Each summer period, ozone concentrations surpass the legislation limits in the north of Portugal, more precisely at the Douro Norte monitoring station. To solve/mitigate this air pollution problem, and respond to the EU requirements, air quality plans were studied and designed. An air quality numerical modelling system was applied, with a source apportionment technique (to the most relevant ozone episodes) to identify the major source regions and the main activity sectors responsible for the ozone values at this site. Modelling results pointed out that the background values contribute with more than 50 % to the ozone concentration measured, followed by transport and residential sectors—the main anthropogenic activities. More regional and transboundary strategies are needed to reduce the background concentrations, besides local actions on road transport and residential combustion at urban regions (e.g., the Porto area).