MetNH3

Getting the measure of ammonia The EU has passed regulations limiting ammonia emissions in Member States, yet it remains difficult to reliably measure levels of the gas in the atmosphere. We spoke to Daiana Leuenberger, Ph.D. and Bernhard Niederhauser about the MetNH3 project’s work goal of improving the metrological infrastructure for ammonia at ambient air amount fractions A gas compound

that plays an important role in atmospheric chemistry, ammonia (NH 3) can have harmful effects on both human health and the environment.

With emissions steadily increasing, accurately measuring atmospheric ammonia amount fractions is an important issue in environmental science, yet it is a major challenge. “Ammonia is very difficult to measure, as it reacts easily with other molecules, for example with water.

In addition, it readily adsorbs on material surfaces,” says Daiana Leuenberger. Based at the Swiss Federal Institute for Metrology (METAS), Leuenberger is acting coordinator and one of the research scientists working on the MetNH3 project, an initiative that brought together 10 partners from national metrology institutes (NMIs) and other research institutions across Europe to address a diverse range of issues around ammonia metrology in a 3-year project that ended in May 2017. A key goal of MetNH3 was to develop SI-traceable reference material, either in the form of reference gas mixtures or as instrumental transfer standards. “It is only via traceability to a common reference, in our case to the International System of Units (SI), that measurements from one place can be made comparable to other measurements elsewhere,” says Bernhard Niederhauser, the coordinator of the project.

The project itself was organised in three technical workpackages, encompassing work in several different areas. Some partners focussed on the traceability of laser based measurement instruments, while others investigated the generation of SI-traceable reference gas mixtures (RGM) at known amount fractions of NH 3 used for the calibration of laser based spectroscopic instruments “A third focus was on the application and validation of the developed reference gas mixtures and laser based optical instruments and their

It is only via traceability to a common reference, in our case to the International System of Units (SI), that measurements from one place can be made comparable to other measurements elsewhere comparison to instruments of collaborators and stakeholders,” continues Leuenberger.

“This has been realised in laboratory and field studies and allowed the assessment of difficulties associated with ammonia measurements, while we have also been able to lower their uncertainty.”

Ammonia emissions This work has been prompted in part by regulatory changes. European Directive 2016/2284/EU sets individual emission ceilings for each Member State to be met by 2020, based on the revised Gothenburg

Provisional results of the NH3 intercomparison study held in South East Scotland.

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Protocol and sets even more ambitious reduction commitments for 2030 so as to cut the health impacts of air pollution by half compared with 2005. Previous directives led to the incorporation of ammonia measurements into national air monitoring networks and considerable funds have been invested in compliance measures, with the agricultural sector a major area of interest. “The main source of ammonia emissions is the agricultural sector, due to fertilizer application and livestock production,” outlines Leuenberger.

Reducing agricultural emissions requires substantial governmental subsidies, making the assessment of their effectiveness all the more important, over both the short-term and the longer-term and with a consistent geographical resolution. The choice of site for measurements is highly important, as the majority of NH 3 is deposited close to the source. “The development in the emissions is very slow. The detection of small trends requires measurements realised with high precision and low measurement uncertainty,” continues Leuenberger.

“Moreover, the datasets have to be coherent over long time periods and consistent in space in order to be comparable. Regular calibration of the instruments with SI-traceable reference gas mixtures, or by comparison to an

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instrumental standard, is imperative in order to achieve the required accuracy of the data.”

“The role of metrology is to provide the measurement community with the link to the SI via traceable reference material, and thus to allow for the comparability of the data obtained with different methods and at different times and places. MetNH3 was realised in the framework of the European Metrology Research Programme EMRP to find solutions for the problems with NH 3 reference materials (Pogány et al. 2016).”

Laser-based methods Many new laser-based methods for measuring atmospheric NH 3 have been developed over the years. The majority of measurements are currently made using passive samplers for reasons of costefficiency, even though indirect quantification is associated with large uncertainties. The alternative approach of applying extractive, laser-based systems operating in the infra-red wavelengths is much more expensive, though it does bring benefits. “The ambient air is analysed at real-time resolution at higher quality - i.e. at lower levels of uncertainty compared to passive samplers. In addition, these analysers can be calibrated with reference gas mixtures making the measurements SI-traceable.

