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4G. Lujanien_e et al. / Journal of Environmental Radioactivity xxx (2012) 1e10137Cs, µBq/m3120010008006004002000131 IR = 0.69 (n= 29)0 2000 4000 6000 80007Be, µBq/m 3137 CsR = 0.75 (n=29)4000300020001000Fig. 3. 137 Cs and 131 I activity concentrations plotted against the 7 Be activity concentrationin aerosol samples collected during the Fukushima plume episode.(e.g. Lujanas and Lujanien_e, 2007). An increase in the 7 Be activityconcentrations in summer and autumn season was explained bya vertically downward transport within the troposphere (Kochet al., 1996) and by a stratosphere-troposphere exchange (Jordanet al., 2003; Land and Feichter, 2003). The high 7 Be activityconcentrations in the surface air were also interpreted by thedownward and upward atmospheric flows in the tropospherecaused by a pair of travelling anticyclone and extra tropical cyclonethat passes over Japan in spring and autumn with a period of a fewdays (Yoshimori, 2005). It was supposed that the air of lowtemperature flows downward from convergence in the Rossbywaves to divergence in the surface anticyclone, and the warm airmoves upward from convergence in the surface cyclone to divergencein the Rossby waves. In these studies an increase in theactivity concentrations in the near-surface atmosphere was associatedwith downward movements of air masses. Thus, the positivecorrelation between the anthropogenic radionuclides and 7 Be(Fig. 3) can be an indication of their arrival from the upper layers ofthe troposphere.On the other hand, short-lived radon decay products, such as theterrigenous 210 Pb, 212 Pb and 214 Pb can also be used as atmospherictracers to study air masses transport (Sheets and Lawrence, 1999).The main source of 212 Pb (half-life of 10.6 h) in the air is 220 Rn (halflifeof 54 s) exhalation from the earth’s surface, therefore 212 Pbactivity concentrations reflect local conditions (at height of about1 km), contrary to 222 Rn (half-life of 3.82 d) progenies 214 Pb(half-life of 26.8 min) and 210 Pb (half-life of 22.3 y). The residencetimes of radon decay products in the ambient air were estimated tobe similar to that of 7 Be (about 8 days). About 76% of the 214 Pbactivity and 67% of the 212 Pb activity were usually associated withaerosol particles in the 0.08e1.4-mm size range, though a small shiftin the aerosol size distribution was observed for214 Pb(Papastefanou, 2009). Seasonal variations of212 Pb and 210 Pbisotopes were found to be distinctly different in the lower atmospherewhile the behaviour of 214 Pb was similar to that of 210 Pb. Itwas concluded therefore that airborne concentrations of 212 Pb,contrary to those of 210 Pb could be strongly influenced by localemissions (Sheets and Lawrence, 1999). This behavior and shift inthe size distribution can be attributed to the variation in half-livesof Pb isotopes and their parents. Both parents are gaseous speciesbut their different half-life under particular local conditions such asexhalation, mixing height and intensity can result in their variousvertical and horizontal transports.222 Rn of longer half-life hasa higher potential to be more widely distributed vertically andhorizontally. On the other hand, due to different half-lives of Pbisotopes, they can serve for indication of events occurring on0131I, µBq/m 3different time scales. Kownacka (2002) reported that activityconcentrations of 210 Pb were almost constantly distributed above1 km, and did not decrease with altitudes. An increase in 210 Pb andother natural radionuclide concentrations in the stratosphere wasalso observed after the large volcanic eruptions. Abe et al. (2010)showed that distributions of the 7 Be and 210 Pb nuclides wereuniform in the range of a few hundred kilometers in the horizontaldirection and up to w1 km height, whereas 212 Pb activity concentrationsvaried greatly depending on the geographical location andaltitude of the observation site. The recent studies indicateda similar behavior of 7 Be and 210 Pb, and that they cannot be used asindependent tracers to study atmospheric processes. Although thebehavior of 212 Pb and 214 Pb is not fully understood yet, they can beused as independent atmospheric traces. A weak correlation wasfound in 7 Be and 212 Pb records (0.39), while no correlation wasobserved for 7 Be and 214 Pb ( 0.16), indicating different sources ofPb isotopes. The 212 Pb and 214 Pb records may therefore indicatedifferent atmospheric processes. Most probably 212 Pb is an indicatorof horizontal transport at lower heights (up to 1 km),however, a weak correlation showed that this transport waslimited. On the other hand, 214 Pb represents short time events atour site. An increase in the activity ratios of 7 Be/ 212 Pb and 7 Be/ 214 Pb,accompanied by an increase in 137 Cs and 131 I activity concentrationsobserved during the studied period may indicate that the dominantsource of Fukushima originated radionuclides at our site was athigher altitudes. Therefore, an increase in the 7 Be/ 212 Pb and7 Be/ 214 Pb activity ratios in this case can be used for an indication ofthe downward air mass transport.The activity concentration of 137 Cs as well as the 134 Cs/ 137 Cs,7 Be/ 212 Pb and 7 Be/ 214 Pb activity ratios in aerosol samples in thestudied episode after the Fukushima accident are presented inFig. 