Inter-comparison of O 2 /N 2 Ratio Scales Among AIST, NIES, TU, and SIO Based on Round-Robin Using Gravimetric Standard Mixtures Round-Robin

. A study was conducted to compare the (cid:303) (O 2 /N 2 ) scales used by four laboratories engaged in atmospheric (cid:303) (O 2 /N 2 ) measurements. These laboratories are the Research Institute for Environmental Management Technology, Advanced Industrial 15 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 O 2 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 (cid:303)(cid:11)(cid:50) 2 /N 2 (cid:12)(cid:3) (cid:86)(cid:70)(cid:68)(cid:79)(cid:72)(cid:86)(cid:15)(cid:3) %, corresponding with the difference of 0.29 Pg yr (cid:237) 1 in the estimates for land biospheric and oceanic CO 2 uptakes. The zero offsets from the NMIJ/AIST scale are (cid:237) 581.0 ± 2.2, (cid:237) 221.4 ± 3.1, (cid:237) 243.0 ± 3.0, and (cid:237)(cid:24)(cid:19)(cid:17)(cid:26)(cid:3)(cid:147)(cid:3)(cid:21)(cid:17)(cid:23) per meg 25 for EMRI/AIST, TU, NIES, and SIO, respectively. T (cid:75)(cid:72)(cid:3)(cid:68)(cid:87)(cid:80)(cid:82)(cid:86)(cid:83)(cid:75)(cid:72)(cid:85)(cid:76)(cid:70)(cid:3)(cid:303)(cid:11)(cid:50) 2 /N 2 ) 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. 2 /N 2 ) NMIJ/AIST values. The high-precision standard mixtures prepared in April and June 2017 were selected from the round-robin standard mixtures. All residuals were within the expanded uncertainties, 30 which were less than 8 per meg, identified that the NMIJ/AIST scale could be reproduced any time by preparing high-precision standard mixtures. Results show that a long-term temporal drift o (cid:73)(cid:3)(cid:72)(cid:68)(cid:70)(cid:75)(cid:3)(cid:79)(cid:68)(cid:69)(cid:82)(cid:85)(cid:68)(cid:87)(cid:82)(cid:85)(cid:92)(cid:182)(cid:86)(cid:3)(cid:303)(cid:11)(cid:50) 2 /N 2 ) scale, which is determined against a reference natural air in a high-pressure cylinder, can be evaluated by comparing the reference air with high-precision standard mixtures by NMIJ/AIST.


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. O2 is exchanged with CO2 with some stoichiometric ratios for terrestrial biospheric activities and fossil fuel combustion. Meanwhile, the ocean CO2 uptake and O2 emissions are decoupled since the ocean acts as a carbon sink by physiochemically dissolving the CO2 (e.g., Keeling et al., 1993). Various laboratories 5 have performed changes in atmospheric O2 since the early 1990s (e.g., Keeling et al., 1996;Bender et al., 2005;Manning and Keeling, 2006;Tohjima et al., 2008Tohjima et al., , 2019Ishidoya et al., 2012a, b;Goto et al., 2017). Recently, 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 global OHC change is a fundamental 10 measure of global warming. Indeed, the ocean uptakes more than 90% of the earth's excess energy and is evaluated based on ocean temperature measurements using Argo float (e.g., Levitus et al., 2012). Thus, the atmospheric O2 measurements are linked to the global CO2 budget and OHC.

