# Variation in airborne 134Cs, 137Cs, particulate 131I and 7Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses

O. Masson, J. Bieringer, E. Brattich, A. Dalheimer, S. Estier, I. Penev, W.

Ringer, C. Schlosser, T. Steinkopff, P. Steinmann, et al.

► To cite this version:

O. Masson, J. Bieringer, E. Brattich, A. Dalheimer, S. Estier, et al. Variation in airborne 134Cs, 137Cs, particulate 131I and 7Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses. Journal of Environmental Radioactivity, 2016, 162-163, pp.14-22. 10.1016/j.jenvrad.2016.05.004. hal-02559797

HAL Id: hal-02559797

https://hal.science/hal-02559797v1

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Variation in airborne <sup>134</sup>Cs, <sup>137</sup>Cs, particulate <sup>131</sup>I and <sup>7</sup>Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses

Olivier MASSON a*, Jacqueline BIERINGER b, Erika BRATTICH c, Axel DALHEIMER d, Sybille ESTIER e, Ilia PENEV f, Wolfgang RINGER g, Clemens SCHLOSSER b, Thomas STEINKOPFF d, Philipp STEINMANN e, Laura TOSITTI c, Pieter VAN BEEK h, Anne de VISMES-OTT i

a Institut de Radioprotection et de Sûreté Nucléaire (IRSN), BP 3, 13115, S'Paul lez Durance, Cedex, France

b Bundesamt für Strahlenschutz (BfS), 79098 Freiburg, Germany

c Laboratorio di Chimica e Radioattività ambientale, University of Bologna, 40126 Bologna, Italy

d Deutscher Wetterdienst (DWD), Referat TI 24, Frankfurterstraße 135, 63067 Offenbach, Germany

e Federal Office of Public Health, (BAG), Schwarzenburgstrasse 165, CH-3003 Bern, Switzerland

f Institute for Nuclear Research and Nuclear Energy (INRNE), 72 Tzarigradsko chaussee, Blvd Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria

g Austrian Agency for Health and Food Safety (AGES), Radon and Radioecology, 4020 Linz, Austria.

h Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS), Observatoire Midi Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France

i Institut de Radioprotection et de Sûreté Nucléaire (IRSN), LMRE, 501, Bois des Rames, 91400 Orsay, France.
Variation in airborne <sup>134</sup>Cs, <sup>137</sup>Cs, particulate <sup>131</sup>I and <sup>7</sup>Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses

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ABSTRACT: The Fukushima-labeled air mass arrival, and later the cesium-134 (<sup>134</sup>Cs), cesium-137

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(<sup>137</sup>Cs) and particulate iodine-131 (hereafter noted <sup>131</sup>I<sub>p</sub>) maximum levels were registered in Europe at

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different dates depending on the location. Most of those data were obtained at low-altitude sampling

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areas. Here, we compare the airborne levels registered at different high-altitude European locations

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(from 850 m to about 3,500 m). The integrated <sup>137</sup>Cs activity concentration was not uniform with

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regard to the altitude even after a long travel time/distance from Japan. Moreover, the relation of

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integrated <sup>137</sup>Cs vs. altitude showed a linear decrease up to an altitude of about 3,000 m. A similar

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trend was noticed for <sup>131</sup>I<sub>p</sub> (particulate fraction) while it increased above 3000m. Comparison with <sup>7</sup>Be

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activity concentration showed that, as far as the high altitude location is concerned, the <sup>137</sup>Cs and <sup>134</sup>Cs

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maximum concentrations corresponded to the <sup>7</sup>Be maximum, suggesting downdraft movements from

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high tropospheric or stratospheric layers to be responsible for <sup>137,134</sup>Cs increase and peak values. This

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was also confirmed by high potential vorticity and low relative humidity registered during the peak

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values.

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Keywords: Fukushima, High-altitude, Vertical distribution, Radionuclide, <sup>134</sup>Cs, <sup>137</sup>Cs, Europe

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# 1. INTRODUCTION

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Most of the sampling locations spread in Europe for monitoring purposes of nuclear facilities (more or

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less dense local networks) as well as for surveillance programmes on a nation-wide scale (sparse

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network), are set up at low-altitude sites or below 500 m (all elevations mentioned in this study are

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above sea level; a.s.l). Most locations fulfil practical and operational reasons (proximity to the

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operator for sampling and maintenance) or can be justified by their representativeness for densely

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inhabited regions. At higher elevations, the monitoring of the activity concentrations resulting from

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the vertical distribution of either routine or accident release of radionuclides remains limited. Most

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information at high-altitude derives either from a handful of high-altitude sampling stations or from

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dispersion models. Europe hosts several high-altitude meteorological and research observatories that

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are valuable sampling locations for radionuclide monitoring (Angelov et al., 2011; Bourcier et al.,

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2010, 2011; Evrard et al., 2012; Flury and Völkle, 2008; Gerasopoulos et al., 2001; Masson et al.,

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2015; Steinkopff et al., 2012; Tositti et al., 2004, 2012, 2014). Among the radionuclides that are

