It is well known that exposure to aerosols exerts a negative impact on human health and that aerosols affect climate and the environment. These effects are dependent on the composition and sources of these fine atmospheric aerosols (particulate matter with aerodynamic diameter below 2.5 µm, PM2.5). The main challenge of the Action is to consistently assess their spatial variability (across Europe), their temporal variability (at a one hour time resolution or better), their seasonality (using long term datasets), their phenomenology (chemical composition) and their sources. To this end many research groups and some air quality monitoring networks in Europe and across the world have acquired recently-developed chemical composition measurement instrumentation. These include the Aerosol Chemical Speciation Monitor (ACSM) (based on Aerosol Mass Spectrometer (AMS) technology), which measures non-refractory ammonium, nitrate, sulfate, chloride, and organic mass, and instruments that measure the refractory black carbon, such as the Aethalometer and Multi Angle Absorption Photometer (MAAP). These new high time resolution techniques, which chemically characterize the aerosols, are capable of operating for long time periods and have only been available in 5-10 years. The processing and interpretation of the data from these instruments has matured to a stage where harmonized across Europe is now possible; this will be achieved by a network built through the present Action to jointly develop the capacity for the interpretation of the measurements gathered using these techniques. The outcomes of the Action will be relevant for air quality modellers and policy makers.