To contribute to a better understanding of the global carbon cycle, a high precision continuous measurement system for atmospheric O2/N2 ratio was developed using a fuel cell oxygen analyzer. To obtain highly precise values of the atmospheric O2/N2 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 O2/N2 ratio. A new set of secondary and working standard gases with specified O2/N2 ratios were also prepared by drying natural air to dew points lower than-80°C using a specially designed H2O traps and then adjusting its amount of O2. The prepared five secondary standard gases were repeatedly calibrated against our primary standard, and their O2/N2 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 CO2 concentration. The analytical precision of the CO2 concentration was estimated to be ±0.03 ppm (1σ). Using the new system, we initiated a systematic observation of the atmospheric O2/N2 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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