Development of a high precision continuous measurement system for the atmospheric O₂/N₂ ratio andits application at Aobayama, Sendai, Japan

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.