EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmos. Chem. Phys., 13, 1425–1438, 2013 Atmospheric Atmospheric www.atmos-chem-phys.net/13/1425/2013/ doi:10.5194/acp-13-1425-2013 Chemistry Chemistry © Author(s) 2013. CC Attribution 3.0 License. and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Modelling the global atmospheric transport and deposition of Open Access from the Fukushima Dai-ichi nuclear accident Biogeosciences Biogeosciences Discussions T. Christoudias1 and J. Lelieveld1,2 1The Cyprus Institute, Nicosia, Cyprus Open Access 2Max Planck Institute of Chemistry, Mainz, Germany Open Access Climate Correspondence to: T. Christoudias ([email protected]) Climate of the Past of the Past Received: 24 August 2012 – Published in Atmos. Chem. Phys. Discuss.: 19 September 2012 Discussions Revised: 7 January 2013 – Accepted: 22 January 2013 – Published: 5 February 2013 Open Access Open Access Earth System Abstract. We modeled the global atmospheric dispersion level 7 events. This causedEarth the release System of large amounts of and deposition of radionuclides released from the Fukushima radionuclides to the atmosphereDynamics (Butler, 2011; Chino et al., Dynamics Dai-ichi nuclear power plant accident. The EMAC atmo- 2011; Stohl et al., 2012). In particular the of io- Discussions spheric chemistry – general circulation model was used, with dine and adversely affect human health through the circulation dynamics nudged towards ERA-Interim reanaly- large-scale contamination of air, water, soils and agricul- Open Access Open Access sis data. We applied a resolution of approximately 0.5 de- tural products (Anspaugh etGeoscientific al., 1988). Although both ra- Geoscientific grees in latitude and longitude (T255). The model accounts dionuclides are releasedInstrumentation as gases, caesium has a low volatil- Instrumentation for emissions and transport of the radioactive isotopes 131I ity and partitions into ambientMethods aerosol and particles, whereas 137 Methods and and Cs, and removal processes through precipitation, par- in our model largelyData remains Systems in the gas phase. Re- ticle sedimentation and dry deposition. In addition, we sim- moval of these compounds from the atmosphere is governed Data Systems ulated the release of 133Xe, a that can be regarded by dry and wet deposition processes. Several initial stud- Discussions Open Access Open Access as a passive transport tracer of contaminated air. The source ies of the atmospheric behaviour and budgets of radioactive Geoscientific terms are based on Chino et al. (2011) and Stohl et al. (2012); species, notably 131I and 137GeoscientificCs, were conducted using chem- 131 Model Development especially the emission estimates of I are associated with istry transportModel models. For Development example, the Austrian Weather a high degree of uncertainty. The calculated concentrations Service (ZAMG) performed a daily forecast of the disper- Discussions have been compared to station observations by the Com- sion of radioactivity in the atmosphere. Morino et al. (2011) Open Access prehensive Nuclear-Test-Ban Treaty Organisation (CTBTO). simulated the transport and deposition of the radionuclidesOpen Access We calculated that about 80 % of the radioactivity from over Japan using an off-lineHydrology regional model.and A numerical Hydrology and Fukushima which was released to the atmosphere deposited simulation for the long-rangeEarth transport System from the plant to the Earth System into the Pacific Ocean. In Japan a large inhabited land area US and even Europe with a global aerosol transport model was contaminated by more than 40 kBqm−2. We also esti- was conducted by Takemura et al.Sciences(2011). Ten Hoeve and Ja- Sciences 137 134 mated the inhalation and 50- dose by Cs, Cs and cobson (2012) quantified worldwide health effects using the Discussions

131 Open Access I to which the people in Japan are exposed. GATOR-GCMOM global model. Open Access The objective of the present study is to model the global Ocean Science atmospheric dispersionOcean of radionuclides Science from the Fukushima accident and compute the deposition patterns using an at- Discussions 1 Introduction mospheric chemistry-general circulation model, initialized by estimated source terms. The emission strengths have Open Access On the 11 March 2011 an earthquake occurred off the Pa- been determined through inverse modelling byOpen Access Stohl et al. cific coast of Tohoku,¯ which triggered a powerful tsunami. (2012). The temporal emission profiles of the radioactive iso- Solid Earth The tsunami damaged the Fukushima Dai-ichi nuclear power topes 133Xe and 137Cs were estimatedSolid Earth at three reference re- plant complex, followed by a series of major accidents, giv- lease heights using the Lagrangian transport model FLEX- Discussions ing rise to three INES (International Nuclear Event Scale) Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. The Cryosphere The Cryosphere Discussions 1426 T. Christoudias and J. Lelieveld: Global atmospheric modeling of Fukushima radioactivity

