Yu Morino

Atmospheric behavior, deposition, and budget of radioactive materials from the Fukushima Daiichi nuclear power plant in March 2011

[1]xa0To understand the atmospheric behavior 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, we simulated the transport and deposition of iodine-131 and cesium-137 using a chemical transport model. The model roughly reproduced the observed temporal and spatial variations of deposition rates over 15 Japanese prefectures (60−400 km from the plant), including Tokyo, although there were some discrepancies between the simulated and observed rates. These discrepancies were likely due to uncertainties in the simulation of emission, transport, and deposition processes in the model. A budget analysis indicated that approximately 13% of iodine-131 and 22% of cesium-137 were deposited over land in Japan, and the rest was deposited over the ocean or transported out of the model domain (700 × 700 km2). Radioactivity budgets are sensitive to temporal emission patterns. Accurate estimation of emissions to the air is important for estimation of the atmospheric behavior of radionuclides and their subsequent behavior in land water, soil, vegetation, and the ocean.

Temporal variations of elemental carbon in Tokyo

[1]xa0Mass concentrations of elemental carbon (EC) in fine mode and mixing ratios of carbon monoxide (CO) were measured at the University of Tokyo campus in Tokyo in different seasons in 2003–2005. Measurements of EC were made using a semicontinuous thermal-optical analyzer. The mass concentrations of nonvolatile aerosol measured by the calibrated scanning mobility particle sizer combined with a heated inlet agreed with the independent EC measurements with a systematic difference of about 4%, demonstrating that the mass concentrations of nonvolatile aerosol well represent those for EC. A majority of the nonvolatile aerosol and therefore EC mass concentration was in volume equivalent diameters between 50 and 200 nm, peaking at around 130 nm. The correlation of EC and CO was generally compact throughout the measurement period because of the similarity in sources. The slope of the EC-CO correlation (ΔEC/ΔCO) is therefore a useful parameter in validating EC emission inventories. The EC concentration and ΔEC/ΔCO showed distinct diurnal variation. On weekdays, EC and ΔEC/ΔCO reached maximum values of about 3 μg m−3 and 9 ng m−3/parts per billion by volume, respectively, in the early morning (0400–0800 local time), when the traffic density of heavy-duty trucks with diesel engines was highest. In addition, these values were lower by a factor of 2 on Sundays. The heavy truck traffic showed similar diurnal and weekday/weekend variations, indicating that exhaust from diesel engines is an important source of EC. Monthly mean ΔEC/ΔCO showed a seasonal variation, reaching broad maximum values in spring-autumn and reaching minimum values in midwinter, following the seasonal variation in temperature, as observed in Maryland, United States (Chen et al., 2001). This temperature dependence is likely due to the temperature dependence of EC emissions from diesel engines on intake air temperature. More stringent regulation of emissions of particles from diesel cars started in the Tokyo Metropolitan Area in October 2003. The ΔEC/ΔCO values did not change, however, exceeding the natural variability (10%) after 1 year from the start of the new regulations, when the temperature dependence is taken into account. This indicates that the regulation of particle emissions in the Tokyo Metropolitan Area was not effective in reducing the EC concentrations after 1 year.

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 ((137)Cs) 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 (137)Cs 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 (137)Cs 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.

Impacts of biomass burning in Southeast Asia on ozone and reactive nitrogen over the western Pacific in spring

[1] Aircraft measurements of ozone (O3) and its precursors (reactive nitrogen, CO, nonmethane hydrocarbons) were made over the western Pacific during the Transport and Chemical Evolution Over the Pacific (TRACE-P) campaign, which was conducted during February–April 2001. Biomass burning activity was high over Southeast Asia (SEA) during this period (dry season), and convective activity over SEA frequently transported air from the boundary layer to the free troposphere, followed by eastward transport to the sampling region over the western Pacific south of 30� N. This data set allows for systematic investigations of the chemical and physical processes in the outflow from SEA. Methyl chloride (CH3Cl) and CO are chosen as primary and secondary tracers, respectively, to gauge the degree of the impact of emissions of trace species from biomass burning. Biomass burning is found to be a major source of reactive nitrogen (NOx, PAN, HNO3, and nitrate) and O3 in this region from correlations of these species with the tracers. Changes in the abundance of reactive nitrogen during upward transport are quantified from the altitude change of the slopes of the correlations of these species with CO. NOx decreased with altitude due to its oxidation to HNO3. On the other hand, PAN was conserved during transport from the lower to the middle troposphere, consistent with its low water solubility and chemical stability at low temperatures. Large losses of HNO3 and nitrate, which are highly water soluble, occurred in the free troposphere, most likely due to wet removal by precipitation. This has been shown to be the major pathway of NOy loss in the middle troposphere. Increases in the mixing ratios of O3 and its precursors due to biomass burning in SEA are estimated using the tracers. Enhancements of CO and total reactive nitrogen (NOy), which are directly emitted from biomass burning, were largest at 2–4 km. At this altitudetheincreasesinNOyandO3were810partspertrillionbyvolume(pptv)and26parts per billion by volume (ppbv) above their background values of 240 pptv and 31 ppbv, respectively. The slope of the O3-CO correlation in biomass burning plumes was similar to those observed in fire plumes in northern Australia, Africa, and Canada. The O3 production efficiency (OPE) derived from the O3-CO slope and NOx/CO emission ratio (ER) is shown to be positively correlated with the C2H4/NOx ER, indicating that the C2H4/NOx ER is a critical parameter in determining the OPE. Comparison of the net O3 flux across the western Pacific region and total O3 production due to biomass burning in

