Transboundary dispersion and deposition of gaseous radioactive iodine revealed by anthropogenic ¹²⁹I in mountainous regions

Gaseous radioactive iodine is the major concern in the radioactive impact on the environment and human health due to its long-range atmospheric transport. ¹²⁹I, as one of the radioisotopes of iodine, can ideally trace the long-distance transport and deposition of radioactive gaseous iodine due to its high volatility and unique source term. This work investigated the vertical distribution of iodine isotopes in soils, air bio-monitors (mosses and pine needles) and seasonal vegetation (wormwoods) along an elevation gradient on Mount Taibai, the highest peak on the eastern edge of the Tibet Plateau in mainland China. A distinct triple-peak pattern of ¹²⁷I and ¹²⁹I concentrations, with pronounced enrichment at 3000 m, was observed across all sample types. Air mass trajectory analysis revealed that the elevated ¹²⁹I levels at high altitudes originate from transcontinental transport of volatile ¹²⁹I via the westerlies and heavy wet deposition, while the higher ¹²⁷I at low elevation is attributed to the release of ¹²⁷I from arid inland of Asia. In addition to the long-range transport of high ¹²⁹I concentration air masses, the unique mountain climate intensifies ¹²⁹I deposition and retention at high elevations through the combined effects of efficient scavenging by orographic precipitation, significant litterfall input, and high organic matter retention. This research highlights the critical role of mountainous regions as effective sinks for transboundary radioactive pollutants and their importance in radiological risk assessment and nuclear emergency preparedness.