PART (Stohl et al., 1998), driven by operational meteoro- freely and consistently according to the model dynamics. logical data from the European Centre for Medium-Range Therefore, the precipitation field is not necessarily identical Weather Forecasts (ECMWF) and the National Center for with that of the ERA-Interim dataset, which is relevant for Environmental Prediction (NCEP) Global Forecast System the transport and wet deposition of soluble tracers, notably (GFS) analyses, and constrained by a large number of avail- 137Cs. In Fig. 1 we present a comparison between the time able concentration and deposition data from Japan, North integrated precipitation from ERA-Interim and our model America and Europe. over the simulated from 11 March onward. It shows Furthermore, we incorporated the preliminary estimates of that precipitation patterns downwind of Japan are very simi- release amounts of 131I by Chino et al. (2011). A reverse lar, whereas in the tropics (e.g., the intertropical convergence estimation of the source term was conducted by coupling zone) differences are more evident. We do not expect that Japanese Environmental monitoring data with regional at- this significantly influences our results. Both ERA-Interim mospheric dispersion simulations. Emission inventories are and EMAC results indicate that during the first week after further discussed in Sect. 2.1. the accident (11–18 March 2011; Fig. 1) some precipitation We briefly describe the model in Sect. 2, including the occurred in the northern part of the main island of Japan, emissions of radionuclides and the representation of removal mainly along the east coast (37◦–40◦ N). mechanisms. The analysis of our results regarding the global The applied model setup comprises the submodels deposition (Sect. 3.2) and a comparison of surface concen- RAD4ALL for radiation and atmospheric heating processes, trations to station measurements (Sect. 3.1) are discussed in CLOUD for cloud formation and microphysical processes in- Sect. 3. Estimates of doses from the inhalation and ground cluding precipitation, and CONVECT for the vertical trans- deposition of radionuclides are given in Sect. 3.3. A sum- ports of trace species associated with convection. The DRY- mary and conclusions are presented in Sect. 4. DEP (Kerkweg et al., 2006) and SCAV (Tost et al., 2006, 2007) submodels were used to simulate aerosol dry and wet deposition processes, respectively. The SEDI submodel was 2 Model simulation used to simulate particle sedimentation, of which the results will be presented below as part of the simulated dry deposi- The global ECHAM5/MESSy Atmospheric Chemistry tion. (EMAC) general circulation model includes sub-models de- We focus on the radionuclides that were emitted as gases scribing tropospheric and middle atmospheric processes and and partly attached to ambient aerosol particles: the semi- their interactions with oceans, land and vegetation, and volatile 131I (with a half-life of 8 days) trace species emissions of natural and anthropogenic origin and caesium 137Cs (with a half-life of ∼30 ). We also (Jockel¨ et al., 2005). It uses the first version of the Modular modeled the emission of the 133Xe (with a half-life Earth Submodel System (MESSy1) to link multi-institutional of 5.25 days). Since xenon is an unreactive noble gas and is computer codes. The core atmospheric model is the 5th gen- not removed by deposition processes, it serves as a passive eration European Centre Hamburg general circulation model tracer of atmospheric transport. 137Cs is modeled as a water- (ECHAM5; Roeckner et al., 2003, 2006). soluble aerosol with a standard lognormal distribution with For the present study we applied EMAC version 1.9 (based mean radius 0.25 µm and a Henry’s law coefficient equal to on ECHAM5 version 5.3) at the T255L31 resolution, i.e., 1.0 molL−1 atm−1 and a density of 1000.0 kgm−3. Due to with a spherical spectral truncation of T255 (corresponding the long decay lifetime of 137Cs compared to the simulation to a quadratic Gaussian grid of approximately 0.5 by 0.5 de- period and the short timescales of the atmospheric removal grees in latitude and longitude) with 31 vertical hybrid pres- processes considered, its is not taken into sure levels up to 10 hPa, a time step of two minutes and three- account in the simulation. hourly output. To test the effects of model resolution on the simulation we repeated the study on the reduced T106L31 2.1 Emissions resolution (corresponding to a quadratic Gaussian grid of about 1.1 by 1.1 degrees). Our simulation spans the period We used the temporal emission patterns for the radioactive of 1 March–31 May 2011. isotopes 133Xe and 137Cs for March and April 2011, esti- The large-scale component of the model circulation dy- mated based on an inverse modelling method, applying the namics was nudged by applying a Newtonian relaxation transport model FLEXPART, by Stohl et al. (2012). The (Jeuken et al., 1996) towards the European Centre for first guess for the a priori emissions was based on fuel Medium-range Weather Forecasts (ECMWF) ERA-Interim events and documented accident events. Measurement data reanalysis data (Simmons et al., 2007), produced at T255 res- of both atmospheric activity concentrations and bulk deposi- olution, to realistically represent the tropospheric meteorol- tion from several stations in Japan, North America and other ogy of the simulated period. The nudged variables are vortic- regions were used to provide subsequently improved esti- ity, divergence, temperature and surface pressure (Lelieveld mates by inverse modeling. Tracers are emitted within three et al., 2007), whereas the hydrological cycle is computed atmospheric layers over the Fukushima Dai-ichi power plant

Atmos. Chem. Phys., 13, 1425–1438, 2013 www.atmos-chem-phys.net/13/1425/2013/ T. Christoudias and J. Lelieveld: Global atmospheric modeling of Fukushima radioactivity 1427 T.Christoudias et al.: Global atmospheric modeling of Fukushima radioactivity 3

Fig. 1. Time integrated precipitation (in m) patterns over the period 11 March–31 May 2011 computed with the EMAC model (upper right) comparedFig. 1. Time to the integrated ERA-Interim precipitation data (upper (in m left).) patterns The lowerover the panels period show 11 March the precipitation – 31 May during2011 computed the first week with the after EMAC the accident model (upper (11–18 Marchright) compared 2011). to the ERA-Interim data (upper left). The lower panels show the precipitation during the first week after the accident (11–18 March 2011).

