To contribute to a better understanding of the global carbon cycle, a high precision continuous measurement system for atmospheric O
2/N
2 ratio was developed using a fuel cell oxygen analyzer. To obtain highly precise values of the atmospheric O
2/N
2 ratio, pressure fluctuations of the sample and standard air were reduced to within ±0.005 Pa, with temperatures stabilized to 32.0 ± 0.1°C. The analytical precision of the system was estimated to be ±1.4 per meg for 24-minute measurement as the standard deviation (1σ) of replicate analyses of the same sample air. This analytical precision is sufficient for clearly detecting very small spatiotemporal variations of the atmospheric O
2/N
2 ratio. A new set of secondary and working standard gases with specified O
2/N
2 ratios were also prepared by drying natural air to dew points lower than −80°C using a specially designed H
2O traps and then adjusting its amount of O
2. The prepared five secondary standard gases were repeatedly calibrated against our primary standard, and their O
2/N
2 ratios were confirmed to be stable with no appreciable trend for over 570 days at least. A non-dispersive infrared analyzer was also installed into the measurement system to allow simultaneous measurements of the atmospheric CO
2 concentration. The analytical precision of the CO
2 concentration was estimated to be ±0.03 ppm (1σ). Using the new system, we initiated a systematic observation of the atmospheric O
2/N
2 ratio at Aobayama, Sendai, Japan in February 2007. The observed measurements clearly showed seasonal and diurnal cycles, along with short-term variations on time scales of several hours to several days, caused by terrestrial biospheric and human activities.
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