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Significance of high-quality calibration and quality assurance demonstrated for effective delivery of pollution control policies, and features in journal

The project

Nitrogen dioxide (NO₂) is produced by burning fuel, for example, in car engines and power stations, and has one of the greatest impacts on human health of all air pollutants. Levels of NO₂ in the atmosphere needed to be limited to improve the quality of life for European citizens and reduce economic burdens of the health issues caused by exposure. However, prior to the work of this project NO2 was the only regulated air pollutant not directly measured, resulting in uncertain and inaccurate measurements.

Completed EMPIR project Metrology for nitrogen dioxide (16ENV05, MetNO₂) has progressed the state of the art to enable the direct calibration and direct measurement of NO₂ in ambient air with implications for how atmospheric amount fractions of this important trace gas could be monitored in the future supporting the direct NO₂ market (speciality gas industry, instrument manufacturers and the atmospheric monitoring community) and working to bring NO₂ in line with other the regulated air pollutants.

Project achievements

Specific achievements include:

• improved understanding in evolution of NO₂ and HNO₃ over time identifying a potential route to develop reference materials that could be certified to provide improved uncertainties for the required stability periods
• identification of nitric acid (HNO₃) as the major impurity formed unintentionally during reference material preparation through the hydrolysis of NO₂, prompting the development of suitable quantification methods and methods to minimise presence of water vapour
• improved understanding in evolution of NO₂ and HNO₃ over time identifying a potential route to develop reference materials that could be certified to provide improved uncertainties for the required stability periods
• developing dynamic reference standards for low amount fractions of NO₂, with less than 3 % uncertainty demonstrated for a portable generator
• advancements in high precision NO₂ measurements by quantum cascade laser absorption spectroscopy
• A comprehensive comparison experiment of direct NO₂ instruments indicated the importance of calibration and quality assurance requirements to ensure measurement data is of the highest quality
• Proposal to the European Commission to establish a new work item in CEN TC264/WG12 for the development of a European Standard Reference Method for the Direct Measurement of NO₂ in Ambient Air

Project research publishes high profile journal article

Project Coordinator Dave Worton from NPL, wrote an opinion article, referring to the work of this EMPIR project, entitled Future Adoption of Direct Measurement Techniques for Regulatory Measurements of Nitrogen Dioxide: Drivers and Challenges published in the Viewpoint section of the American Chemical Society’s Journal of Environmental Science and Technology.

The article describes the importance of atmospheric NO₂ and that as a result of the chemiluminescence measurement technique typically used to demonstrate regulatory compliance it remains the only regulated air pollutant that is not directly measured or calibrated. With the chemiluminescence method the NO₂ proportion is estimated as the difference between two channels. One channel measures NO only and the other channel measures total nitrogen oxides (NO_x = NO + NO₂) after NO₂ is either catalytically or photolytically converted to NO prior to detection. Interferences in the NOx channel from other reactive nitrogen compounds (NO_y) result in inaccuracies in the estimated NO₂ amount fractions, which limits the applicability of this data to improve atmospheric models and satellite retrievals. With recent advancements in adsorption spectroscopy, commercial instruments capable of direct NO₂ measurements are now readily available and the challenges to the widespread use of direct NO₂ instruments for regulatory compliance are identified and discussed in the article.

Comment from EMPIR Project Coordinator and article author

Dave Worton said

‘This project has made progress towards reaching the ambitious goal of improved atmospheric monitoring of NO₂ to reduced population level exposure by outlining the needs for future adoption of direct NO₂ monitoring for regulatory compliance and by advancing the metrological infrastructure to enable the direct calibration of selective NO₂ measurements that are becoming essential as regulatory limits decrease’.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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