SI-traceable standards for atmospheric monitoring of essential climate variables

Céline Pascale, METAS (CH)

To support atmospheric monitoring of essential climate variables (ECV) such as water vapour and greenhouse gases, we have developed a method to produce reference gas mixtures at nmol/mol (ppb) to pmol/mol levels (ppt). This method is dynamic and SI-traceable. This work is conducted in the framework of the EMRP projects HIGHGAS and KEY-VOCs as well as METAS’ AtmoChemECV project.
The method has been already applied to water vapour in the range 5-20 µmol/mol (ppm), HFC-125 (pentafluoroethane, widely used in air conditioners), HFC-1234yf (2,3,3,3-tetrafluoropropene, a car air conditioner fluid of growing importance) and SF6 (insulant in electric switch-gears). It is particularly suitable for gas species and/or concentration ranges that are not stable in cylinders and it can be applied to a large variety of molecules related to air pollution and climate change (e.g., NO2, volatile organic compounds such as BTEX, NH3, CFCs, HCFCs, HFCs). The expanded uncertainty is less than 3% (k=2).
The generation process is composed of four successive steps. In the first step the matrix gas, nitrogen or synthetic air is purified. Then this matrix gas is spiked with the pure substance, using a permeation device which contains a few grams of the pure substance (e.g., HFC-125) in the liquid form and loses it linearly over time by permeation through a membrane. This mass loss is precisely calibrated in our lab in Bern, using a magnetic suspension balance. In a third step the desired concentration is reached by dilution of the high concentration mixture exiting the permeation chamber with a chosen flow of the matrix gas in one or two subsequent dilution steps. All flows are piloted by mass flow controllers. All parts in contact with the gas mixture – including the balance – are passivated using coated surfaces, to reduce adsorption/desorption processes as much as possible. In the last step the mixture can be i) directly used to calibrate an analyser, ii) sampled on sorbent tubes or iii) pressurized into Silconert2000-coated stainless steel cylinders by cryo-filling.
We present here the method, the uncertainty budget as well as first results of intercomparisons to other references for various compounds.


Pen Profile

I finished my Master in Chemical Engineering in 2011 at the polytechnic school of Lausanne, in Switzerland. I began to work straight away at METAS, the Swiss federal Institute of Metrology, in the Gas Laboratory.
Since 6 years, I am in charge of the production of reference gas mixtures at ambient level concentrations for reactive gases. I am experienced in the dynamic production of reactive gases, such as NO, NO2, and VOC, using permeation and dilution methods. I also have expertise in uncertainty budget assessment.
Since November 2014, I am co-managing an interne new R&D project aiming at improving the reference gas mixtures for Essential Climate Variables defined by GCOS in 2010.Those new certified reference materials will improve the quality of the actual measurements of air monitoring stations giving them traceability to national standard and a well-defined uncertainty. This will lead to international data recognition.

As responsible for the dissemination of standards, METAS takes part in the GAS2017 Symposium to present the new developments and calibration possibilities of the Gas Laboratory.


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