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monitored on a routine basis, <sup>7</sup>Be and <sup>210</sup>Pb have provided comprehensive information about air mass

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movements. Before the Fukushima accident, traces of <sup>137</sup>Cs originating from resuspension of fallout

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from the Chernobyl accident and nuclear test global fallout was still measurable at trace levels; below

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1 µBq m<sup>-3</sup> in Central Europe and less (<0.3 µBq m<sup>-3</sup>) in northern and southern Europe (Masson et al.,

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2011). On the contrary, <sup>134</sup>Cs, also released by the Chernobyl accident, had not been measured in the

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atmosphere since the middle of the 1990s due to its fairly short half-life of 2.06 y. Apart from

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Switzerland or Germany, almost all European laboratories abandoned high tropospheric and

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stratospheric surveillance in the early 1980s (Corcho Alvarado et al., 2014). In the event of a nuclear

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accident, high-altitude stations could provide a broader representativeness (at least regional) compared

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with lowland stations, in addition to vertical dispersion observations. In a previous study (Masson et

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al., 2011) we concluded that maximum activity concentration of Fukushima-derived <sup>134,137</sup>Cs and <sup>131</sup>I

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fraction attached onto aerosol particles (hereafter noted <sup>131</sup>I<sub>p</sub>) over Europe showed fairly homogeneous

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values with respect to elevation while noticeable differences up to a factor of 6 were detected in the

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horizontal plane thanks to the large number of sampling locations (about 150). In this work we present

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a reanalysis of the previous dataset supplemented with observations from additional high-altitude

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stations that make it possible to point out details also on the vertical distribution of both natural and

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artificial γ-emitters. More specifically this study focuses on the overall lower troposphere, from the

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planetary boundary layer up to about 3,500 m based on an unique European monitoring platform for

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the large scale distribution of radioactivity. Most of the data presented here were provided by

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organizations or laboratories involved in the Ring of Five (Ro5) European network dedicated to
radionuclide trace determinations in the atmosphere (Masson et al., 2011). After the nuclear accident at the Fukushima Daiichi nuclear power plant (FDNPP), aerosols were also collected by Tiger F-5E/F military airplanes up to about 8,000 m above Switzerland, measuring <sup>131</sup>I at 2.3 mBq m<sup>-3</sup>, at the end of March 2011(Estier and Steinmann, 2012). Two specific aircraft missions were also conducted over northern Germany by the Deutsches Zentrum für Luft und Raumfahrt (DLR) on March 23 and 30 and April 14, 2011. During the second mission, activity concentrations clearly exceeded the detection limit. During these experiments it was also possible to detect <sup>133</sup>Xe from the FDNPP above the tropopause (Schlager et al., 2014; Simgen et al., 2014).

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## 2. MATERIAL AND METHODS

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During the Fukushima accident crisis, a dozen high-altitude aerosol sampling stations, located between 850 and 3,454 m, provided airborne activities across Europe (Table 1 and Fig. 1). The most widely reported Fukushima-labeled radionuclides were <sup>134</sup>Cs, <sup>137</sup>Cs and <sup>131</sup>I (half-lives of 2.06 y, 30.17 y and 8.02 d, respectively). Despite the fact that only a vertical profile and in-depth local climatology analysis for each site could provide the free troposphere extent (Tositti et al., 2014; Venzac et al., 2009), it can be assessed that in March/April all the high-altitude European sites mentioned in this study were above the free troposphere. The Jungfraujoch research observatory (3,454 m a.s.l) is the highest European location where radionuclide determinations were performed (Flury and Völkle, 2008). The number of high-altitude sampling sites devoted to radionuclide monitoring represented at most 5% of the total number of aerosol sampling locations that provided airborne activities (at least one result) in Europe in connection with the FDNPP accident. Apart from the Modane sampling equipment which corresponds to the ventilation and filtration system of the LSM underground measurement facility (Loaiza et al., 2012) and the Pic du Midi station equipped with a low flow sampler and Teflon® membrane filter (Evrard et al., 2012), the other sampling stations were typically equipped with a high-volume sampler (flow rate > 60 m<sup>3</sup> h<sup>-1</sup> and up to 1,800 m<sup>3</sup> h<sup>-1</sup>) that collected aerosols on filters of different kinds (glass, glass plus cellulose, polyvinyl chloride or polypropylene fibre filters). All volumes of filtered air were corrected to normalized temperature and pressure (NTP) conditions, i.e. 0°C and 1013 hPa. The sampling duration varied from twice a day at most (Moussala Peak) to daily sampling (Schauinsland, Sonnblick, Modane) or according to adaptive sampling duration of some few days to one week at other locations (Jungfraujoch, Puy de Dôme, Kahler Asten, Wasserkuppe, Zinnwald, Zugspitze). Monte Cimone had bi-daily sampling while Pic du Midi maintained its weekly sampling period. After sampling, filters were pressed into pellets of various diameters depending both on filter and detector sizes. Standards for detector calibration were prepared from the blank filters of each type using spikes of standard mixed radionuclide solutions in suitable geometry. All measurements were performed using coaxial or well-type Hyper Pure Germanium (HPGe) gamma spectrometers. A coincidence summing correction (also called peak sum correction) was applied for the <sup>134</sup>Cs determination. For maximum activity concentrations, uncertainties were typically ranging from 5 to ~ 20% depending on detector and sample features. The Pic du Midi time series exhibited only one value above the detection limit for both cesium isotopes. In this case, the single value (10 µBq m<sup>-3</sup> for <sup>137</sup>Cs; 7 µBq m<sup>-3</sup> for <sup>134</sup>Cs, given at 2-sigma level) was considered as the maximum value.

