Emission source term is one of the strong controlling factors for the air quality simulation capability, particularly over the urban area. Ulsan is an industrial area and frequently required to simulate for environmental assessment. In this study, two CAPSS (Clean Air Policy Support System) emission data; CAPSS-2003 and CAPSS-2010 in Ulsan, were employed as an input data for WRFCMAQ air quality model for emission assessment. The simulated results were compared with observations for the local emission dominant synoptic conditions which had negative vorticities and lower geostrophic wind speed at 850hPa weather maps. The measurements of CO, NO2, SO2 and PM10 concentrations were compared with simulations and the ‘scaling factors’ of emissions for CO, NO2, SO2, and PM10 were suggested in in aggregative and quantitative manner. The results showed that CAPSS-2003 showed no critical discrepancies of CO and NO2 observations with simulations, while SO2 was overestimated by a factor of more than 12, while PM10 was underestimated by a factor of more than 20 times. However, CAPSS-2010 case showed that SO2 and PM10 emission were much more improved than CAPSS-2003. However, SO2was still overestimated by a factor of more than 2, and PM10 underestimated by a factor of 5, while there was no significant improvement for CO and NO2 emission. The estimated factors identified in this study can be used as‘scaling factors’for optimizing the emissions of air pollutants, particularly SO2 and PM10 for the realistic air quality simulation in Ulsan.