Impact on industrial and other user communities This project will develop clear tangible outputs (new static and dynamic reference materials, instrumentation, methods, best practice guides and recommendations) to address the requirement for the maintenance of short and long‑term stable values of NO2 at unprecedented levels of precision and accuracy to meet the DQOs established by the WMO‑GAW programme. This project will: Improve the long‑term atmospheric monitoring of NO2 through the promotion of more accurate direct measurement techniques and direct calibration through the provision of accurate and stable NO2 reference standards. Support data harmonisation across Europe through providing an accurate SI traceable infrastructure which is needed to ensure stable, comparable and coherent datasets. Improved evidence based legislation for motor vehicles based on better trend and concentration analysis leading to more effective mitigation strategies. Ensure the most cost effective compliance with EU legislation through lower uncertainty and more comparable measurements. Enable the enhanced competiveness of European NMIs and companies based in Europe with institutes outside of Europe by considerably improving European capabilities and positioning Europe at the frontiers of metrology. Improve quality of life for European citizens and reduced economic burden of health effects of NO2 exposure through facilitating the rapid evaluation of the effectiveness of pollution abatement strategies. Impact on the metrology and scientific communities The direct involvement of representatives from the key atmospheric monitoring communities (WMO‑GAW, EMEP, AQUILA, LAQN, NABEL) will ensure the rapid dissemination of new reference materials, measurement guidelines and recommendations ensuring the early impact of the project’s outputs. These outputs will be used to provide updates to the EMEP ‘Manual for Chemical Analysis’ and the WMO-GAW measurement guidelines for ‘Global long-term measurements of nitrogen oxides and recommendations for GAW nitrogen oxides network’, which are used to train air quality monitoring station personnel in the standard operating procedures. Activities in this project are well coordinated with existing members of the Horizon 2020 funded Aerosol, Clouds and Trace Gases Infrastructure (ACTRIS) II and In‑service Aircraft for a Global Observing System (IAGOS) research infrastructure projects ensuring that it will complement and contribute to efforts within these communities to improve reliability and traceability of NO2 measurements. This project will contribute positively to ongoing data harmonisation activities improving the comparability of future NO2 measurements in Europe by strengthening and enhancing the impact of the European funded ACTRIS II and IAGOS projects. The provision of new more accurate reference materials will support the very extensive network of NO2 measurement sites within Europe allowing Europe to maintain its position at the forefront of NOx measurement research. The development of reference materials will support future key comparisons (e.g., CCQM:K74.2018 10 µmol/mol NO2 in N2) allowing for new and/or improved calibration and measurement capability claims for NO2. This project will also support the work of the World Calibration Centre (WCC) for NOx of the WMO‑GAW located at Research Center Jülich, Germany. Impact on relevant standards The developments in the project will be used to revise reference methods and documentary standards under ISO/TC158 (Gas Analysis) and CENTC/264 (Air Quality). New methods developed in this project for the preparation of reference materials will be proposed for inclusion in the next revisions of ISO 6142 and ISO 6145 (production and certification of reference materials) and ISO 19229 (purity analysis). A new work item or a revision of EN 14211:2012 will be proposed to develop a standard reference method for the direct measurement of ambient NO2.