The overall objective of this project is to deliver a highly accurate SI traceable measurement infrastructure to underpin direct measurements of NO2 concentrations in the atmosphere that meet the Data Quality Objectives (DQOs) established by the World Meteorological Organisation Global Atmospheric Watch (WMO‑GAW) Scientific Advisory expert group. The project will increase the accuracy and stability of NO2 reference standards, which is challenging due to the highly reactive nature of NO2. This will require a more comprehensive characterisation of the impurities formed unintentionally during the preparation and development of new methods that are designed to suppress the formation of impurities. New selective spectroscopic NO2 measurement techniques will be developed, characterised and validated with reference to the standard chemiluminescence method (EN 14211:2012). The project has been structured into the following objectives: To develop high concentration traceable static reference standards for NO2 (1 µmol/mol ‑ 10 µmol/mol) with a target uncertainty of ≤ 0.5 % and stability of ≥ 2 years. To achieve these challengingly low uncertainties will require the full characterisation of critical impurities and the development of new methods to minimise their formation during static standard preparation. To develop high accuracy traceable dynamic reference standards for low amount fractions of NO2 (10 nmol/mol – 500 nmol/mol) with a target uncertainty of ≤ 1 %. To achieve these challengingly low uncertainties will require the full characterisation of critical impurities and the development of new methods to minimise their formation during dynamic standard preparation. To develop analytical methods to quantify the main impurities formed unintentionally during the preparation of static and dynamic NO2 reference standards. To validate selective spectroscopic methods for directly measuring NO2 and compare them with the standard reference method as described in EN 14211:2012, using field trials at an atmospheric simulation chamber. To engage with stakeholders to ensure the uptake of the reference standards, calibration methods and devices developed in this project by standards development organisations, atmospheric monitoring networks, speciality gas companies, instrument manufacturers and other measurement infrastructures.