Both passive sampling and extractive systems were investigated in the project,” continues Leuenberger. “The chemical uptake rates of various types of passive samplers have been re-evaluated in a test chamber (CATFAC) using projectdeveloped NH 3 RGMs. This will lead to lower levels of uncertainty associated with measurements with these types in the future due to the fact that the new findings are integrated in a European standard document.”

“Furthermore, the project managed to realise a so-called optical transfer standard (OTS) for NH 3. This is an IR spectrometric analyser performing amount fraction absolute NH 3

measurements.

Such instrumental standards do not rely on a calibration with NH 3 gas standards but use stable molecular parameters (so-called spectral line parameters) to describe the light absorption properties of ammonia and to deduce the amount fraction in air samples.

This opens a complementary route to SItraceability and instrument calibration and circumvents difficulties in the temporal stability of NH 3 reference gas standards normally used for instrument calibration.”

Where not all of the relevant input parameters of an instrument are very stable and traceable to the SI, optical analysers ought to be calibrated regularly with SI-traceable reference gas mixtures to detect potential drifts and instrumental offsets in measurements over time. In the course of such tests with reference gas mixtures, a non-negligible crosssensitivity of a commercial optical NH 3 analyser was detected (Martin et al. 2016).

In collaboration with the instrument manufacturer, a water correction algorithm has been developed which has now been incorporated in all the new NH 3 analysers.

Reference gas mixtures The previous paragraph highlights the importance of SI-traceable reference gas mixtures (RGM). “The project was, however, strongly motivated by significant discrepancies between different NMIs depending on the techniques applied to prepare NH 3 reference gas mixtures and associated uncertainties,” says Niederhauser.

RGMs have to be available at ambient air NH3 amount fractions (0.5 – 500 nmol mol-1, corresponding to 10 -9 mol per mol also referred to as ppb, parts per billion), or dynamically diluted to those fractions, in order to cover the measurement range of the analysers which are to be calibrated.

Moreover, the required relative expanded uncertainty is U NH3 ≤ 3 percent (k = 2 at the 95 percent confidence interval).

Yet, RGMs produced gravimetrically in pressurised stainless steel or aluminium

Photo of the test chamber (CATFAC). © Nicholas A. Martin, NPL. cylinders are subject to the aforementioned adsorption of NH 3 on their surfaces (Vaittinen et al. 2014). Thus, NH 3 amount fractions released from the cylinder are lower than their initially assigned gravimetrical value. As pressure in the cylinder decreases over time, adsorbed molecules are desorbed and NH 3 amount fractions increase. This instability over time results in high uncertainty in the RGM. Moreover, the mixtures in cylinders are commercially available in amount fractions NH 3 ≥ 20 µmol mol-1 with U NH3 ≥ 3 percent, i.e. at amount fractions 40 to 40000 times higher than in the atmosphere, thus they require dynamic dilution, further adding to the uncertainty of the resulting RGM. Thus, MetNH3 has investigated surface materials which show reduced adsorption of NH 3 molecules. Apart from polymer surfaces, a silica-based coating which is commercially applied by chemical vapour deposition has shown outstanding adsorption-minimising properties when applied on stainless steel cylinders, as well as to other gas-wetted stainless steel surfaces.

This considerably increases the stability of the RGM, with NH3 = 10 µmol mol-1, and reduces the relative expanded uncertainty of the RGM to U NH3 ≤ 1 percent. Furthermore, stabilisation times of measurements are significantly reduced, due to considerably decreased adsorption on instrument inlets and cavities.

An altogether different approach to producing reference gas mixtures avoids the problem of adsorption losses by dynamically generating NH 3 at ambient amount fractions in real-time. NH 3 stored in a tube in its pure form permeates through a selective polymer membrane as

MetNH3 field intercomparison at CEH Edinburgh, Scotland.

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At a glance Full Project Title Metrology for ammonia in ambient air (MetNH3) http://metnh3.eu Project Partners 8 European metrology institutes (METAS, BAM, DFM, MIKES VTT, NPL, PTB, VSL) 2 Researcher Excellence Grant holders (Dr. Christine Braban, NERC CEH Edinburgh, UK. Dr. Olavi Vaittinen, University of Helsinki, Finland) Project Funding MetNH3 is a Joint Research Project (JRP) ENV55 running for three years starting 1st of June 2014 under the European Metrology Research Project (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Publications Vaittinen et al. 2014.

