The specific objectives are to 1. Use test aerosol particles in test chambers, to develop traceable methods for the calibration of optical and aerodynamic particle size spectrometers to determine mass and number concentration, as well as size distribution. In addition, to characterise aerosol bipolar charge conditioners in mobility particle size spectrometers and based on this the pre-normative definition of their performance criteria. 2. Develop new calibration procedures for automatic pollen monitoring instruments based on fluorescent polystyrene latex and real pollen particles (at concentrations < 1 /cm3, and particle sizes of 0.5 μm up to > 20 μm, with target uncertainties of < 10 %). In addition, comparisons to reference instruments in the field will be used to characterise instrument responses to pollen. 3. Develop certified reference substrates and traceable measurement techniques for the quantification of regulated and unregulated substances in ambient air with cascade impactor sampling and X-Ray Fluorescence analysis (XRF). 4. Develop portable instruments and software for the measurement of ambient aerosol particle concentrations under variable environmental conditions. Knowledge gained will be applied to other appropriate classes of portable instruments by producing suitably adapted procedures. 5. Facilitate the take up of the technology and measurement infrastructure developed in the project by: the measurement supply chain (accredited laboratories), standards developing organisations (ISO and CEN) and end users (e.g. the Network of European Air Quality Reference Laboratories (AQUILA) , the European Monitoring and Evaluation Programme (EMEP), the European Research Infrastructure (Aerosol, Clouds and Trace Gases Infrastructure) and its future European Research Infrastructure Consortium (ERIC)).