Through analyses on the synoptic systems, pollution characteristics of O₃ precursors, and modeling of local O₃ formation and processes influencing O₃ level, we found that this O₃ pollution event was induced by a uniform pressure field over the Shandong Peninsula and also aggravated by a low-pressure trough in the last few days. This finding indicated that the NCP might be an O₃ source region, which exported photochemical pollution to the adjoining regions or even to the neighboring countries.