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

The vertical distribution of ozone (O) 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 and its precursors, especially volatile organic compounds (VOCs), have led to a scarcity of research on O formation sensitivity and mechanisms. Online measurements for O, nitrogen oxides (NO ), 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 and photochemical reaction rates on the mountainside, marking it as a NO -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 peaks and higher concentrations of hydroperoxy radicals (HO) and organic peroxy radicals (RO). The arrival of fresh air masses rich in NO accelerated OH radical cycling, enhanced atmospheric oxidization, and significantly impacted surface O concentrations though vertical transport. Moreover, NO reduction scenario simulations show that when considering vertical transport, the peak O production rate at the surface is lower due to differences in O formation sensitivity vertically. This study highlights the significant sensitivity of O formation to NO within the BL, underscoring the potential impact of vertical in situ O formation above the ground on surface-level O concentrations through vertical exchange, particularly in cities with mountainous terrain.