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Abstract. Precise monitoring of changes in atmospheric O2 levels wasimplemented by preparing primary standard mixtures with less than1  µ mol mol −1 standard uncertainty for O2 molar fractions. In thisstudy, these mixtures were crafted in 10 L high-pressure aluminium alloycylinders using a gravimetric method in which unknown uncertainty factorswere theoretically determined and subsequently reduced. Molar fractions ofthe constituents ( CO2 , Ar, O2 , and N2 ) in the primarystandard mixtures were mainly resolved using masses of the respective sourcegases ( CO2 , Ar, O2 , and N2 ) that had been filled into thecylinders. To precisely determine the masses of the source gases, thedifference in mass of the cylinder before and after filling the respectivesource gas was calculated by comparison with an almost identical referencecylinder. Although the masses of the cylinders filled with the source gas withrespect to the reference cylinder tended to deviate in relation totemperature differences between the source-gas-filled cylinder andsurrounding air, the degree of the deviation could be efficiently reduced bymeasuring the two cylinders at the exact same temperature. The standarduncertainty for the cylinder mass obtained in our weighing system wasdetermined to be 0.82 mg. The standard uncertainties for the O2 molarfractions in the primary standard mixtures ranged from 0.7to 0.8  µ mol mol −1 . Based on the primary standardmixtures, the annual average molar fractions of atmospheric O2 and Arin 2015 at Hateruma island, Japan, were found to be 209339.1±1.1 and 9334.4±0.7 µ mol mol −1 ,respectively. The molar fraction for atmospheric Ar was in agreement withprevious reports.

atmospheric measurement techniques

Preparation of primary standard mixtures for atmospheric oxygen measurements with less than 1 µmol mol&amp;lt;sup&amp;gt;−1&amp;lt;/sup&amp;gt; uncertainty for oxygen molar fractions