(which extend up to 50, 300 and 1000 m). Emission invento- Ministry of Education, Culture, Sports, Science and Tech- riesonly are deposition, provided by basedStohl on et Japanese al. (2012 and) and non-Japanese used in our study data, stantnology radioactivity (MEXT, 2011 ratios),for was the used different to calculate radionuclides dose rates based and betweenthe sensitivity the 10 scaling Marchand factors the of20 the April a posteriori 2011. total emis- onatmospheric iodine and concentrations. caesium concentrations in rain, snow and veg- sionsThe are estimates reported are to reported be 55%, to 59% be sensitive and 150% to replacing respectively. the etationThe in emission the area. inventory By taking compiled into account by Chino these factors et al. ( and2011 a ) GFSThe totalmeteorological amount of data137Cs withemitted ECMWF is 36.7 data (20.1–53.1) with a sensitiv-PBq, numberused Japanese of assumptions environmental used in monitoring their simulation, station Chino data within et al. ityand equal of 133 toXe scaling15.3 (12.2–18.3) the a posterioriEBq total(Stohl emissions et al., 2012). estimate Al- (2011)a regional estimate simulation that their domain. emission The calculations inventory is assumed associated con- 134 bythough 104 % we for did133 notXe explicitly and 68 % include for 137Cs. the When emissions using of onlyCs de-, withstant an radioactivity error of at ratios least a for factor the different of five. Thisradionuclides uncertainty based is position,in Sects. based 3.2 and on 3.3Japanese we will and discuss non-Japanese the deposition data, the of sen- this takenon iodine into accountand caesium in our concentrations model of the deposition in rain, snow of radionu- and veg- sitivitycompound scaling by assuming factors of a the scaling a posteriori factor to total 137Cs emissions suggested are clidesetation by in conducting the area. By a taking sensitivity into studyaccount with these 5 times factors higher and a reportedby Ten Hoeve to beand 55 % Jacobson , 59 % and (2012). 150 % respectively. The to- emissions,number of discussed assumptions in Sec. used 3.2. in their simulation, Chino et al. tal amount of 137Cs emitted is 36.7 (20.1–53.1) PBq, and of (2011) estimate that their emission inventory is associated We also incorporated in our model preliminary emission The preliminary initial emission estimates by Chino et al. 133Xe 15.3 (12.2–18.3) EBq (Stohl et al.131, 2012). Although we with an error of at least a factor of five. This uncertainty is estimates of the released amounts of I (in total 150 PBq) (2011) were superceded by revised estimates published by did not137 explicitly include the emissions of 134Cs, in Sects. 3.2 taken into account in our model of the deposition of radionu- and Cs (in total 13 PBq) by Chino et al. (2011), based Katata et al. (2012) based on additionally disclosed environ- and 3.3 we will discuss the deposition of this compound by clides by conducting a sensitivity study with 5 times higher on the reverse estimation of the source term by coupling mental monitoring data for air dose rates and concentrations assuming a scaling factor to 137Cs suggested by Ten Hoeve emissions, discussed in Sect. 3.2. Japanese environmental monitoring data with regional atmo- of radionuclides from four stations during the early phase andspheric Jacobson dispersion(2012 simulations.). The System for Prediction of theThe accident, preliminary from initial the morning emission of estimates March 12 by toChino late night et al. ofWe Environmental also incorporated Emergency in our Dose model Information preliminary (SPEEDI; emission March(2011) 14. were Major superseded releases byof high-concentration revised estimates publishedplumes dur- by estimatesChino et al., of the 1993) released network amounts system, of operated131I (in totalby the 150 Japanse PBq) Katata et al. (2012) based on additionally disclosed environ- 137 ing this period, which were not taken into account in Chino andMinistryCs of (in Education, total 13 PBq) Culture, by Chino Sports, et Science al. (2011 and), Tech-based etmental al. (2011), monitoring significanlty data for increased air dose the rates amount and concentrations of dry depo- onnology the reverse (MEXT, estimation 2011), was of used the to source calculate term dose by rates coupling and sitionof radionuclides and air dose from rates. four The stations preliminary during theand early revised phase esti- of Japaneseatmospheric environmental concentrations. monitoring data with regional atmo- matesthe accident, of the source from the term morning were verified of 12 March and refined to late in night Terada 14 sphericThe emission dispersion inventory simulations. compiled Theby System Chino for et Predictional. (2011) etMarch. al. (2012) Major and releases found to of be high-concentration reasonable, within plumes the large during un- ofused Environmental Japanese environmental Emergency monitoring Dose Information station data (SPEEDI; within certaintiesthis period, (order which of were factor not 5), taken over theinto period account when in Chino the plume et al. Chinoa regional et al. simulation, 1993) network domain. system, The calculations operated by assumed the Japanese con- dispersed(2011), significantly over land, which increased could the not amount be verified of dry over deposition the pe- www.atmos-chem-phys.net/13/1425/2013/ Atmos. Chem. Phys., 13, 1425–1438, 2013 1428 T. Christoudias and J. Lelieveld: Global atmospheric modeling of Fukushima radioactivity T.Christoudias et al.: Global atmospheric modeling of Fukushima radioactivity 5

