Resuspension of coarse particles in the region of Chernobyl

Atmospheric Environment 33 (1999) 3313}3323

Resuspension of coarse particles in the region of Chernobyl F. Wagenpfeil , H.G. Paretzke , J.M. Peres
, J. Tschiersch * GSF-National Research Center for Environment and Health, Institute of Radiation Protection, D-85764 Neuherberg, Germany
Institut de Protection et de SuL rete& Nucle& aire, De& partement de Protection de l+Environnement, F-92265 Fontenay-aux-Roses Cedex, France Received 30 October 1997; received in revised form 3 September 1998; accepted 8 September 1998

Abstract Measurements of resuspension into air in the coarse particle range ('10 lm in aerodynamic diameter) were performed as part of a "eld measurement experiment at four sites around Chernobyl during wind-driven and anthropogenic enhanced resuspension (agricultural activities). Caesium-137 was used to quantify the magnitude of resuspension. The mean resuspension factor of coarse particles was between 1;10\ and 6;10\ m\ for wind-driven resuspension. During agricultural activities, the resuspension increased by up to three orders of magnitude. Coarse particles comprised about one-third of the total amount of resuspended Caesium-137. The activity size distribution of Caesium-137 was not uniform in the coarse particle range: approximately 44% of the activity was found in the range 10}20 lm aerodynamic diameter. The determination of the number concentration of particles '20 lm aerodynamic diameter showed a linear relation between particle number and activity: the mean activity per particle was 0.75$0.15 lBq at the site Novozybkov, Russia. The resuspension factor was found to decrease exponentially with increasing relative soil moisture content. At higher soil humidity, the portion of coarse particles of the total resuspended activity was larger.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Resuspension; Caesium-137; Particle size; Soil humidity; Field experiment

1. Introduction During the Chernobyl accident a large quantity of radionuclides was released into the troposphere, transported partially over long distances and contaminated a wide geographical area (IAEA, 1986; Tschiersch and Georgi, 1987; IAEA, 1991). Radionuclides were removed from the atmosphere by wet deposition (rain out and wash out) and dry deposition. Radionuclides, once deposited on the ground, can be re-entrained into the atmosphere by the action of the wind or by anthropogenic disturbances, for example road tra$c or agricultural practices (Sehmel, 1973).

*Corresponding author. Tel.:#49 (0) 89 31872763; fax:#49 (0) 89 31873363; e-mail: [email protected]

The stratosphere, although still contaminated by radionuclides from nuclear weapon tests and Chernobyl debris, contributes only a negligible part of the tropospheric activity burden (Kownacka and Jaworowski, 1994; Bunzl et al., 1995). Thus, the measured activity in the atmospheric surface layer in the region of Chernobyl has its origin in the surrounded soil. There are only few measurements about the contribution of coarse particles (aerodynamic diameter d '10  lm) to the total atmospheric particle concentration. One reason is that coarse particles are di$cult to quantitatively sample. Another reason is that many measurements of atmospheric particle concentrations focus on air pollution problems associated with small particles from combustion sources. The sampling instruments generally used, for example EPA samplers, do not allow particles with d *15 lm to enter. The inlets of other samplers 

