Yutaka Kanai

Model study on particle size segregation and deposition during Asian dust events in March 2002

[1] A size-segregated aerosol model that includes most of the major physical processes (generation, transport, and dry and wet deposition) is developed. This model is coupled with a Regional Air Quality Model (RAQM) and is applied to simulate Asian dust storms during the 10-day period of 15-24 March 2002. A nonhydrostatic mesoscale model (MM5) is used to provide meteorological fields. Model results are verified by available observational data including surface weather observations and size-segregated particle concentrations. The validation demonstrates a good capability of this model system in capturing most of the key features of dust evolution and reproducing the particle mass size distribution along the transport pathway of soil dust. An apparent feature has been both observed and reproduced by the model, showing a shift of size range with peak mass concentration from coarse mode to finer mode on the pathway from source regions to distant downwind areas. The maximum dust concentration averaged over 10 days is simulated to be 3000 μg m over the southern China-Mongolia border. Total dry deposition of soil dust for 10 days is up to 30 g m -2 in the Gobi desert along the southern China-Mongolia border. Distribution and magnitude of particle deposition are strongly dependent on both concentration and size-segregated dry deposition velocity and scavenging rate. While dry deposition dominates the removal of dust particles in or in the vicinity of source regions, the influence of wet deposition increases along the transport pathway of soil dust, with high removal efficiency for coarser particles (>2 μm) and very low efficiency for particles in the accumulation mode. Of the total dust emission (43.2 megatons), about 71% is redeposited onto the underlying surface by the dry deposition process, 6% is removed by the wet deposition process, and the remaining 23% is suspended in the atmosphere or subject to long-range transport.

Time trends of perfluorinated compounds from the sediment core of Tokyo Bay, Japan (1950s-2004).

Perfluorinated compounds (PFCs) were detected in sediment core samples collected in Tokyo Bay to reveal their time trends. The core sample deposited during 1950s-2004 was divided into two- to three-year intervals and the concentrations of 24 types of PFCs were determined. Perfluorooctane sulfonate (PFOS) decreased gradually from the early 1990s and its precursor decreased rapidly in the late 1990s, whereas perfluorooctanoic acid (PFOA) increased rapidly. The observed trends were regarded as a reflection of the shift from perfluorooctyl sulfonyl fluoride (PFOSF)-based products to telomer-based products after the phaseout of PFOSF-based products in 2001. The branched isomers of perfluoroundecanoic acid (PFUnDA) and perfluorotridecanoic acid (PFTrDA) were detected in the sample with its ratio of linear-isomer/branched-isomer concentrations decreasing. In this study, we revealed that the sediment core can serve as a tool for reconstructing the past pollution trend of PFCs and can provide interesting evidence concerning their environmental dynamics and time trend.

Monitoring of aerosols in Tsukuba after Fukushima Nuclear Power Plant incident in 2011.

Artificial radionuclides were released into the atmosphere by the Fukushima Dai-ichi Nuclear Power Plant incident after a strong earthquake on 11 March 2011. Aerosol monitoring at the Geological Survey of Japan, Tsukuba, was started 20 d after the incident. Radionuclides such as (99)Mo/(99m)Tc, (132)Te/(132)I, (129 m)Te/(129)Te, (131)I, (137)Cs, (136)Cs, (134)Cs, (140)Ba/(140)La, (110 m)Ag, and (95)Nb were observed and, with the exception of (137)Cs and (134)Cs, these radionuclides decreased to below the limit of detection in the middle of June. The activity ratio of atmospheric (134)Cs/(137)Cs in aerosols decreased over time almost following physical decays. Therefore, the (134)Cs/(137)Cs activity ratio in the averaged air mass in this study could be regarded as homogeneous although those of several reactors in the Nuclear Power Plant were not ascertained. A further research on the released (137)Cs and (134)Cs would be necessary for the sedimentology of lake sediment.

