In this study, NO2 and SO2 levels during a specific winter episode in the city of Istanbul were researched with regard to the respective impact of weather conditions. The city under study has the largest population and is one of the most polluted cities of Turkey. Furthermore, Istanbul is vulnerable to trans-boundary transport of air pollutants from Europe, because its location is on the eastern end of the Continent, where westerly winds prevail. However, there has neither been any comprehensive study nor any regulation to fix and solve the air quality problem in the city so far. A framework was prepared to model air quality using MM5 for meteorological modeling and CMAQ for transport and chemistry modeling. This study was supported by the tracer and the trajectory investigations over the area of interest. The model results underestimated the concentration of air pollutants over Istanbul as expected. But, in spite of the coarse emission inventory, the similarity of the time series between the model and observation values during the simulation period is very encouraging for future studies. That is to say, the tracer and the trajectory studies indicated that trans-boundary sources are responsible for the poor air quality of Istanbul under specific weather conditions. It must be pointed out, though, that a high-resolution national emission inventory is needed to get more accurate results.