The diagram above illustrates the workflow for Work Package 1 with partners’ responsibilities colour coded. This diagram is regularly updated as activities are completed (marked with ticks). The aim of this work package is to develop traceable methods for measuring particle number concentration and produce monodisperse, spherical reference materials with known number concentration to calibrate industrially relevant techniques. In the following, the term ‘particle’ is used to cover both nanoparticles (<100 nm diameter) and sub-micron particles (>100 nm diameter), since both types of particle are required for this work. Currently, certified reference nanomaterials for the absolute particle number concentration are not available, nor are uncertainty analyses for particle concentration measurements using particle sizing techniques which are sensitive to the concentration. In this work package, a set of highly monodisperse particle samples of different chemical composition (metallic, metal oxide and organic) and covering a size range from 1 nm to 1000 nm will be synthesised and sourced from commercial suppliers to serve as internal reference materials in the project (task 1.1). Two potential primary methods (SAXS and spICPMS) together with mass analysis (TGA) will be employed to determine the absolute particle number concentration of these reference materials with target uncertainties better than 10 % (task 1.2). The two potential primary methods are not applicable to all particles: in spICPMS the size detection limit depends on the element (e.g. ~15 nm diameter for Au particles) and is limited to inorganic particles of known stoichiometry, density and geometry; SAXS is an accurate method but is limited to spherical, monodisperse particles. The uncertainty analysis of these two potential primary methods will be facilitated by a significant overlap in the range of applicability. Therefore, the methods can be compared to ensure that they agree to within their respective uncertainties and any discrepancy can be identified and dealt with. Additionally, TGA measurements will be performed to supplement and validate the measurements via the mass balance. Following validation the potential primary methods will be used to determine the number concentration of the reference materials used in the project and the reference materials can then be used to calibrate other methods. The calibrated reference materials will finally be used to assess the suitability of widely-available laboratory methods, such as DLS and DCS, and emerging techniques (NTA, TRPS, electrospray-DMA-CPC) for particle number concentration measurements (task 1.3). This will include studies of the sensitivity, linearity, and the applicability to binary mixtures of the reference materials. The latter will also be carried out using the validated potential primary methods.