Effect of dry deposition, washout and resuspension on radionuclide ratios after the chernobyl accident

The Science of the Total Environment, 90 (1990) 1-12 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

1

EFFECT OF DRY DEPOSITION, WASHOUT AND RESUSPENSION ON RADIONUCLIDE

RATIOS

AFTER

THE CHERNOBYL

ACCIDENT

G. ROSNER, H. HOTZL and R. WINKLER

GSF-Institut fi~r Strahlenschutz, D-8042 Neuherberg (Federal Republic of Germany) (Received March 20th, 1989; accepted April 18th, 1989)

ABSTRACT

The temporal variations of radionuclide ratios in air and deposition samples collected simultaneously at Munich-Neuherberg (F.R.G.) after the Chernobyl accident have been studied. Until 8 May 1986, the radionuclides investigated were ~Mo, l°3Ru, l°6Ru, 11°mAg,125Sb,l ~ T e , 13~Te, 131I, 134Cs, 13VCs,14°Ba, raCe and raCe. After 8 May, ~Mo, 11°~Ag, '2~Sb, and the Ce isotopes were below the detection limits. Considerable temporal variations of the above radionuclides, relative to 137Cs, were observed in air as well as in deposition. In air the temporal variations reflect the arrival of different parts of the reactor plume with different elemental composition. In deposition, the temporal patterns were quite different from those in air for a given radionuclide. This is explained by varying contributions of dry and wet deposition. Until 8 May, the washout ratios of the above radionuclides covered a range from 240 to 5600, with smaller variations for all radionuclides within one event (e.g. 460-910), and larger variations from one event to another (e.g. 460-3300 for ~3~Cs). The dry deposition velocity of ~37Cswas found to be 0.27 cm s-2, similar to that of 11°~Ag, aerosol 13'I and 14OBa(0.37, 0.13 and 0.15 cm s-'). Another group of radionuclides includes ~°3Ru, l°~Ru, 125Sb, total ~a~Iand ~32Tewith dry deposition velocities of 0.08, 0.10, 0.07, 0.03 and 0.08cms -~ and with temporal variations in deposition which are quite different from those of the first group. From 8 May to the end of June, the washout ratios increased to values between 1500 and 24000, with the exception of iodine, which had considerably lower washout ratios of between 37 and 4400. These later effects are explained by resuspension and, in the case of iodine, by remobilization of gaseous species. INTRODUCTION The Chernobyl accident has led to a significant increase of the environmental radioactivity in many parts of Europe. The temporal development of artificial radioactivity in air has been followed at several places from which s o m e a c t i v i t y v a l u e s i n d e p o s i t i o n h a v e a l s o b e e n r e p o r t e d (e.g., L a r s e n e t al., 1986; T h o m a s a n d M a r t i n , 1986; B a l l e s t r a e t al., 1987; C a m b r a y e t al., 1987; D e n s c h l a g e t al., 1987; M f i c k , 1988). H o w e v e r , t h e r e s e e m t o e x i s t n o s e q u e n t i a l simultaneous measurements of fallout in air and deposition over extended p e r i o d s o f t i m e . I n t h e p r e s e n t p a p e r w e g i v e a d e t a i l e d a n a l y s i s o f (i) t h e temporal patterns of the fallout radionuclides in air and in simultaneously c o l l e c t e d t o t a l ( w e t a n d d r y ) d e p o s i t i o n s a m p l e s , a n d (ii) t h e r e l a t i o n s b e t w e e n the radionuclides in air and in deposition, which are linked by their respective deposition velocities and washout ratios, all as observed at Neuherberg near M u n i c h (F.R.G.). A n y t e m p o r a l c h a n g e s o f t h e r a d i o n u c l i d e c o m p o s i t i o n w i l l

