Account
Precise measurements of the atmospheric O2/N2 ratio by gas chromatographic method
2004 Volume 53 Issue 12 Pages 1389-1398
Details
2004 Volume 53 Issue 12 Pages 1389-1398
A gas chromatographic method for measuring the changes in the atmospheric O2/N2 ratio was developed. An aliquot of air sample was injected into a column filled with molecular sieve 5 A with H2 carrier gas, and the separated O2 + Ar and N2 peaks were detected by a thermal conductivity detector (TCD). The change in the (O2 + Ar)/N2 ratio, which was the value actually measured, was easily converted to the change in the O2/N2 ratio without any significant error, because the variation in the atmospheric Ar/N2 ratio is negligible. Air samples were collected in 2 l or 2.5 l glass flasks at a pressure of about 0.2 MPa (0.1 MPa above ambient pressure). To improve the precision statistically, alternate analyses of the sample and reference gas were repeated fourteen times. The reproducibility for the flask samples was about 5 per meg (about 1.1 ppm O2 in air). O2/N2 measurements of the gravimetric mixtures of pure O2 and N2 indicated that the response of the gas chromatographic system is almost linear within 2%. The reference scale for the O2/N2 determination was based on the air derived from high-pressure cylinders that showed no systematic changes in the O2/N2 ratio. Results from samples collected in the urban area (Tsukuba) and in the forest canopy (Tomakomai) showed clear diurnal O2/N2 variations. The O2/N2 ratios for samples collected at Hateruma Island from 1997 to 2003 showed clear seasonality and decreasing trend.
