Observational Evidence of the Vertical Exchange of Ozone within the Urban Planetary Boundary Layer in Shanghai, China

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 (NOx) 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 NOx-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.