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The vertical mass exchange of ozone (O₃) plays an important role in determining surface O₃ air quality, the understanding of which, however, is greatly limited by the lack of continuous measurements in the vertical direction. Here, we characterize O₃ variations at a high-altitude monitoring site at the top of Shanghai Tower (SHT) and discuss the potential impacts of the vertical exchange of air pollutants on O₃ air quality within the urban planetary boundary layer (PBL) based on continuous measurements during 2017–2018. During the daytime, two distinct patterns of vertical O₃ gradient are detected. In summer, the daytime O₃ formation at SHT is observed to be more limited by nitrogen oxides (NO_x) than the surface, which, together with the efficient vertical mixings, results in higher O₃ levels in the upper mixing layer. In cold months, the opposite vertical gradient is observed, which is associated with weak vertical exchange and NO_x-saturated O₃ formation. A nighttime O₃ reservoir layer and consistent morning O₃ entrainments are detected all year round. These results provide direct evidence of the vertical mixings within the urban PBL, underscoring the pressing need for improving vertical resolution in near-surface layers of air quality models.