Critical role of vertical exchange in the boundary layer on urban photochemical pollution

Ozone dynamics in the boundary layer differ significantly from those in the free troposphere, with vertical exchange playing a key role in ozone pollution. However, two major issues remain unresolved: the contribution of the nighttime residual layer to surface ozone levels and the role of daytime vertical mixing in ozone formation. This review summarizes observational methods for vertically measuring meteorological factors and pollutants, providing insights into their distributions in the boundary layer and evaluating the influence of boundary layer dynamics on ozone and its precursors. Multiple studies have indicated that residual layer transport contributes 20-50 % of surface ozone, whereas daytime free convection enhances surface ozone concentrations by promoting vertical mixing and accelerating photochemical reactions. These results emphasize the crucial role of NO_x in photochemical pollution and underscore the need for more aggressive NOₓ emission reductions. Given the importance of vertical transport in the boundary layer, reducing industrial NOₓ emissions from elevated sources can more effectively suppress ozone formation aloft and its downward transport, leading to lower surface O₃ levels.