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## Table 1

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## Figure 1

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## 3. RESULTS AND DISCUSSION

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Time series of <sup>134</sup>Cs, <sup>137</sup>Cs and <sup>131</sup>I<sub>p</sub> are shown on Fig. 2. Airborne <sup>137</sup>Cs activity concentrations were multiplied at most by about 100 to 1000 times depending on the “old” <sup>137</sup>Cs background (i.e., found prior to the accident). It is thus assessed that “old <sup>137</sup>Cs” did not significantly contribute to the ambient concentrations at the time of the peak detection. Based on 117 pairs of <sup>134</sup>Cs and <sup>137</sup>Cs activity concentrations, the <sup>134</sup>Cs/<sup>137</sup>Cs ratio at high altitude locations was quite the same (average 0.88 and median 0.81) than for the ratio obtained at low altitude sites and corresponding to 662 pairs of <sup>134</sup>Cs

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105 and ¹³⁷Cs data used in Masson et al. (2011) (Fig. 3). This confirms that most of the ¹³⁷Cs was coming
106 from the damaged power plant.

107 All arrival dates of the Fukushima-labeled air masses must be cautiously considered since they
108 corresponded to rising activities and thus depended on the low-level sensitivity of measurement
109 capabilities of each participant and on the sampling duration. For maximum activities, the amount of
110 radionuclides collected on the filters clearly exceeded the detection limit regardless of the detector
111 sensitivity. Regarding the sampling duration, weekly sampling is expected to have a major impact on
112 the maximum concentration values; the longer the sampling period, the weaker the maximum value.
113 Accordingly, it was not possible to investigate properly the variation of the maximum values with
114 regard to the altitude sites. We also tried to compare activity concentrations at high-altitude stations
115 with those from closest low-altitude sites. The distance to the closest site ranged between 12 and 170
116 km but even for short distances, this approach was not conclusive, demonstrating a high horizontal
117 variability similarly to what observed at low-altitude stations (Masson et al., 2011; Steinhauser et al.,
118 2013). This results from atmospheric circulation complexity induced by orographic influence of
119 mountain sites, below-cloud precipitation scavenging and to the decoupling of the planetary boundary
120 layer from the free troposphere in the considered season. Apart from ⁷Be activities which were clearly
121 higher at high altitude sites, this comparison was not conclusive for the FDNPP-labeled radionuclides,
122 i.e. the ¹³¹Iₚ maximum activity was sometimes either lower in altitude or similar at both sites and even
123 lower at the low-altitude location. Gaseous and particulate iodine behave differently with respect to
124 solar radiation resistance (Kulyukhin et al., 2005) or scavenging process (Masson et al., 2013).
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126 Figure 2

127 Figure 3

128 To cope with the sampling duration effect, we finally considered the cumulative amount of Becquerels
129 of a given radionuclide. This corresponds to the integrated plume concentration (i.e., the area under
130 the time series curve) that went by each high-altitude sampling site during the whole detection period
131 (Fig. 4). Both cumulative radionuclide variations show a decrease with the altitude at least for altitudes
132 below 3,000 m which indicates that the total amount of a given radionuclide differed with regard to the
133 altitude, i.e. high-altitude sites were not exposed to the same amount of Fukushima-derived
134 radionuclide despite the long distance or travel time from Japan.
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137 Figure 4

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139 3.1. Radioactive plume overview

140 During and after the FDNPP accident, the radioactive plumes spread throughout the global
141 atmosphere, mostly in the Northern Hemisphere following different routes at different average wind
142 speeds and different transport altitudes. While most of the radionuclides spread eastward over the
143 Pacific Ocean by prevailing winds, one route was south-westward. Before leaving Japan the
144 radioactive plume met with high-altitude locations, providing an idea of the elevation to which it
145 extended at medium distances from the damaged plant. Maximum gamma-ray dose rates ranging from
146 0.3 to 0.5 µSv h⁻¹ were found in April 2012 between approximately 900 and 1,100 m above sea level
147 on the slopes of Mount Nakimushi-yama in the Nikko mountain area, at the northern rim of the Kanto
148 Plain and about 160 km southwest of the FDNPP (Hososhima and Kaneyasu, 2015). These authors
149 showed that on March 15 afternoon, the corresponding altitudes were embedded in a cloud layer
150 suggesting that cloud/fog deposition was the likely explanation for the observed maximum. Cesium
151 isotopes were also investigated along the slopes of the Mt. Fuji at about 310 km south west to the
152 FDNPP showing that the plume stayed below about 2,500 m (Saito et al., 2014).

