Preparation of gravimetric standards for measurements of atmospheric oxygen and reevaluation of atmospheric oxygen concentration

Abstract

Fourteen standard mixtures composed of ambient levels of CO2, Ar, O2, and N2 have been prepared in 10‐L high‐pressure aluminum cylinders by a gravimetric technique for atmospheric O2 measurements. A highly precise balance with a precision of 2.5 mg is used to determine the masses of individual components in the cylinders. To balance the buoyant forces on both sides of the balance beam during weighing the gravimetric standard cylinder, a same sized cylinder is placed on a pan on the opposite side of the beam. In addition, the cylinders of the gravimetric standards and a tare cylinder are alternately weighed to compensate for the drift of the zero‐point of the balance. To determine the mole fractions accurately, the mass of each component is corrected for the buoyancy changes caused by the expansion of the cylinder, and the molecular masses of the source O2 and N2 gases are corrected for their isotopic compositions. The differences in the O2 mole fractions of the 14 gravimetric standards range about 100 ppm (μmol mol−1). The gravimetric mole fractions are compared with the analyzed values of CO2, Ar, O2 + Ar, and (O2 + Ar)/N2. The reproducibility of the gravimetric technique is determined from the standard deviations of the differences between the gravimetric and analyzed values, and is quantified as 15.5 per meg for the O2/N2 ratio and 2.9 ppm for the O2 mole fraction. The gravimetric scale is applied to the measurements from air samples collected at Hateruma Island, Japan. The average Ar mole fraction for the air samples collected from June 2003 through June 2004 is 9333 ± 2 ppm. On the basis of this Ar mole fraction, the annual average mole fractions of O2 and N2 in 2000 are evaluated to be 209392 ± 3 ppm and 780876 ± 2 ppm, respectively.