1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 9 3 - 3

3314

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

are speci"cally designed to restrict the entry of particles above a certain size, for example, PM or PM sam   plers. However, according to the new lung model of the International Commission on Radiological Protection (ICRP, 1994) particles up to a size of 100 lm in aerodynamic diameter have to be considered for inhalation dose assessment. Although the long-range transport of coarse particles might be expected to be limited, they may contribute signi"cantly to local deposition. A preliminary assessment of the radiological situation in the environs of the Chernobyl region indicated that a signi"cant fraction (in terms of activity) of resuspended particles would be in the coarse particle range (Paretzke and Garland, 1992). Previous measurements of atmospheric concentration in an urban area indicated that between 30 and 70% of the total particulate matter in air is of particle size d '15 lm (Pires et al., 1993). During  agricultural activities, resuspension measurements mostly focus on the assessment of respirable dust particles (d (4}5 lm) inside cabs of farming machines. How ever, in studies by Atiemo et al. (1980) and NoreH n (1985) size distribution measurements of coarse particles in ambient air were made additionally. Atiemo et al. (1980) measured mass median diameters between 13.3 and 18.8 lm with geometric standard deviation of 1.7}2.0 during soil management within the dust plume of the operating machine. This means that 70}81% of the particle mass was in the coarse size range. NoreH n (1985) found approximately 43% of resuspended mass (sampled on the roof of the cab) in the particle range 10}22 lm. The very high proportion of coarse particles in both studies is a consequence of the sampling location which was situated on the operating machine. Resuspension by wind and during soil management was investigated in a large scale "eld measurement programme (ECP1) in the contaminated region of Chernobyl from 1992 to 1994 using radionuclides as tracers (HollaK nder and Garger, 1996; Garger et al., 1997b). The aim of the present study was, according to the design of ECP1, to quantify the magnitude of resuspension of Chernobyl derived radioactive coarse particles. Measurements were made of the wind-driven resuspension of Cs, and resuspension of Cs during simulated (at Zapolie and Kopachy) and real agricultural activities (at Novozybkov). The dependence of resuspension on a range of parameters was examined. These parameters included meteorological parameters (vertical wind pro"le, moisture content, wind direction), soil characteristics (soil type, particle size, moisture content) and the height above ground.

2. Materials and methods 2.1. Field measurement sites and soil contamination The resuspension measurements were carried out at di!erent locations, with di!erent soil types and coverings,

in the environs of Chernobyl during 1992}1994 (Fig. 1, Table 1). To calculate the resuspension parameters, the surface soil contamination, A , of Cs had to be determined. In this work, material in the "rst 5 cm of the upper soil layer was considered to be available for resuspension. The soil at the Pripyat-Beach site is mostly sand, with a low clay and organic matter content. At this site, as expected, the migration velocity of Cs was higher compared to the other sites where podsolic soils with a higher clay and organic matter content dominated. The upper soil layers of the sites had not been disturbed since the Chernobyl accident in 1986. The agricultural activities carried out as part of this work programme altered the depth pro"le of Cs. After harrowing at Novozybkov, the Cs activity was homogeneously distributed up to a depth of 25 cm, declined slowly and no Cs was detected below 30 cm. 2.2. Wind proxle and soil moisture measurements The atmospheric turbulence and the stability category of the atmospheric boundary layer was determined. The horizontal wind velocity at the ground surface is zero and increases as a function of height, z. The vertical gradient of the horizontal wind velocity, v , can be expressed as V (Roedel, 1992) dv u V" * U, dz iz

(1)

where u is the friction velocity (m s\), i the von Karman * constant [0.4], and U the dimensionless stability function. The friction velocity, u , is a characteristic velocity in * the atmospheric boundary layer and is proportional to tangential rotational velocity of the eddies. The stability function, U, gives the ratio between the actual vertical gradient of the horizontal wind velocity and the gradient of the horizontal wind velocity for neutral conditions. In neutral conditions, the wind speed as a function of height is de"ned as




u z v (z)" * ln V i z 

(2)

where z is the roughness length (m).  Measurements of wind speed and temperature were made at four heights (up to approximately 4 m) using cup anemometers and platinum resistance thermometers, respectively (HollaK nder and Garger, 1996). The wind speed and temperature were averaged over 10 min periods. Besides the vertical wind pro"le, the resuspension process depends on the soil moisture content. Therefore samples of the upper 5 cm soil layer were taken. The soil moisture was determined by the di!erence in mass before and after drying in an oven for 24 h at a temperature of 603C.

