This source apportionment modeling study shows that around 70 % of ground-level O₃ in Madrid (Spain) is transported from other regions. Nonetheless, emissions from local sources, mainly road traffic, play a significant role, especially under atmospheric stagnation. Local measures during those conditions may be able to reduce O₃ peaks by up to 30 % and, thus, lessen impacts from high-O₃ episodes in the Madrid metropolitan area.