Abstract. Atmospheric trace gas measurements of greenhouse gases are critical in their precision and accuracy. In the past 5 years, atmospheric measurement and gas metrology communities have turned their attention to possible surface effects due to pressure and temperature variations during a standard cylinder's lifetime. This study concentrates on this issue by introducing newly built small-volume aluminum and steel cylinders which enable the investigation of trace gases and their affinity for adsorption and desorption on various surfaces over a set of temperature and pressure ranges. The presented experiments are designed to test the filling pressure dependencies up to 30 bar and temperature dependencies from −10 ∘C up to 180 ∘C for these prototype cylinders. We present measurements of CO2, CH4, CO and H2O using a cavity ring-down spectroscopy analyzer under these conditions. Moreover, we investigated CO2 amount fractions using a novel quantum cascade laser spectrometer system enabling measurements at pressures as a low as 5 mbar. This extensive dataset revealed that for absolute pressures down to 150 mbar the enhancement in the amount fraction of CO2 relative to its initial value (at 1200 mbar absolute) is limited to 0.12 µmol mol−1 for the prototype aluminum cylinder. Up to 80 ∘C, the aluminum cylinder showed superior results and less response to varying temperature compared to the steel cylinder. For CO2, these changes were insignificant at 80 ∘C for the aluminum cylinder, whereas a 0.11 µmol mol−1 enhancement for the steel cylinder was observed. High-temperature experiments showed that for both cylinders irreversible temperature effects occur especially above 130 ∘C.

21Keywords 22 ammonia in ambient air, traceability, reference gas standards, optical transfer standard, validation and testing 23 infrastructure 24 Abstract 25The environmental impacts of ammonia (NH 3 ) in ambient air have become more evident in the recent decades, 26 leading to intensifying research in this field. A number of novel analytical techniques and monitoring 27 instruments have been developed, and the quality and availability of reference gas mixtures used for the 28 calibration of measuring instruments has also increased significantly. However, recent inter-comparison 29 measurements show significant discrepancies, indicating that the majority of the newly developed devices and 30 reference materials require further thorough validation. There is a clear need for more intensive metrological 31 research focusing on quality assurance, intercomparability and validations. MetNH3 (Metrology for ammonia in 32 ambient air) is a three-year project within the framework of the European Metrology Research Programme 33 (EMRP), which aims to bring metrological traceability to ambient ammonia measurements in the 0.5 -34 500 nmol/mol amount fraction range. This is addressed by working in three areas: 1) improving accuracy and 35 2 stability of static and dynamic reference gas mixtures, 2) developing an optical transfer standard and 3) 36 establishing the link between high-accuracy metrological standards and field measurements. In this article we 37 describe the concept, aims and first results of the project. 38

