A Eulerian grid-based photochemical airshed model was used to simulate the formation of ozone and other photochemical compounds in the Metropolitan Area of São Paulo, Brazil, thus updating previous preliminary local study. Ambient concentrations of the criteria pollutants (NO2, CO, SO2, and O3) were forecasted and compared to the official air quality records (CETESB). Comparing the two present scenarios (Base Case (BC) and Refined Emission Inventory (REI) Case), the initial perceptions were that the percent contribution of vehicular-bound atmospheric emissions might be overestimated within the local air pollution dynamics. VOC emissions attributable to vehicles might be oversized by a factor of 40%, when comparing the official inventory for this source category for total hydrocarbons (CETESB) with the VOC emissions (exhaust + fugitive) presented in the REI scenario. In terms of NOx emissions, the initial reduction was of 50% applied on the local emission inventory, moving from BC to REI case. This is an abstract of a paper presented at the AWMA's 99th Annual Conference and Exhibition (New Orleans, LA 6/20-23/2006).