Gas Reference Materials for Underpinning Atmospheric Measurements of Stable Isotopes of Nitrous Oxide

The precise measurement of the amount fraction of atmospheric nitrous oxide (N2O) is required to understand global emission trends. Analysis of the site-specific stable isotopic composition of N2O provides a means to differentiate emission sources. The availability of accurate reference materials of known N2O amount fractions and isotopic composition is critical for achieving these goals. We present the development of nitrous oxide gas reference materials for underpinning measurements 15 of atmospheric composition and isotope ratio. Uncertainties target the World Metrological Organisation Global Atmosphere Watch (WMO-GAW) compatibility goal of 0.1 nmol mol and extended compatibility goal of 0.3 nmol mol, for atmospheric N2O measurements in an amount fraction range of 325-335 nmol mol. We also demonstrate the stability of amount fraction and isotope ratio of these reference materials and present a characterisation study of the cavity ring down spectrometer used for analysis of the reference materials. 20

(1) sampling with a pressure transducer (Omega PXM 319) and data was recorded via LabVIEW.
In a second approach, nominally 325 nmol mol -1 N2O in synthetic air reference materials were prepared from the same 500 µmol mol -1 N2O reference material in 10 L cylinders with three different commercially available internal passivation processes.
The cylinders were sampled into the CRDS analyser following the same procedure as for the 0.85 L cylinders. 160

Analytical methods
A cavity ring-down spectrometer (Picarro G5131-i) was used for the analysis of the ambient amount fraction N2O mixtures.
The instrument allows simultaneous monitoring of N2O amount fraction and isotopic composition through measurement of the bulk δ 15 N, δ 18 O and the site-specific δ 15 N α and δ 15 N β . Bulk δ 15 N is calculated as the average of the site-specific δ 15 N α and δ 15 N β . 165 The instrument comprises an internal pump and a critical orifice to reduce the gas flow into the cavity of the analyser. An excess flow was provided to the instrument (0.5 L min -1 ) and the excess vented to the atmosphere to ensure stable (atmospheric) inlet pressure and no contamination with ambient air.
Analysis of the amount fraction of argon in the nominally 30 % argon in nitrogen pre-mixture cylinders was performed by gas 170 chromatography with thermal conductivity detector (GC-TCD; Agilent 6890) using a capillary column (Molsieve 5A, 30 m x 0.53 mm x 0.50 µm) operated isothermally at 30 ± 1 °C.

Allan deviation 175
A 325 nmol mol -1 N2O in synthetic air reference material was analysed continuously over 25 hours, collecting temporal trends of N2O amount fractions and isotope delta values. The Allan deviation was calculated, to assess the optimum averaging time  The Allan deviation initially decreases with an increase in the averaging time and reaches a minimum for N2O amount fractions (0.036 nmol mol -1 ) and delta values δ 15 N (0.37 ‰), δ 15 N α (0.67 ‰), δ 15 N β (0.33 ‰) and δ 18 O (0.89 ‰) for averaging times of 185 around 15 minutes. For longer averaging times, an increase in the Allan deviation is shown and likely to be a result of analyser drift. An averaging time of 10 minutes was adopted to ensure both minimal uncertainty for comparing the reference gas to a sample gas, and efficient use of the reference material. Achieved precisions for N2O amount fraction and isotope ratios are in agreement with the typical precisions reported by Picarro in the instrument specification of < 0.05 nmol mol -1 N2O and < 0.7 ‰ for δ 15 N, δ 15 N α , δ 15 N β , δ 18 O for a 10-minute averaging period (Picarro, 2017). 190

Characterisation of the CRDS for reported delta values with N2O amount fraction.
The characterisation of the CRDS for reported delta values with N2O amount fraction was assessed with both statically and dynamically generated reference materials. Dynamic reference materials were produced in the amount fraction range 150-1100 nmol mol -1 by dilution from a nominally 320 µmol mol -1 N2O in synthetic air reference material with synthetic air using a 195 dynamic dilution device comprising one diluent and three standard critical flow orifices (Hill-Pearce et al., 2018). The static https://doi.org/10.5194/amt-2021-45 Preprint. Discussion started: 25 February 2021 c Author(s) 2021. CC BY 4.0 License. and dynamic reference materials were generated alternately for 4 iterations, with synthetic air measured between each set. Due to the large number of measurements recorded, a reduced sampling time of 5 minutes was adopted for each measurement interval resulting in a slightly lower standard deviation of 0.03 nmol mol -1 for amount fractions.

