Episode Analysis of Deposition of Radiocesium from the Fukushima Daiichi Nuclear Power Plant Accident

Chemical transport models played key roles in understanding the atmospheric behaviors and deposition patterns of radioactive materials emitted from the Fukushima Daiichi nuclear power plant after the nuclear accident that accompanied the great Tohoku earthquake and tsunami on 11 March 2011. However, model results could not be sufficiently evaluated because of limited observational data. We assess the model performance to simulate the deposition patterns of radiocesium (¹³⁷Cs) by making use of airborne monitoring survey data for the first time. We conducted ten sensitivity simulations to evaluate the atmospheric model uncertainties associated with key model settings including emission data and wet deposition modules. We found that simulation using emissions estimated with a regional-scale (∼500 km) model better reproduced the observed ¹³⁷Cs deposition pattern in eastern Japan than simulation using emissions estimated with local-scale (∼50 km) or global-scale models. In addition, simulation using a process-based wet deposition module reproduced the observations well, whereas simulation using scavenging coefficients showed large uncertainties associated with empirical parameters. The best-available simulation reproduced the observed ¹³⁷Cs deposition rates in high-deposition areas (≥10 kBq m^–2) within 1 order of magnitude and showed that deposition of radiocesium over land occurred predominantly during 15–16, 20–23, and 30–31 March 2011

Source–Receptor Relationship Analysis of the Atmospheric Deposition of PAHs Subject to Long-Range Transport in Northeast Asia

The source–receptor relationship analysis of PAH deposition in Northeast Asia was investigated using an Eulerian regional-scale aerosol chemical transport model. Dry deposition (DD) of PAH was controlled by wind flow patterns, whereas wet deposition (WD) depended on precipitation in addition to wind flow patterns. The contribution of WD was approximately 50–90% of the total deposition, except during winter in Northern China (NCHN) and Eastern Russia (ERUS) because of the low amount of precipitation. The amount of PAH deposition showed clear seasonal variation and was high in winter and low in summer in downwind (South Korea, Japan) and oceanic-receptor regions. In the downwind region, the contributions from NCHN (WD 28–52%; DD 54–55%) and Central China (CCHN) (WD 43–65%; DD 33–38%) were large in winter, whereas self-contributions (WD 20–51%; DD 79–81%) were relatively high in summer. In the oceanic-receptor region, the deposition amount decreased with distance from the Asian continent. The amount of DD was strongly influenced by emissions from neighboring domains. The contributions of WD from NCHN (16–20%) and CCHN (28–35%) were large. The large contributions from China in summer to the downwind region were linked to vertical transport of PAHs over the Asian continent associated with convection

Sensitivities of Simulated Source Contributions and Health Impacts of PM_2.5 to Aerosol Models

Chemical transport models are useful tools for evaluating source contributions and health impacts of PM_2.5 in the atmosphere. We recently found that concentrations of PM_2.5 compounds over Japan were much better reproduced by a volatility basis set model with an enhanced dry deposition velocity of HNO₃ and NH₃ compared with a two-product yield model. In this study, we evaluated the sensitivities to organic aerosol models of the simulated source contributions to PM_2.5 concentrations and of PM_2.5-related mortality. Overall, the simulated source contributions to PM_2.5 were similar between the two models. However, because of the improvements associated with the volatility basis set model, the contributions of ammonia sources decreased, particularly in winter and spring, and contributions of biogenic and stationary evaporative sources increased in spring and summer. The improved model estimated that emission sources in Japan contributed 35%–48% of the PM_2.5-related mortality in Japan. These values were higher than the domestic contributions to average PM_2.5 concentrations in Japan (26%–33%) because the domestic contributions were higher in higher population areas. These results indicate that control of both domestic and foreign emissions is necessary to reduce health impacts due to PM_2.5 in Japan

Emission and Atmospheric Transport of Particulate PAHs in Northeast Asia

The emission, concentration levels, and transboundary transport of particulate polycyclic aromatic hydrocarbons (PAHs) in Northeast Asia were investigated using particulate PAH measurements, the newly developed emission inventory (Regional Emission inventory in ASia for Persistent Organic Pollutants version, REAS-POP), and the chemical transport model (Regional Air Quality Model ver2 for POPs version, RAQM2-POP). The simulated concentrations of the nine particulate PAHs agreed well with the measured concentrations, and the results firmly established the efficacy of REAS/RAQM2-POP. It was found that the PAH concentrations in Beijing (China, source region), which were emitted predominantly from domestic coal, domestic biofuel, and other transformations of coal (including coke production), were approximately 2 orders of magnitude greater than those monitored at Noto (Japan, leeward region). In Noto, the PAH concentrations showed seasonal variations; the PAH concentrations were high from winter to spring due to contributions from domestic coal, domestic biofuel, and other transformations of coal, and low in summer. In summer, these contribution were decrease, instead, other sources, such as the on-road mobile source, were relatively increased compared with those in winter. These seasonal variations were due to seasonal variations in emissions from China, as well as transboundary transport across the Asian continent associated with meteorological conditions

Mapping the relative risk of surface water acidification based on cumulative acid deposition over the past 25 years in Japan

<p>Sensitivity maps of atmospheric acid deposition in Japan have not been updated in 20 years. Here, we propose new relative risk maps of surface water acidification in forests based on a weighted overlay of cumulative potential acid deposition (CPAD) simulated for a 25-year period (1981–2005), including the sensitivities of soil and bedrock to acidification. We assumed that relative acidification risk is high in areas that exhibit high sensitivities of soil and bedrock to acid and have received a large amount of cumulative acid deposition over the past several decades. We aggregated fine soil and bedrock maps into a 20-km mesh for overlay onto an 80-km mesh map of CPAD by considering their spatial structures in Japan. Allocation of the weights among CPAD and soil and bedrock sensitivities was performed based on observational trends in river pH over the past 30 years. The resulting risk map for surface water acidification showed that large areas of western Japan, as well as smaller areas of Hokkaido, Tohoku, Kanto, and Kyushu, are at high risk of surface water acidification. Seventy-seven percent of all rivers for which a declining trend in pH was observed from 2001 to 2009 were also high-risk areas. Acid deposition might be one factor controlling surface water acidification in areas with high bedrock sensitivity, in addition to high CPAD and soil sensitivity, although the risk of soil acidification remains unclear.</p