153 Outside Japan but still in the Asian continent, samples collected at low-altitudes and at a high-
154 mountain site (2,862 m) indicated that the radionuclides were transported to Taiwan via two pathways
155 at different altitudes (Huh et al., 2012): one within the planetary boundary layer (typically from ground
156 level to several thousand meters high) by the northeast monsoon wind directly to Taiwan and the other
157 in the free troposphere (i.e. altitudes typically above 1,500 – 2,000 m) by the prevailing westerly large
158 scale circulation. Similarly, Fukushima-labeled radionuclides were first conveyed to south-western
159 China in 13 days by high-altitude westerlies followed by high-altitude downdraft to lower altitudes by
160 an easterly pathway (Wan et al., 2014). Similar analyses performed by Liu and co-workers (2013) found that the Fukushima-contaminated air masses tended to be quickly transported westward at altitudes higher than 3,000 m. The same westerlies were responsible for the arrival of the Fukushima-derived radionuclides in South Korea after their travel over Europe through high-altitude tropospheric downdraft (Hong et al., 2012).

165 The plume reached the eastern US and Canada between 15 and 18 March (Biegalski et al. 2012; Chang et al. 2011; Diaz Leon et al., 2011; Zhang et al. 2011). To our knowledge, no field data about the vertical distribution of Fukushima-derived radionuclides or at high-altitude sites over North America has been reported. Europe was reached by north-westerlies from March 20 to 25 depending on the location (Masson et al., 2011). Apart from continental Europe, aerosols sampled at 300 m and 2,400 m, in Tenerife Island (North Atlantic Ocean 1,200 km southwest from Spain) showed higher maximum activities of <sup>131</sup>I, <sup>137</sup>Cs and <sup>134</sup>Cs at low altitude (López-Pérez et al., 2013). Surface air activity measurements around the globe offered the opportunity to track the mid-latitude surface westerlies, covering the Earth circumference in less than 21 days (Hernández-Ceballos et al. 2012). Far away from the emission site some significant differences in the arrival time of the contaminated air masses may be a priori expected between ground-level and high-altitude sites due to intense free troposphere winds. Aircraft measurements over Switzerland and Germany eventually detected <sup>131</sup>I<sub>p</sub>, <sup>137</sup>Cs and <sup>134</sup>Cs at 8,000 m (Estier and Steinmann, 2012) as well as <sup>133</sup>Xe across the tropopause (Schlager et al., 2014; Simgen et al., 2014) demonstrating that the whole troposphere thickness (at least in the Northern Hemisphere) was labelled by the radioactive releases. It must be noted that, as noble gases, xenon and krypton are not affected by wet removal processes and may have spread at higher altitudes than cesium isotopes which remains attached to aerosols and thus are instead highly sensitive to precipitation scavenging and sedimentation processes (Nitta et al., 2014; Stohl et al. 2012). It can be assumed that this scheme also stands for mid- and upper tropospheric altitudes as for the altitude range covered by the high-altitude European locations. The Fukushima-derived plume reached Mt. Lulin, Taiwan, located within the free troposphere (2862 m) at an average wind speed of about 22 m s<sup>-1</sup> by westerlies after a round trip across the globe, whereas the average wind speed at ground level was only 3 m s<sup>-1</sup> for low-altitude winds blowing directly from Japan to Taiwan (Huh et al., 2012).

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189 3.2. Reported numerical simulations

190 Information for the pathway altitudes of the Fukushima plume was also provided by dispersion models reported in various publications. Based on backward trajectory computations and a Lagrangian model, Povinec et al. (2013a, 2013b; 2013c) found that contaminated air masses travelled to Europe at an average wind speed of 50-70 km h<sup>-1</sup> (~ 1,200-1,700 km d<sup>-1</sup>) consistently with the reported value cited above. Depending on the location of interest, transport height differed but the FDNPP-labeled air masses tended to be quickly transported above 3,000 m (Liu et al., 2013) and at mid- (~ 5 km) or upper troposphere by jet stream (Takemura et al. 2011). Based on <sup>133</sup>Xe measurements near or above the tropopause and model simulations, it was found that a west-Pacific warm conveyor belt lifted rapidly the eastern Asian planetary boundary layer up to the tropopause and above, followed by relatively fast eastward intercontinental and quasi horizontal advection of the contaminated plume (Schlager et al., 2014; Simgen et al., 2014). Air mass trajectories at various pressure levels have shown that particles released in different times and altitudes had various trajectories (Povinec et al., 2013a, 2013c). Some of these pressure-level dependent trajectories affected Europe; some did not, depending on the considered date. As for <sup>133</sup>Xe observations, those simulations also indicate that the lower the pressure level, the sooner the arrival date in Central Europe. Using Lagrangian dispersion model, it was shown that the dominant air mass transport for Central Europe preferentially occurred at a 500 hPa pressure level (about 5,500 m) (Povinec et al., 2013a, 2013c). It can be summarized that soon after the release, contaminated air masses rose quickly then travelled faster at high-altitude and that the whole air column up to the tropopause was affected.

