High-precision gravimetric primary standards for atmospheric Ar/N₂ measurements and redetermination of atmospheric Ar abundance

Abstract. Accurate determination of atmospheric Ar/N₂ ratios requires primary standard gases with well-characterized uncertainties. However, an initial preparation approach based on the transfer of premixed gases introduced systematic deviations caused by transfer-induced composition changes, which limited the achievable accuracy. In this study, we developed an improved gravimetric preparation method for highly precise Ar standards (HPAs), in which CO₂, Ar, O₂, and N₂ are introduced independently from source gases. This approach eliminates transfer-induced compositional changes and allows the Ar amount to be determined directly with high precision. The resulting uncertainty in Ar/N₂ was reduced to approximately 5 per meg, corresponding to an Ar molar fraction of 0.05 µmol mol⁻¹. Validation confirmed that the gravimetric and measured values for CO₂, O₂/N₂, and Ar/N₂ agreed within their uncertainties. Using the validated HPAs, the absolute Ar/N₂ ratio of the AIST reference air was redetermined as 0.01195567 ± 0.000000011, a 8-fold reduction in uncertainty relative to the previous value. Application to atmospheric observations yielded an absolute Ar/N₂ ratio of 0.01195497 ± 0.00000013 and an Ar mole fraction of 9335.71 ± 0.10 µmol mol⁻¹ in 2021. The numbers following the symbol ± represent the standard uncertainty. These results establish a metrologically traceable framework for high-precision atmospheric Ar measurements.