25
In the equation, n depicts the molar amount of each substance, and the subscripts sam and ref represent sample and reference 2/N2) value multiplied by 10 6 is expressed in per meg units. The O2 molar fractions in air are 20.946% (Machta and Hughes, 1970). Therefore 2 2/N2) by 4.8 per meg. Each laboratory has typically employed its own O2/N2 reference based on natural air compressed and stored in high-pressure cylinders. Each laboratory has also assumed responsibility for calibrating the relationship between the measured instrument 30 response and the reported change per meg units (span sensitivity). Therefore, the reported trends in O2/N2 are potentially biased by any long-term drift in the O2/N2 ratio of the reference cylinders (zero drift) or errors in the assumed span sensitivity the instrument (span error). Note that an uncertainty below 5 per meg is required for the global CO2 budget analyses based on https://doi.org /10.5194/amt-2020-481 Preprint. Discussion started: 6 January 2021 c Author(s) 2021. CC BY 4.0 License. e 20.946% (Machta and Hughes, 1970 Table 2 in Keeling et al. (1993)]. Challenges in achieving this precision include fractionations of O2 and N2 induced by pressure, temperature, and water vapour gradients (Keeling et al., 2007), adsorption/desorption of the constituents on the cylinder's inner surface , and permeation/leakage of the constituents from/through the valve (Sturm et al., 2004;Keeling et al., 2007). Tohjima et al. (2005) developed high-precision O2 standard mixtures with 2/N2) to resolve these problems by preparing gravimetric standard mixtures of pure N2, O2, Ar, 5 and CO2. Their study was significant, but the uncertainties remain larger than those recommended by Keeling et al. (1993), as mentioned above.
Recently, a technique was developed for preparing high-precision primary standard mixtures with standard uncertainties less than 5 per meg for 2/N2) at the National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) (Aoki et al., 2019). The high-precision standard mixtures allow us to evaluate scale zero and 10 span offsets accurately and precisely. In this study, we conducted inter-comparison experiments to compare span sensitivities among the O2/N2 scales of Research Institute for Environmental Management Technology, Advanced Industrial Science and Technology (EMRI/AIST), National Institute for Environmental Studies (NIES), Tohoku University (TU), and Scripps Institution of Oceanography (SIO) using the developed high-precision standard mixtures. Following this, a regression analysis is applied to the inter-comparison results to investigate the relationship between the individual laboratory O2/N2 scales. Results 15 showed a slight but significant difference in the span sensitivities of the individual scales. Finally, we compare the atmospheric 2/N2) values observed on the EMRI/AIST scale with those on the NIES scale for the air samples collected at Hateruma Island (HAT; 24°03'N, 123°49'E), Japan, using the relationship between the individual laboratory scales obtained in this study.

NMIJ/AIST Scale and Round-Robin Standard Mixtures 20
In this study, five high-precision standard mixtures with standard uncertainties 2/N2) were used as round-robin standard mixtures. The NMIJ/AIST previously mixed them gravimetrically following ISO 6142-1:2015 (Aoki et al., 2019), which were contained in 10 L aluminium-alloy cylinders (Luxfer Gas Cylinders, UK) with a diaphragm valve (G-55, Hamai Industries Limited, Japan). Table 1 shows the gravimetrically determined molar fractions for N2, O2, Ar, CO2, as well as 2/N2) in the round-robin mixtures. However, the gravimetric values of N2, O2, Ar, and CO2 molar fractions were 25 recalculated based on the cylinders' updated expansion rate. The value was determined as 1.62 ± 0.06 ml Mpa (unpublished data), which was determined by measuring expansion volume of a cylinder with an increase of inner pressure of the cylinders sunk in water since the previous expansion rate (2.2 ± 0.2 ml Mpa ) was provided by a cylinder supplier. The source gases used are pure CO2 (>99.998%, Nippon Ekitan Corp., Japan), pure Ar (99.9999%, G1-grade, Japan Fine Products, Japan), pure O2 (99.99995%, G1-grade, Japan Fine Products, Japan), and pure N2 (99.99995%, G1-grade, Japan Fine Products, Japan). 30 Impurities in the source gases were identified and quantified via several techniques, including gas chromatography (GC). GC https://doi.org /10.5194/amt-2020-481 Preprint.  equipped with a thermal conductivity detector (GC/TCD) was used to analyse N2, O2, CH4, and H2 in pure CO2. O2 and Ar in pure N2 and N2 in pure O2 were analysed using GC, equipped with a mass spectrometer. A Fourier-transform infrared spectrometer was used to detect CO2, CH4, and CO in pure N2, O2, and Ar. A galvanic cell O2 analyser was used to quantify O2 in pure Ar. A capacitance-type moisture sensor measured H2O in pure CO2, and a cavity ring-down moisture analyser measured H2O in pure N2, O2, and Ar. 5 In this study, the absolute O2/N2 scale determined using the round-robin standard mixtures is hereafter the NMIJ/AIST scale.

