Ammonia (NH3) in the atmosphere is the major precursor for neutralising atmospheric acids and is thus affecting not only the long-range transport of sulphur dioxide and nitrogen oxides but also stabilies secondary particulate matter. These aerosols have negative impacts on air quality and human health. Moreover, they negatively affect terrestrial ecosystems after deposition.
NH3 has been included in the air quality monitoring networks and emission reduction directives of European nations. Atmospheric concentrations are in the order of 0.5-500 nmol/mol. However, the lowest substance amount fraction of available certified reference material (CRM) is 10 µmol/mol.
Moreover, analytical techniques to which said materials are to be applied are very diverse and cause challenges for the production and application of CRM. The Federal Institute of Metrology METAS has developed, partially in the framework of EMRP JRP ENV55 MetNH3, an infrastructure to meet with the different requirements. A magnetic suspension balance monitors the specific temperature-dependent mass loss of the substance in a permeation tube (here NH3) by permeation through a membrane into a carrier gas, which is connected to a dynamic dilution system. This system consists of two subsequent steps to enable the dilution to ambient amount fractions. The permeation tube with known mass loss over time can be transferred into the temperature-regulated permeation chamber of a newly developed mobile reference gas generator (ReGaS1) which consists of the same components as afore mentioned, stationary system. Analysers can be connected directly to both, stationary and mobile systems for calibration. Moreover, the resulting CRM can also be pressurised into coated cylinders by cryo-filling. The mobile system as well as these cylinders can be applied for calibrations in other laboratories and the field. Considerable effort has been made to minimise adsorption on the gas-wetted surfaces by applying a coating substance. All components are fully traceable to SI standards. The characteristics of individual components lead to the uncertainty estimation for the generated NH3 gas mixture according to GUM <3 %. In addition, a traceable system based on a cascade of critical orifices has been established to dilute NH3 mixtures in the order of µmol/mol for the participation in the international key-comparison CCQM K117. It is planned to establish this system to calibrate and re-sample gas cylinders since it’s very economical in gas use. Here we present insights into the development of said infrastructure and results of the first performance tests. Moreover, we include results of the study on adsorption/desorption effects in dry as well as humidified matrix gas into the discussion on the generation of reference gas mixtures.
Daiana Leuenberger started working as research scientist in the gas analysis laboratory at METAS in 2014. Her tasks are primarily related to advancement and development of techniques and infrastructure in the dynamic generation of NH3 reference gas mixtures at atmospheric amount fractions combining the permeation method and the dynamic dilution with thermal mass flow controllers and reducing the uncertainties therein.
In addition, she was the acting coordinator of EMRP JRP ENV55 MetNH3 “Metrology for Ammonia in Ambient Air”.
Prior to her research in metrology, Daiana obtained her MSc and PhD in Climate Sciences from the University of Bern during which she analysed polar ice cores for impurities at trace levels and investigated the resulting high-resolution time series for extreme values. She spent several field seasons in Greenland in the framework of the North Greenland Eemian Ice Core Drilling Project.