Preparation of primary standard mixtures for atmospheric oxygen measurements with uncertainty less than 1 ppm for oxygen mole fractions

Primary standard mixtures with less than 1 ppm or 5 per meg standard uncertainty for O2 mole fractions or for O2/N2 ratios were prepared to monitor changes, which occurred in atmospheric oxygen. These mixtures were crafted in 10 L high-pressure aluminum cylinders using a gravimetric method in which unknown uncertainty factors were identified and subsequently reduced. The mole fractions of the constituents, CO2, Ar, O2, and N2, were mainly determined using the masses of the respective source gases that had been filled into the cylinders. To precisely determine the masses of the source gases used in each case, the differences in the masses of the cylinders before and after filling were calculated and compared to nearly identical reference cylinders. Although the mass of the cylinder with respect to the reference cylinder tended to vary in relation to temperature differences between both cylinders, the degree of change could be reduced by measuring both cylinders at the same temperature. The standard uncertainty for the cylinder mass was determined to be 0.82 mg. The standard uncertainties for the O2 mole fractions and O2/N2 ratios in the primary standard mixtures ranged from 0.7 ppm to 0.8 ppm and from 3.3 per meg to 4.0 per meg, respectively. Based on the primary standard mixtures, the mole fractions of atmospheric O2 and Ar on Hateruma Island, Japan. In 2015, the O2 and Ar mole fractions were found to be 209 339.1 ± 1.1 ppm and 9334.4 ± 0.7 ppm.