4. The 134 Cs/ 137 Cs activity ratio in Vilnius was close to 1 (N ¼ 30,Mean ¼ 0.782, S.D. ¼ 0.345, Median ¼ 0.938). In samples collectedon 24 March, and from 26 to 27 March, the activity concentration of134 Cs was below the detection limit. In the most active samplecollected on 3e4 April the 134 Cs/ 137 Cs activity ratio was equal to1.00 0.05.Fig. 5 shows the wind speed and wind vectors indicating the jetstream at 500 hPa for 24e25 March which affected the transport ofthe Fukushima plume to Europe. The strong meanders on the jetstream resulted in the downward air mass transport, as it is indicatedby an increase in the 7 Be activity concentrations (Fig. 2), aswell as by an increase in the activity ratios of 7 Be/ 212 Pb and7 Be/ 214 Pb (Fig. 4). A similar increase in the activity concentrations of131 I,137 Cs and 7 Be, together with enhanced activity ratios of7 Be/ 212 Pb and 7 Be/ 214 Pb observed on 23, 24, 27 and 31 March, as137Cs µBq/m310001001010.1137 Cs7 Be/ 212 Pb134Cs/ 137 Cs7 Be/ 214 Pb24 26 28 30 1 3 5 7 9 11 13March April 20111001010.10.01Fig. 4. Activity concentration of 137 Cs and 134 Cs/ 137 Cs, 7 Be/ 212 Pb and 7 Be/ 214 Pb activityratios in aerosol samples in Vilnius in 2011.Ratio of radionuclidesPlease cite this article in press as: Lujanien_e, G., et al., Radionuclides from the Fukushima accident in the air over Lithuania: measurement andmodelling approaches, Journal of Environmental Radioactivity (2012), doi:10.1016/j.jenvrad.2011.12.004
G. Lujanien_e et al. / Journal of Environmental Radioactivity xxx (2012) 1e10 5Fig. 5. The wind speed, wind vectors and relative humidity at 500 hPa for 24e25March and 3 April, 2011.well as on 4 April, can be interpreted as a downward transport of airmasses carrying the Fukushima plume radionuclides from higherlayers of the troposphere (Figs. 2 and 4). The low relative humidityover the sampling site (Fig. 5) observed at 500 hPa on 4 April canserve as an additional confirmation of air flow from the upper levelsof the atmosphere.A slightly different pattern of the radionuclide record observedfrom 28 to 31 March can be explained by the effect of precipitation(Fig. 2) that could remove the Fukushima derived radionuclides and/or preferably wash out aerosol particles carrying cosmogenic 7 Bedue to their different chemical composition and size distribution(Lujanien_e et al., 1998; Lujanien_e, 2000, 2003). Another possibleexplanation can be variations of the transport altitudes and arrivaltime of the Fukushima radioactive particles. The NOAA HYSPLITmodel (Draxler and Rolph, 2011) was used to assess the transportpattern and to explain the deviation in radionuclide activityconcentrations found in Vilnius. A large number of air mass backwardtrajectories were calculated over the time of interest. The mosttypical trajectories, capable to provide a proper interpretation of theobserved radionuclide variations (Fig. 6), show backward air masstransport starting at 500 (red triangles), 3000 (blue squares), and7000 m (green circles) above the ground level (AGL). Trajectories arelabelled every 24 h by a filled symbol. The vertical projection of thetrajectories with time is shown in the panel below the map. The airmass backward trajectories calculated for 30 March can serve as anexample of complicated pathway of air masses (Fig. 6A). The backwardtrajectories were calculated for three 500, 3000 and 5000 mAGL for 315 h. The air masses at 500 m were caught up into a cyclonicsystem, while air masses at 3000 and 5000 m were lifted and rapidlytransported over the North America to Europe. It seems thatradioactive particles have had a greater chance of being transportedat higher atmosphere levels. They can be removed in the lower layerof the atmosphere due to various reasons, e.g. rainfall characteristics,fog formation or growth of aerosol particles and their deposition.Thus, there was a higher probability that activityconcentrations of radionuclides found on 28e31 March werediluted by clean air masses, and/or they were reduced by precipitationin the near-surface level and/or marine boundary layer(w1 km) during their long-range transport from Japan.The air masses which arrived on 1e2 April were affected both bycyclone and anticyclone systems, and they brought rather clean airto Europe (Fig. 6B). During these days the activity concentrations of131 I and 137 Cs dropped to 150e190 mBq/m 3 and 8e16 mBq/m 3 ,respectively (Fig. 2), however, on 4 April they again rose up to2280e3690 mBq/m 3 and 609e1026 mBq/m 3 , respectively. It shouldbe noted that an increase in radionuclide activities was detected inthe most European countries (Masson et al., 2011).In order to explain the origin of the second maximum in theradionuclide courses (Fig. 2), the air mass backward trajectorieswere calculated for 1000, 3000 and 5000 m heights. The resultsshowed (i) a direct transfer from Fukushima across the PacificOcean, (ii) a transport through the North Pole, and (iii) a pathwaythrough the Greenland and Iceland (Fig. 6. C). The air masses at1000 and 5000 m were rapidly transported, while the air masses at3000 m exhibited rather slow transport, and most probably theseair masses provided a transfer of contaminated air already presentover the Greenland. These results are in good agreement with theprognoses made by the CTBTO (Wotawa, 2011) explaining twomaxima of the Fukushima plume observed over Europe.We can conclude that the measured activity concentrations at thesite of investigation resulted from a complicated air mass transport,arrival time, arrival height, meteorology and downward air masstransport. The downward transport was found to be an importantfactor affecting activity concentrations in the surface air. Higheractivities can be transported over long distances at higher altitudesPlease cite this article in press as: Lujanien_e, G., et al., Radionuclides from the Fukushima accident in the air over Lithuania: measurement andmodelling approaches, Journal of Environmental Radioactivity (2012), doi:10.1016/j.jenvrad.2011.12.004
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