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The behaviors of atmospheric 129I and the global 129I cycle remain incompletely understood because the spatiotemporal resolution of monitoring is insufficient, and few measurement-based models have been reported. This study aims to quantitatively understand the global 129I budget. When quantifying, we conducted global atmospheric 129I dispersion simulations covering from the period 2007 to 2010. To achieve this goal, the present study newly incorporates the iodine chemical processes of two gas-phase chemical reactions, six photolysis reactions, and two heterogeneous reactions into an existing atmospheric 129I transport model (GEARN–FDM). In addition to the aerial release of 129I from nuclear fuel reprocessing facilities, the model includes the volatilization processes of 129I compounds from the Earth’s surface. The net 129I exchange fluxes from the atmosphere to the Earth’s oceans and land were estimated to be 5.3 GBq/y and 18.0 GBq/y, respectively. The global 129I emission from the oceans was estimated as 7.2 GBq/y, and nearly half of the emission totals are emitted from the English Channel (3.2 GBq/y). In addition, the global 129I emissions from land are estimated to be 1.7 GBq/y. The remarkable 129I emission levels from land mainly appear in Europe, Russia, and North America, and the emission distribution is impacted by the activities of past and ongoing nuclear fuel reprocessing facilities. The total 129I emissions from the oceans and land are lower than the 129I emissions from the model-included nuclear fuel reprocessing facilities (23.3 GBq/y), and show that the aerial releases from the nuclear fuel reprocessing facilities in operation are still important 129I sources.

The long-lived radioactive iodine ($^{129}$I) is a useful geochemical tracer in the atmospheric environment. We recently observed clear seasonal trends in air concentration and deposition of $^{129}$I in Japan. Using these data, we developed a global atmospheric $^{129}$I transport model to reveal key processes for the global atmospheric $^{129}$I cycle. The model generally reproduced the observed seasonal change in air concentration and deposition of $^{129}$I in Japan, and the global distribution of $^{129}$I concentration in rain as presented in past literature. Numerical experiments changing the intensity of anthropogenic and natural sources were conducted to quantify the impact of anthropogenic sources on the global $^{129}$I cycle. The results indicated that the atmospheric $^{129}$I from the anthropogenic sources was deposited in winter and can be accumulated mainly in the northern part of Eurasia. In contrast, the atmospheric $^{129}$I from the natural sources dominated the deposition in summer. These results suggested that the re-emission process of $^{129}$I from the Earth's surface may be important as a secondary impact of $^{129}$I in the global-scaled environment. Furthermore, although wet deposition dominated the total deposition in the Northern hemisphere, dry deposition regionally and seasonally contributed to the total deposition over arctic and northern part of Eurasia in winter, suggesting that the dry deposition may play a key role in the seasonal change of $^{129}$I deposition in the Northern hemisphere high latitudes., 長寿命放射性ヨウ素($^{129}$I)は、大気環境における放射性核種の有用な地球化学トレーサである。本研究では、$^{129}$Iの大気濃度および沈着の観測を実施し、観測データから大気濃度および沈着の明瞭な季節変動を得た。さらに、大気中の$^{129}$I循環を支配する要因を明らかにすることを目的として、得られた観測データを用いて、移流、乱流拡散、大気沈着、光化学、ガス粒子変換、核燃料再処理工場からの$^{129}$Iの排出、海洋および陸域からの$^{129}$Iの揮発の各物理・化学過程を考慮した全球ヨウ素輸送モデルを開発した。全球ヨウ素輸送モデルは、我々が観測した$^{129}$Iの大気濃度および沈着の季節変動、そして既往文献の$^{129}$Iの降水中濃度の全球分布を良好に再現した。開発した全球ヨウ素輸送モデルを用いて人為起源と自然起源の$^{129}$Iインベントリの強度を変化させる数値実験を実施し、地球全体の$^{129}$I循環に対する人為起源の$^{129}$Iの影響を評価した。その結果、冬季においては、人為起源の$^{129}$Iが主にユーラシアの北部に沈着する可能性があることが示された。一方で、夏季においては、自然起源の$^{129}$Iが北半球中高緯度の沈着に支配的であった。これらの結果は、地球表面からの$^{129}$Iの再飛散過程が全球規模での$^{129}$I循環に重要であることを示唆している。さらに、冬季のユーラシア北部や北極域においては局所的に乾性沈着が寄与しており、乾性沈着が環境中の$^{129}$Iの季節変化に重要な影響を及ぼすことが示唆された。

