Intercomparison of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; ∕ N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; ratio scales among AIST, NIES, TU, and SIO based on a round-robin exercise using gravimetric standard mixtures

Shinji Morimoto,Adam Cox,Shuichiro Takebayashi,Shigeyuki Ishidoya,Ralph F Keeling,Shohei Murayama,Nobuyuki Aoki,Yasunori Tohjima

doi:10.5194/amt-14-6181-2021

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.

Highlights

Observing the long-term change in atmospheric O2 molar fraction combined with CO2 observation enables us to estimate terrestrial biospheric and oceanic CO2 uptakes separately (Manning and Keeling, 2006; Tohjima et al, 2008; Ishidoya et al, 2012a, b)
The intercepts of the lines represent the differences between individual laboratory scales and the NMIJ/AIST scale corresponding to δ(O2/N2)NMIJ/AIST = 0: 65.8 ± 2.2, 425.7 ± 3.1, 404.5 ± 3.0, and 596.4 ± 2.4 per meg for EMRI/AIST, Tohoku University (TU), National Institute for Environmental Studies (NIES), and Scripps Institution of Oceanography (SIO), respectively
The relative deviations in span sensitivity of the 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 %, which were quantified for the first time

Summary

Introduction

Observing the long-term change in atmospheric O2 molar fraction combined with CO2 observation enables us to estimate terrestrial biospheric and oceanic CO2 uptakes separately (Manning and Keeling, 2006; Tohjima et al, 2008; Ishidoya et al, 2012a, b). Resplandy et al (2019) introduced a method to estimate the global ocean heat content (OHC) increase based on atmospheric O2 and CO2 measurements. They extracted solubility-driven components of the atmospheric potential oxygen (APO = O2 +1.1×CO2) (Stephens et al, 1998) by combining their observational results with climate and ocean models. The atmospheric O2 measurements are linked to the global CO2 budget and OHC

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Conclusion