Procedure of Inter-comparison
The EMRI/AIST, NIES, TU, and SIO conducted the inter-comparison experiment. scales to the NMIJ/AIST. The subscript round-robin is hereafter the round-robin standard mixture. Each lab analysed air delivered from the cylinders after placing them horizontally for more than five days after their transport to avoid the change 2/N2)round-robin values in the standard mixtures by thermal diffusion and gravitational fractionation 2/N2)round-robin values determined by individual laboratories using 2/N2)NMIJ/AIST values. EMRI/AIST and TU used mass spectrometry, NIES used GC, and SIO used the interferometric method, as summarised in 20 Table 2. The stability of O2/N2 ratios in the round-robin standard mixtures during the inter-comparison experiment was evaluated by analysing 2/N2)round-robin values using a mass spectrometer (Delta-V, Thermo Fisher Scientific Inc., USA) (Ishidoya and Murayama, 2014) at EMRI/AIST before and after the inter-comparison experiment.
Ar molar fractions in the round-mol , much more variable than ) (Keeling et al., 2004). 17  Author: stephens Subject: Highlight Date: 5/21/2021 4:15:30 PM Why not use a more modern O2 mole fraction estimate? If it does not matter, consider saying so. Since you have gravimetric determinations, I'm guessing the assignment of zero on the scale is arbitrary, so it might help to say "we arbitrarily assign zero on the NMIJ/AIST scale to correspond to a ratio of 0.26825" and also maybe that this corresponds to the late 1960s. We applied the 2/N2)round-robin values from the individual laboratories by considering the deviations of Ar molar fraction and isotopic ratios in the round-robin standard mixtures from the tropospheric air. The 5 2/N2)round-robin values reported by EMRI/AIST and TU were corrected based on the deviation in the isotope ratio from the atmospheric level using isotopic ratios of N and O measured simultaneously at EMRI/AIST. This is because they measured   (Junk and Svec, 1958;Baertschi, 1976;Li et al., 1988;Barkan and Luz, 2005).

NIES
2/N2)round-robin 2+Ar)/N2}round-robin values measured using a GC/TCD (Tohjima,15 2+Ar)/N2} round-robin values were calculated against the reference air on the NIES scale, which is natural air filled in a 48 L aluminium cylinder. A column separates the (O2 + Ar) and N2 in the air sample, and a TCD detected the individual peaks. The reference and sample air were repeatedly measured using the GC/TCD, 2+Ar)/N2} round-robin values were calculated based on the ratios of the (O2 + Ar) peak area to N2 peak area using Eq. (9). 20 2/N2) round-robin value is given by Eq. (10).
where the coefficient a is defined by a = k(Ar/O2)ref. k represents the TCD sensitivity ratio of Ar relative to O2, and the value was evaluated as 1.13 by comparing gravimetric mixtures of O2 + N2 and Ar + O2 + N2 (Tohjima et al., 2005). Natural air is used for the reference gas. Therefore, the value of a is calculated as 0.050 (Ar = 0.93% and O2 = 20.94%). In this study, the 2)round-robin value was calculated based on N2 molar fractions in the round-robin standard mixtures calculated based on 30 2+Ar)/N2}round-robin values from the GC/TCD and CO2 molar fractions from non-dispersive infrared spectroscopy and gravimetric Ar molar fractions in the round-robin standard mixtures.
In this study, the 30 2) value was calculated based on N2 molar fractions in the round-robin standard mixtures calculated based on )round-robin 2+Ar)/N2} values from the GC/TCD and CO2 molar fractions from non-dispersive infrared spectroscopy and }round-robin gravimetric Ar molar fractions in the round-robin standard mixtures. The NIES O2/N2 scale is related to a set of 11 primary reference air. The NIES O2/N2 scale's long-term stability has been maintained within ±0.45 per meg yr 1 by analysing the relative differences in the O2/N2 ratios in the primary and working reference air (Tohjima et al., 2019). Details of the analytical methods and the NIES O2/N2 scale are given in Tohjima et al. (2005Tohjima et al. ( , 2008 The isotopic ratios in the round-robin standard mixtures were calculated using Eqs. (6), (7), and (12). 14 N 14 N/ 15 N 14 14 N 14 N/ 15

