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SI-traceable reference gas mixtures for halogenated VOCs at atmospheric amount of substance fractions
by Dr. Tobias Bühlmann, Mrs./Ms. Céline Pascale


Many synthetic halogenated volatile organic compounds (VOCs) are potent greenhouse gases emitted by anthropogenic sources and are found in the atmosphere in the low pmol/mol range. Moreover, those compounds containing chlorine or bromine contribute to the depletion of the ozone layer. Hydrofluoroolefins (HFOs) are new replacement products with reduced atmospheric lifetimes, but their degradation products can have a negative impact on the environment. It is of great importance to monitor halogenated VOCs in the atmosphere to assess and mitigate their impacts on the environment. To compare measurement results of these substances among measuring stations around the world, analyzers need to be calibrated using SI-traceable reference gas mixtures with amount fractions close to atmospheric levels. The consortium of the EURAMET-EMPIR project "Metrology for climate relevant volatile organic compounds" (MetClimVOC, 19ENV06) identified and prioritized halogenated VOCs that are lacking a SI-traceable reference. At METAS, we developed SI-traceable primary standards for these substances: HFC-32, HFC-365mfc, CH2Cl2, CCl4, 1,2-dichloroethane and HFO-1336mzzZ with a challenging expanded measurement uncertainty of <3%. We dynamically generated these SI-traceable primary reference gas mixtures using permeation devices calibrated in magnetic suspension balances and diluted the generated reference gas dynamically down to the pmol/mol level. Losses due to adsorption on surfaces are largely reduced by coating the critical parts with SilcoNert®2000. During the entire production process, it is crucial to avoid contamination from the carrier gas, the entire system and the permeation devices. For storage and transportation the dynamically generated primary standards need to be filled in SilcoNert®2000 coated cylinders. Because no suitable filling system was available commercially, METAS developed an improved cryo-filling system that precisely controls the mass flow, pressure and filling time. The system is automated and is coated with SilcoNert®2000, is free of PTFE and therefore suitable for reactive and halogenated gases. To meet the needs of the stakeholders and to support the WMO-GAW implementation plan 2016-2023, the infrastructure at METAS will be used to produce more reference gas mixture for halogenated VOCs that are found in the atmosphere but are lacking a SI-traceable reference. To omit matrix effects and to offer fit-for-purpose standards, METAS is planning to set up whole air reference gases with both an SI-traceable and a best-fit value.


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Topic : Theme 1: Atmospheric Chemistry and Physics.
Reference : T1-A13

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