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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.

In order to study temporal and spatial variations of atmospheric CH4 quantitatively, we originally improved a measurement system for carbon and hydrogen isotopic ratios (δ13C and δD) of CH4 to attain high-precision measurements. By analyzing 100 mL aliquots of an ambient air sample, the precision of our system is 0.080‰ for δ13C and 2.20‰ for δD(1σ), which are one of the highest precisions reported so far. The system consists mainly of aCH4 preconcentration device and a continuous-flow gas chromatograph isotope ratio mass spectrometer equipped with a combustion furnace and a pyrolysis furnace for measurements of δ13C and δD. The preconcentration trap temperature was maintained at -130 ± 1°C during collection of CH4 from the air sample by passing it through the trap, then at -83 ± 1°C while remaining air components such as N2 and O2 except for CH4 escaped, and finally at 100 ± 1°C for CH4 elusion. The isotopic values are measured on a mass spectrometer, relative to respective reference gases. For this study, the δ13C and δD values of the reference gases were calibrated against our primary standards provided by the IAEA: our δ13C primary standard is NBS18, whereas our δD primary standards are V-SMOW and SLAP. To ensure the long-term stability and reproducibility of our measurement system, a calibrated whole air stored in a high-pressure cylinder, which was called “test gas,” was measured at least twice on each day when sample measurements were made. To measure small air samples, such as those extracted from ice cores, we also examined the relation between the sample size and the measured value of δ13C and δD: gradual enrichment of the δ13C occurred with decreasing CH4 content less than 8 nmol whereas no such effect could be seen for the δD. Furthermore, preliminary results of latitudinal distributions of δ13C and δdD were discussed along with CH4 concentrations obtained by our shipboard air-sampling program.

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.