Harbours are important hubs for economic growth in both tourism and commercial activities. They are also an environmental burden being a source of atmospheric pollution often localized near cities and industrial complexes. The POSEIDON 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.