Abstract: Three scenarios were designed in which catalysts for selective catalytic reduction (SCR) of NO_x were applied to diesel exhaust (Scenario 1), catalysts for SCR were applied to both diesel exhaust and natural gas boilers according to the Chinese energy plan for 2015 (Scenario 2), and the catalysts were applied as in Scenario 2 but the Chinese energy plan for 2030 was used (Scenario 3). Simulations were performed with the WRF-CAMx model in Beijing, Tianjin, and Hebei Province in China (the BTH region) for January, April, July, and October of 2007, representing winter, spring, summer, and autumn, respectively. The results indicate that Scenario 1 can reduce the surface NO_x concentrations by 20% in Beijing and Tianjin and by 5% in Hebei Province, and decrease the PM2.5 (particulate matter with diameters less than or equal to 2.5 μm) concentrations by 10% in the BTH region. Scenarios 2 and 3 lead to decreases of more than 20% and more than 30%, respectively, in the surface concentrations of NO_x and PM2.5 over the BTH region. This suggests that decreases in the surface concentrations of NO_x and PM2.5 depend significantly on the amount of the catalyst used for SCR in diesel exhaust and natural gas boilers over the BTH region. The chemical process plays a key role in the formation of nitrates, sulfates, and ammonium salts, which are major components of PM2.5 over the BTH region. The surface concentrations of nitrates, sulfates, and ammonium salts contribute more than 60% in winter, spring, and autumn, more than 70% in summer and autumn, and approximately 25% in all four seasons, respectively, to the surface PM2.5 concentration. This implies that a large reduction in the emissions of major precursors of PM2.5, e.g., NO_x, SO₂, NH₃, volatile organic compounds, and CO, can effectively reduce surface concentrations of PM2.5.