The source term of 137Cs from the Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident was estimated from the results of local-scale atmospheric dispersion simulations and measurements. To confirm the source term's validity for reproducing the large-scale atmospheric dispersion of 137Cs, this study conducted hemispheric-scale atmospheric and oceanic dispersion simulations. In the dispersion simulations, the atmospheric-dispersion database system Worldwide version of System for Prediction of Environmental Emergency Dose Information (WSPEEDI)-DB and oceanic dispersion model SEA-GEARN-FDM that were developed by the Japan Atomic Energy Agency were used. Compared with the air concentrations of 137Cs measured by the Comprehensive Nuclear-Test-Ban Treaty Organization, overall, the WSPEEDI-DB simulations well reproduced the measurements, whereas the simulation results partly overestimated some measurements. Furthermore, the validity of the deposition of 137Cs by WSPEEDI-DB was investigated using SEA-GEARN-FDM and concentrations of 137Cs in seawater sampled from the North Pacific. Seawater concentrations of 137Cs by the oceanic dispersion simulation, in which the deposition flux of 137Cs by WSPEEDI-DB was used as input from the atmosphere to oceans, were statistically consistent to the measurement. However, the simulated seawater concentrations of 137Cs were underestimated regionally in the North Pacific. Both the overestimation of air concentrations and underestimation of seawater concentrations could be attributed to the less amounts of 137Cs deposition by less precipitation over the North Pacific. The overestimation and underestimation could be improved without contradiction between the air and seawater concentrations of 137Cs using more realistic precipitation in atmospheric dispersion simulations. This shows that the source term validated in this study could reproduce the spatiotemporal distribution of 137Cs from the FDNPS accident in both local and large-scale atmospheric dispersion simulations.

To assess the radiological dose to the public resulting from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in Japan, especially for the early phase of the accident when no measured data are available for that purpose, the spatial and temporal distributions of radioactive materials in the environment need to be reconstructed through computer simulations using the atmospheric transport, dispersion, and deposition model (ATDM). For the ATDM simulation, the source term of radioactive materials discharged into the atmosphere is essential and has been estimated in many studies. In the present study, we further refined the source term estimated in our previous study and improved the ATDM simulation with an optimization method based on Bayesian inference, which used various measurements such as air concentration, surface deposition, fallout, and newly released hourly air concentrations of 137Cs derived by analyzing suspended particulate matter (SPM) collected at air pollution monitoring stations. This optimization improved not only the source term but also the wind field in meteorological calculation, which led to the reduction of discrepancies in plume passage time at monitoring points to less than 3 h between calculations and measurements, by feeding back comparison results between the dispersion calculations and measurements of radionuclides. As a result, the total amounts of 137Cs and 131I by the present study became 1.0 × 1016 and 1.2 × 1017 Bq, respectively, and decreased by 29% and 20%, respectively, in comparison with those by previous study. The ATDM simulation successfully reproduced both the air concentrations at SPM monitoring points and surface depositions by airborne monitoring. FA10 for total samples of air concentrations of 137Cs at SPM monitoring points increased from 35.9% by the previous study to 47.3%. The deposition amount on the land decreased from 3.7 × 1015 Bq by the previous study to 2.1 × 1015 Bq, which was close to the measured amount of 2.4 × 1015 Bq. We also constructed the spatiotemporal distribution of some major radionuclides in the air and on the surface (optimized dispersion database) by using the optimized release rates and ATDM simulations. The optimized dispersion database can be used for comprehensive dose assessment in tandem with behavioral patterns of evacuees from the FDNPS accident by collaborating research group in the Japanese dose assessment project. The improvements in the present study lead to the refinement of the dose estimation.