Free convection exacerbates surface ozone pollution by enhancing the vertical exchange of ozone

Although it is known that downward transport from the upper boundary layer contributes significantly to the surface ozone (O₃) concentration, the factors influencing vertical O₃ exchange remain unclear. To solve this problem, a platform for observing the vertical exchange flux of O₃ in the urban canopy was established in Beijing. The observation results revealed that the city functioned as an O₃ sink. The daily variation in the O₃ flux was highly consistent with the development of the boundary layer structure, peaking at -15.2 nmol·m^-2·s^-1 at midday, when the level of the convection was the highest. Combining ground-based remote sensing data, the observation days were divided into forced and free convection days, and the free convection days were further subdivided into low-, medium-, and high-O₃ days on the basis of the maximum 8-h moving average O₃ concentration. Intense free convection promoted the upward exchange of surface nitrogen dioxide (NO₂), the chemical reaction process of generating O₃ at high altitudes and the downward exchange of O₃ in the upper boundary layer, thereby exacerbating surface O₃ pollution. This research highlights the importance of NO_X emission reduction, which reduces the vertical circulation of NO₂-O₃ driven by free convection, thus effectively controlling O₃ pollution.