Fig.Fig. 2. 2. Total atmospheric content (in kg) of 137CsCs(left) and 133Xe (middle) and 131II(right) (right) that that originated originated from from Fukushima Fukushima Dai-ichi Dai-ichi based based onon the the emissions emissions inventories by Stohl et al. ( (2012)2012) and Chino et al. (2011).(2011). The red bands indicate the reported range of the the emission emission source source estimates,estimates, and and the the dashed dashed lines correspond to the results of the T106 horizontal resolution simulation. andis critical air dose for rates. the simulation The preliminary of wetand deposition. revised estimatesIt is notable of tionThe and subset most of model stations results with fall available within measurementsthe factor of five used range in thethat source the results term of were the higher verified resolution and refined T255 in calculationsTerada et are al. ofour the study measurements can be seen (Fig. in Figs. 4). However,3 and 4. our model shows the (not2012 systematically) and found to more be reasonable, consistent with within the the measurements large uncer- arrivalThe of stations high concentrations operate highvolume of radioactive aerosol aerosols samplers in the us- taintiesthan the (order lower of resolution factor 5), T106 over results. the period The when lower the panels plume of Asia-Pacificing collection region filters (see with for a example minimum USP80 detection at Upi, capability Guam, dispersedFigs. 3, 4,over and land, 5 present which could the root-mean-square-deviations not be verified over the pe- in(MDC) the supplement, range typically and between PHP52 in 1–10 Tanay, µBqm the− Philippines,3 for Cs. Since in riod(RMSDs) when between the plume the travelled measurements over the and ocean. model The results refine- for Fig.iodine 4), largely which travelsare not in reflected the gas in phase, the station the filter observations. measure- mentsboth resolutions. of the results We by alsoChino computed et al. the(2011 normalised) are not RMSDs,included Therements isrepresent clearly reducedonly a small agreement fraction compared of airborne to the131 resultsI, thus inand our find study. that they are typically below 50%, irrespective of the forassociated the noble with gas a133 relativelyXe, which large we measurementattribute to the uncertainty additional distanceA comparison between of the the station estimate and the of released emission137 source.Cs by Stohl uncertainties(Stoehlker et related al., 2011 to). the Only simulation 20–50 % of ofremoval131I is processes, collected et al.For(2012 the eight) and stationsChino et available al. (2011) with can varyingbe seen innumber the total of in particularthe samples, by sinceprecipitation. the IMS These stations types are of using uncertainties paper fil- modelmeasurements, calculated there atmospheric is good agreement content (Fig. between2) and the for model indi- alsoters through affect the which source part estimates of the iodine obtained passes by unattenuated inverse mod- (as vidualresults station and the measurements measurements (Figs. for the3– noble5). The gas two133 emissionXe, both elling,reported e.g. by by the Stohl CTBTO et al. Virtual(2012). Data Exploitation Centre). estimatesfor the higher differ and greatly, lower resolutionboth in magnitude (Fig. 3). Nevertheless,and time profile. the NobleTo determine gas concentration the total measurement level of airborne stations131 areI concentra- also been 133 Wemodeled find that concentrations the spatial resolution at JPX38 of(Takasaki, our model Gunma, has little Japan) im- tions,set up both worldwide, the gaseous with andXe particulate being the fractions most important of 131I must and pactpeak on in the March global at atmosphericvalues well outside content the of radionuclides. dynamic range of beprevalent accounted isotope. for. It Sampling is measured gaseous with typical radio-iodine MDC requiresof about −3 the measurement instruments. All measurements at the other activated0.2 mBqm charcoal. The CTBTO traps, which station are measurements not implemented of radionu- in the stations are below 100 mBqm−3, the reported upper range of IMSclide network,concentrations as they can are be not directly required compared for CTBT to the monitor- concen- 3 Results the observation accuracy. The model not only represents the ingtrations (Stoehlker simulated et al., by 2011). our model. Therefore, the iodine measure- There are some additional aspects that affect the CTBTO Themeasurements comparison well of our in the model vicinity results of the with source, station but measure- also in ments should be understood as a lower bound, as only the station measurements, which should be taken into account mentsNorth, is Middle discussed and Southin Sect. America3.1. The (stations modeled USX75, global PAX50, deposi- aerosol phase can be collected on particle filters (Winiarek when comparing to the model simulations (Stoehlker et al., tionFRX31), of radionuclides and even as is far presented downwind in Sect. as Germany3.2. The (DEX33). doses to et al., 2012). Based on data by the “Ring of Five (Ro5)”, 2011). The highest concentrations of Xe after the accident theAgain, public there from are deposition no indications and inhalation that the higher based on resolution our re- an informal network of European national authorities (which version of the model performs systematically better. Even compriseswere outside more the than dynamic 150 sampling range of systems the noble of high gas volumesystem, sults are calculated in Sect. 3.3. − though the ERA-Interim data, used to nudge the model to the samplersso detections and somethat are with higher activated than coal100 Bqm traps),3 theare average not ex- 3.1actual Comparison meteorology, with are produced Station Measurements at T255 resolution, the re- gaseous/totalpected to be ratio accurate. for 131 ItI isis also77.2 known± 13.6% that(Masson the Takasaki et al., gridding of the data to the coarser T106 resolution seems to 2011).(JPX38) The noble U.S. gas Environmental station in Japan Protection was affected Agency significantly (EPA) Thehave little Comprehensive effect on the Nuclear-Test-Banquality of the dynamics Treaty simulations. (CTBT), RadNetby dead stationtime in measurementsthe first three weeks detected after 81% the accident.of the ambi- The 131 adoptedFor the by comparison the United of Nations the model General simulation Assembly, results foresees to ob- entJapaneseI in particulate the gas and station 19% JPP38, in the also particle at Takasaki, phase (Ten was Ho- af- aservations ban of all of nuclear the surface explosions. concentrations For compliance, of 137Cs a global(Fig.In- 4), evefected and by Jacobson, radioactive 2012). leaks These into the values measurement are close to room the andav- ternationalreduced though Monitoring nevertheless System reasonable (IMS) of measurement agreement can tech- be eragepower reported outages between value for 14 the and Chernobyl 16 March accident 2011. Contamina- by Hilton nologies has been established by the CTBT Organization ettion al. at (1991) JPP38 and the to average an overestimation of 71±11% ofreported137Cs concentra- from the claimed, taking into account the inherent uncertainties based −3 (CTBTO),on the multitude with data and deliveredthe complexity to the of International the simulated Data removal Cen- Fukushimations by about site 1 from mBqm 22 Marchuntil to August 4 April 2011. 2011 Similar (Stoehlker con- tre for of the CTBTO Preparatory commission in Vienna, ettamination al., 2011). occurred Therefore, at USP71 a factor at of Sand 4 gaseous Point, Alaska, to particulate USA, processes (sedimentation, dry and wet deposition). In most −3 Austria.cases there The is IMS close comprises coincidence between the measurement modeled and sta- ob- fractionleading toseems an overestimation an appropriate inestimate the range for the of partitioning.3 µBqm for tionsserved with time continuous of arrival coverage,of the first distributed radioactive around plume the at the globe. sta- As we do not account for the removal of 131I via wet de-

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133 Fig. 3. Top:Fig. CTBTO3. Top: CTBTO measurement measurement network network station station location location and and code for for the the noble noble gas gas Xe133forXe the for subset the subset of stations of stations with measurements with measurements available used in our study. available used in our study. 133 Middle: Observed (black points) and modeled (solid lines) surface concentrations of 133Xe at the eight stations. T255 horizontal resolution Middle: Observedis shown in (black blue and points) T106 in and red. modeled The time (solidaxis represents lines) surface days after concentrations the accident. of Xe at the eight stations. T255 horizontal resolution is shownBottom: in blue and Modeled T106 versus in red. observed The time surface axis concentrations represents days of 133 afterXe at the the accident. eight stations. The diagonal lines represent the 1:1 ratio and the Bottom: Modeledfactor of 5versus over- and observed underestimates. surface concentrations of 133Xe at the eight stations. The diagonal lines represent the 1:1 ratio and the factor of 5 over- and underestimates.

