Over the past ten years (2005–2014), ground-level O<sub>3</sub> in Hong Kong has consistently increased in all seasons except winter, despite the yearly reduction of its precursors, i.e., nitrogen oxides (NO<sub>x</sub> = O + NO<sub>2</sub>), total volatile organic compounds (TVOCs) and carbon monoxide (CO). To explain the contradictory phenomenon, an observation-based box model (OBM) coupled with CB05 mechanism was applied in order to understand the influence of both locally-produced O<sub>3</sub> and regional transport. The simulation of locally-produced O<sub>3</sub> showed an increasing trend in spring, a decreasing trend in autumn and no changes in summer and winter. The O<sub>3</sub> increase in spring was caused by the net effect of more rapid decrease of NO titration and unchanged TVOC reactivity despite decreased TVOC mixing ratios, while the decreased local O<sub>3</sub> formation in autumn was mainly due to the reduction of aromatic VOC mixing ratios and the TVOC reactivity and much slower decrease of NO titration. However, the decreased in-situ O<sub>3</sub> formation in autumn was overridden by the regional contribution, resulting in elevated O<sub>3</sub> observations. Furthermore, the OBM-derived relative incremental reactivity indicated that the O<sub>3</sub> formation was VOC-limited in all seasons, and the long-term O<sub>3</sub> formation was more sensitive to VOCs and less to NO<sub>x</sub> and CO in the past 10 years. In addition, the OBM results found that the contributions of aromatics to O<sub>3</sub> formation decreased in all seasons of these years, particularly in autumn, likely due to effective control of solvent-related sources. In contrast, the contributions of alkenes increased, suggesting a continuing need to reduce traffic emissions. The findings provided updated information on photochemical pollution and its impact in Hong Kong.