SIO 25 SIO
2/N2) values based on measurements using a two-wavelength interferometer . The SIO O2/N2 2/N2) = 0) is based on a suite of 18 primary reference gases stored in high-pressure cylinders (aluminium or steel, volumes ranging from 29 to 47 L) filled with natural air (Keeling et al., 2007). Differences between the round-robin cylinders and the SIO reference were determined from  cylinders, which may differ in their Ar/N2 ratios from natural air and which lack constituents other than N2, O2, Ar, and CO2.
These corrections require estimates of the molar Ar/N2 ratio and other gases' abundances in typical background air. Notably, the primary reference gases are relevant in Eq. (13) as references for relative refractivity. Therefore, the exact Ar/N2 ratio and abundances of other gases in the SIO reference are not directly relevant. For background air, the following values were adopted: Ar/N2 = 0.0119543, Ne/N2 = 2.328 × 10 5 , He/N2 = 6.71×10 6 , Kr/N2 = 1. refractivity data for the pure gases and natural air (Keeling, 1988 using Xe data from Kronjäger (1936) (also see Keeling et al., 2020). 2 2) = ((Ar/N2)grav/0.0119543 1). 20 The Ar/N2 interference ( / (Ar/N )) ranges from 55 to + 24 per meg, depending on the round-robin cylinder. The sum of the remaining interferences, other than for CO2 (-other interferences), is effectively constant at 14.3 per meg. The largest individual contributions are from Ne ( 32.8 per meg) and CH4 (+11.9 per meg). refractivity data for the pure gases and natural air (Keeling, 1988 using Xe data from Kronjäger (1936) (also see Keeling et al., 2020  The temporal drifts analysed in March 2018 (before shipment) ranged from 5.9 to 5.5 per meg. This range was within the expanded uncertainty (6.4 per meg) of measurement using the mass spectrometer of EMRI/AIST. Here the expanded uncertainty (a a 95% level of confidence. The temporal drifts analysed in March 2019 (after 10 the cylinder's return from SIO) ranged from 16.4 per meg to 2.9 per meg. This range was larger than the expanded uncertainty of measurement.

Results and Discussion
We also analysed the round-robin standard mixtures in March 2020 (a year after return) and found that the temporal drifts ranged from 18.3 per meg to 5.6 per meg.
2/N2)round-robin values decreased slightly with time in all cylinders, 2/N2)round-robin values in the cylinders, except for 15 CPB16379, was 3.2 ± 1.1 per meg yr 1 . Meanwhile, that of the CPB16379 cylinder was 6.7 ± 2.1 per meg yr 1 . The decreasing rates and standard deviations were calculated from least-square fitting. The decrease in 2/N2)round-robin values during the inter-comparison experiment are thought to be caused by O2 consumption by the oxidation of residual organic material, oxidation of the inner surface of the cylinders, and selective O2 desorption on the inner surface of the cylinders rather than the fractionation of O2 and N2 since of the escape of gas from the cylinder generally increases the O2/N2 in a cylinder 20 (Langenfelds et al., 1999). We corrected the temporal drifts during the inter-comparison experiment by linearly interpolating 2/N2)NMIJ/AIST value of the data analysed by individual laboratories using the temporal drifts measured before and after the analysis of individual laboratories. Following this, we compared the int 2/N2)NMIJ/AIST value with the measured 2/N2)round-robin value. We evaluated the NMIJ/AIST scale's reproducibility using nine high-precision standard mixtures prepared in different periods 25 (from April 2017 to February 2020). Figure 2 shows the relations 2/N2)NMIJ/AIST values gravimetrically 2/N2) values measured using the mass spectrometer at EMRI/AIST. The lines in Figure  2a represent the Deming least-square fit to the data, and Figure 2b Table 3 summarises the 2/N2)round-robin values measured by individual laboratories. Notably, 2/N2)round-robin shown in Table   3 are the corrected values for the deviations in Ar/N2 ratios and isotopic ratios of N2 and O2 in the round-robin standard mixtures from the atmospheric values and determined against their scales, as described in Section 2.3. The differences in the intercepts between SIO and other laboratories were 530.3 ± 3.3, 170.7 ± 3.9, and 192.4 ± 3.9 per meg for EMRI/AIST, TU, and NIES, respectively. The differences of NIES and TU from SIO were consistent with those 15 obtained from past inter-comparison experiments (the GOLLUM comparison, 2015) (Table 4) although the difference of TU from SIO was slightly bigger. Figure 3b shows the residuals from the fitting lines. All of them fall within expanded uncertainties on the measurement for individual laboratories.