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210 3.3. Arrival dates of maximum activity concentrations

211 Overall <sup>134</sup>Cs or <sup>137</sup>Cs maximum coincided with <sup>131</sup>I<sub>p</sub> maximum especially when the sampling period was long (typically one week). However, when looking at daily information when available and taking into account uncertainties associated with <sup>134,137</sup>Cs and <sup>131</sup>I<sub>p</sub> maximum values, it was noticed that at four locations out of twelve, <sup>131</sup>I<sub>p</sub> maximum occurred sometimes slightly ahead (Table 2) compared
with the date of the ¹³⁴,¹³⁷Cs maxima. This time-lag was at most two days and only detectable for high time resolution samplings. This time-lag cannot be ascribed to the fact that ¹³¹I has a higher (~ 2 ) detection efficiency than ¹³⁷Cs since ¹³⁷Cs was already detectable from time to time at most sampling locations before the arrival of the Fukushima-labeled air masses. Moreover from the first ¹³¹Iₚ detection date and as far as ¹³¹Iₚ was visible at a given high-altitude location, the ¹³¹Iₚ / ¹³⁷Cs ratio was always higher than 1. Based on 110 couples of ¹³¹Iₚ (not decay-corrected) and ¹³⁷Cs values obtained at the presented high-altitude locations, the mean and median values of ¹³¹Iₚ / ¹³⁷Cs ratio were 16.8 and 11.2, respectively. However, owing to the limited number of high-altitude stations, this finding was tested against the results from low-altitude stations with a consistency of about 100 sites (Masson et al., 2011). Most of maximum activities were obtained simultaneously for both cesium isotopes and ¹³¹I. However, in 30% of cases, the ¹³¹Iₚ maximum activity was again found ahead of the ¹³⁷Cs maximum activity. This finding may result from the multiple release scenarios including a sequence of venting and/or explosions with different ¹³¹I / ¹³⁷Cs ratios during several weeks (Katata et al., 2015). Another explanation may be linked to the effect of highly variable meteorological conditions especially rains, encountered during the long time span of FDNPP atmospheric emissions and during transport. The behavior of Fukushima-labeled particles and gaseous species may especially be different regarding wet scavenging. Simgen and co-workers (2014) also demonstrated that ¹³³Xe at very high altitude over Germany appeared several days before the ground-level plume showing evidence for a faster transport at such high altitudes. Additionally, it can be seen that the dates of the maximum activities varied from 28-30 March to 5 April, thus about one-week shift depending on the location.

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237 Table 2

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In order to gain a deeper insight on high altitude influence, ¹³¹I and ¹³⁷Cs were compared for the European locations investigated in this study with those obtained at about one hundred low-altitude sites from the data set mentioned in Masson et al. (2011). Despite it corresponds to the dominant fraction, no gaseous ¹³¹I determination was performed at high-altitude sites due to the lack of gaseous sampler; thus only the particulate ¹³¹I fraction (¹³¹Iₚ) was considered here for the sake of comparison. High-altitude locations seem to exhibit on average higher ¹³¹Iₚ than for the whole low altitude data set. This potentially indicates a possible relative enrichment of particulate iodine in comparison with ¹³⁷Cs at high altitudes (Fig.5). Elsewhere from Europe, observations in Dalat, Vietnam, at 1,500 m were also characterized by a ¹³¹Iₚ / ¹³⁷Cs ratio higher than those in Hanoi and Ho Chi Min city, both located at low-altitudes (Long et al., 2012). A proper ¹³¹I vs. ¹³⁷Cs relationship standing for high-altitude sites would have required the knowledge of the dominant gaseous ¹³¹I component. Since this component was missing for high-altitude sites, such possible higher ¹³¹I/ ¹³⁷Cs ratio at high-altitude remains to be investigated.

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253 Figure 5

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It should be reminded that ¹³¹I data shown here, including those obtained at low-altitude sites, correspond solely to the particulate fraction.

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3.4. Comparison between ¹³⁷Cs, ¹³¹Iₚ and ⁷Be time series

