Development of a global transport model for airborne iodine-129 including atmospheric photolysis and gas-particle conversion processes

Iodine-129 (129I) has been shown as a useful isotope for dating of water, tracing of marine sediments and investigating the geochemical cycle of iodine. Main sources of atmospheric 129I are volatilization from ocean and discharge from nuclear fuel reprocessing facilities. Although released 129I is globally transported in the atmosphere and deposited to the Earth's surface, spatial and temporal distributions of atmospheric 129I are still not well understood. In this study, we developed an atmospheric global transport model of 129I which includes the processes such as advection and turbulent diffusion, dry and wet deposition, discharge from nuclear fuel reprocessing facility, volatilization from ocean and atmospheric chemical reactions (atmospheric photolysis and gas-particle conversion). Input meteorological fields of three-dimensional components of wind, air temperature, atmospheric pressure, and turbulent diffusion coefficient were calculated using WRF (Weather Research and Forecasting) with ERA-Interim dataset. The simulation period was set to be from 1 Jan 2006 to 31 Dec 2010. For model validation, we used air concentration and deposition of gaseous and particulate forms of 129I measured at Rokkasho in Japan from 2006 to 2010 and past measurements of 129I concentration in rain water in Europe, Asia, and North America. The model successfully reproduced the seasonal variations of measured air concentration and deposition of 129I at Rokkasho as maximum and minimum values during the wintertime and summertime, respectively. Furthermore, spatial patterns of simulated 129I concentration in globe were similar to those of measurements. In the presentation, key factors in controlling the spatiotemporal distribution of airborne 129I and its cycle in the atmosphere suggested by model results will be discussed.