Partitioning of HNO3 and particulate nitrate over Tokyo : Effect of vertical mixing

[1]xa0Ground-based measurements of gas-phase nitric acid (HNO3) and particulate nitrate (NO3−) were performed in Tokyo during 2003–2004. These measurements provide a comprehensive data set for investigating the diurnal and seasonal variations of gas-phase HNO3 and particulate NO3− and the thermodynamic equilibrium of these compounds. HNO3 and NO3− have distinct diurnal and seasonal variations, especially in summer. This study shows that the thermodynamic equilibrium of HNO3 and NO3− and the production rate of total nitrate (TNO3 = HNO3 + NO3−) are the major controlling factors affecting these seasonal and diurnal variations. A thermodynamic equilibrium model (ISORROPIA) is newly coupled with a one-dimensional (1-D) model to take into account the effect of vertical mixing during daytime on the partitioning of HNO3 and NO3− by constraining the TNO3 concentrations to the observations. The 1-D model reproduces the NO3−/TNO3 ratios observed during daytime, whereas the equilibrium model significantly underestimates these ratios. The agreement between the observed and calculated NO3−/TNO3 ratios is improved over the observed temperature range (1°–34°C) and relative humidity (18–95%) by the 1-D model. These results suggest the importance of vertical mixing in determining HNO3-NO3− partitioning in the boundary layer. It is also found that the mass accommodation coefficient for HNO3 needs to be approximately 0.1 to explain the observed HNO3-NO3− partitioning at the surface.

Formation and transport of oxidized reactive nitrogen, ozone, and secondary organic aerosol in Tokyo

[1]xa0Measurements of the major reactive nitrogen species (NOy)i (NOx, peroxyacyl nitrates, HNO3, and particulate nitrate (NO3−)), total reactive nitrogen (NOy), volatile organic compounds, OH and HO2, and organic aerosol were made near the urban center of Tokyo in different seasons of 2003–2004 to study the processes involving oxidized forms of reactive nitrogen and O3. Generally, NOx constituted the dominant fraction of NOy throughout the seasons. The NOx/NOy and HNO3/NOy ratios were lowest and highest, respectively, in summer, owing to the seasonally high OH concentration. The fraction of NOy that remained in the atmosphere after emission (RNOy) decreased with the decrease in the NOx/NOy ratio in summer and fall. It is likely that the median seasonal-diurnal variations of Ox = O3 + NO2 were controlled by those of the background O3 levels, photochemical O3 formation, and vertical transport. Ox showed large increases during midday under stagnant conditions in mid-August 2004. Their in situ production rates calculated by a box model were too slow to explain the observed increases. The high Ox was likely due to the accumulation of Ox from previous days in the upper part of the boundary layer (BL) followed by transport down to near the surface by mixing after sunrise. Considering the tight correlation between Ox and secondary organic aerosol (SOA), it is likely that SOA also accumulated during the course of sea-land breeze circulation in the BL.

Significant alteration in the hygroscopic properties of urban aerosol particles by the secondary formation of organics

[1]xa0The temporal variation in the hygroscopicity of urban aerosol particles was investigated in Tokyo based on the hygroscopic growth factor (g) measurement in the summer of 2004. For 100 nm particles, sporadic appearance of intermediately hygroscopic (1.11 ≤ g < 1.29) particles was observed when the oxygenated organic mass increased (r = 0.76), suggesting that the formation of secondary organics governs the hygroscopicity of these particles. During a photochemically active period in the daytime, the mode hygroscopicity of 100 nm particles above g = 1.11 rapidly decreased in a few hours. This is explained by the condensation of secondary organics on pre-existing particles that are highly hygroscopic (g ≥ 1.29) and are transported from outside the Tokyo metropolitan area. The observed rapid change in particle hygroscopicity suggests that the condensation of secondary organics increase the dry particle diameter at rates of 8–17 nm h−1.

