Tropospheric ozone (O3) pollution in high-altitude urban systems presents unique challenges due to intensified photochemistry, complex topography-driven stagnation, and cross-boundary transport - a critical yet underexplored nexus in atmospheric science. This study pioneers an integrated framework combining 5-year monitoring (2018–2022), high-resolution trajectory modeling (HYSPLIT-PSCF/CWT), and WRF-CAMx with OSAT module simulations to dissect the interplay between local emissions, regional transport, and altitudinal meteorology in driving O3 extremes. This system consists of an emission source model, the Weather Research and Forecasting (WRF) mesoscale meteorological model, and the Comprehensive Air Quality Model with extensions (CAMx). The OSAT (Ozone Source Apportionment Technology) method is a key extension of the CAMx model, serving as an integrated approach combines sensitivity analysis and process analysis. Contrary to lowland cities where summer dominates O3 peaks, our results reveal a paradigmatic springtime surge (March–May) linked to pre-monsoon stagnation and valley wind inversions, amplifying precursor accumulation by 40–60 %. Source apportionment uncovers natural sources (45 % annually; 82 % in summer) as the dominant contributor - a striking deviation from industrial/urban narratives - attributed to biogenic VOC emissions enhanced by high-altitude UV radiation. Transboundary transport from southwestern regions contributes 35 % of episodic O3, demonstrating the vulnerability of mountainous basins to cross-jurisdictional pollution. Crucially, we identify VOC-limited regimes as the linchpin for mitigation, with industrial coatings and architectural paints accounting for 52 % of anthropogenic precursors. This work establishes a mechanistic template for O3 management in global mountain cities, emphasizing the need for altitude-sensitive policies and transregional governance frameworks.
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