Barcelona is the second-largest city in Spain (4.3 million inhabitants in the metropolitan area), the capital of the Catalonia region. The city hosts the largest European harbour for cruises (the 11th for goods) and the 7th largest airport. The air quality in Barcelona is managed by the regional administration.
Milan is located in one of the most polluted areas (Po Valley) and the hotspot for secondary aerosols formation in Europe. The comparison between coastal cities (Barcelona, Stockholm, and Thessaloniki) and the inland city of Milan is crucial for understanding the role of stagnant atmospheric conditions and the influence of agriculture and biomass burning in the formation and toxicity of different nanoparticles sources and processes.
Stockholm is the capital of Sweden, with 2.3 million inhabitants in the metropolitan area. The city’s environmental administration has coordinated or been involved in several EU projects related to transport emissions, for example, Zero and low Emission Vehicles in Urban Society (ZEUS), TRENDSETTER setting trends for sustainable urban mobilityTrendsetter, BioEthanol for sustainable transport (BEST) and GrowSmarter. The city area hosts two airports, several harbours for goods and cruises, several railway stations for personal transport and cargo.
The Thessaloniki Metropolitan Area has around 1.1 million inhabitants, and it is the second major economic, industrial, commercial centre in the country and an important transportation hub for Greece and south-eastern Europe, notably through the Port of Thessaloniki. Thessaloniki air pollution is characterized by high concentrations of airborne particulate matter (PM) of carbonaceous particles and photochemical smog.