Antimony Stable Isotopes Decipher PM2.5-Bound Antimony Source Origins and Cross-Boundary Transport in the Himalayan Atmosphere

Antimony (Sb), a carcinogenic pollutant, poses significant health risks via particulate matter with aerodynamic diameter <2.5 μm bound Sb (Sb_PM2.5). However, their sources and transboundary dynamics in ecologically fragile regions such as the Himalayas remain poorly resolved. Here, we combined antimony stable isotopes (ε¹²³Sb), air-mass trajectory analysis, and multivariate mixing modeling to elucidate the sources and seasonal transport mechanisms of Sb_PM2.5 at Mount Qomolangma Station (QOMS) on the Tibetan Plateau, based on year-round samples collected during the premonsoon, monsoon, postmonsoon, and winter seasons. The annual mean Sb concentration (0.88 ± 0.64 ng m^-3) was comparable to values reported for European urban sites, indicating a substantial transboundary influence. Sb isotopic signatures (ε123Sb: -1.92 to +1.61 ε) exhibited clear seasonality: lighter values prevailed in winter and premonsoon samples and were associated with South Asian coal combustion (28 ± 10%) and biomass burning (29 ± 10%), whereas heavier values in the postmonsoon period reflected an increased contribution from desert dust (41 ± 11%). Monsoon-driven precipitation modulated the observed isotopic composition with wet scavenging preferentially removing lighter isotopes. A multivariate mixing model that integrates Sb isotopes and Rb/Sr ratios resolved three primary sources, desert dust (23-64%), coal combustion (13-46%), and biomass burning (14-45%), whose influences are linked to westerly circulation and glacial-valley winds. This study represents the first application of Sb isotopes to trace atmospheric Sb at a high-altitude site and demonstrates monsoon-regulated transport and a dominant anthropogenic signal from South Asia. These findings provide critical insights into trans-boundary heavy-metal pollution in high-altitude ecosystems and inform strategies for mitigating environmental risks in the Himalayas and comparable regions worldwide.