Dynamic-gravimetric preparation of metrologically traceable primary calibration standards for halogenated greenhouse gases
Atmospheric Measurement Techniques Discussions
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
... years, this anchoring method is highly relevant as measurement reproducibility is currently much better (<&thinsp;1&thinsp;%, k&thinsp;=&thinsp;2 or 95&thinsp;% confidence interval) than the expanded uncertainty of a reference gas mixture (usually >&thinsp;2&thinsp;%). 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. <br><br> To support atmospheric monitoring of halogenated gases, we create new primary calibration scales for SF<sub>6</sub> (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 (Système International d'Unités) and is assigned an uncertainty estimate following international guidelines (JCGM 100:2008), ranging from 0.6&thinsp;% for SF<sub>6</sub> to 1.3&thinsp;% (k&thinsp;=&thinsp;2) for all other substances. The smallest uncertainty obtained for SF<sub>6</sub> is mostly explained by the high substance purity level in the permeator as well as low SF<sub>6</sub> contamination of the matrix gas. The measured internal consistency of the suite ranges from 0.23&thinsp;% for SF<sub>6</sub> to 1.1&thinsp;% for HFO-1234yf (k&thinsp;=&thinsp;1). The expanded uncertainty after verification (i.e. measurement of the cylinders vs each others) ranges from 1&thinsp;% to 2&thinsp;% (k&thinsp;=&thinsp;2). <br><br> 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 allows to maximise the compatibility within the network while linking all reference values to the SI and assigning carefully estimated uncertainties. <br><br> For SF<sub>6</sub>, 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&thinsp;% lower than SIO-14, and for HFO-1234yf 9&thinsp;% 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&thinsp;% higher that the interim calibration scale (Interim-98) in use within the Advanced Global Atmospheric Gases Experiment (AGAGE) network.