The comparison of nitrous oxide in air standards from ten laboratories has demonstrated reference value uncertainties of 0.1 nmol/mol to 0.3 nmol/mol, for the third most important greenhouse gas in the atmosphere. Globally averaged atmospheric amount fractions have surpassed 334.5 nmol/mol and annual growth rates exceed 1 nmol/mol.

The results are published by the BIPM in the report of the key comparison CCQM-K68.2019 and its associated Pilot Study CCQM-P206. Participating laboratories included National Metrology Institutes and the Earth System Research Laboratory of the National Oceanic and Atmospheric Administration (NOAA/ESRL). NOAA/ESRL has been the Central Calibration Laboratory for the Global Atmosphere Watch (GAW) programme of the World Meteorological Organisation (WMO) since 2000. The Scripps Institution of Oceanography of the University of California San Diego (SIO/UCSD) participated as the calibration laboratory of the Advanced Global Atmospheric Gases Experiment (AGAGE) global measurement network.

Nitrous oxide is the third most important individual contributor to radiative forcing by long lived greenhouse gases (LLGHGs) and accounting for about 7%. It is emitted into the atmosphere from both natural sources (approximately 57%) and anthropogenic sources (approximately 43%), including oceans, soils, biomass burning, fertilizer use and various industrial processes. WMO has reported that recent increases in nitrous oxide amount fractions exceed the average annual growth rates over the past decade.

The comparison has demonstrated that standards that are currently used in monitoring networks for nitrous oxide agree within their stated uncertainties and SI traceable values, as determined by the key comparison reference values. The highest levels of internal consistency of standards used within monitoring networks are maintained through traceability schemes relying on one central laboratory maintaining an individual set of standards for N₂O in air. Networks report values on the same scale, to avoid uncertainties arising from the use of different standards, and with conversion and reporting of values on a common scale then possible. More information regarding the development, maintenance and comparison of GHG Scale Standards is available through the link below.

Key comparison reference function based on the amount fractions reported by the participants and the corresponding response ratios measured by the BIPM. The dark blue line with light blue shading represents the Bayesian EIV regression and associated uncertainty.