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137Cs. Therefore, for these locations we expect our model to For the eight stations available with varying number of underestimate the reported observations. measurements, there is good agreement between the model The locations of the CTBTO measurement stations for ra- results and the measurements for the noble gas 133Xe, both dionuclide gases and particulates used for comparison, in- for the higher and lower resolution (Fig. 3). Nevertheless, the cluding station codes, can be seen in Fig. 4. In total, eight modeled concentrations at JPX38 (Takasaki, Gunma, Japan) stations are available for noble gas observations and 37 for peak in March at values well outside the dynamic range of particulates. The availability of 3 months of measurements the measurement instruments. All measurements at the other allows for the comparison over a wide range of meteorologi- stations are below 100 mBqm−3, the reported upper range of cal conditions and radionuclide concentrations at each station the observation accuracy. The model not only represents the location. measurements well in the vicinity of the source, but also in We focus on the concentrations of radionuclides in the North, Middle and South America (stations USX75, PAX50, lowest layer of our model and in the grid boxes where the FRX31), and even as far downwind as Germany (DEX33). stations are located, and compare with the measured concen- Again, there are no indications that the higher resolution trations at each station at the nearest model output time step version of the model performs systematically better. Even to the station collection start time. The comparison for all though the ERA-Interim data, used to nudge the model to 133Xe stations is presented in Fig. 3 and selected stations for the actual meteorology, are produced at T255 resolution, the 137Cs and 131I in Figs. 4 and 5, respectively. The entire set regridding of the data to the coarser T106 resolution seems to of measurements and concomitant model results is presented have little effect on the quality of the dynamics simulations. in the electronic supplement. Our model does not take into For the comparison of the model simulation results to ob- account the resuspension of particles after they have been servations of the surface concentrations of 137Cs (Fig. 4), deposited, though we expect the effect to be small. reduced though nevertheless reasonable agreement can be Overall, there is good coincidence of the time of first ar- claimed, taking into account the inherent uncertainties based rival of the radionuclide plume at the station and the time of on the multitude and the complexity of the simulated re- first detection of radioactivity at a station, especially for sta- moval processes (sedimentation, dry and wet deposition). In tions in the Asia-Pacific and North American regions, taking most cases there is close coincidence between the modeled into account coincidence timing uncertainties. As all the sta- and observed time of arrival of the first radioactive plume tions collect samples over a period of typically 12 h or more at the station and most model results fall within the factor for each measurement performed, there is an associated un- of five range of the measurements (Fig. 4). However, our certainty of the same order of magnitude over the time of model shows the arrival of high concentrations of radioactive first arrival of radionuclides from Fukushima at each mea- aerosols in the Asia-Pacific region (see for example USP80 surement location. Furthermore, the measurement record is at Upi, Guam, in the supplement, and PHP52 in Tanay, the not continuous for all stations, resulting in cases where there Philippines, in Fig. 4), which are not reflected in the station is no observational coverage of the model prediction of the observations. There is clearly reduced agreement compared arrival of radionulcide plumes. Good agreement is obtained to the results for the noble gas 133Xe, which we attribute to for the inert noble gas 133Xe (although only a limited num- the additional uncertainties related to the simulation of re- ber of stations is available). The agreement is not as good moval processes, in particular by precipitation. These types for 137Cs and 131I. This is to be expected as the greater ex- of uncertainties also affect the source estimates obtained by tend to which different removal processes are involved (dry inverse modelling, e.g. by Stohl et al. (2012). and wet deposition and sedimentation), creates larger uncer- To determine the total level of airborne 131I concentra- tainties in the source estimates and in the modelling of at- tions, both the gaseous and particulate fractions of 131I must mospheric transport and removal. These removal processes be accounted for. Sampling gaseous radio-iodine requires take place on a sub-grid scale and are therefore parameter- activated charcoal traps, which are not implemented in the ized and thus less explicitly resolved by the model than at- IMS network, as they are not required for CTBT monitor- mospheric dynamical and transport processes. Especially the ing (Stoehlker et al., 2011). Therefore, the iodine measure- model representation of precipitation is critical for the simu- ments should be understood as a lower bound, as only the lation of wet deposition. It is notable that the results of the aerosol phase can be collected on particle filters (Winiarek higher resolution T255 calculations are not systematically et al., 2012). Based on data by the “Ring of Five (Ro5)”, more consistent with the measurements than the lower res- an informal network of European national authorities (which olution T106 results. The lower panels of Figs. 3, 4, and 5 comprises more than 150 sampling systems of high volume present the root-mean-square-deviations (RMSDs) between samplers and some with activated coal traps), the average the measurements and model results for both resolutions. We gaseous/total ratio for 131I is 77.2 ± 13.6 % (Masson et al., also computed the normalised RMSDs, and find that they are 2011). The US Environmental Protection Agency (EPA) typically below 50 % , irrespective of the distance between RadNet station measurements detected 81 % of the ambi- the station and the emission source. ent 131I in the gas and 19 % in the particle phase (Ten Ho- eve and Jacobson, 2012). These values are close to the

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T.Christoudias et al.: Global atmospheric modeling of Fukushima radioactivity 11

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✂ ✂

137 Fig. 4. Top:Fig. CTBTO 4. Top: CTBTOmeasurement measurement network network station station location location and and code code for for 137CsCsfor for the the subset subset of stations of stations with measurements with measurements available used available used in our study. Stations shown in our study are highlighted in red. in our study.Middle: Stations Observed shown (black in our crosses) study and are modeled highlighted (solid inlines) red. surface concentrations of 137Cs at the eight selected stations. T255 horizontal 137 Middle: Observedresolution (black is shown crosses) in blue and and T106 modeled in red.(solid The time lines) axis represents surface concentrations days after the accident. of Cs at the eight selected stations. T255 horizontal resolution isBottom: shown Modeled in blue (Stoehlker and T106 et al.in(2011) red. The – solid time circles; axis Chinorepresents et al. (2011) days after– open the rhombi) accident. versus observed surface concentrations of 137Cs Bottom: Modeledat the eight (Stoehlker stations. The et al. diagonal(2011 lines) – solid represent circles; the 1:1Chino ratio andet al. the(2011 factor) of – 5open over- rhombi) and underestimates. versus observed surface concentrations of 137Cs at the eight stations. The diagonal lines represent the 1:1 ratio and the factor of 5 over- and underestimates.

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Fig. 5. Fig.Top: 5. ObservedTop: Observed (black (black points) points) and and modeled modeled (solid (solid lines) lines) surface concentrations concentrations of 131 of 131I atI eight at eight selected selected stations. stations. The station The stationlocations locations 131 are theare same the as same in Fig. as in4 Fig.. Due 4.Due to the to limitedthe limited collection collection efficiency efficiency of of the filter filter sampling sampling of of 131I, theI, the observations observations represent represent about about20-50% 20–50 of % of the ambient concentrations. T255 horizontal resolution is shown in blue and T106 in red. The time axis represents days after the accident. the ambient concentrations. T255 horizontal resolution is shown131 in blue and T106 in red. The time axis represents days after the accident. Bottom: Modeled versus observed surface concentrations of131 I at the eight stations. The diagonal lines represent the 1:1 ratio and the Bottom:factor Modeled of 5 over- versus and observed underestimates. surface concentrations of I at the eight stations. The diagonal lines represent the 1:1 ratio and the factor of 5 over- and underestimates. average reported value for the Chernobyl accident by Hilton As we do not account for the removal of 131I via wet et al. (1991) and the average of 71 ± 11 % reported from the deposition, we would expect our model to overestimate the Fukushima site from 22 March to 4 April 2011 (Stoehlker actual 131I concentrations. Since only 20–50 % of the 131I et al., 2011). Therefore, a factor of 4 gaseous to particulate is captured on the filters, (as reported by the CTBTO Vir- fraction seems an appropriate estimate for the partitioning. tual Data Exploitation Centre) we would also expect that the model more strongly overestimates the measured 131I

