Barcelona, Spain. 
The site is Palau Reial, 41º23´14´´N, 2º6´56´´E. This is an AQ monitoring co-operated by partner CSIC and the Regional Gobernment of Catalonia (Spain). This is an urban background site, but highly influenced by emissions from the Diagonal Avenue loacted at around 300m with a traffic flow of around 100000 cars/working day.

Porto, Portugal. The site is Praça Francisco Sá Carneiro, Urban Station, 41º 09´ 46.10´´ N; 8º 35´ 26.95´´ W. The station is part of the National Air Quality Network, QualAr, controlled by the CCDRN. It is located in the eastern side of the Porto city, next to the Fernão de Magalhães Avenue and at 600 meters from the Inner Circular Motorway.

Florence, Italy. The site is called Bassi, an AQ monitoring site of the Environmental Protection Agency of Tuscany. It is a urban background site placed in Bassi street, influenced by traffic (it is close to the Florence stadium).

Athens, Greece. The sampling site is the DEM-Athens Urban background Monitoring station for Aerosol properties established at the grounds of the National Centre for Scientific Research “Demokritos” since 2007. The area lies in the North East corner of the Greater Athens Metropolitan Area and at an altitude of 270 m.a.s.l on the hillside of Hemetus mountain. The site is partly influenced by the Urban area and partly by incoming air from the North East representative of regional atmospheric aerosol conditions.

Milan, Italy. The MLN-UB (Milano Pascal) urban background station is part of the ARPA Lombardia Air Quality Network, and it is one of the Italian Supersites for the D. Lgs. 155/2010 (Italian transposition of 2008/50/CE). It is located in the eastern side of Milan, the University area called “Città Studi”, in a playground about 130 m far from the road traffic.