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

3315

Fig. 1. Map of the 30 km exclusion zone indicating the locations of the measurement sites Pripyat-Beach, Zapolie and Kopachy around the Chernobyl Nuclear Power Plant (NPP) modi"ed after Garger et al. (1997b). The measurement site Novozybkov, Russia, is situated 150 km Northeast of the NPP. Table 1 Characteristics of the measurement sites. Nearly all Cs was in the "rst "ve centimetres of soil at the sites Zapolie, Kopachy and Novozybkov. However, at the Pripyat-Beach site only about 50% of the total activity was in the uppermost 5 cm. Detailed information about the soil properties at the measurement sites are given in Besnus et al. (1997). A : surface contamination calculated from the uppermost 5 cm layer of the soil ($one standard deviation); D : local surface gamma dose rate; (a): surface contamination of the * undisturbed "eld; (b): surface contamination of the prepared areas in the "eld for anthropogenic resuspension Site

Period of experiment

A (Cs) 5-cm layer (Bq m\)

Percentage of A in 2-cm layer (%)

D * (lSv h\)

Soil cover (mean height)

Pripyat-Beach Zapolie

(1.0$0.9);10 (4.9$2.6);10 (5.4$1.5);10 (2.2$0.4);10

40 65

10 0.6

Kopachy

August 1991 (a) July 1991 (b) May 1993 July 1993

70

3

Novozybkov

May 1994

(4.4$0.5);10

50

0.5

Sand Grass (0.5 m) Grass (0.2 m) Bare soil

2.3. Measurement of CS atmospheric concentration using the Rotating Arm Impactor Atmospheric concentration of Cs were determined using a Rotating Arm Impactor (RAI) (Fig. 2). The RAI

Distance from Chernobyl NPP (km) 3.8 14 3 150

Direction from the NPP

NW S S NE

collects coarse particles by impaction on a slide mounted on the rotating arm. The RAI is relatively easy to construct in comparison to other aerosol sampling devices capable of sampling coarse particles and several analytical methods can be used to examine the

3316

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

Fig. 2. The rotating arm impactor, RAI, with three impaction surfaces (slides). A prototype of the RAI is described by Jaenicke und Junge (1967). To minimize particle bounce-o! and blow-o!, the slides were made sticky by coating with a thin layer (about 80}100 lm) of silicon oil. The rotating arm was secured to a speed controllable three phase current motor which was controlled by a static frequency changer. The adjusted frequency will be hold with an accuracy of about 1. Behind the motor there was a wind vane. This vane ensured that the rotating plane of the RAI was at 903 to the wind direction and optimised the particle capture on the impaction surfaces. Three masts of di!erent heights were used to allow resuspension to be assessed as a function of height (from 1.6 up to 6.4 m). Table 2 The characteristics of the RAI are given by the cut-o! diameter, d , sampling rate,
Slide

1 2 3

1 4 4

Radius, R (cm)

30 30 15

;"5 Hz

;"20 Hz

d (lm) 


d (lm) 


10 20 28

8.82 35.3 17.6

5 10 14

35.3 141 70.6

impaction surfaces without special preparation of the samples. The cut-o! diameter, d (lm) (aerodynamic particle  diameter at which the impactor has a collection e$ciency of 50%), of particles captured by the impaction surfaces of the RAI was estimated to be (Jaenicke und Junge, 1967; Wagenpfeil, 1996)



B d 121 (3)  R; for atmospheric aerosol particles, where B is the width of the slide (cm), R the distance of the slide from the rotating axis (cm), and ; the frequency of rotation (Hz). To allow the aerosol particles to be size fractionated, a RAI was constructed with three sampling slides (di!erent R or B, respectively). Each slide collects particles of di!erent size ranges (Table 2). 2.4. Gamma-spectrometry of the slides from the RAI The Cs-activity from the particles on the slides was measured using Germanium detectors (Canberra Inc., Frankfurt), shielded by 6 cm lead. Signals from the de-

tectors were recorded by a multi-channel analyser (Canberra Inc., Series 90) and analysed with the program &&SpektranF'' (Canberra Inc.). Calibration slides were prepared to determine the detector e$ciency for the #at geometry used in this study (Wagenpfeil, 1996). The Cs-activity of the slides from the "eld measurement campaigns varied from 20 mBq to several hundred mBq. Acceptable counting statistics were de"ned in this work as a 1p-error (10%. Therefore, the slides with the lowest activities had to be counted for 2}3 days. If there was no Cs peak detected after one day, the measurement was stopped. From background measurements, the detection limit of Cs for the slides was estimated to be about 15 mBq for a measurement period of one day. 2.5. Image analysis system The size distribution of the particles collected on the slides was determined using image analysis. The measured particle size was the equivalent diameter of the two-dimensional projection, i.e. the diameter of a circle with the same area than the projection. To obtain the size distribution of the coarse particles, twelve size class inter-