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Fig. 7. Regional total deposition of 137Cs and dry deposition of Fig. 7. Regional total deposition of 137Cs and dry deposition of gaseousFig. 7.131RegionalI in Bq at total different deposition scales. of The137Cs blackand point dry deposition marks the of lo- gaseous 131I in Bq at different scales. The black point marks the gaseous 131I in Bq at different scales. The black point marks the Fig. 6. Global distribution of dry (upper panel) and wet (middle locationcation of of the the Fukushima Fukushima nuclear nuclear power power plant plant and and the the rectangle rectangle the Fig. 6. Global distribution of137 dry (upper panel) and wet (middle Tokyolocation metropolitan of the Fukushima area. Contours nuclear are power on a plant logarithmic and the scale.rectangle The panel)Fig. deposition 6. Global (in distribution Bq) of ofCs dry and (upper total panel) deposition and wet (bottom (middle the Tokyo metropolitan− area. Contours are on a logarithmic scale. panel) deposition137 (in Bq) of 137Cs and total deposition131 (bottom 40the kBqm Tokyo2− metropolitan2contour lines area. signify Contours areas are that on received a logarithmic total deposi- scale. panel) of Cs and dry deposition137 of gaseous I. Contours are The 40 kBqm contour−2 lines signify areas that received total de- panel)137 deposition (in Bq) of Cs and total131 deposition (bottom The 40 kBqm contour lines signify areas that received total de- panel) of Cs137and dry deposition of gaseous I.131 Contours are positiontion greater greater than than the contamination contamination activity activity level level defined defined by In- by Inter- on a logarithmicpanel) of scale.Cs and dry deposition of gaseous I. Contours are position greater than the contamination activity level defined by In- on a logarithmic scale. ternationalnational Atomic Atomic Energy Energy Agency Agency (IAEA) (IAEA) (2005,(2005 2009)., 2009). on a logarithmic scale. ternational Atomic Energy Agency (IAEA) (2005, 2009). concentrations. On the contrary, the model tends to generally underestimates of the actual emissions of 131I by at least a underestimate the measurements of the atmospheric concen- factor two to five. trations of 131I based on the emission inventory by Chino et al. (2011) (Fig. 5). Chino et al. (2011) estimate a total re- 3.2 Deposition of radionuclides lease of 150 PBq between 12 March and 6 April 2011, re- vised in Terada et al. (2012) to 128 PBq between 12 March In this section we assess the atmospheric budget and deposi- and 1 May 2011. Both results are considered by their respec- tion patterns of radioactivity by calculating the contributions tive authors to be lower estimates and to have an error of of different removal processes, i.e., particle sedimentation, at least a factor of five. Winiarek et al. (2012) estimate the dry and wet deposition; the latter through large-scale and lower bound of the total activity of 131I released into the at- convective precipitation. Since the contribution of sedimen- mosphere between 11 March and 26 March 2011 to be be- tation is only small we present it as part of the dry deposition. tween 190 and 700 PBq (and 190–380 PBq when using the The global distribution of deposited 137Cs radionuclides is most reliable methods and the Japanese observations only), shown in Fig. 6 for dry and wet deposition processes sepa- comparable to the first estimates of 150 PBq by the Nuclear rately (top and middle panel, respectively) based on the emis- Safety Commission of Japan. Considering that the model sion inventory of Stohl et al. (2012). Following the definition typically underestimates measured 131I concentrations, the by the International Atomic Energy Agency (IAEA) (2005, lower bound estimates mentioned above probably represent 2009), any area with activity ≥40 kBqm−2 is considered to www.atmos-chem-phys.net/13/1425/2013/ Atmos. Chem. Phys., 13, 1425–1438, 2013 1434 T. Christoudias and J. Lelieveld: Global atmospheric modeling of Fukushima radioactivity be contaminated (the most intense red scale). Formally, con- 14 T.Christoudias et al.: Global atmospheric modeling of Fukushima radioactivity tamination means the presence of a radioactive substance on a surface in quantities in excess of ≥40 kBqm−2 for beta and gamma emitters. Both, 137Cs and 131I are beta and gamma emitters. Since we integrate the deposition over the period after the accident until the end of May 2011, our results rep- resent the accumulated contamination of these two radionu- clides. The prevailing winds during the emission period after 11 March transported most of the radionuclides in easterly di- rection, away from Japan and the continent. Precipitation events caused the deposition to occur predominantly over the ocean. Figure 6 illustrates that the dry deposition patterns are smoother than of wet deposition, i.e., less dependent on indi- vidual meteorological events. It also shows that dry deposi- tion of 137Cs is mostly confined to the Pacific Ocean region because this process only acts upon particles in the bound- ary layer. Only the radioactive particles that escape towards the free troposphere can be transported over much larger dis- tances. Since exchange between the boundary layer and free troposphere is mostly dependent on convection events, the particulate contaminants are to a large extent removed by the associated precipitation. This is much less the case for 131I, which has a low solubility, so that convection effectively re- distributes this gas into the free troposphere where the wind speed is typically higher and transport distances larger. Therefore, even though 131I has a relatively short half- life, meteorological conditions associated with convection and vertical mixing over the Pacific Ocean promote its long- distance transport so that it contributes to radioactivity de- position worldwide (Fig. 6, bottom panel). Our model re- sults suggest that the plumes that traversed the Pacific Ocean caused significant deposition of radioactivity over conti- nental North America, in particular western USA, western Canada and eastern USA (>100 Bqm−2). Our model results also show substantial deposition of radionuclides in regions southwest of Japan, e.g., around the Philippines (Fig. 7, up- per panel). Two weeks after the accident, all operational CTBTO stations in the northern hemisphere had reported at least one 131I detection (Stoehlker et al., 2011). Detailed budget analysis, based on the T255 resolution model results, shows that 29.3 PBq of the nominal total of 36.7 PBq 137Cs released by Fukushima was deposited to the ocean (equivalent to ∼80 % ) and the rest over land, mostly in Fig. 8. Global (top), and regional (middle: Asia Pacific; Bottom: Japan)Fig. 8. combinedGlobal (top), total and deposition regional of (middle:137Cs ( AsiaStohl Pacific; et al., 2012 Bottom:) and Japan. This is in agreement to the budget analysis performed 137 137 131Japan)I in Bq, combined based on total a sensitivity deposition simulation of Cs (Stohl with et 5 times al., 2012) enhanced and by Morino et al. (2011) which indicated that 22 % of Cs 131 131II emissionsin Bq, based (750 on PBq; a sensitivityChino et simulation al., 2011 with), also 5 times including enhanced134Cs was deposited over land in Japan, and the rest deposited 131I emissions (750131 PBq; Chino et al. (2011)),137 also including 134Cs 2 and particulate I deposition scaled to Cs. The black point over the ocean (or transported outside of their 700×700 km 131 137 marksand particulate the locationI ofdeposition the Fukushima scaled nuclear to Cs power. The plant black and point the model domain) between 11–30 March 2011 and the budget rectanglemarks the the location Tokyo of metropolitan the Fukushima area. nuclear Contours power are plant on a and logarith- the analysis performed by Ten Hoeve and Jacobson (2012), in- micrectangle scale. The the Tokyo 40 kBqm metropolitan−2 contour area. lines Contours signify areasare on that a logarith- received 137 mic scale. The 40 kBqm−2 contour lines signify areas that received dicating that 19 % of the total worldwide deposited Cs total deposition greater than the contamination activity level defined total deposition greater than the contamination activity level defined was over land and the remaining 81 % over the oceans. We by the International Atomic Energy Agency (IAEA) (2005, 2009). find that only a small fraction (less than 1 % ) was deposited by the International Atomic Energy Agency (IAEA) (2005, 2009). within the Arctic circle (at latitudes higher than 67◦ N). We estimate that of the total emitted amount of radioactivity,