Heterogeneous distribution of radiocesium in aerosols, soil and particulate matters emitted by the Fukushima Daiichi Nuclear Power Plant accident: retention of micro-scale heterogeneity during the migration of radiocesium from the air into ground and river systems

We analyzed 137Cs in aerosols, rock, soil and river suspended sediment collected after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Based on the results, we discuss the post-event behavior and transportation of radiocesium in the environment from the air into ground and river systems. First, radionuclides were emitted from the FDNPP as airborne ‘hot’ particles, which contained water-soluble fractions of radiocesium. Radiocesium was still present in a water-soluble fraction after deposition on the ground. Subsequent interaction of the ‘hot’ particles with water (e.g. rainfall) dissolved and strongly fixed the radiocesium on rock and soil particles, thus changing the radiocesium into insoluble forms. The distribution of ‘hot spots’ was possibly controlled by the initial position of deposition on the ground. Consequently, ‘hot spots’ were studded on the rock surface rather than being uniformly distributed. The distribution of radiocesium in river suspended particles was not homogeneous during water transportation, reflecting the heterogeneity of radiocesium in rock and soil. Leaching experiments demonstrated that radiocesium in rock, soil and river suspended sediment was fairly insoluble, showing that the adsorption reaction is irreversible. The micro-scale heterogeneous distribution of radiocesium in aerosols, soil and suspended particles was due to the presence of ‘hot’ particles in aerosols. Dissolution of radiocesium in the ‘hot’ particles in the aerosols and subsequent irreversible adsorption onto the soil particle complex are responsible for the preservation of the heterogeneity both in soil and in river suspended particles.

Seasonal characterization of dust days, mass concentration and dry deposition of atmospheric aerosols over qingdao, china

The seasonal characterization of dust days, mass concentration and dry deposition of atmospheric aerosols were investigated using the historical data of dust days observed over Qingdao during the period from 1961 to 2001 and ground-based aerosol sampling data collected in the period from May 2001 to November 2002. In Qingdao most of the dust days occurred in spring and in winter and no dust days existed in summer. The seasonal variation of the uplifting dust day over Qingdao was in phase with that analyzed over North China. The mean mass concentration of the total suspended particles (TSP) in spring, summer, autumn and winter was respectively 300, 110, 113 and 185 μg·m−3. The values and phase of the seasonal oscillation for fine particles were very similar to those for coarse particles except for the month March, during which the concentration of the coarse particles was about 4 times as high as that of the fine particles. Comparison between seasonal variation of mass concentration of aerosols and dust days indicated that high frequency of uplifting dust day was accompanied by high TSP mass concentration. The TSP mass concentration measured by the low-volume instrument was about 30% lower than that measured by high-volume instrument, though the two data sets are highly correlated (correlation coefficient=0.89). The dry deposition flux during the observation period from May 2001 to November 2002 ranged from 0.06 to 0.2 g·m−2·d−1 with a mean value of 0.13 g·m−2·d−1, which was about 1/3 as that over Beijing.

Retrospective analysis by data processing tools for comprehensive two-dimensional gas chromatography coupled to high resolution time-of-flight mass spectrometry: A challenge for matrix-rich sediment core sample from Tokyo Bay

Data processing tools for non-target analysis using comprehensive two-dimensional gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC×GC-HRTOFMS) were developed and applied to a sediment core in Tokyo Bay, focusing on chlorinated compounds in this study. The processing tools were classified in two different methods: (1) the consecutive use of mass defect filter followed by artificial neutral loss scan (MDF/artificial NLS) as a qualitative non-target screening method and (2) Entire Domain Combined Spectra Extraction and Integration Program (ComSpec) and two-dimensional peak sentinel (T-SEN) as a semi-quantitative target screening method. MDF/artificial NLS as a non-target screening approach revealed that PCBs, followed by octachlorodibenzo dioxin (OCDD), were the main chlorinated compounds present in all sediment layers. Furthermore, unknown peaks thought to be chlorinated compounds were found in increasing numbers, some in increasing amounts. T-SEN and ComSpec as a target screening approach were adapted for automatic semi-quantitative analysis showed that, in decreasing concentration order, PCBs, OCDD, and dichlorodiphenyltrichloroethane and its metabolites (DDEs, DDDs) were the main chlorinated pollutants in the sediments. The complementary use of both techniques allows us to extract significant chlorinated pollutants, including non-targeted compounds. This retrospective analysis by this approach performed well even on matrix-rich sediment samples and provided us an interesting insight of historical trends of pollution in Tokyo Bay.

