Stratosphere-monsoon superposition drives summertime surface ozone extremes on the Tibetan Plateau

Ozone (O₃) is a pivotal trace gas at the core of the Earth's coupled climate-chemistry-biosphere system with significant implications for global public health, ecosystem stability, and the Earth's radiative balance. This study integrates a high-density surface O₃ monitoring network deployed along key inflow corridors in the southwestern Tibetan Plateau (TP), with EAC4 reanalysis and Lagrangian transport diagnostics to quantify the contributions of different mechanisms driving summertime surface O₃ formation. By applying an iterative discrete wavelet transform (IDWT), we decompose hourly O₃ concentrations into intra-day (∼8 h), synoptic (1-2 days), and seasonal (∼4 months) components, corresponding to local photochemical production, regional transport, and a quasi-constant background. Results show that regional transport accounts for approximately 78% of the total surface O₃ during the monsoon season (June-September 2024). Decomposition further indicates that, beyond a relatively constant background, stratospheric intrusion (SI) is the primary driver during polluted days (MDA8 O₃ > 160 μg/m³), contributing 50.2% to surface O₃ exceedances, followed by long-range anthropogenic transport (LRAT) (28.7%) and local production (LP) (21.1%). Dry conditions and enhanced solar radiation act as critical local amplifiers of O₃ pollution over the plateau. A representative event from 11 to 14 June 2024 reveals a dual-channel "stratosphere-monsoon superposition" mechanism: a descending stratospheric O₃ plume entering from northern Xinjiang superimposes on a monsoon-transported South Asian pollution plume crossing the Himalayas, producing widespread exceedances of 160 μg/m³ across all surface sites in southwestern TP. Weighted airflow trajectories combining observations and simulations identify that the upwind regions along the northern South Asian pollution belt as primary potential source areas. These findings challenge the notion of the southwestern TP as a predominantly natural O₃ background, instead considering it as a dynamic mixing gateway where stratospheric air and monsoon outflow mix and interact to shape surface oxidant levels.