2/N2) Data Between the Laboratories and Its Implication to the Global CO2 Budget Analysis 20
This study shows that the inter-comparison results allow us to compare the observation data of individual laboratories directly.
We compared the O2/N2 ratios measured by EMRI/AIST and NIES based on flask samples collected at HAT from October 2015 to December 2019 (Tohjima et al., 2008). The values of NIES after March 2018 are preliminary data. The air samples were collected twice monthly into two Pyrex glass flasks arranged in series (one for AIST and the other for NIES). We confirmed that the isotopic ratios of N2 and O2 did not significantly differ from the atmospheric values for the HAT air samples. 25 Therefore, 16 O 16 O/ 14 N 14 2+Ar)/N2} which were measured using the mass spectrometer and GC/TCD equal 2/N2) in Eq. (1). Figure 4a shows the 2/N2) values reported on the NIES and EMRI/AIST scales. The average difference in 2/N2) between the two scales was 329.3 ± 6.9 per meg. The uncertainty represents the standard deviation of the differences. B 2/N2) were converted to the NMIJ/AIST scale using Eq. (14), 30 where an and bn are the slope and intercept of each laboratory's line (n) obtained in Section 3.2. Figure 4b shows the converted 2/N2) values. This scale conversion reduced the bias between the 2/N2) values of EMRI/AIST and NIES to 6.6 ± 6.8 2/N2) values of EMRI/AIST from those of NIES. The bias dropped within the uncertainty, representing the standard deviation of the differences. Figures 5a and 5 2/N2) before and after the scale 5 conversion, confirming the compatibility between the span sensitivities on the EMRI/AIST and NIES scales. The lines represent a Deming least-square fit to the scatter plots. The slope of the line before scale conversion and its standard deviation is 0.956 ± 0.015, consistent with the difference in the span sensitivity between both scales (0.9983/1.0329 = 0.967) within uncertainty. After the scale conversion, the slope and its standard deviation is 0.990 ± 0.015, identifying that the scale conversion corrected the difference in the span sensitivity between the EMRI/AIST and NIES scales to the NMIJ/AIST scales. 10 Observing the long-2/N2) provides critical information on the global CO2 budget (Manning and Keeling, 2006). Recently, Tohjima et al. (2019) estimated the land biospheric and oceanic CO2 uptakes using the average secular changi 2/N2 2/N2) on the NIES scale to that on the NMIJ/AIST scales and recalculated the global CO2 budgets from 2000 to 2016 using the converted rates. Table   5 summarises the CO2 budgets reported by Tohjima et al. (2019)  estimated an increase in the global OHC based on the atmospheric O2 and CO2 measurements. They reported that the largest single source of uncertainty in their estimation is the scale error from the span calibration of the O2/N2 analyser. They also mentioned that the error would be reduced via within-lab and inter-lab comparisons. Therefore, the span sensitivities of the EMRI/AIST, TU, NIES, and SIO scales against the NMIJ/AIST absolute scale obtained from the inter-comparison experiment in this study should improve the accuracy of the OHC increase estimate significantly. 25

Conclusions
The inter-comparison experiment was used to evaluate the relationship between 2/N2) values and span sensitivities of the individual laboratory scales from the NMIJ/AIST scale using gravimetrically prepared high-precision   243.0 ± 3.0, and per meg, respectively. The differences between individual absolute values were consistent with the results from the GOLLUM round-robin cylinder comparison. However 2/N2) values in the round-robin standard mixtures decreased at rates of 6.7 ± 2.1 per meg yr 1 for one cylinder and 3.2 ± 1.1 per meg yr 1 for the other four cylinders.
The decrease was caused by O2 consumption by oxidation of residual organic material, oxidation of the cylinders' inner surface, and selective O2 desorption on the inner surface of the cylinders rather. The fractionation of O2 and N2 did not cause it because 5 of the escape of gas from the cylinder. The O2/N2 ratios in high-precision standard mixtures prepared in different periods by NMIJ/AIST are reproduced within the O2/N2 ratios' uncertainty, identifying that the NMIJ/AIST scale can be reproduced any time by preparing high-precision standard mixtures. Further, a long-term temporal drift of each laboratory's scale can be evaluated by comparing the reference air with high-precision standard mixtures prepared by NMIJ/AIST. Finally, we 2/N2) on the EMRI/AIST and NIES scales in flask samples collected at 10 HAT became comparable by converting both scales to the NMIJ/AIST scale, although the bias is not negligible. The results obtained in this study should improve the estimation method of carbon budgets and OHC increase.

Acknowledgments
We thank the Global Environmental Forum (GEF) staff for their work in collecting the air samples at the Hateruma station.

Figure 1
The temporal 2/N2)round-robin values from the initial values were measured using a mass spectrometer at EMRI/AIST after preparing the round-robin standard mixtures before the shipment of the cylinders to SIO, after the return of 20 the cylinders from SIO, and a year after the return.
Numbers are given in the unit of per meg. The numbers following the symbol ± denote the standard uncertainty of 30 measurement for individual laboratories.   Table 4. Slopes and intercepts of the lines obtained by the Deming least-2/N2)round-robin values for individual laboratories, and deviation in the individual scales from SIO in this study and the GOLLUM 15.