Metrology for Ammonia in
Ambient Air

Project summary


Measuring ammonia (NH3) in ambient air is a sensitive and priority issue due to its harmful effects on human health and ecosystems. The European Directive 2001/81/EC on “National Emission Ceilings for Certain Atmospheric Pollutants (NEC)” regulates ammonia emissions in the member states. However, there is a lack of regulation to ensure reliable ammonia measurements namely in applicable analytical technology, maximum allowed uncertainty, quality assurance and quality control (QC/QA) procedures as well as in the infrastructure to attain metrological traceability.

The discrepancies among European NMIs are substantial for the analysis of certified reference material (CRM) in spite of them having been analysed at 30 to 50 µmol/mol and thus at amount of substance fractions far above those of ambient air. Moreover, the performance of the multitude of applied measurement techniques varies in both resolution as well as precision. This threatens not only the credibility of instrumental performance due to insufficient means provided for calibration, but also the scientific value of extensive ambient air monitoring by national networks as well as the comparability of data across countries.

Need for the project

Ammonia emissions are estimated to have at least doubled over the last century across Europe. This development is concurrent with the employment of intensive farming practices and the associated use of nitrogen-based fertilisers. The primary causes for ammonia emissions are intensive agriculture (particularly fertilization with urea), together with various non-agricultural sources such as sewage treatment, catalytic converters, anaerobic digesters (rapidly increasing since 2010) and industrial processes. Emissions from diesel vehicles will increase in the future, particularly in urban environments, due to the application of urea as a selective catalyst for reducing NOx emissions.

Ammonia is the major precursor for neutralising atmospheric acids and thus, affects the long-range transport of SO2 and NOx as well as the stabilisation of secondary particulate matter (PM10, PM2.5 and aerosols). Depending on their location in the atmosphere, PM have either a negative or a positive radiative forcing potential by scattering or absorbing radiation, respectively. By additionally acting as cloud condensation nuclei, this potential is further enhanced. The eventual deposition of ammonia bound in various molecules contributes to eutrophication and acidification of land and fresh water and thus to a reduction in soil and water quality. This has negative effects on biodiversity and ecosystems. Moreover, high PM levels in the lower troposphere negatively affect human health by being closely related to a clustered occurrence of respiratory and cardiovascular diseases.

Air pollution spreads across national borders and over long distances. Ammonia has been included under the UNECE 1999 Gothenburg Protocol (revised in 2012), which is part of the convention on Long-Range Transboundary Air Pollution. The protocol has been designed to reduce acidification, eutrophication and ground-level ozone by setting emissions ceilings to be met by 2010. Moreover, the aforementioned EU National Emissions Ceiling Directive (NEC) 2001/81/EC, sets individual emission ceilings for each member state, based on the Gothenburg Protocol. This has led to the incorporation of ammonia measurements into national air monitoring networks.

Currently, ammonia concentrations in ambient air are determined mainly by the application of diffusive samplers and denuders. This method is quantitative and reliable but ammonia is measured indirectly and in insufficient temporal resolution. There are instruments available applying spectroscopic techniques and allowing for direct measurements. However, they lack traceability and neither inlet systems nor relative humidity controls are sufficient. Moreover, acquisition costs are considerable and they still require calibration with CRM. For ammonia, such gas mixtures are not commercially available in amount of substance fractions corresponding to those of ambient air. This is a consequence of ammonia’s high reactivity with various surfaces and with water leading to adsorption and desorption of ammonia on surfaces and thus, to unstable ammonia amount fractions. Such effects occur at all surfaces and therefore, affect the traceability of ammonia measurements not only indirectly via reference gas mixtures in bottles but also directly by occurring at inlet and sampling systems of on-line instruments and diffusive samplers.

CRM for ammonia is commercially available in amount of substance fractions >30 µmol/mol. However, ammonia in the ambient air occurs at amount of substance fractions between 0.5 and 500 nmol/mol, i.e. 60 to 60 000 times lower concentrations of CRM are required for calibration. This discrepancy has to be overcome by the development of new analysis methods and novel approaches for the production of CRM.

Scientific and technical objectives

The JRP aims to develop metrological traceability for the measurement of ammonia in air from the level of primary gas mixture and instrumental standards to the field leve

  • WP1 will develop improved gas mixture standards by static gravimetric and dynamic methods.
  • WP2 will develop and characterise laser based optical spectrometric standards applying extractive and open-path (sampling free) approaches.
  • WP3 will establish the transfer from high-accuracy standards to field applicable methods by applying characterised exposure chambers and field sites for validation and comparison experiments.

Expected results and potential impact

The results of this JRP will enable NMIs to offer and disseminate SI-traceable and concordant calibration and measurement infrastructure at ammonia amount fractions required for measurements in ambient air (0.5 to 500 nmol/mol). Such accurate, stable and traceable standards and validated measurement methods are the bases for achieving comparability and standardisation of measurement results across networks and among various analytical techniques.

Validated ammonia measurement data of high quality from air monitoring networks are vitally important for identifying changes due to implementations of environment policies, for minimising the believed uncertainties in current emission inventories and for providing independent verifications of atmospheric model predictions.

All JRP-Partners are closely linked to stakeholders and to end-users of this project by being members or service providers of key national or international environmental monitoring networks and by participating in standardisation activities: CEN/TC264/WG11 Ambient air quality - Diffusive samplers for the determination of gases and vapours - Requirements and test methods and other Working Groups, ISO/TC146 Air Quality and ISO/TC158 Gas Analysis, or by collaborating with National Environmental Institutes. The impact of this project is ensured by the extensive network of the JRP-Consortium members, which also includes the leading SMEs developing analytical instruments for measuring ammonia in ambient air.

The research leading to these results has received funding from the European Union on the basis of Decision No 912/2009/EC. Last update: 10/04/2018