December 14, 2017 In the Autumn of 2017 Michael Ward (NPL Higher Research Scientist) undertook a 3 month secondment at the the Chemistry Department of Bureau International des Poids et Mesures (BIPM). The secondment was an intensive course (part of the Capacity Building and Knowledge Transfer programme) aimed at providing core training on Fourier Transform InfraRed (FT-IR) spectroscopy, a technique that can be applied to gas metrology. In particular, Michael learnt how to correctly measure nitrogen dioxide and nitric acid using FT-IR spectroscopy and devising uncertainty budgets for such measurements. These newly learnt methodologies are now being used at NPL in measurements of reference materials containing nitrogen dioxide, nitric acid and water vapour. These measurements are needed in order to understand the conversion of NO2 to HNO3 within reference materials and the parameters involved. This information will then be used to maximize the stability of NO2 reference materials which will ultimately improve the accuracy of atmospheric measurements.