DOI:10.1007/s00340-013-5590-3 Pogány et al. 2016.

DOI:10.1088/0957-0233/27/11/115012 Martin et al. 2016.

DOI:10.1007/s00340-016-6486-9 Contact Details Daiana Leuenberger, Ph.D. Research scientist E: daiana.leuenberger@metas.ch Bernhard Niederhauser Head of Laboratory / Coordinator MetNH3 E: bernhard.niederhauser@metas.ch Laboratory for Gas Analysis Federal Institute of Metrology METAS Lindenweg 50, 3003 Bern-Wabern Switzerland W: www.metas.ch/gases

Daiana Leuenberger, Ph.D. (Left) Bernhard Niederhauser (Right)

Daiana Leuenberger is a research scientist in the gas analysis laboratory at METAS, a position she has held since 2014. Her tasks are primarily related to the advancement and development of techniques and infrastructure in the dynamic generation of reference gas mixtures for reactive compounds at atmospheric amount fractions.

Bernhard Niederhauser is head of the gas analysis laboratory at METAS. He holds deep expertise in high accuracy low inert gas volume flows and the dynamic preparation of traceable low concentration reactive gas mixtures, as well as primary ozone measurements.

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Summary of the mean of the reported NH3 concentrations for diffusive and pumped samplers tested in the atmosphere test chamber CATFAC, expressed as a percentage deviation from the reference values. a function of temperature and pressure into a precisely controlled flow of NH 3free matrix gas. The amount of NH 3 added to the matrix gas is determined by continuous weighing of the mass loss of the permeation tube (of the order ng min-1, i.e. 10 -9 g min-1) in a magnetic suspension balance. The initial NH 3 mixture can then be diluted in further steps to required amount fractions. The drawback of this method is that the RGMs cannot be preserved for on-site calibrations of instruments in measurement stations. To fill this void, MetNH3 has produced a reference gas generator combining the permeation method with two dilution steps for the on-site production of RGMs at NH 3 amount fractions (0.5 – 500 nmol mol-1) with U NH3 ≤ 3 percent (k = 2).

Inter-comparison event Two inter-comparison events were held under field conditions in Scotland in the summer of 2016 to assess the different developments of the project, as well as to offer collaborators and stakeholders the opportunity to test the performance of their instruments and passive samplers.

The different instruments were calibrated beforehand, with the RGMs prepared in cylinders as well as with the mobile reference gas generator, with the aim of ensuring that they all generated consistent measurements and performed well. “In this kind of exercise it can be seen which instruments manage to measure with the required precision, detection level and within a reasonable level of uncertainty,” says Leuenberger. “Fertiliser was applied to the field and NH 3 concentrations increased correspondingly, giving researchers the opportunity to assess the instruments at both background and elevated conditions. We were interested in

seeing whether the analysers could capture both concentrations as well as temporal resolution of the event.”

“At a different site nearby, a second inter-comparison was held exposing passive samplers which had been previously tested in the test chamber two times over the course of four weeks. This allowed for the validation of the newly developed, diffusive uptake rates under field conditions.”

This work formed an important part of the project’s third workpackage, which centred around the validation of field measurement techniques. The wider goal in the project has been to improve metrological traceability for measuring NH 3 in air, which will enable rigorous monitoring of the impact of efforts to reduce emissions. “It would be very positive if the results of the project were taken up by the respective end-user communities,” says Leuenberger. The project is involved in organisations on both the European and national levels.

The moment of truth for researchers developing reference gas mixtures at individual NMIs will come next year when, in a key comparison, they will assess their concordance with the SI on the highest instance. “We’ve seen in the past that the uncertainty estimations with respect to different realisations of primary gas mixtures were too optimistic,” outlines Niederhauser. “The effects of adsorption were not taken into account and this resulted in significant discrepancies between mixtures prepared with different methods, though within the respective methods, there has been good agreement. Considering all the lessons learnt since and from the MetNH3 project, the potential for much better agreement is high.”

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