A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean by two major pathways: direct release from the accident site and atmospheric deposition. A 1 yr, regional-scale simulation of ¹³⁷Cs activity in the ocean offshore of Fukushima was carried out, the sources of radioactivity being direct release, atmospheric deposition, and the inflow of ¹³⁷Cs deposited into the ocean by atmospheric deposition outside the domain of the model. Direct releases of ¹³⁷Cs were estimated for 1 yr after the accident by comparing simulated results and measured activities adjacent to the accident site. The contributions of each source were estimated by analysis of ¹³¹I/¹³⁷Cs and ¹³⁴Cs/¹³⁷Cs activity ratios and comparisons between simulated results and measured activities of ¹³⁷Cs. The estimated total amounts of directly released ¹³¹I, ¹³⁷Cs, and ¹³⁷Cs were 11.1 ± 2.2 PBq, 3.5 ± 0.7 PBq, and 3.6 ± 0.7 PBq, respectively. Simulated ¹³⁷Cs activities attributable to direct release were in good agreement with measured ¹³⁷Cs activities not only adjacent to the accident site, but also in a wide area in the model domain, therefore this implies that the estimated direct release rate was reasonable. Employment of improved nudging data by JCOPE2 improved both the offshore transport result and the reproducibility of ¹³⁷Cs activities 30 km offshore. On the other hand, simulated ¹³⁷Cs activities attributable to atmospheric deposition were low compared to measured activities. The rate of atmospheric deposition into the ocean was underestimated because of a lack of measurements of deposition into the ocean when atmospheric deposition rates were being estimated. Simulated ¹³⁷Cs activities attributable to the inflow of ¹³⁷Cs deposited into the ocean outside the domain of the model were in good agreement with measured activities in the open ocean within the model domain after June 2012. The consideration of inflow is important to simulate the ¹³⁷Cs activity in this model region in the later period of the simulation. The contribution of inflow increased with time and was dominant (more than 99%) by the end of February 2012. The activity of directly released ¹³⁷Cs, however, decreased exponentially with time and was detectable only in the coastal zone by the end of February 2012.

Acidifying species in precipitation can have severe impacts on ecosystems. The chemical composition of precipitation is directly related to the amount of precipitation; accordingly, it is difficult to identify long-term variation in chemical concentrations. The ratio of the nitrate (NO₃⁻) to non-sea-salt sulfate (nss-SO₄²⁻) concentration in precipitation on an equivalent basis (hereinafter, Ratio) is a useful index to investigate the relative contributions of these acidifying species. To identify the long-term record of acidifying species in precipitation over East Asia, the region with the highest emissions worldwide, we compiled ground-based observations of the chemical composition of precipitation over China, Korea, and Japan from 2001 to 2015 based on the Acid Deposition Monitoring Network in East Asia (EANET). The spatial coverage was limited, but additional monitoring data for Japan, southern China, and northern China around Beijing were utilized. The period of analysis was divided into three phases: Phase I (2001–2005), Phase II (2006–2010), and Phase III (2011–2015). The behaviors of NO₃⁻ and nss-SO₄²⁻ concentrations and hence the Ratio in precipitation were related to these precursors. The anthropogenic NO_x and SO₂ emissions and the NO_x ∕ SO₂ emission ratio were analyzed. Further, satellite observations of the NO₂ and SO₂ column density to capture the variation in emissions were applied. We found that the long-term trend in the NO₃⁻ concentration in precipitation was not related to the variation in NO_x emission and the NO₂ column. In comparison, the nss-SO₄²⁻ concentration in precipitation over China, Korea, and Japan was partially connected to the changes in SO₂ emissions from China, but the trends were not significant. The long-term trends of Ratio over China, Korea, and Japan were nearly flat during Phase I, increased significantly during Phase II, and were essentially flat again during Phase III. This variation in Ratio in East Asia clearly corresponded to the NO_x ∕ SO₂ emission ratio and the NO₂ ∕ SO₂ column ratio in China. The initial flat trend during Phase I was due to increases in both NO_x and SO₂ emissions in China, the significantly increasing trend during Phase II was triggered by the increase in NO_x emissions and decrease in SO₂ emissions in China, and the return to a flat trend during Phase III was caused by declines in both NO_x and SO₂ emissions in China. These results suggest that emissions in China had a significant impact not only on China but also on downwind precipitation chemistry during the 15-year period of 2001–2015. In terms of wet deposition, the NO₃⁻ wet deposition over China, Korea, and Japan did not change dramatically, but the nss-SO₄²⁻ wet deposition declined over China, Korea, and Japan from Phase II to III. These declines were caused by a strong decrease in the nss-SO₄²⁻ concentration in precipitation accompanied by a reduction in SO₂ emission from China, which counteracted the increase in precipitation. These findings indicated that the acidity of precipitation shifted from sulfur to nitrogen.

The inter-comparison of regional air quality models is an effective way to understand uncertainty in ambient pollutant concentrations simulated using various model configurations, as well as to find ways to improve model performance. Based on the outcomes and experiences of Japanese projects thus far, a new model inter-comparison project called Japan’s study for reference air quality modeling (J-STREAM) has begun. The objective of J-STREAM is to establish reference air quality modeling for source apportionment and effective strategy making to suppress secondary air pollutants including PM_2.5 and photochemical ozone in Japan through model inter-comparison. The first phase focuses on understanding the ranges and limitations in ambient PM_2.5 and ozone concentrations simulated by participants using common input datasets. The second phase focuses on issues revealed in previous studies in simulating secondary inorganic aerosols, as well as on the three-dimensional characteristics of photochemical ozone as a new target. The third phase focuses on comparing source apportionments and sensitivities under heavy air pollution episodes simulated by participating models. Detailed understanding of model performance, uncertainty, and possible improvements to urban-scale air pollution involving secondary pollutants, as well as detailed sector-wise source apportionments over megacities in Japan are expected.

Emission source contributions of tropospheric ozone (O₃) were comprehensively investigated by using the higher‐order decoupled direct method (HDDM) for sensitivity analysis and the ozone source apportionment technology (OSAT) for mass balance analysis in the comprehensive air‐quality model with extensions (CAMx). The response of O₃ to emissions reductions at various levels in mainland China, Korea, and Japan were estimated and compared with results calculated by the brute force method (BFM) where one model parameter is varied at a time. Emissions were assessed at three receptor sites in Japan that experienced severe pollution events in May 2009. For emissions from China, HDDM assessed O₃ response with a bias of only up to 3 ppbv (a relative error of 4.5%) even for a 50% reduction but failed to assess a more extreme reduction. OSAT was reasonably accurate at 100% reduction, with a −4 ppbv (−7%) bias, but was less accurate at moderate ranges of reduction (∼50–70%). For emissions from Korea and Japan, HDDM captured the nonlinear response at all receptor sites and at all reduction levels to within 1% in all but one case; however, the bias of OSAT increased with the increasing reduction of emissions. One possible reason for this is that OSAT does not account for NO titration. To address this, a term for potential ozone (PO; O₃ and NO₂ together) was introduced. Using of PO instead of O₃ improved the performance of OSAT, especially for emissions reductions from Korea and Japan. The proposed approach with PO refined the OSAT results and did not degrade HDDM performance.