Urban air pollution is affected by both emissions and meteorology. Using the recent 18 yr long measurements of air pollutants in the East Asian megacity of Seoul, we quantitatively investigated effects on short-term and long-term air quality. Specific synoptic weather patterns were important for episodic PM₁₀ and O₃ events, while the long-term decrease in wind speed and the 2008 economic recession played roles in the recent improvement of PM₁₀ and deterioration of O₃ air quality in Seoul.