Cross-regional NO₂ transport over the Tibetan Plateau (2005–2024): Bidirectional flux dynamics, seasonal drivers, and environmental implications

Abstract. Tropospheric NO₂ over the Tibetan Plateau (TP) reflects the combined influence of local emissions and long-range transport. We characterize the spatiotemporal variability of tropospheric NO₂ columns, surface concentrations, and transport boundary fluxes during 2005–2024 by integrating OMI and TROPOMI satellite data, ground-based observations (CNEMC), and flux diagnostics based on a closed-loop integral method. The TP shows a marked spatial gradient in tropospheric NO₂ columns, with overall levels substantially lower than those over South Asia. During the study period, NO₂ in urban areas of the plateau increased, with the most pronounced rises observed in Lhasa and Qamdo. Flux analysis shows that tropospheric NO₂ transport over the TP is quasi-symmetric across segments, manifesting as a bidirectional transport structure, with external influx dominating the southwestern segment and internal efflux toward the northeastern segment. The northeastern segment shows both a higher net flux and more rapid increases in internal efflux and external influx relative to the southwestern segment, highlighting its growing contribution to eastern China. Random forest (RF) and SHAP analyses reveal distinct dynamical controls, with winter–spring transport dominated by the upper-level westerly jet (200–400 hPa) and summer external influx primarily linked to the Indian summer monsoon (450–550 hPa). Overall, this study emphasizes the important role of the TP in cross-regional nitrogen oxide transport and provides a reference for understanding its potential impacts on regional air quality and environmental conditions.