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Within the framework of this paper an attempt is made in order to assess the relative contribution of the main diesel vehicle classes; namely, buses, taxis and heavy goods vehicles (HGVs) to the air pollution of the Thessaloniki (Greece) urban area based on an emission factors approach. Data sets refer to the situation before and after the construction of the city ring road. Findings show the significant contribution of the ring road to the urban air pollution decrease. This reduction could be mainly attributed to the observed significant traffic decrease of HGVs in the city center between 1988 and 1998. Small changes have been observed during the same period for the bus and taxi traffic in the city center. Also, the improvement of the diesel fuel quality and the temporal renewal of the diesel vehicles are considered as important factors for the observed temporal decreasing trend of the air pollutants with diesel oil origin.