Intercomparison of O₂ ∕ N₂ ratio scales among AIST, NIES, TU, and SIO based on a round-robin exercise using gravimetric standard mixtures

Abstract

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 the O2 molar fraction gravimetrically prepared by the National Metrology Institute of Japan, AIST (NMIJ/AIST) with a standard uncertainty of less than 5 per meg (0.001 ‰) were used as round-robin standard mixtures. EMRI/AIST, NIES, TU, and SIO reported the analyzed 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 intercomparison experiment from May 2017 to March 2020 was corrected based on the δ(O2/N2) values analyzed before and after the laboratory measurements by EMRI/AIST. The scales are compared based on offsets in zero and span. The relative span offsets of EMRI/AIST, TU, NIES, and SIO scales against the NMIJ/AIST scale were -0.11%±0.10%, -0.10%±0.13%, 3.39 %±0.13 %, and 0.93 %±0.10 %, respectively. The largest offset corresponded to a 0.30 Pg yr−1 decrease and increase in global estimates for land biospheric and oceanic CO2 uptakes based on trends in atmospheric CO2 and δ(O2/N2). The deviations in the measured δ(O2/N2) values on the laboratory scales from the NMIJ/AIST scale are 65.8±2.2, 425.7±3.1, 404.5±3.0, and 596.4±2.4 per meg for EMRI/AIST, TU, NIES, and SIO, respectively. The difference between atmospheric δ(O2/N2) values observed at Hateruma Island (HAT; 24.05∘ N, 123.81∘ E), Japan, by EMRI/AIST and NIES were reduced from -329.3±6.9 to -6.6±6.8 per meg by converting their scales to the NMIJ/AIST scale.