Dynamic-gravimetric preparation of metrologically traceable primary calibration standards for halogenated greenhouse gases

Abstract

Abstract. For many years, the comparability of measurements obtained with various instruments withing a global-scale air quality monitoring network has been ensured by anchoring all results to a unique suite of reference gas mixtures, also called '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 those halogenated gases which have been measured over the last forty years, this anchoring method is highly relevant as measurement reproducibility is currently much better (< 1 %, k = 2 or 95 % 5 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 sub-pmol/mol 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 levels.To support atmospheric monitoring of halogenated gases, we create new primary calibration scales for SF 6 (sulfur hexafluo-10 ride), 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 (Système 15International d'Unités) and is assigned an uncertainty estimate following international guidelines (JCGM 100:2008), ranging from 0.6 % for SF 6 to 1.3 % (k = 2) for all other substances. The smallest uncertainty obtained for SF 6 is mostly explained by the high substance purity level in the permeator as well as low SF 6 contamination of the matrix gas. The measured internal consistency of the suite ranges from 0.23 % for SF 6 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). 20This 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 allows to maximise the compatibility within the network while linking all reference values to the SI and assigning carefully estimated uncertainties.

Atmospheric histories and emissions of chlorofluorocarbons CFC-13 (CClF&lt;sub&gt;3&lt;/sub&gt;), ΣCFC-114 (C&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;F&lt;sub&gt;4&lt;/sub&gt;), and CFC-115 (C&lt;sub&gt;2&lt;/sub&gt;ClF&lt;sub&gt;5&lt;/sub&gt;)

“…This was, among other reasons, due to the use of an interim calibration scale, which was not well defined as it was based on a dilution of a commercial reference gas. The present study prompted the creation of a primary calibration scale for CFC-13 in the parts per trillion range by the Swiss Federal Institute of Metrology, METAS (Guillevic et al, 2018). A suite of 11 primary standards was created using a technique that combines permeation tube substance loss determination by a magnetic suspension balance, dynamic dilution through mass flow controllers, and cryogenic collection in containers.…”