become more evident from a study of radionuclide ratios than from air concentration and ground deposition values themselves. Therefore, we studied the ratios of 99Mo, l°3Ru, '°~Ru, ~l°mAg, '25Sb, 129mTe, 132Te, '3'I, '4°Ba, raCe and '~Ce relative to the long-lived '37Cs, as well as the ratios between air concentration and ground deposition values of most of these radionuclides. EXPERIMENTAL The sampling site is located at Neuherberg, 10km north of Munich in a semi-rural region. Deposition samples are collected in several tubs of 0.6m 2 area, 1 m above grassland, and include also dry fallout since the bottom of the tub is kept covered with water during dry periods. Aerosols are sampled on 20cm diameter Microsorban filters at an air flow rate of about 30m3h -1. For trapping gaseous iodine, a charcoal impregnated filter is placed beneath the aerosol filter. For gamma-ray spectrometry with high resolution germanium detectors, aliquots or entire deposition samples are transferred to I l Marinelli beakers, and the air filters are pressed to 6 cm diameter discs. The experimental methods have been described in more detail earlier (HStzl et al., 1983). Repeated measurements in 1987 and 1988 allowed the determination of the less abundant long-lived radionuclides '°6Ru, H°mAg and 12~Sbin some samples where they had previously been below the detection limits (HStzl et al., 1987). The reference date for all radionuclide ratios, relative to '37Cs, is 6 May 1986, in accordance with the reference date for the releases from the reactor, as given by the Soviet Union (INSAG, 1986). The errors due to counting statistics (one standard deviation) in radionuclide ratios in air and deposition relative to '37Cs, dry deposition velocities and washout ratios, range from 1-3% for most radionuclides to 10% for some less abundant radionuclides in the period to 8 May. In the following period to the end of June they range from 3-5% to ~ 40% when the activity values were near the detection limits. RESULTS AND DISCUSSION The artificial radioactivity from Chernobyl arrived at Munich-Neuherberg during the night of 29/30 April 1986 and reached its first peak maximum on 30 April. The main activity deposition to ground ( ~ 70% of the total) occurred within about 30min during a thundershower (5.6 l m 2) in the afternoon of 30 April. In Fig. 1, we give as an illustration the development of 137Cs air concentration, of '37Cs ground deposition and of the precipitation to the end of June 1986. Generally, the total amount of activity deposited to ground increased in sampling intervals which included wet deposition. This behaviour was common for all radionuclides observed. The radionuclide ratios relative to 137Cs in Chernobyl fallout were far from being uniform, as evident from Figs 2 and 3: (i) they change with time and (ii) their temporal patterns in air are very different from those in deposition.

%

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z

1~ ~

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101

lm t__

100

J

3o

i

~20-

i°t 0

'10 MAY

:~0

30 '

lb

1o

30

JUNE

Fig. 1. Variation of 137Cs air concentration, 137Cs ground deposition and precipitation at Neuherberg to the end of June, 1986.

However, it is possible to identify groups of radionuclides with similar temporal patterns in the air as well as in the deposition. This will be discussed in more detail in the following sections. Ratios in air

From the arrival of the plume during the night of 29/30 April, most of the radionuclide ratios in air increase to a first maximum in the air filter taken from 30 April (afternoon) to 1 May (morning) (Fig. 2). The amplitude of this maximum varies from a factor of ~ 2 for SgMo, aerosol 131I, 12~Te, 132Te, to 3-4 for l°3Ru and l°6Ru. For 125Sb, the maximum is less pronounced, and for gaseous

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.

to.o. j 0.04

1

5 MAY

'

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1

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5

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MAY

Fig. 2. Temporal variation of radionuclide ratios, relative to ~37Cs,in ground-level air and in total (wet and dry) deposition at Neuherberg to mid-May, 1986(all data decay-corrected to 6 May 1986). iodine, as well as for 11°mAg, 14°Ba, raCe and l"Ce (not shown in the figure), it is absent. A distinct d e c r e a s e in the r a d i o a c t i v i t y of the filter t a k e n s u b s e q u e n t to the a f t e r n o o n of 1 M a y is observed. This decrease of the r a t i o s does not seem to be due to a selective s c a v e n g i n g effect from the s t r o n g t h u n d e r s h o w e r on 30 April, since the ratios in air r e m a i n e d high until the m o r n i n g of 1 May, while the a c t i v i t y c o n c e n t r a t i o n s decreased by a f a c t o r of 2-3. M o r e likely the decrease of the r a t i o s seems to be due to the a r r i v a l of a n o t h e r p a r t of the r a d i o a c t i v e cloud, c h a r a c t e r i z e d by a different r a d i o n u c l i d e composition. F r o m the a f t e r n o o n of 1 May, all r a d i o n u c l i d e r a t i o s in air, except u°mAg/ 137Cs, i n c r e a s e to a m a x i m u m on 8 May. Overall, from the arrival of the plume on 29 April to 8 May, the d e c a y - c o r r e c t e d r a d i o n u c l i d e ratios r e l a t i v e to ~37Cs

