The precise measurement of the amount fraction of atmospheric
nitrous oxide (N

Nitrous oxide (N

Anthropogenic N

Isotopic abundances are given in the delta notation (

In summary, this results in a limited compatibility of laboratory analyses
for N

Atmospheric N

Recent advances in spectroscopic instrumentation have improved N

Advances in instrumentation must be coupled with advances in high-precision
isotope ratio reference materials, particularly for the calibration of the
site-specific isotopic composition

Crucial for the development of reference materials is the stability of the
N

We present work on the development of N

All primary reference materials (PRMs) were prepared by gravimetry, in
accordance with ISO 6142-1:2015, in 10 L aluminium cylinders (Luxfer) with a
range of outlet diaphragm valves (Ceodeux): BS341 no. 14, DIN 477 no. 1 and
DIN 447 no. 8. The cylinders have SGS™ internal surface (Luxfer) or
were treated internally with a range of proprietary passivation processes
including SpectraSeal™ (BOC), Megalong™ and
Aculife IV/III™ (Air Liquide). Cylinders were evacuated using
an oil-free pump (Scrollvac SC15D, Leybold Vacuum) and turbo molecular pump
with magnetic bearing (Turbovac 340M, Leybold Vacuum) to a pressure of

The reference materials were produced gravimetrically by the addition of
N

Atmospheric amount fraction reference materials in the range 300–360 nmol mol

Gravimetric uncertainties associated with the preparation of N

The total gravimetric uncertainty of the reference material combines
gravimetric uncertainty from the uncertainty in mass added in each addition with uncertainty in the
amount of N

There is
ongoing research to improve the accuracy of the quantification of trace
N

Aluminium cylinders (0.85 L, Luxfer) were filled to 30–35 bar with a 325 nmol mol

In a second approach, 325 nmol mol

A cavity ring-down spectrometer (Picarro G5131-

Analysis of the amount fraction of argon in the 30 % argon-in-nitrogen pre-mixture cylinders was performed by gas chromatography with a
thermal conductivity detector (GC–TCD; Agilent 6890) using a capillary
column (Molsieve 5A, 30 m

A 325 nmol mol

Typical Allan deviation plot for a CRDS N

The Allan deviation initially decreases with an increase in the averaging
time and reaches a minimum for N

The characterisation of the CRDS for reported delta values with N

The

CRDS analyser response for

Similarly, the

The agreement in

Uncertainty in the amount fraction of N

To assess the uncertainty in the amount fraction from production of N

The combined contribution to the uncertainty due to gravimetry and purity of
the components for the ambient amount fraction N

Sources of uncertainty and their relative contribution to the
combined expanded uncertainty (

The sources of uncertainty and their contribution to the combined
gravimetric uncertainty (

The amount fraction of N

Expanded analytical uncertainties of 0.07 % (

The WMO-GAW has published an amount fraction range of 325–335 nmol mol

The gravimetrically prepared reference materials were validated against a
reference material prepared at 325 nmol mol

Residuals of the linear regression of the certified amount
fraction as a function of the gravimetric amount fraction for N

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 N

The stability of a 325 nmol mol

Percentage difference between gravimetric and certified N

Figure 5 shows 5 min averages for amount fraction,

Temporal change in

Greenhouse gas reference materials are prepared at NPL in passivated
cylinders to inhibit the adsorption of target components. While adsorption
of N

Figure 6 shows the 5 min average of the response of N

Difference from initial N

The effects of the production and storage of ambient amount fraction
N

Residuals from the linear regression of analyser response with
increasing gravimetric amount fraction of the dynamic reference materials.
Static (triangles) and dynamic (squares) reference materials in the amount
fraction range 300–1500 nmol mol

The generated dynamic reference materials were validated against static
reference materials of a similar amount fraction. The residuals of the
static and dynamic linear regression of analyser response as a function of
increasing gravimetric amount fraction of dynamic reference materials are
shown in Fig. 7. Agreement within 0.05 % (0.16 nmol mol

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.

In a first approach, five reference materials were prepared by Empa
containing N

The reference materials were certified at NPL via CRDS against NPL in-house
PRMs static reference materials in the amount fraction range of 325–360 nmol mol

Certified NPL amount fraction minus the certified Empa amount
fraction of five N

In a second approach, a 325 nmol mol

Agreement within the combined gravimetric uncertainty (

Gravimetric amount fraction (filled circle) and amount fraction
certified by Empa (open triangle) against the NOAA/GMD reference scale for a 325 nmol mol

The expanded uncertainty (

N

The amount fraction of a prepared N

The change in amount fraction of the mixtures with decreasing cylinder pressure was shown to be smaller than measurement uncertainty regardless of cylinder passivation chemistry, and the stability of the mixtures over 3 years was within the expanded WMO-GAW DQO for compatibility. The isotopic composition of the reference mixtures was also demonstrated to be stable with reducing pressure, and agreement of delta values was achieved for static reference materials with dynamic dilutions within the analytical uncertainty. The next steps towards producing reference materials for source apportionment will be to produce reference materials with a range of isotopic values and to verify the assignment of their delta values.

The data underlying the figures in this paper are available upon request to the corresponding author.

The standard preparation and analysis work was undertaken by REHP, AH, EMW and KC, with JM and CZ comparing reference materials to existing scales. All authors (REHP, AH, EMW, KC, SO'D, JM, CZ, DRW, PJB) contributed to discussions and writing of the paper.

The authors declare that they have no conflict of interest.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

This research has been supported by the European Metrology Programme for Innovation and Research (EMPIR) programme co-financed by the participating states and the European Union’s Horizon 2020 research and innovation programme.

This paper was edited by Frank Keppler and reviewed by Stefan Persijn and one anonymous referee.