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Fig.Fig. 9. 9. Effective dose from the inhalation of airborne radionuclides for the the nominal nominal emission emission dataset dataset (left); (left); inhalation inhalation dose dose sensitivity sensitivity test test 131 131 134 137 forfor the the 131II emissionemission scaled by factor 5 (middle); and fifty-year dose from the the ground ground deposition deposition of of 131II,, 134CsCsand and 137CsCs(right) (right) based based on on dosedose factors factors by by the theIAEA IAEA( (2009).2009). approximately 40 % , 60 % , and 75 % were deposited in re- in the atmosphere. If this were correct, the model underes- gions at distances of 5◦, 10◦ and 20◦ in latitude and longitude timate of 131I concentrations compared to the CTBTO mea- from the location of the source. surements would even be larger, providing additional justifi- The regional deposition distribution of the sum of 137Cs cation for a much larger source than 150 PBq. Further study and 131I at different scales is shown in Fig. 7. We esti- will be needed to resolve this important issue. mate that the land area affected by the deposition of ra- dioactivity (by both compounds) in excess of 40 kBqm−2 is approximately 34,000 km2. Using the population data from 3.3 Doses from Inhalation and Ground Deposition (CIESIN/CIAT/SEDAC, 2005) (referring to the year 2010), this part of Japan is inhabited by ∼9.4 million people. The We estimated the cumulative doses due to inhalation over the surface area that received a total deposition greater than simulation period (11 March–31 May 2011) as well as the 10 kBqm−2 encompasses parts of the Tokyo metropolitan effective doses over 50 years from ground contamination by area, and approximately covers 60,000 km2, being inhabited applying conversion factors for 137Cs, 134Cs and 131I recom- by ∼46 million people. We emphasize that this is based mended by the International Atomic Energy Agency (IAEA, on the emission of 150 PBq 131I estimated by Chino et al. 2009, Appendix I). The 134Cs concentration and deposition (2011), which might actually be a factor of five too low. rate in each grid cell was scaled from 137Cs, assuming the To test the potential of a larger source than 150 PBq 131I, 134Cs to 137Cs activity ratio of 0.9, based on CTBTO obser- and the contribution of particulate 131I deposition and other vations (Ten Hoeve and Jacobson, 2012). The contribution radionuclides we performed a sensitivity test by applying 5 by noble gases, including 133Xe, can be neglected as the dose times higher emissions, based on the emissions estimate un- ratios relative to 131I are effectively zero. certainty by Chino et al. (2011). In the sensitivity test 134Cs The inhalation doses are converted from model calculated and particulate 131I concentrations and deposition rates were concentrations (Bqsm−3) into Sv (Sievert), applying the fac- scaled with 137Cs, assuming a 134Cs to 137Cs activity ratio of tors 1.29 × 10−11, 6.60 × 10−12 and 2.44 × 10−12 for 137Cs, 0.9 (Ten Hoeve and Jacobson, 2012) and a factor of 4 gaseous 134Cs and 131I, respectively. The results are shown in Fig. 9. to particulate 131I in the atmosphere. Figure 8 shows the re- Our calculations do not comprise dosages to individuals from sults, indicating significantly larger and perhaps more realis- the ingestion of radionuclides, based on the assumption that tic deposition of radioactivity worldwide (upper panel). The food intervention levels prevent such exposure. The maxi- regional and national deposition patterns are illustrated by mum cumulative effective dose from inhalation in Japan is the middle and lower panel, respectively. If this would be cor- found to exceed 10 mSv for the nominal emissions (36.7 PBq rect, a large inhabited area would be contaminated by more 137Cs and 150 PBq 131I) and 20 mSv for the sensitivity test than 40 kBqm−2, including the Tokyo metropolitan area. Al- with five times higher 131I emissions, i.e., the upper bound of though this large 131I source is speculative, it seems more the emissions uncertainty indicated by Chino et al. (2011). realistic than the low estimate of 150 PBq by Chino et al. These estimates apply to adult members of the general pub- (2011). It should be noted that the assumption of 25 % 131I in lic and not to workers who received higher doses in contam- the particulate phase to a reduction of the 131I lifetime inated facilities. www.atmos-chem-phys.net/13/1425/2013/ Atmos. Chem. Phys., 13, 1425–1438, 2013 1436 T. Christoudias and J. Lelieveld: Global atmospheric modeling of Fukushima radioactivity

