Ozone production sensitivity in the highland city of Lhasa: a comparative analysis with Beijing

As Lhasa experiences accelerated urbanization and increased pollutant emission intensity, controlling ozone pollution becomes more challenging due to increased photochemical reactions—a problem exacerbated by the city’s already high solar UV radiation and background ozone levels. In order to reveals the intricacies of ozone formation, this study conducted direct ozone production rate (OPR) measurements in Lhasa in June 2021, comparing them to findings in Beijing in similar climatic conditions. Despite lower NO_x levels, Lhasa exhibited a higher OPR with reduced variations (12.8 ± 6.0 ppbv h^-1) due to stronger UVR and OH reactivity (VOC-k_OH) influenced by oxidized volatile organic compounds when compared to Beijing (9.4 ± 11.0 ppbv h^-1). When adjusting for comparable UVR conditions, Lhasa demonstrated a less pronounced NO turning point in OPR compared to Beijing. Lhasa’s unique bridge-shaped VOC-k_OH diel variation favored daytime peroxy radical production, pushing the NO turning point higher from morning to noon, resulting in a NO_x-limited regime except in the early morning. Furthermore, the accumulation of OVOCs in the Lhasa River valley, coupled with rapid photochemical production despite relatively low VOCs emissions, explained the elevated VOC-k_OH levels. This suggests that OVOCs and, consequently, O₃ pollution could worsen with future VOCs emission increases, possibly triggered by factors like rapid warming and local afforestation initiatives. Our findings highlight the complex and sensitive nature of O₃ pollution in Lhasa and other Tibetan Plateau cities, emphasizing the need for continued monitoring and in-depth research to develop effective control strategies.