Delta 15 N
The δ 15 N values analysed by the G5131-i analyser were recorded for each static and dynamic reference material for four repetitions of five minutes. The mean value of the stable response was calculated. The change in reported delta value with amount fraction was assessed and found to vary with a linear function with respect to the reciprocal of N2O amount fraction 205 as reported by (Harris et al., 2020) for the same CRDS model, with a different year of manufacture. Figure 2 shows the CRDS analyser response to δ 15 N for static and dynamic reference materials prepared from the same pure N2O source in the amount fraction range of 300-1500 nmol mol -1 . (Winther et al., 2018) reported the same trend for dependence of reported δ 15 N on N2O amount fraction, attributing the amount fraction dependence to offsets in the measurement of 14 N 15 N 16 O and 15 N 14 N 16 O. The agreement between static and dynamic reference materials is discussed in the results section.

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The agreement in δ 15 N and δ 18 O between static and dynamic reference materials (shown in Figure 2) indicates minimal fractionation of isotopocules on dilution through a critical flow orifice based dynamic system or on production of the reference materials by filling though an intermediate vessel and dilution. No variation in reported delta values beyond the measurement uncertainty for N2O amount fractions over the range of 300-1500 nmol mol -1 was observed between the analyser response of the static and dynamic reference materials ( Figure 2). However, the large uncertainty makes comparisons of the delta value 230 between similar amount fractions challenging. The uncertainty would be reduced by increasing the averaging time.

Uncertainty in N2O amount fraction
Uncertainty in the amount fraction of N2O in a reference material has several sources including: uncertainty due to gravimetric preparation (weighing uncertainties), uncertainty in the purity of the gases used (e.g. amount fraction of N2O in the matrix), 235 cylinder effects such as adsorption of the gas molecules onto the walls of the cylinder and valve, uncertainties in amount fraction due to the stability of the gas reference material and analytical precision of the measurement technique. Each uncertainty contribution is discussed below.

Uncertainty and reproducibility in the amount fraction of reference materials due to gravimetric production. 240
To assess the uncertainty in the amount fraction from production of N2O reference materials, 8 reference materials were produced by two separate operators from two separate 500 µmol mol -1 N2O in nitrogen reference materials but the same matrix gases and pure N2O source. Four of the reference materials were produced at nominally 337 nmol mol -1 and four were produced at nominally 326 nmol mol -1 .

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The combined contribution to the uncertainty due to gravimetry and purity of the components for the ambient amount fraction N2O in synthetic air reference materials produced, as detailed above, is 0.08 % (k=2) 0.28 nmol mol -1 . This uncertainty is within the WMO-GAW extended compatibility goals of ± 0.3 nmol mol -1 .
The combined expanded uncertainty is dominated by the uncertainty in the mass of parent gas additions. There is a 0.3 mg 250 uncertainty on the mass of N2O added in the indirect transfer vessel additions to prepare the 500 µmol mol -1 N2O intermediate and 325 nmol mol -1 N2O reference materials, which combine to 73.07 % of the combined expanded uncertainty. There is a 3 mg uncertainty on the mass added for each direct gas addition (N2, O2, Ar), which provides a negligible contribution to the expanded uncertainty. The uncertainty in the N2O impurity in O2 and N2 provide contributions of 6.15 % and 17.87 % respectively to the combined expanded uncertainty. The uncertainty contribution from the N2O impurity in the matrix gas 255 scales with the amount fraction of the matrix component. The N2O impurity in Ar, and the uncertainty in relative molar masses provides negligible contributions to the combined expanded uncertainty. Input quantities (X1, X2) have associated uncertainties that are combined to give a combined standard uncertainty for the measurement of N2O amount fraction derived from each validation. The standard uncertainty associated with the gravimetric 270 amount fraction ( 1 ) is provided by the software Gravcalc2 (Brown, 2009). The standard uncertainty in the ratio measurement ( 2 ) is the standard deviation of the mean of the four ratios. Both input quantities were modelled with normal distributions and sensitivity coefficients (c1, c2) were taken as the partial derivatives with respect to each input quantity. ) was applied to each, providing equal weighting to the final analytical uncertainty.

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Expanded analytical uncertainties of 0.07 % (k=2) were demonstrated using this approach. The final combined expanded uncertainty contains the contributions from gravimetric and analytical uncertainty. The combined expanded uncertainty of ambient amount fraction N2O reference materials is calculated to be 0.11 % (k=2) or 0.36 nmol mol -1 .

