Cosmogenic ³⁵ S Tracer Evidence for Seasonal Stratosphere‐to‐troposphere Transport Over the Southeastern Tibetan Plateau

Abstract Stratosphere‐to‐troposphere transport (STT) is a key process that influences atmospheric chemical composition and radiative balance. The Tibetan Plateau (TP), the highest and most extensive plateau in the world, has been identified as a global hotspot for stratospheric intrusions. However, direct observational evidence remains limited, particularly in the southeastern TP—a region traditionally considered to have minimal STT influence. Stratospheric 35 S is a powerful tracer for directly identifying STT processes. Here, we present the first measurements of 35 SO 4 2− concentrations and their seasonal variability in aerosols collected at Yaze, a remote site in the southeastern TP. The highest 35 S activity (840 ± 90 atoms m −3 ) occurred in spring and coincided with elevated surface ozone levels. Backward trajectory and vertical motion analyses indicate that this spring enhancement was attributable to descending stratospheric air masses originating from the Himalayan region, highlighting the prominent influence of frequent STT events during this season. A secondary 35 S activity peak in November (810 ± 80 atoms m −3 ) was associated with stratospheric intrusions from the northwestern TP. These findings provide direct observational evidence of pronounced seasonal variability in stratospheric intrusions over the southeastern TP and enhance our understanding of regional stratosphere–troposphere exchange processes. Furthermore, we report 35 SO 4 2− concentrations measured in spring glacier snowpits and summer precipitation from the nearby glaciers, which are associated with atmospheric 35 SO 4 2− and serve as representative end‐member values for glacier runoff, establishing essential baseline data for future assessments of glacier melt and hydrological responses in this climatically sensitive region.