An estimate of the global budget of atmospheric iodine-129 during the 2000s with a global transport model

Since the atomic era has started in the 20^th century, a long-lived radioactive iodine (¹²⁹I) has been continuously released into the atmosphere via human nuclear activities. Airborne ¹²⁹I compounds behave as gaseous and particulate forms, and the air concentration of ¹²⁹I compounds is determined by transport, deposition, chemistry and sources. However, the behavior of atmospheric ¹²⁹I and its global cycle have never been fully understood. In the present study, we newly introduced chemistry processes of two gas-phase chemical reactions, six photolytic reactions and six heterogeneous reactions into ¹²⁹I transport model GEARN-FDM that is validated in our previous study, and quantitively evaluated global emission and deposition for ¹²⁹I during the 2000s. In addition to aerial released ¹²⁹I from nuclear fuel reprocessing facilities, as a ¹²⁹I source, we used volatilization of ¹²⁹I from terrestrial and ocean, which were calculated from flux of stable iodine (¹²⁷I) emission and atomic ratio of ¹²⁹I/¹²⁷I, respectively. From the simulation result, the oceanic ¹²⁹I emission flux was estimated to be 7.4 GBq/y. In particular, the English Channel was the largest marine source of ¹²⁹I. In addition, we estimated the ¹²⁹I emission flux totals from typical terrestrial sources of environmental iodine (rice paddy, biomass burning, wood fuel) to be 0.6 GBq/y, in which the largest emission was rice paddy (0.3 GBq/y). This result shows that the impact of the oceanic emission on global budget of ¹²⁹I is currently larger than the above-mentioned terrestrial emission. Meanwhile, the current flux of aerial released ¹²⁹I from nuclear fuel reprocessing facilities (34.2 GBq/y) is larger than the fluxes from ocean and terrestrial, showing that aerial released ¹²⁹I from the facilities in operation dominates the source of ¹²⁹I. The airborne ¹²⁹I emitted from the facilities was mainly deposited nearby the facilities due to dry deposition, and the deposition amount totals was estimated to be 26.2 GBq/y. The ¹²⁹I emission flux from the deposition areas via volatilization was approximated to be 1.4 GBq/y, which is larger than the ¹²⁹I emission flux totals from the other terrestrial sources. This suggests that the ¹²⁹I emission from the vicinity of the facilities would be a dominant land-based source of ¹²⁹I.