Beryllium-7 (⁷Be; half-life = 53.3 d) is recognized as a powerful tracer of tropospheric downdrafts or stratosphere-troposphere air exchanges (STE) (Cristofanelli et al., 2006, 2009; Danielsen, 1968; Gerasopoulos and Zanis, 2006; Jasiulionis and Wershofen, 2005; Papastefanou, 2008; Wan et al., 2014). Beryllium-7 is produced in the upper atmospheric layers (roughly 1/3 in the upper troposphere and 2/3 in the stratosphere (Papastefanou, 2008) by spallation reactions of light atmospheric nuclei such as carbon, oxygen, nitrogen and argon with the high-energy nucleonic component of the galactic cosmic ray-induced atmospheric cascade (Lal and Peters, 1967; Usoskin et al., 2009). More generally, ⁷Be activity concentration at a given location provides integrated information on both production areas and transport (Leppänen et al., 2010). However, considering high-altitude sites, it is assessed that as a result of the proximity to the production layers, the higher the altitude the higher the ⁷Be activity concentration (Steinmann et al., 2013). Here we compare the time series of ⁷Be with that of ¹³⁷Cs and
¹³¹Iₚ to check for the arrival of Fukushima-labelled radionuclides by downwind air masses already mentioned in modelling studies and test the assumption of a ¹³⁴,¹³⁷Cs and ⁷Be maximum concomitance. For readability purpose, ¹³⁷Cs activity concentrations were multiplied by 10 in Fig. 6. Iodine-131 data in this work refers only to the aerosol-borne fraction as previously explained. Less emphasis is put instead on the total ¹³¹I values (gaseous + particulate), since the central focuses of this work are the patterns of times series and dates of maximum activity. All maximum activities of ¹³⁴,¹³⁷Cs and ¹³¹Iₚ match with a ⁷Be increase, except for the Jungfraujoch results showing a small shift between maximum periods for ⁷Be and ¹³⁴,¹³⁷Cs. This finding indicates that the increases leading to the maximum values of the Fukushima-labelled radionuclides were due to downward transport from the upper atmospheric layers. Following the dates for ¹³⁷Cs and ¹³¹Iₚ maximum activities and especially after April 7, other ⁷Be maximums were detected but the reduction of the releases and the atmospheric dispersion led to decreasing airborne activities that did not coincide anymore with ⁷Be maximums. A positive correlation between the Fukushima-derived radionuclides and ⁷Be was also found in Vilnius, Lithuania, as an indication of their arrival from the upper tropospheric layers (Lujanienė et al., 2012). Backward trajectory computations and Lagrangian model simulations performed by Povinec and co-workers (2013a, 2013b, 2013c) made it possible to show that the transport altitude of contaminated air masses travelling to Europe was first at a pressure level of about 500 hPa (~ 5500 m) on March 20 and then reached a lower altitude (700 hPa on March 30; 850 hPa on 13 April). This finding matches with our observations. However, since ⁷Be concentrations are largely determined by the fate of its carrier aerosol, strongly affected by wet/dry scavenging, and exhibit strong seasonal and inter-annual variations driven by different controlling factors (e.g., solar activity, circulation patterns, and meteorology), ⁷Be cannot be considered as an unambiguous stratospheric tracer (Cristofanelli et al., 2009; Steinmann et al., 2013). For this reason, we investigated other stratospheric tracers, such as potential vorticity (PV) as initially proposed by Danielsen (1968), and we visually investigated NCEP/NCAR reanalysis maps (Kalnay et al., 1996) of relative humidity, omega, and vector wind values. The omega parameter is used in meteorology to represent the rate of change of pressure over time (dp/dt) (in Pa/s). It is therefore a vertical velocity in pressure-coordinates and is negative for ascending motion and positive for descending motion. Thresholds to detect stratospheric intrusions usually include at least one other stratospheric tracer together with ⁷Be (Cristofanelli et al., 2006, 2009) such as PV (rapidly increasing with altitude and reaching typical values from 1.0 pvu to 3.5 pvu at tropopause level, where 1 pvu = 1 potential vorticity unit = $1 \times 10^{-6} \text{ m}^2 \text{ K kg}^{-1} \text{ s}^{-1}$), relative humidity (as known stratospheric layers are characterized by extremely low water vapour contents), or tropopause height. PV values (and corresponding heights) were analysed along the 6-day three dimensional FLEXTRA back-trajectories (Stohl et al., 1995) arriving at Jungfraujoch, Moussala Peak and Mt. Cimone. This analysis confirmed that a stratospheric signal (PV > 1.6 pvu and heights above 5000 m) was detected at all sites, with different arrival dates: 18 March, 20 March, 04-05 April, and 08 April 2011 at Jungfraujoch, 20 March, 27 March, 02 April and 05 April at Mt. Cimone, and 06-09 April 2011 at Moussala Peak. The analysis of vector winds (maps provided as supplementary data) indicate that the jet stream appeared more strongly over Europe in the period 01-09 April 2011 (particularly over the region of the Moussala Peak), whereas relative humidity and omega values (maps provided as Supporting Information) showed stratospheric signals over large parts of Europe on 02 April, and 07-09 April 2011, supporting the thesis that ⁷Be and ¹³⁴,¹³⁷Cs maximums during that period were due to downwind transport from upper troposphere-lower stratosphere levels. The correlation between ¹³⁴,¹³⁷Cs and ⁷Be activities at ground level was regularly observed during the nuclear weapon test (NWT) era (1945-1980) and until the beginning of the 1990s. Depending on the bomb yield, most radionuclides spread either within the stratosphere or at least at high tropospheric altitudes. Until the beginning of the 1990s, usual ⁷Be increases encountered at ground level in spring and summer in Europe (Ajtic et al., 2013; Błazej et al., 2014; Gerasopoulos et al., 2001; Kulan et al., 2006; Steinmann et al., 2013; Tositti et al., 2014) due to downdrafts coincided with ¹³⁷Cs increases (Bouisset et al., 2004). Since, it is generally considered that ¹³⁷Cs is no more correlated with ⁷Be at ground level. After the Chernobyl accident, Kownacka and Jaworowski (1994) identified that a small fraction of its releases entered the lower stratosphere. The concentrations of radiocesium in the stratosphere between 1986 and 1991 were higher than prior to the Chernobyl accident, and since the end of 1987 they were always higher in the stratosphere than at any tropospheric altitude. It has been recently shown that residual plutonium traces are still present in the stratosphere (at higher
concentrations than at ground level) and could be injected in tropospheric layers during STE events (Corcho Alvarado et al., 2014). Since the stratosphere is thermally stratified and separated from the troposphere by the tropopause, radioactive aerosols have a longer residence time in the stratosphere (Corcho Alvarado et al., 2014). Moreover, the same authors recently upgraded the mean residence time to 2.5–5 years for plutonium and ¹³⁷Cs in the stratosphere.

