Gravimetric Standard Gas Mixtures for Global Monitoring of Atmospheric SF 6

In this study, standard gas mixtures of SF in synthetic air were gravimetrically developed as a suite consisting of 6 mixtures with mole fractions of SF ranging from 5 to 15 pmol/mol. For precision in weighing the gas fills, an automatic weighing system coupled with a high sensitivity mass balance was used and a gravimetry precision of 3 mg (2σ) was achieved. Impurity profiles of the raw gases were determined by various analyzers. In particular, sub pmol/mol levels of SF in the matrix components (N , O , and Ar) were carefully measured, since the mole fraction of SF in the final step can be significantly biased by this trace amount of SF in the raw gases of the matrix components. Gravimetric dilution of SF by purity-assessed N was performed in 6 steps to achieve a mole fraction of 440 pmol/mol. In the final step, O and Ar were added to mimic the atmospheric composition. Gravimetric fractions of SF and the associated standard uncertainty in each step were computed according to the ISO 6142 and JCGM 100:2008, respectively, and validated experimentally. Eventually, the SF fraction uncertainty of the standard gas mixtures combined by uncertainties of gravimetric preparation and verification measurements were found to be nominally 0.08% at a 95% confidence interval. A comparison with independent calibration standards from NOAA shows agreement within 0.49%, satisfying the extended WMO compatibility goal, 0.05 ppt.