Dynamic–gravimetric preparation of metrologically traceable primary calibration standards for halogenated greenhouse gases
Authors: Guillevic Myriam , Vollmer Martin K. , Wyss Simon A. , Leuenberger Daiana , Ackermann Andreas , Pascale Céline , Niederhauser Bernhard , Reimann Stefan 
Published: June 2018

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.