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Fig. 3. Temporal variation of radionuclide ratios, relative to 137Cs,in air and deposition to the end of June, 1986 (all data decay-correctedto 6 May 1986). increase by factors between 2-4 (99M0, z25Sb, 129roTe, Z32Te, aerosol 131I, 14°Ba) and 5-6 (l°3Ru, 1°6Ru, total 1311). On 8 May a sharp decrease of most of these radionuclide ratios occurs, while 1311/137Cs increases suddenly by a factor of 5 (aerosol) to 8 (gaseous). According to the local meteorological data measured at the site, the wind direction changed to the west on this day, so the changes of the radionuclide ratios might be due to a change of the air masses. When the time scale is extended to the end of J u n e 1986, the change of the ratios on 8 May is followed by a new increase of the ruthenium, iodine and tellurium isotope ratios relative to ~37Cs (Fig. 3). This observation will be discussed below. The temporal variations of radionuclide ratios in air until 8 May reflect the arrival of different air masses with varying elemental composition. In the first maximum of the radionuclide ratios until 1 May, the volatile elements iodine, ruthenium and tellurium were predominant. The ratios of the refractory elements barium and cerium increased distinctly, but only during the night of 3/4 May, in parallel with the second increase of the ratios of iodine, ruthenium and tellurium. The Soviet Union reported that, after the initial release, the Chernobyl reactor core cooled down until 1 May and then heated up again with release rates approaching 70% of the initial value (INSAG, 1986). The rising contribution of refractory elements in air at Munich-Neuherberg might be due to this new heat-up period. The sharp drop in air concentrations and in most radionuclide ratios at Neuherberg on 8 May, however, seems not to be associated directly with the sharp drop in the reactor release reported for 5 May, since at Neuherbeg on 8 May the main wind direction changed to the west, from the north and northeast on the preceding days.

Similar observations were made at Vienna, where a sharp decrease of the ratios 99Mo/137Cs, l°3Ru/~37Cs and 132Te/~37Cson 1 May and a subsequent increase were reported, while the ratio ~4°Ba/~37Cs was approximately constant until 1 May, and then increased parallel to the other ratios; on 7 and 8 May, ~32Te/137Cs and 99Mo/137Cs were again decreasing (Mfick, 1988). At Chilton, U.K., the ratio ~°3Ru/~37Cs decreased strongly from 6 to 7 May and increased from the middle to the end of May, in keeping with our observations (Cambray et al., 1987). It is remarkable that the ratio ~°mAg/~37Cs in air shows much less variation in time than any other of the ratios discussed above. Apparently, silver and cesium behaved very similarly on their passage from Chernobyl to Western Europe.

Ratios in deposition In deposition, the temporal patterns of the radionuclide ratios relative to 137Cs (Fig. 2) are quite different from their counterparts in air. Until 7 May, a common feature of most radionuclide ratios is that the values are high in sampling periods during which wet precipitation occurred. However, the ratios of l~°~Ag and ~t°Ba to ~37Csremain nearly unchanged. In the thunder shower of 30 April, which delivered ~ 70% of the total activity deposition at Neuherberg, most radionuclide ratios in deposition had their maximum or near maximum values. In the following period with decreasing amounts of precipitation recorded (0.4, 0.1, 0.1, 0ram) most radionuclide ratios also decreased continuously, while on the following 2 days with distinct precipitation (5 May, 0.7 mm; 7 May, 2.5 mm) most radionuclide ratios had maxima similar to their initial values. Most activity ratios in deposition decreased strongly on 8 May, similar to the ratios in air. They were then similar to those observed in the preceding dry periods. Later rainfalls (10-12 May, 12.4 mm; 14-16 May, 7.6 mm) led to a significant increase of the activity deposition to ground (Fig. 1), but not to an increase of the activity ratios. An enrichment of several radionuclides in rainwater, relative to ~37Cs, as compared with air, soil and vegetation, until about 9 May, was also observed at Mainz, F.R.G. (Denschlag et al., 1987). When the time scale is extended to the end of June (Fig. 3), a further increase of the activity ratios of I, Ru and Te becomes evident.