Reproducibility of reference gas production 285
The WMO-GAW has published an amount fraction range of 325-335 nmol mol -1 representative of the unpolluted troposphere, while the range of N2O amount fractions covered by the WMO scale is somewhat broader (260-370 nmol mol -1 ). The linearity of the CRDS analyser response to changes in amount fraction and the influence of amount fraction on apparent isotope delta values were investigated in the amount fraction range 320 to 360 nmol mol -1 using a set of gravimetric prepared reference The gravimetrically prepared reference materials were validated against a reference material prepared at nominally 325 nmol mol -1 . Figure 3 shows the residual of the linear regression of the certified amount fraction as a function of the gravimetric amount fraction for each reference material. The deviation from the linear regression does not show any obvious trend with 295 gravimetric amount fraction and falls within the extended WMO-GAW compatibility goal for all reference materials of ± 0.3 nmol mol -1 , demonstrating the suitability and linearity of the CRDS analytical technique for certifying N2O reference materials in this range and the reproducibility of the reference materials produced.

Stability of N2O reference materials for amount fraction and isotopic composition
The demonstration of stability is important to achieve measurements of amount fraction and isotope ratio in the field with low uncertainty and also safeguards against drift in measurements as a result of changes in the reference material. The effect of storage of reference materials of N2O in synthetic air, with and without other greenhouse gas components in cylinders with 310 different surface treatments was investigated.

Stability of reference materials for extended storage times
The stability of a nominally 325 nmol mol -1 N2O in synthetic air reference material was assessed over a three year period by comparison with freshly prepared binary reference materials comprising N2O in synthetic air prepared in the amount fraction 315 range 300-360 nmol mol -1 and reference materials containing N2O in synthetic air and trace gases CO2 (290-800 µmol mol -1 ), CH4 (1.8-3.0 µmol mol -1 ) and CO (0.07-1.00 µmol mol -1 ) (Figure 4). All validations within this period demonstrated agreement of amount fraction within the extended WMO compatibility goal of ± 0.3 nmol mol -1 and thus demonstrates the stability of the 325 nmol mol -1 N2O reference material. The linearity of response of the CRDS in this amount fraction range is detailed above.
No clear distinction between agreement of validation of N2O in synthetic air and N2O within multi-component gas mixtures 320 was observed, suggesting minimal interference of these gases on the CRDS analyser response to the total reported amount fraction of N2O. The findings show agreement with those of (Erler et al., 2015;Harris et al., 2020) where no significant effect of CH4, CO or CO2 at atmospheric amount fraction was found on the reported N2O amount fraction. In contrast, the authors reported a strong effect of O2 amount fractions on apparent N2O amount fraction, which they attribute to changes in the pressure broadening. Similar effects on the CO2 and CH4 reported amount fractions with changing matrix composition when using 325 CRDS have been observed earlier by (Nara et al., 2012).    Figure 6 shows the 5-minute average of the response of N2O amount fraction with pressure relative to initial amount fraction for the three cylinders with different passivation processes. There is no difference in the reported N2O amount fraction between the three passivation processes. The data demonstrates that the internal passivation process causes negligible changes to the 355 N2O analyser response for amount fraction with changes in cylinder pressure. The water vapour content of similar mixtures in synthetic air was measured to be around 0.5 µmol mol -1 (Hill-Pearce et al., 2018).

Comparison with existing scales
Two comparisons of amount fraction were carried out between NPL and the WMO/GAW World Calibration Centre (WCC-Empa) on reference materials prepared at Empa and NPL. 385 In a first approach, five reference materials were prepared by Empa containing N2O in the amount fraction range 290-370 nmol mol -1 and certified against reference materials on the WMO-X2006A calibration scale (Hall et al., 2007;Noaa/Esrl, 2011) via quantum cascade laser absorption spectroscopy (QCLAS, model: QC-TILDAS-CS, 2200 cm -1 , Aerodyne Inc., USA). The Empa reference materials contained greenhouse and reactive gas components CO2 (360-800 µmol mol -1 ), CH4 (1.7-3.2 µmol within the WMO -GAW compatibility goal was achieved for amount fractions at nominally 330 nmol mol -1 (Figure 8) and within the extended WMO -GAW compatibility goal at nominally 337 nmol mol -1 . In a second approach, a nominally 325 nmol mol -1 N2O in synthetic air reference material containing nominally 526 nmol mol -405 material (Figure 9). The uncertainty in the analytical amount fraction certified by Empa combines uncertainty contributions from traceability to the NOAA scale, scale propagation and repeatability the analytical system from analyser drift and pressure changes. For the Aerodyne analyser used in the comparison to validate the nominally 325 nmol mol -1 N2O reference material, the sources of uncertainty were combined to the combined standard uncertainty as shown in equation 8.