term storage. These flasks have been in use since 2004.
Brand, W. A.: O2/N2 Storage Aspects and Open Split Mass Spectrometric Determination. In: Proceedings of the 12th IAEA/WMO meeting of CO2 experts, Toronto, Sept. 2003, pp. 146 - 151 (Eds. Worthy, D.; Huang, L.) (2005)