Deposition velocities The deposition behaviour of airborne material is often described empirically in the literature by dry deposition velocities, and for wet deposition by washout ratios or washout coefficients (Peirson and Keane, 1962; Slinn, 1978; McMahon and Denison, 1979). Dry deposition velocities (cm s- 1) were calculated for those 24 h sampling periods in which no precipitation had been recorded, from the mean observed deposition flux (Bqm-2s -1) and the mean air concentration (Bq m- 3) of the respective nuclide. The results are shown in Table 1. The deposition velocities of 137Csand H°~Ag

7 TABLE 1 Dry deposition velocities at the w a t e r surface of Chernobyl radionuclides at M u n i c h - N e u h e r b e r g (cm s - ~ _+ s t a n d a r d deviation) Nuclide

'O3Ru

Vd, cms- ' + SD a

1O6Ru

0.08

0.I0

_+0.02

+ 0.02

11OmAg

0.37 +_0.04 b

12~Sb

,3,I (total)

,311 (aerosol)

132Te

,~TCs

14OBa

0.07

0.03

0.13

0.08

0.27

0.15

_+0.01

_+0.01

+ 0.06

_+0.03

_+0.11

_+0.03

a F r o m t h r e e 24-h samples (3, 4 and 6 May) w i t h no precipitation. b Two samples.

are higher than the deposition velocities of the other radionuclides by a factor of 2-5, and higher than that of total iodine by a factor of ~ 10. This is consistent with the observation that, in dry sampling intervals, the ratios of most other radionuclides relative to '37Cs in deposition were low, as discussed above. For a pair of radionuclides with identical deposition properties a timeindependent ratio in deposition should be expected, and this was observed in practice for the ratio 134Cs/'~Cs (not shown in Fig. 2). Among the other radionuclides considered on 8 May, the deposition behaviour of '4°Ba and of "°mAg seems to be very similar to that of 137Cs, since their ratios to 137Csin deposition show much less variation with time than do the other radionuclide ratios. Based on their similar temporal variation in deposition, and on their similar dry deposition velocities, it seems possible to divide the radionuclides considered above, roughly, into two groups, namely 137Cs(and l~Cs), "°mAg and 14°Ba on the one hand, and ~°3Ru, ~°6Ru, ~25Sb, la'I, ~29mTeand ~32Te on the other. For 99Mo, 141Ce and 't4Ce, only few deposition data are available; hence, these nuclides are more likely to belong to the first group of radionuclides. Deposition velocities are a complicated function of the aerosol size (e.g. Slinn, 1978; Maqua et al., 1987). In principle, differences in dry deposition velocities might be explained by different aerosol sizes. Airborne Chernobyl radionuclides, however, have been reported to have very similar aerosol size distributions, except iodine, during the first days (e.g. Jost et al., 1986; Georgi and Tschiersch, 1987). For '3~I, slower deposition relative to 137Cs, and also remobilization is easily explained by the presence of gaseous forms. A common property of I as well as of Ru, Sb and Te is their quite complex chemistry compared with the metal ions Cs, Ag and Ba. Hence, it cannot be excluded that the observed differences of the deposition velocities may be influenced by differences in chemical properties. It should be noted that the values of, and the relations between, the dry deposition velocities of the elements refer to the collector type used in the present work, namely a water surface in a sampling tub. There are few literature data available on dry deposition velocities of Chernobyl fallout

radionuclides. As expected, they seem to depend on the history of the aerosols (and therefore, also on the geographical location) and on the roughness of the collector surface. The dry deposition velocity of 137Cs (Table 1) is similar to other values from Central Europe (Mfick, 1988; Bunzl et al., 1989). From England and Denmark, values ranging from 0.01 to 0.12cms 1 have been reported on grass (Cambray et al., 1987; Aarkrog, 1988) and on urban materials (Nicholson, 1987; Roed, 1987). An exceptionally low value of 0.0042 cm s ~was reported from a rain collector in Denmark (Aarkrog, 1988). On the other hand, the dry deposition velocities of Ru, Te and Sb recorded at Munich are similar to the 137Csvalues from England and Denmark. The mean deposition velocities of iodine were similar to that of cesium on grass in Central Europe (Maqua et al., 1987; Mfick, 1988), and up to one order of magnitude higher than that of cesium in England and Denmark on grass and construction materials (Cambray et al., 1987; Roed, 1987; Clark and Smith, 1988). Washout ratios

