Vertical ozone formation mechanisms resulting from increased oxidation on the mountainside of Mount Tai, China.

The vertical distribution of ozone (O 3 ) within the boundary layer (BL) and its ground-level effects have been extensively studied. However, observational limitations in obtaining high-resolution, real-time data on O 3 and its precursors, especially volatile organic compounds (VOCs), have led to a scarcity of research on O 3 formation sensitivity and mechanisms. Online measurements for O 3 , nitrogen oxides (NO x ), and VOCs were made on the mountainside of Mount Tai (∼550 m a.s.l.) in China during the summer of 2022 and were compared with the data from a ground-level site. The Master Chemical Mechanism (V3.3.1) was used to uncover a positive correlation between NO x and photochemical reaction rates on the mountainside, marking it as a NO x -limited regime in contrast to the VOC-limited regime identified at surface. On the mountainside, lower NO levels limited hydroxyl radicals (OH) recycling reactions, resulting in earlier O 3 peaks and higher concentrations of hydroperoxy radicals (HO 2 ) and organic peroxy radicals (RO 2 ). The arrival of fresh air masses rich in NO accelerated OH radical cycling, enhanced atmospheric oxidization, and significantly impacted surface O 3 concentrations though vertical transport. Moreover, NO x reduction scenario simulations show that when considering vertical transport, the peak O 3 production rate at the surface is lower due to differences in O 3 formation sensitivity vertically. This study highlights the significant sensitivity of O 3 formation to NO within the BL, underscoring the potential impact of vertical in situ O 3 formation above the ground on surface-level O 3 concentrations through vertical exchange, particularly in cities with mountainous terrain.