In this paper, the efect of utilizing urban buses with either outdated or modern environmental technologies on the air pollution of Arak city is modeled. Arak is an industrial city with population and trafc congestion; hence, such modeling is of paramount signifcance. Here, the roads with heavy trafc are modeled and the PM2.5 pollutants emitted by light and heavy vehicles in the rush hours over the entire 365 days of the year were contemplated. Three diferent cases of only light vehicles, 2 light vehicles and EURO3 urban buses, and 3 light vehicles and EURO4 urban buses were considered. All required data including synoptic and upper air meteorological data, topography data and mobile source data are utilized for modeling. For modeling vehicle-caused emission, all trafc data including type of the vehicle, number of vehicles and quality of traffc were used. Results showed that urban busses are highly infuential on the PM2.5 pollutants in Arak city. In the presence of EURO3 and EURO4 urban buses, the concentration exceeds 5 and 10 times the standard level, respectively. Moreover, with the replacement of EURO3 urban buses with their EURO4 rivals, the number of polluted days over the period of a year would decrease from 26 to 10 days.