Temporal variations of nitrogen wet deposition across Japan from 1989 to 2008

[1]xa0To evaluate temporal variations in nitrogen wet deposition across Japan during 1989–2008, we analyzed results of a chemical transport model (the Models-3 Community Multiscale Air Quality) and observational data. The model successfully reproduced the general patterns of spatial and temporal variations of observed NO3− wet deposition rates. Wet deposition rates of NO3− across Japan increased during 1989–2008, with rates of increase of 2–5%/yr. Sensitivity simulations indicated that the increase of NO3− wet deposition rates was mostly (61%–94%) explained by the increased emissions of atmospheric pollutants in China. Contributions of Chinas emissions increased from 29%–35% during 1989–1993 to 43%–61% during 2004–2008, suggesting that transboundary pollution had a large impact on NO3− wet deposition in Japan. The contribution of observed NO3− to total nitrogen wet deposition (i.e., NO3− + NH4+) increased in southwestern Japan, and currently, NO3− and NH4+ make similar contributions to nitrogen wet deposition across Japan. Interannual variation of NO3− wet deposition was further evaluated using a meteorological index, area-weighted surface pressure anomaly (ASPA). When ASPA was negative, air masses from the Asian continent were more directly transported to Japan, and NO3− concentrations across Japan became high. Thus, anomalies of NO3− concentrations were negatively correlated with ASPA. Anomalies of NO3− wet deposition rates, however, showed a weak positive correlation with ASPA, reflecting a positive correlation between anomalies in precipitation rates and ASPA. This result strongly suggests that precipitation patterns have a large impact on the interannual variation of NO3− wet deposition across Japan.

Radiocarbon (14C) Diurnal Variations in Fine Particles at Sites Downwind from Tokyo, Japan in Summer

The radiocarbon ((14)C) of total carbon (TC) in atmospheric fine particles was measured at 6 h or 12 h intervals at two sites, 50 and 100 km downwind from Tokyo, Japan (Kisai and Maebashi) in summer 2007. The percent modern carbon (pMC) showed clear diurnal variations with minimums in the daytime. The mean pMC values at Maebashi were 28 ± 7 in the daytime and 45 ± 16 at night (37 ± 15 for the overall period). Those at Kisai were 26 ± 9 in the daytime and 44 ± 8 at night (37 ± 12 for the overall period). This data indicates that fossil sources were major contributors to the daytime TC, while fossil and modern sources had comparable contributions to nighttime TC in the suburban areas. At both sites, the concentration of fossil carbon as well as O(3) and the estimated secondary organic carbon increased in the daytime. These results suggest that fossil sources around Tokyo contributed significantly to the high daytime concentration of secondary organic aerosols (SOA) at the two suburban sites. A comparison of pMC and the ratio of elemental carbon/TC from our particulate samples with those from three end-member sources corroborates the dominant role of fossil SOA in the daytime.

Comprehensive source apportionment of volatile organic compounds using observational data, two receptor models, and an emission inventory in Tokyo metropolitan area

[1]xa0Source contributions of volatile organic compounds (VOCs) were comprehensively evaluated using an observational data set, two receptor models, and an emission inventory. Hourly concentrations of C2–C8 nonmethane hydrocarbons (NMHCs) were measured at Saitama, which is near the northern edge of Tokyo, throughout 2007. Estimates of background NMHC concentrations at the Saitama site corresponded well with median NMHC concentrations at a remote island in Japan in winter and spring. Source contributions of ΔNMHCs (differences between ambient and background concentrations) calculated by the chemical mass balance (CMB) model and positive matrix factorization (PMF) corresponded with each other within a factor of 2. The two receptor models estimated that vehicle exhaust, gasoline vapor, liquefied natural gas and liquefied petroleum gas (LPG), and other evaporative sources contributed 14%–25%, 9%–16%, 7%–10%, and 49%–71%, respectively, to total VOC concentrations on a mass basis. These values agreed with the emission inventory except for the LPG values. In addition, the CMB and PMF results explained at least two thirds of the observed total ΔNMHC values. These results suggest that the current emission inventory roughly captures the individual contributions and total amount of VOC emissions. However, characterization of background NMHCs is necessary to fully understand the VOC budget.