Introduction
This study derives from the need to evaluate the impact on the urban air quality of two-wheeler vehicular class, mostly
used as popular means of daily moving in the major cities of southern Europe (Vasic and Weilemann2006), where mopeds and motorcycles represent a great proportion of vehicles. In the last years, powered two-wheelers accounted for over 32 million vehicles in EU-27, representing about 8 % of passenger mobility fleet (European Commission2010); in Italy, where traffic congestion and parking difficulty influence the choice of transportation mode, the share of the two-wheelers to the whole passenger vehicle fleet is about 25 % (Iodice and Senatore2013a). With over nine million two-wheelers, Italian fleet contributed for 28 % to EU-27 figures (Prati et al. 2011). Always with reference to the Italian situation, powered two-wheeler fleet included about 33.3 % of mopeds (vehicles with an engine capacity lower than 50 cm 3), while motorcycles (engine displacement higher than 50 cm 3 ) accounted for about 66.7 % (ACI2012; ANCMA 2011).