Abstract. For many years, the comparability of measurements obtained with various instruments within a global-scale air quality monitoring network has been ensured by anchoring all results to a unique suite of reference gas mixtures, also called a “primary calibration scale”. Such suites of reference gas mixtures are usually prepared and then stored over decades in pressurised cylinders by a designated laboratory. For the halogenated gases which have been measured over the last 40 years, this anchoring method is highly relevant as measurement reproducibility is currently much better (< 1 %, k = 2 or 95 % confidence interval) than the expanded uncertainty of a reference gas mixture (usually > 2 %). Meanwhile, newly emitted halogenated gases are already measured in the atmosphere at pmol mol−1 levels, while still lacking an established reference standard. For compounds prone to adsorption on material surfaces, it is difficult to evaluate mixture stability and thus variations in the molar fractions over time in cylinders at pmol mol−1 levels. To support atmospheric monitoring of halogenated gases, we create new primary calibration scales for SF6 (sulfur hexafluoride), HFC-125 (pentafluoroethane), HFO-1234yf (or HFC-1234yf, 2,3,3,3-tetrafluoroprop-1-ene), HCFC-132b (1,2-dichloro-1,1-difluoroethane) and CFC-13 (chlorotrifluoromethane). The preparation method, newly applied to halocarbons, is dynamic and gravimetric: it is based on the permeation principle followed by dynamic dilution and cryo-filling of the mixture in cylinders. The obtained METAS-2017 primary calibration scales are made of 11 cylinders containing these five substances at near-ambient and slightly varying molar fractions. Each prepared molar fraction is traceable to the realisation of SI units (International System of Units) and is assigned an uncertainty estimate following international guidelines (JCGM, 2008), ranging from 0.6 % for SF6 to 1.3 % (k = 2) for all other substances. The smallest uncertainty obtained for SF6 is mostly explained by the high substance purity level in the permeator and the low SF6 contamination of the matrix gas. The measured internal consistency of the suite ranges from 0.23 % for SF6 to 1.1 % for HFO-1234yf (k=1). The expanded uncertainty after verification (i.e. measurement of the cylinders vs. each others) ranges from 1 to 2 % (k = 2). This work combines the advantages of SI-traceable reference gas mixture preparation with a calibration scale system for its use as anchor by a monitoring network. Such a combined system supports maximising compatibility within the network while linking all reference values to the SI and assigning carefully estimated uncertainties. For SF6, comparison of the METAS-2017 calibration scale with the scale prepared by SIO (Scripps Institution of Oceanography, SIO-05) shows excellent concordance, the ratio METAS-2017 / SIO-05 being 1.002. For HFC-125, the METAS-2017 calibration scale is measured as 7 % lower than SIO-14; for HFO-1234yf, it is 9 % lower than Empa-2013. No other scale for HCFC-132b was available for comparison. Finally, for CFC-13 the METAS-2017 primary calibration scale is 5 % higher than the interim calibration scale (Interim-98) that was in use within the Advanced Global Atmospheric Gases Experiment (AGAGE) network before adopting the scale established in the present work.

In the wake of the redefinition of the kilogram, the last unit of the International System of Units (SI) that is still based on a man-made artefact, discussions were launched on the necessity of redefining other units, amongst other the unit mole. Since 1971 the mole is defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. The symbol of the unit is 'mol'. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. The definition is based on the pre-existing choice to set the relative atomic mass of carbon 12 equal to 12 exactly. In the proposed new definition the mole is the amount of substance containing exactly 6.022 141 79 × 1023 atoms or molecules, ions, electrons, other particles, or specified groups of such particles, i.e. the Avogardo constant would have a fixed value without an uncertainty. This contribution critically examines the submitted arguments to justify the proposed redefinition of the unit mole by 2011 for their persuasive power to change a scientific and cultural good such as a unit of measurement. As shown, there are no convincing scientific arguments for a redefinition of the mole that stand a closer examination. The current definition is well understood, established in science and technology for almost 50 years and is still up to date.

We report a pilot study organized within the Consultative Committee for Amount of Substance (CCQM), in which the ozone reference standards of 23 institutes have been compared to one common reference, the BIPM ozone reference standard, in a series of bilateral comparisons carried out between July 2003 and February 2005. The BIPM, which maintains as its reference standard a standard reference photometer (SRP) developed by the National Institute of Standards and Technology (NIST, United States), served as pilot laboratory. A total of 25 instruments were compared to the common reference standard, either directly (16 comparisons) or via a transfer standard (9 comparisons). The comparisons were made over the ozone mole fraction range 0 nmol/mol to 500 nmol/mol.Two reference methods for measuring ozone mole fractions in synthetic air were compared, thanks to the participation of two institutes maintaining a gas-phase titration system with traceability of measurements to primary gas standards of NO and NO2, while the 23 other instruments were based on UV absorption.In the first instance, each comparison was characterized by the two parameters of a linear equation, as well as their related uncertainties, computed with generalized least-squares regression software. Analysis of these results using the Birge ratio indicated an underestimation of the uncertainties associated with the measurement results of some of the ozone standards, particularly the NIST SRPs.As a final result of the pilot study, the difference from the reference value (BIPM-SRP27 measurement result) and its related uncertainty were calculated for each ozone standard at the two nominal ozone mole fractions of 80 nmol/mol and 420 nmol/mol.Main text. To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCQM.