Inter-comparison of O₂/N₂ Ratio Scales Among AIST, NIES, TU, and SIO Based on Round-Robin Using Gravimetric Standard Mixtures

Abstract. A study was conducted to compare the δ(O₂/N₂) scales used by four laboratories engaged in atmospheric δ(O₂/N₂) measurements. These laboratories are the Research Institute for Environmental Management Technology, Advanced Industrial Science and Technology (EMRI/AIST), the National Institute for Environmental Studies (NIES), Tohoku University (TU), and Scripps Institution of Oceanography (SIO). Therefore, five high-precision standard mixtures for O₂ molar fraction gravimetrically prepared by the National Metrology Institute of Japan (NMIJ), AIST (NMIJ/AIST) with a standard uncertainty of less than 5 per meg were used as round-robin standard mixtures. EMRI/AIST, NIES, TU, and SIO reported the analysed values of the standard mixtures on their own δ(O₂/N₂) scales, and the values were compared with the δ(O₂/N₂) values gravimetrically determined by NMIJ/AIST (the NMIJ/AIST scale). The δ(O₂/N₂) temporal drift in the five standard mixtures during the inter-comparison experiment was corrected based on the δ(O₂/N₂) values analysed before and after the experiments by EMRI/AIST. The scales are compared based on offsets in zero and span. The span offsets from the NMIJ/AIST scale ranged from −0.17 % to 3.3 %, corresponding with the difference of 0.29 Pg yr⁻¹ in the estimates for land biospheric and oceanic CO₂ uptakes. The zero offsets from the NMIJ/AIST scale are −581.0 ± 2.2, −221.4 ± 3.1, −243.0 ± 3.0, and −50.7 ± 2.4 per meg for EMRI/AIST, TU, NIES, and SIO, respectively. The atmospheric δ(O₂/N₂) values observed at Hateruma Island (HAT; 24.05° N, 123.81° E), Japan, by EMRI/AIST and NIES became comparable by converting their scales to the NMIJ/AIST scale.