Washout ratios W, defined by: W = activity per unit mass of rain activity per unit mass of air were calculated for the periods in which wet deposition exceeded 0.1 mm per sampling interval. The results are shown in Table 2. Until 8 May, the washout ratios are given separately for each event. Radioactivity in the first deposition sample from 29 to 30 April cannot be assigned unambiguously as being mainly due to dry or wet deposition, since at least a part of the precipitation fell before the arrival of the plume. However, its deposition velocity as well as its washout ratio values look more likely to be due to wet, rather than dry fallout. The second sample is from the strong thunder shower on 30 April. The washout ratios until the morning of 8 May show much more variation from one sampling interval to another than do the dry deposition velocities. This is not unexpected since it is well known that the individual meteorological parameters strongly influence the deposition behaviour in each individual rainfall event. On the other hand, in each individual ~'wet" sampling interval until 8 May the washout ratios of the individual radionuclides were much more similar to each other than were the dry deposition velocities in dry periods. In general, the washout ratios of individual radionuclides, except iodine, differed by not more than 30% from the washout ratio of 137Cs, when the first deposition sample (see preceding paragraph) is not considered. Thus, the complex temporal pattern of radionuclide ratios in ground deposition (Fig. 2) until 8 May can be explained by the different proportions of the individual radionuclides in wet and dry deposition. In particular, the characteristic maxima of most radionuclide ratios in deposition on 5 and 7 May show the effect of the increased wet deposition of most radionuclides, relative to 137Cs, as compared with dry periods.

“Sample change at 8a.m.

20.5.p30.6. Range Mean f SD

15062700 2100 f 370

(n = 6)

7200 (n = 4) 330&9600 6800 f 1900

600 580 1500 2400 510

f 1 h, unless otherwise

155.5

1.9 37.3

8.5.-9.5. 8.5.-20.5.

Range Mean f SD

21.2 5.6 0.4 0.7 2.5

“sRu

mentioned.

(n = 6) 16093900 2600 + 1100

> 8100

730 570 1600 2800 610

l”Ru

> 6800

260 550 1800 3600 750

‘lomAg

per kg air)

at Munich-Neuherberg

W = (Bq per kg rain)/(Bq

radionuclides

Precipitation (lm-‘)

29.4.-30.486 30.4.(9:0&17:00) 30.4.(17:00~1.5.(9:00) 5.5.6.5. 75-8.5.

Date”

Washout ratios, W, of Chernobyl

TABLE 2

> 1800

450 680 2500 3600 730

r=Sb

37-210 110 f 90

(n = 3)

210

660 360 1500 1000 270

10800 (n = 4) 1080622000 14000 f 4000

240 460 1800 3300 710

‘=cs

(n = 6) (n = 6) 11064400 27067400 2100 f 1300 4100 f 1900

1700 (n = 4) 55&1700 1100 * 550

1750 910 3600 5600 1500

1311 (aerosol)

1311 (total)

16000

560 500 1600 1900 490

r3’Te

(n = 4) 260&9100 5800 f 2800

13600 (n = 3) 1360&24000 18000 f 5200

730 600 1400 3300 600

ltiBa

10 The above considerations do not change, when instead of the washout ratios, the wet deposition velocities in cm s 1 (Peirson and Keane, 1962; Slinn, 1978) are used. We also measured the rainfall durations at our site and obtained wet deposition velocities ranging from 12 to 140 cm s ' in the rainfall events until 8 May for all radionuclides except iodine. Observations at later dates

