The POSEIDON (Pollution Monitoring of ship emissions: an Integrated approach for harbours in the Adriatic basin) project investigates the impact on air quality of four major harbours in the Adriatic/Ionian Seas, using a common state-of-the-art methodology based on emission inventories, numerical modelling and experimental results that will produce comparable information useful to plan future actions and controls of emissions in the Adriatic/Ionian macroregion.