Abstract. Over the past 10 years (2005–2014), ground-level O₃ in Hong Kong has consistently increased in all seasons except winter, despite the yearly reduction of its precursors, i.e. nitrogen oxides (NO_x =  NO + NO₂), total volatile organic compounds (TVOCs), and carbon monoxide (CO). To explain the contradictory phenomena, an observation-based box model (OBM) coupled with CB05 mechanism was applied in order to understand the influence of both locally produced O₃ and regional transport. The simulation of locally produced O₃ showed an increasing trend in spring, a decreasing trend in autumn, and no changes in summer and winter. The O₃ increase in spring was caused by the net effect of more rapid decrease in NO titration and unchanged TVOC reactivity despite decreased TVOC mixing ratios, while the decreased local O₃ formation in autumn was mainly due to the reduction of aromatic VOC mixing ratios and the TVOC reactivity and much slower decrease in NO titration. However, the decreased in situ O₃ formation in autumn was overridden by the regional contribution, resulting in elevated O₃ observations. Furthermore, the OBM-derived relative incremental reactivity indicated that the O₃ formation was VOC-limited in all seasons, and that the long-term O₃ formation was more sensitive to VOCs and less to NO_x and CO in the past 10 years. In addition, the OBM results found that the contributions of aromatics to O₃ formation decreased in all seasons of these years, particularly in autumn, probably due to the effective control of solvent-related sources. In contrast, the contributions of alkenes increased, suggesting a continuing need to reduce traffic emissions. The findings provide updated information on photochemical pollution and its impact in Hong Kong.