Dependence of column ozone on future ODSs and GHGs in the variability of 500-ensemble members.

State-of-the-art chemistry-climate models (CCMs) have indicated that a future decrease in ozone-depleting substances (ODSs) combined with an increase in greenhouse gases (GHGs) would increase the column ozone amount in most regions except the tropics and Antarctic. However, large Arctic ozone losses have occurred at a frequency of approximately once per decade since the 1990s (1997, 2011 and 2020), despite the ODS concentration peaking in the mid-1990s. To understand this, CCMs were used to conduct 24 experiments with ODS and GHG concentrations set based on predicted values for future years; each experiment consisted of 500-member ensembles. The 50 ensemble members with the lowest column ozone in the mid- and high latitudes of the Northern Hemisphere showed a clear ODS dependence associated with low temperatures and a strong westerly zonal mean zonal wind. Even with high GHG concentrations, several ensemble members showed extremely low spring column ozone in the Arctic when ODS concentration remained above the 1980-1985 level. Hence, ODS concentrations should be reduced to avoid large ozone losses in the presence of a stable Arctic polar vortex. The average of the lowest 50 members indicates that GHG increase towards the end of the twenty-first century will not cause worse Arctic ozone depletion.

Validity of the source term for the Fukushima Dai-ichi Nuclear Power Station accident estimated using local-scale atmospheric dispersion simulations to reproduce the large-scale atmospheric dispersion of 137Cs.

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.

Refinement of source term and atmospheric dispersion simulations of radionuclides during the Fukushima Daiichi Nuclear Power Station accident.

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.