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 radionuclides 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 isotopes of io- Discussions spheric chemistry – general circulation model was used, with dine and caesium 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 iodine 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 noble gas 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-year 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 period 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 isotopes of iodine 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 years). We also (Jockel¨ et al., 2005). It uses the first version of the Modular modeled the emission of the isotope 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 radioactive decay 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 lead 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 radionuclide 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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