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Figure 6

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## 4. Conclusions

This study shows that despite the long travel distance from Japan to Europe, the vertical distribution of Fukushima-labeled ¹³⁷Cs activities was not homogeneous, this being at least true between 800 and 3454 m. Four out of twelve high-altitude sites shows a ¹³¹Iₚ maximum activity concentration prior to the ¹³⁷Cs maximum activity concentration. Such finding was also found in 30% of low-altitude sites. This finding remains to be investigated more deeply but is probably induced by the dominant gaseous ¹³¹I fraction that was unfortunately not monitored at high-altitude sites and only in about 30% of the low-altitude sites and that is less sensitive to scavenging process than the particulate fraction. High-altitude European locations seemed to exhibit similar ¹³¹Iₚ/¹³⁷Cs ratios compared with ratios obtained at low-altitude locations. It should have been more interesting to investigate the ¹³¹Iₜotal/¹³⁷Cs ratio. This was not possible again because of the lack of gaseous sampler at high-altitude sampling sites. In the light of this radiological event, this lack should be reconsidered by implementing gaseous sampler that could be triggered on in such situations.

Additionally, it has been demonstrated that maximum activities of both ¹³⁷Cs and ¹³¹Iₚ fraction were induced by downwind air movements from upper atmospheric layers since the maxima of the FDNPP-radionuclides coincided with the ⁷Be maximum activity concentration, together with other stratospheric tracers such as potential vorticity (PV).

As a general consideration, high-altitude sites are prone to intercept a radioactive plume spreading in altitude (Al-Rayyes and Mamish, 1999; Flury and Völkle, 2008) leading to higher airborne activities or deposition in altitude, especially at short or medium distance from a source point. This was also demonstrated for Saharan dust outbreaks that can travel in atmospheric layers well above the planetary boundary layer over the Mediterranean Sea and Europe (Bourcier et al., 2011). The present study confirms the need to consider or reinforce high-altitude aerosol sampling during emergency situations. When possible, high-altitude radionuclide monitoring will complete low-altitude monitoring, providing valuable information on the vertical distribution of airborne activities and on the question whether the plume reached the cloud-base or went beyond. High-altitude radionuclide monitoring is expected to be less dependent on regular lowland activities and thus leads to a broader representativeness, i.e. on regional to nation-wide scale. A faster detection of the contaminated air masses can also be expected at high-altitude locations than at low-altitude due to higher wind speeds at high altitude. However, this demonstration would have required higher sampling frequencies (typically daily since arrival lapse time may be of the order of days). This study also points out the need for increasing the number of gaseous ¹³¹I determinations in the case of a nuclear accident.

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## Notes

The authors declare no competing financial interest.

369

## Acknowledgments

We wish to thank colleagues from European institutes, most of them participating in the Ring of Five (Ro5) network, who took part in the data acquisition and for fruitful discussions. We thank the NOAA/ESRL Physical Sciences Division, Boulder Colorado for providing NCEP/NCAR Reanalysis maps through their Web site http://www.esrl.noaa.gov/psd/. We also wish to express our deep consideration to all Japanese scientists for the huge work performed to improve the overall knowledge about atmospheric dispersion and deposition of Fukushima-derived radionuclides.

