Vertical ozone formation mechanisms driven by deep convection within the boundary layer over the Loess Plateau, China.

The vertical distribution of ozone and its precursors within the boundary layer in plain urban agglomerations have been extensively studied. However, the vertical evolution of precursors and its impact on photochemical ozone formation mechanisms remain unclear under the conditions of intense solar radiation and deep convective boundary layer over plateaus. In this study, we conducted a 15-day intensive sounding campaign using an unmanned aerial vehicle (UAV) platform in Yulin, a typical energy and chemical industrial city on the Loess Plateau in China, during summer. Combined with an observation-based box model, we quantitatively evaluated the vertical variations in ozone production rates and sensitivity. The results indicate that ozone pollution episode was characterized by high temperature, low humidity, weak winds, strong radiation, and deep convection. Alkanes showed a uniform vertical distribution, while alkenes and aromatics decreased with height. In contrast, the proportion of oxygenated VOCs (OVOCs) increased with altitude due to chemical oxidation during vertical mixing. Aromatics and OVOCs were the dominant contributors to ozone formation. The column ozone concentration in the residual layer (RL) during pollution episode was twice that during non-pollution periods, with RL transport contributing 44.4% to the midday mixing layer (ML) ozone and photochemical production accounting for 47.6%, together serving as key drivers of pollution episode. Ozone formation throughout the boundary layer was VOCs-limited. Ozone production near the surface was dominated by aromatics, while sensitivity to OVOCs increased significantly at 1000 m height. NOx accumulation at higher levels suppressed ozone formation. This study elucidates the ozone pollution mechanism under the effect of deep convective boundary layer over the Loess Plateau, providing a scientific basis for targeted regional ozone control.