Characterization and geochemical behaviour of uranium series nuclides in sandy sediments at Kanamaru area, Yamagata and Niigata Prefectures, Japan

Uranium series nuclides in the Tertiary arkosic sands from a high rainfall area in Japan were determined and their characteristics and behaviour were studied. Uranium was concentrated in the upper (c. 241 ppm) and lower (c. 581 ppm) layers although no specific U mineral was observed. The filtration experiment showed that U in the groundwater is in the dissolved form rather than colloidal or particulate forms. Most U in the concentrated layers was in easily soluble fractions (such as AcONa/AcOH soluble) while the U content in the water around the site was low. The fact that the 230Th/234U activity ratios of a sample in the upper layer were much greater than unity suggests that U migration occurred within 0.3 Ma. However, the 234U/238U and 230Th/234U activity ratios of samples in the lower layer were close to unity, suggesting that the material has not been strongly leached. Using chemical leaching techniques, a small disequilibrium among the fractions is observed, even though the bulk sample was near equilibrium. It is inferred that sorption of U is more effective than precipitation in this area.

Geochemical behavior and activity ratios of Fukushima-derived radionuclides in aerosols at the Geological Survey of Japan, Tsukuba, Japan

Aerosols from the Fukushima Dai-ichi Nuclear Power Plant accident were collected at the Geological Survey of Japan. The temporal patterns of the 134Cs/137Cs, 136Cs/137Cs, and 132Te/137Cs activity ratios were consistent with expectations based on their half-lives. Those for 131I and 129mTe decreased with some fluctuations. 110mAg/137Cs declined more rapidly than expected and 95Nb/137Cs increased with time. The results suggest that the temporal patterns depended not only on their half-lives but also on many factors such as chemical speciation, the diversity of sources, changes in the temperature and discharge rate from the reactors, and the processes of transport.

Uranium redistribution implied by 234U/238U disequilibrium study on apatite-bearing sedimentary rocks at Nakamaruke district, central Japan

Abstract Diagenetic behaviour of uranium in sedimentary rocks associated with apatite deposits at Nakamaruke, central Japan, has been studied to examine the processes of redistribution of uranium under the mild climate of the temperature zone. Uranium content (4–89 ppm) in the strata showed two peaks, the most prominent one in the phosphatic layer and another small one in the lower part of the outcrop, which suggests some amount of uranium has vertically migrated through the strata. 234U/238U activity ratios (ARs) of total U were 1 in the lower part (>0.3 m away from the phosphatic layer) of the outcrop. “U(IV)”, “U(VI)” and “residue” fractions were obtained, about 50% of total U was in “U(VI) fraction”. The ARs of “U(VI) fraction” were >1 in all the samples, so hexavalent U (U(VI)) is inferred to be bound relatively weakly and easily moved. The ARs of “U(IV) fraction” and “residue fraction” showed the same trend as total U. The ARs of the “residue fraction” composed of silicate and clay minerals suggest that some uranium moved and was adsorbed by phyllosilicates.

Characterization of U series nuclides in geological materials by selective leaching method

Geological and geochemical events were elucidated in by using U series nuclides in combination with chemical leaching technique. Two examples were examined: (1) in the apatite-bearing sediment column samples U(VI) is moving, while U(IV) in the samples is almost retained, and (2) the redistribution of U occurred along the column under the monsoon climate. On the other hand, U in the conglomerate sample is divided into 5 categories by the leaching technique, which are related with exchangeable, carbonate, iron oxide, organic/sulfide and silicate components, respectively. Uranium is shown to be abundant in the carbonate and iron rich fractions of conglomerate sediment. Appropriate models are proposed in each case study.