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<figure><img src="image_2.png" /><figcaption>Fig. 1: High-altitude sampling locations considered for the Fukushima-labeled air mass monitoring.</figcaption></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2: Time series of airborne particulate <sup>134</sup>Cs, <sup>137</sup>Cs and particulate <sup>131</sup>I concentrations at European high-altitude aerosol sampling sites after the Fukushima-labeled air mass arrival.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3: <sup>134</sup>Cs/<sup>137</sup>Cs ratio at high altitude locations (this study) compared with ratio from low altitude sites (from Masson et al., 2011).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig.4: Variation in cumulative airborne <sup>137</sup>Cs and particulate <sup>131</sup>I activities at high altitude locations across Europe, during the detection period of the Fukushima-labeled air mass.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig.5: Particulate <sup>131</sup>Ip vs. <sup>137</sup>Cs. Comparison between high-altitude sites with low-altitude locations across Europe.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6: $^{7}$Be, $^{134}$Cs, $^{137}$Cs and $^{131}$I$_{p}$ increases at high-altitude locations across Europe. For readability purpose, $^{134}$Cs and $^{137}$Cs values have been multiplied by 10 (see on the right Y-axis). $^{131}$I$_{p}$ data correspond only to the fraction attached onto aerosols.</figcaption></figure>
<table><caption>Table 1: European high-altitude sites with aerosol samplers and radionuclide determinations.</caption><thead><tr><th>N°</th><th>High-altitude site</th><th>Altitude (m a.s.l.)</th><th>Latitude; longitude</th><th>Country</th></tr></thead><tbody><tr><td>1</td><td>Puy de Dôme</td><td>1465</td><td>45°46′N; 02°58′E</td><td>France</td></tr><tr><td>2</td><td>Pic du Midi</td><td>2877</td><td>42°56′N; 00°85′E</td><td>France</td></tr><tr><td>3</td><td>Modane</td><td>1950</td><td>45°17′N; 06°61′E</td><td>France</td></tr><tr><td>4</td><td>Schauinsland</td><td>1200</td><td>47°55′N; 07°54′E</td><td>Germany</td></tr><tr><td>5</td><td>Zugspitze</td><td>2960</td><td>47°25′N; 10°59′E</td><td>Germany</td></tr><tr><td>6</td><td>Kahler Asten</td><td>858</td><td>51°11′N; 08°29′E</td><td>Germany</td></tr><tr><td>7</td><td>Wasserkuppe</td><td>925</td><td>50°30′N; 09°57′E</td><td>Germany</td></tr><tr><td>8</td><td>Zinnwald</td><td>882</td><td>50°44′N; 13°46′E</td><td>Germany</td></tr><tr><td>9</td><td>Jungfraujoch</td><td>3454</td><td>46°32′N; 07°59′E</td><td>Switzerland</td></tr><tr><td>10</td><td>Monte Cimone</td><td>2165</td><td>44°12′N; 10°42′E</td><td>Italy</td></tr><tr><td>11</td><td>Sonnblick</td><td>3105</td><td>47°03′N; 12°58′E</td><td>Austria</td></tr><tr><td>12</td><td>Moussala Peak</td><td>2925</td><td>42°11′N; 23°35′E</td><td>Bulgaria</td></tr></tbody></table>
<table><caption>Table 2: Sampling date or period corresponding to the maximum ¹³⁴,¹³⁷Cs and ¹³¹Iₚ activities at high-altitude locations in Europe, in 2011. Rows underlined in grey indicate ahead maximum ¹³¹Iₚ than for maximum ¹³⁷Cs.</caption><thead><tr><th rowspan="2">N°</th><th rowspan="2">Site</th><th rowspan="2">Sampling frequency</th><th colspan="2">Sampling date or period</th></tr><tr><th><sup>137</sup>Cs max.</th><th><sup>131</sup>I<sub>p</sub> max.</th></tr></thead><tbody><tr><td>1</td><td>Puy de Dome</td><td>weekly</td><td>4-7 April</td><td>4-7 April</td></tr><tr><td>2</td><td>Pic du Midi</td><td>weekly</td><td>29 March-5 April</td><td>22-29 March</td></tr><tr><td>3</td><td>Modane</td><td>daily</td><td>2-3 April</td><td>2-3 April</td></tr><tr><td>4</td><td>Schauinsland</td><td>daily</td><td>5 April</td><td>5 April</td></tr><tr><td>5</td><td>Zugspitze</td><td>variable</td><td>30 March -1 April</td><td>28-30 March</td></tr><tr><td>6</td><td>Kahler Asten</td><td>variable</td><td>28 March -4 April</td><td>24-28 March</td></tr><tr><td>7</td><td>Wasserkuppe</td><td>variable</td><td>28-30 March</td><td>28-30 March</td></tr><tr><td>8</td><td>Zinnwald</td><td>variable</td><td>28-30 March</td><td>28-30 March</td></tr><tr><td>9</td><td>Jungfraujoch</td><td>variable</td><td>29 March -5 April</td><td>29 March-5 April</td></tr><tr><td>10</td><td>Monte Cimone</td><td>bi-daily</td><td>20-22 April</td><td>11-15 April</td></tr><tr><td>11</td><td>Sonnblick</td><td>daily</td><td>3 April</td><td>2 April</td></tr><tr><td>12</td><td>Moussala Peak</td><td>twice a day</td><td>4 April</td><td>4 April</td></tr></tbody></table>
<figure><img src="image_10.png" /></figure>

Jungfraujoch

Pic du Midi

Modane

Zinnwald

Sonnblick

Zugspitze

Moussala Peak

Monte Cimone

Wasserkuppe

Kahler Asten