More accurate measurements of nitrogen dioxide (NO2) are needed in order to understand population level exposure, to improve air quality models and emission inventories, to better discern long‑term trends in NO2 concentrations and to enforce air quality and vehicle emission legislation. This is essential for the timely evaluation of air pollution mitigation policies, and to improve our understanding of the influence of anthropogenic emissions on the climate system. This project will achieve the necessary accuracy by developing capabilities for the direct measurement of NO2 using newly available selective NO2 techniques and direct calibration with more accurate and stable primary reference standards of NO2. Need Nitrogen dioxide (NO2) is the air pollutant considered to have one of the greatest impacts on human health. A major source of NO2 in cities is from fossil fuel combustion in motor vehicles. Diesel powered vehicles emit twenty times more NO2 compared to petrol powered vehicles. In Europe, NO2 in the air we breathe is becoming a massive issue due to large increases in diesel vehicle ownership, resulting from government driven tax incentives, in conjunction with emission standards not delivering the expected reductions under real world driving conditions. This was highlighted by the Volkswagen emissions scandal, recent health reports linking NO2 exposure with adverse health outcomes and the continuing breach of European Union (EU) legislation (2008/50/EC), which sets both annual and hourly limit values (NO2 concentration maxima), in the majority of EU member states. NO2 emissions in Europe are not decreasing fast enough and lower NO2 concentrations are needed in the future to improve the quality of life for European citizens and to reduce the economic burden of detrimental health outcomes from NO2 exposure. To enable this requires greater confidence in measured trends in emissions and ambient air leading to better evidence based policy and more effective mitigation policies, which are strongly dependent on measurement accuracy. To achieve the necessary improvements in measurement accuracy requires the: Direct measurement of NO2, because previous methodologies to measure it indirectly as the difference between NO and total NOx are no longer fit for purpose because of their high uncertainties, e.g., > 10 %. Calibration of instruments with highly accurate NO2 calibration gases at atmospherically relevant concentrations, e.g., 10 nmol/mol – 500 nmol/mol. Current NO2 reference standards are not sufficiently accurate or stable enough to fulfil the requirements of the monitoring community. Full characterisation and minimisation of impurities in reference standards such as water vapour and reactive nitrogen compounds (e.g., nitric acid), which increase uncertainty and decrease long‑term stability.