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, NO₂ was the only regulated air pollutant not directly measured, resulting in uncertain and inaccurate measurements.

This project applied outcomes from the EMRP project ENV01 MACPoll to develop capabilities to directly measure NO₂.

The project developed high-concentration static reference standards for NO₂ with uncertainties of under 0.5 %, and identified a potential route to develop reference materials that could be certified to provide more practical stability times.

Dynamic reference standards for low concentrations of NO₂ were also developed, with less than 3 % uncertainty demonstrated for a portable generator.

Nitric acid and water vapour were identified as the main impurities formed unintentionally during preparation of static and dynamic NO₂ reference standards, prompting the development of suitable quantification methods. 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.