Abstract. In this study, the ozone (O₃) formation in China's northwest city of Xi'an and surrounding areas is investigated using the Weather Research and Forecasting atmospheric chemistry (WRF-Chem) model during the period from 22 to 24 August 2013, corresponding to a heavy air pollution episode with high concentrations of O₃ and PM_2.5. The model generally performs well compared to measurements in simulating the surface temperature, relative humidity, and wind speed and direction, near-surface O₃ and PM_2.5 mass concentrations, and aerosol constituents. High aerosol concentrations in Xi'an and surrounding areas significantly decrease the photolysis frequencies and can reduce O₃ concentrations by more than 50 µg m⁻³ (around 25 ppb) on average. Sensitivity studies show that the O₃ production regime in Xi'an and surrounding areas is complicated, varying from NO_x to VOC (volatile organic compound)-sensitive chemistry. The industrial emissions contribute the most to the O₃ concentrations compared to biogenic and other anthropogenic sources, but neither individual anthropogenic emission nor biogenic emission plays a dominant role in the O₃ formation. Under high O₃ and PM_2.5 concentrations, a 50 % reduction in all the anthropogenic emissions only decreases near-surface O₃ concentrations by about 14 % during daytime. The complicated O₃ production regime and high aerosol levels pose a challenge for O₃ control strategies in Xi'an and surrounding areas. Further investigation regarding O₃ control strategies will need to be performed, taking into consideration the rapid changes in anthropogenic emissions that are not reflected in the current emission inventories and the uncertainties in the meteorological field simulations.