The increase of the activity ratios of I, Ru and Te to Cs in air and deposition at such late dates as between mid-May and the end of June (Fig. 3) is difficult to explain. Several possibilities exist. (i) A continuous, although small, activity release from Chernobyl up to the end of September has been discussed (Cambray et al., 1987). An enhancement of the more volatile elements I, Ru (as the tetroxide) and Te over Cs would not be unlikely at elevated reactor temperatures. The Soviet Union reports a sudden drop in the release rate around 5 May to < 1% of the initial rate and a continuing decline thereafter, but it does not indicate the duration and composition of the continuing releases (INSAG, 1986). (ii) It is known that after the initial phase of activity deposition, resuspension began to play an important role (Aarkrog, 1988; HStzl et al., 1989). In addition, remobilization was certainly important in the case of iodine, of which gaseous species such as elemental iodine or organic iodine compounds are known to occur under environmental conditions. Remobilization of iodine is a possible explanation for the increase of its ratio to 137Cs as well as for the low values of the washout ratio. However, it is difficult to understand which species of Ru and Te could be responsible for remobilization, although ruthenium tetroxide and organic compounds of Ru and Te might be candidates. Clearly, more field data on the behaviour of Ru and Te under environmental conditions are needed. (iii) The increase of the activity ratios of I, Ru and Te to Cs might also be due to a depletion in Cs. In fact, we find for 137Cs the highest dry deposition velocities, and to the end of June the highest washout ratios among the elements considered. The increase of activity ratios would then be due to a successive fractionation of the still present airborne activity. At present, it is not possible to decide between the explanations given above. As noted above, since the morning of 8 May, the air concentrations of most radionuclides decreased by about two orders of magnitude. The decrease of the deposition values was less pronounced (by a factor of 3-6). For '37Cs this is shown in Fig. 1. Consequently, the washout ratio values increased strongly. However, because several air concentration and deposition values fell below the detection limits, washout ratios could not be calculated in all cases. The available values are listed in sections 2 and 3 of Table 2. As shown in section 2, the washout ratios of ~°3Ru, '32Te (only one value), ~37Cs and 14°Ba remained high for the following 12 days. This can be explained by an increasing contribution of resuspended material to the airborne activity after 8 May. Resus-

11 pended material is known to contain many particles of relatively larger size, and high deposition velocities and washout ratios can therefore be expected to be characteristic for resuspended material. Further, if particles with sizes > 20 #m are present in ground-level air, such material will be collected in the deposition samplers, but not quantitatively in the air samplers, thus leading to an additional apparent increase of the washout ratio (Pattenden et al., 1980; HStzl et al., 1989). From 20 May onwards (Table 2, section 3), the washout ratios decreased again. They were, however, still significantly higher than the values of 6001000 observed after nuclear bomb tests to the end of the 1970s (Peirson and Keane, 1962; Peirson and Cambray, 1965; HStzl et al., 1983), but similar to the values of 2500-3900 observed for 137Cs between 1982 and 1985 at MunichNeuherberg. In the latter period, no atmospheric nuclear weapons tests were carried out, and resuspension can therefore be assumed to have been the dominant mechanism in the redistribution of artificial radioactivity in the atmosphere. The same seems to be true for the long-term effects after Chernobyl. CONCLUSIONS For the initial phase of activity deposition (until 8 May), our results may be summarized as follows. (i) The considerable temporal variations of the radionuclide ratios, relative to '37Cs, in air reflect the arrival of different parts of the reactor plume with different elemental composition. The temporal variation of a given radionuclide ratio, relative to 137Cs, in deposition is quite different from its counterpart in air, and reflects varying contributions of dry and wet deposition. (ii) There exist groups of radionuclides with similar deposition properties. The ratios of l°3Ru, l°6Ru, 125Sb, '29roTe, 132Teand '3~I related to ~37Csin deposition have maxima in wet periods. The dry deposition velocities of these nuclides are similar, and significantly lower than that of'37Cs. On the other hand, the ratios of H°mAg and '4°Ba (and 134Cs) to ~37Cs in deposition show much less temporal variation than the former nuclides. The dry deposition velocities of 'l°mAg and '4°Ba are similar to that of '37Cs. For all radionuclides, the washout ratios are similar to each other within one sampling interval. The differences in dry and wet deposition explain the complex temporal patterns of radionuclide ratios in deposition. (iii) The differences in the dry deposition velocities of the various radionuclides seem not to be due to differences in their aerosol size distributions, since the latter have been reported to be similar, except for iodine. Differences in the chemical properties could play a role. Later effects after the initial phase of activity deposition from 8 May to the end of J u n e were characterized by resuspension and, in the case of iodine, probably by the presence of gaseous species.

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