Further, we calculated the effective dose to the public from Organisation (CTBTO) global monitoring network for a se- exposure to ground contamination due to the deposition of lected number of stations. We obtained good agreement with 137Cs, 134Cs and 131I. The 50-year ground deposition doses measurements of 133Xe, at both T255 and T106 resolution. are derived from the conversion of deposited radioactivity The comparison for 137Cs is less favorable, though not sys- in Bqm−2 into an effective dose in Sv, applying the fac- tematically biased based on the 137Cs emission estimate of tors 1.30 × 10−7, 5.10 × 10−9 and 2.70 × 10−10 for 137Cs, 36.7 (20.1–53.1) PBq by Stohl et al. (2012). This is equiva- 134Cs and 131I, respectively (IAEA, 2009). The cumulative lent to somewhat less than half the 137Cs source by Cher- effective dose from radioactive compounds remaining on the nobyl (85 PBq, International Atomic Energy Agency (IAEA) ground for a period of 50 years is shown in the right panel (2006)). The estimate of 13 PBq by Chino et al. (2011) (re- of Fig. 9. The 50-year effective dose is dominated by the ef- vised to 8.8 PBq by Terada et al. (2012) and confirmed as a fects of 137Cs and is less sensitive to the large uncertainty lower bound of 12 PBq by Winiarek et al. (2012), with an up- of the deposited amounts of 131I and 134Cs. The sensitiv- per bound of 210 PBq, was also included in our comparison. ity of these dose estimates to the radionuclide 132Te is ex- The model calculated deposition patterns show that most pected to be low, because even though the radioactivity ratio of the radioactivity which was released to the atmosphere of 131I:(131I+132Te):134Cs:137Cs=1:2:0.1:0.1 (Katata et al., ended in the Pacific Ocean (about 80 % ), in agreement with 2011), the dose factors from ground deposition and inhala- other studies (Morino et al., 2011; Ten Hoeve and Jacobson, tion are at least 3 orders of magnitude lower. 2012). Although the prevailing wind direction during the ac- The geographic distribution of dose estimates within the cident was in easterly direction, some of the atmospheric ra- land area of Japan is associated with a high degree of un- dioactivity was transported toward the west and southwest, certainty, due to the spatial resolution of our model. A nested and deposited in Japan and to a lesser extent reached the regional model can be used in a future study to improve upon Philippines. Based on the above mentioned emission esti- the estimates by the use of more detailed land model and a mates, we calculated that an area of Japan of 34,000 km2 was finer grid. In Fig. 9 we show the horizontal resolution to em- contaminated by more than 40 kBqm−2 of 137Cs and 131I, to phasize the relatively coarse grid of our model, about 50 km, which ∼9.4 million people were exposed. Our model results which for example cannot reflect the complex terrain around indicate that a surface area of 60,000 km2 received a total de- the Dai-ichi accident site. Nevertheless, since the 133Xe sim- position greater than 10 kBqm−2, a region inhabited by ∼46 ulations have shown that transport over large distances is million people. simulated accurately by our model, we may assume that this The source term of 150 PBq 131I applied in our model, also applies to shorter distances of 50 km upward. based on Chino et al. (2011), is associated with a sys- tematic underestimate of simulated concentrations compared to the CTBTO measurements. Therefore, we performed an 4 Conclusions 131I emissions sensitivity test, with a factor of five stronger source, i.e., the upper bound of the uncertainty range indi- The release of radioactivity from the Fukushima Dai-ichi cated by Chino et al. (2011). If this would be realistic, our nuclear power plant accident and the atmospheric transport model indicates that a relatively large and densely populated and deposition patterns between March and May 2011 were part of Japan including parts of the Tokyo metropolitan area simulated using the EMAC atmospheric chemistry – general has been contaminated by more than 40 kBqm−2. Additional circulation model at different resolutions (T255 and T106, work will be needed to estimate the 131I source more accu- equivalent to ∼50 km and ∼110 km Gaussian grid resolu- rately. tion respectively). The atmospheric dynamics were nudged Finally, we estimated the cumulative doses to the pub- towards ERA-Interim reanalysis data of the ECMWF, avail- lic due to the inhalation of radionuclides over the simulated able at T255 resolution. time period (11 March–31 May 2011) and the effective doses We initialized our model by applying emissions of the inert over a 50-year period from ground contamination. The max- noble gas 133Xe and the semi-volatile gases 131I and 137Cs. imum inhalation dose around the Fukushima nuclear acci- In our model, ceasium attaches to ambient aerosol particles, dent site is calculated to be >10 mSv for the nominal emis- whereas the iodine remains in the gas phase. The source sions case and >20 mSv for the sensitivity test to the emis- terms are based on estimates from the literature, produced by sions uncertainty by Chino et al. (2011) (five times higher inverse modelling and reverse estimation techniques. There 131I emissions). These estimates only include 137Cs and 131I, are significant differences and large uncertainties in the to- and neglect other isotopes of caesium and iodine and other tal amount and temporal release of radionuclides between radionuclides such as 132Te. The maximum 50-year dose due the presently available estimates. Removal processes through to ground contamination in the area around the Fukushima precipitation, particle sedimentation, dry deposition and ra- nuclear power plant is estimated to be >125 mSv. This result dioactive decay were taken into account. is less sensitive to the high uncertainty of the 131I emissions Modeled surface concentrations were compared to station since it is dominated by the deposition of caesium. observations by the Comprehensive Nuclear-Test-Ban Treaty

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Supplementary material related to this article is global model for the purpose of model validation, J. Geophys. available online at: http://www.atmos-chem-phys.net/13/ Res., 101, 16939–16950, 1996. 1425/2013/acp-13-1425-2013-supplement.pdf. Jockel,¨ P., Sander, R., Kerkweg, A., Tost, H., and Lelieveld, J.: Tech- nical Note: The Modular Earth Submodel System (MESSy) - a new approach towards Earth System Modeling, Atmos. Chem. Phys., 5, 433–444, doi:10.5194/acp-5-433-2005, 2005. Katata, G., Terada, H., Nagai, H., and Chino, M.: Numerical recon- Acknowledgements. The research leading to these results has struction of high dose rate zones due to the Fukushima Daiichi received funding from the European Research Council under the Nuclear Power Plant accident, J. Environ. Radioactiv., 111, 2–12, European Union’s Seventh Framework Programme (FP7/2007- 2012. 2013) / ERC grant agreement no 226144. Computational resources Katata, G., Ota, M., Terada, H., Chino, M., and Nagai, H.: At- were provided by the LinkSCEEM project. We thank the Interna- mospheric discharge and dispersion of radionuclides during the tional Data Centre (IDC) of the Provisional Technical Secretariat Fukushima Dai-ichi Nuclear Power Plant accident. Part I: Source (PTS) for the CTBTO Preparatory Commission for producing and term estimation and local-scale atmospheric dispersion in early providing the radionuclide measurement data. We are most grateful phase of the accident, J. Environ. Radioactiv., 109, 103–113, for comments received by the CTBTO PTS on how to improve our 2012. manuscript. The Preparatory Commission for the Comprehensive Kerkweg, A., Buchholz, J., Ganzeveld, L., Pozzer, A., Tost, H., and Nuclear-Test-Ban Treaty Organization and its Provisional Techni- Jockel,¨ P.: Technical Note: An implementation of the dry removal cal Secretariat are not responsible for the views of the authors. 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