High Precision Measurements of the Atmospheric O2/N2 Ratio on a Mass Spectrometer

Employing a mass spectrometry method, a high precision measurement system was developed for analysis of the atmospheric O2/N2 ratio. Sample air and reference air were introduced into the mass spectrometer through thermally-insulated thin fused silica capillaries from an inlet system. Interference by CO generated in the ion source of the mass spectrometer from CO2 in the sample air, and the O2/N2 ratio biased due to pressure imbalance between the sample air and the reference air during their introduction into the mass spectrometer were experimentally corrected. Deterioration of sampled air during storage in flasks, as well as air sampling procedures, was also examined. The precision of our measurement system was estimated to be 5.4 per meg for the O2/N2 ratio, which corresponds to 1.1 ppmv of the atmospheric O2 concentration. Our standard air with 6 different O2/N2 ratios were prepared by drying the atmosphere and then stored in 47 L high-pressure cylinders; their O2/N2 ratios were confirmed to be stable within 20.0 per meg over the last 2 years. This system has been used for actual measurements of the atmospheric O2/N2 ratio since May 1999. Preliminary results of the measurements made in the suburbs of Sendai, Japan showed clear evidence for the seasonal cycle and the secular trend of the atmospheric O2/N2 ratio, which are opposite in phase with those of the CO2 concentration.