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

3317

vals in the range from 4 to 256 lm were chosen such that the geometric mean of the interval limits were equidistant in a logarithmic plot. For each picture of the slide area, the number of particles per size class is measured. When N stands for G the number of detected particles in the size class i then the size distribution referred to the volume, <, is given by

size range. The IAS (Fig. 3) is a high-volume sampler and was designed to aspirate as close as possible to isokinetic conditions for an ambient wind speed from 0 to 15 m s\. The Cs concentration detected by IAS sampling was compared to that obtained by RAI sampling, allowing the assessment of the contribution of Cs associated with the di!erent particle size ranges.




2.7. Resuspension calculations



N dN 1 1 4 " G , < d log d e (d) d log d G   G G

(4)

where (dN /d log d) is the particle concentration per 4 G diameter interval d log d (cm\), d log d the logarithmic G G width of the size class (log (d /lm)!log (d /lm)),  G>  G < the volume corresponding to the evaluated slide   area, and e (d) the detection e$ciency of particles G (diameter d) in the image frame. If particles lie very close or even touch each other, they cannot be separated by the image analysis system and were detected as one large particle. This potential error source must be avoided by adjusting the sampling time. In general, before a slide was assessed, it was necessary to check the particle loading on the slide. Slides were rejected if the particle density for microscopic analysis was too high. 2.6. Measurement of Cs atmospheric concentration using the Isokinetic Air Sampler An automatic Isokinetic Air Sampler (IAS) was used for particle sampling, integrating over the total particle

The measurement of the Cs concentration in air and soil around Chernobyl was used to quantify resus pension. The resuspension factor, RF (m\), is de"ned as A (5) RF" 4, A where A is the concentration in air (Bq m\) and A is 4 the mean total surface contamination (Bq m\). The RF implies a homogeneously contaminated surface with an equilibrium between the resuspended and deposited aerosol. This condition is not achieved in our experiments (especially during anthropogenic enhanced resuspension) and RF should be considered as airborne concentration normalized by the surface contamination. RF depends on the particle size range of the airborne concentration measurement and the thickness of the soil surface layer available for resuspension. A more detailed discussion on the resuspension factor is given in Garger et al. (1997a).

Fig. 3. Schematic view of the isokinetic air sampler, IAS. The inlet of IAS was speci"cally designed to allow large particles to enter. Small particles were captured on an upright cellulose "lter. Large particles which had insu$cient momentum to penetrate deep in to the sampler, sedimented out on to a horizontal collection tray. Both, the vertical "lter and the horizontal collector, must be analysed. The aspiration rate of the sampler was automatically regulated according to the ambient wind speed. A cup anemometer and a wind vane measured the wind speed and direction and these data were recorded continuously by a computer. The air velocity inside the inlet of the sampler was measured with a hot wire anemometer and the computer compared the aspiration rate of the sampler with the ambient wind speed. Near isokinetic sampling was maintained by the computer regulating the speed of the motor drawing air through the sampler.

3318

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

The amount of material transported from the soil surface layer into the atmospheric surface layer per time unit is expressed by the resuspension rate, RR (s\), Q RR" A -

(6)

where the vertical #ux of the contamination, Q (Bq m\ s\), is derived from the gradient set-up for turbulent di!usion assuming a #at, homogeneously distributed stationary source, without particle sedimentation (Garger et al., 1990) iu (A (z )!A (z )) 4  Q" * 4  ln(z /z )#correction  

(7)

where z , z are heights above ground (z (z ). The     dimensionless correction term depends on the stability of the atmospheric surface layer and is zero for neutral conditions.

3. Results and discussion Results are presented from measurements during wind-driven resuspension and anthropogenic enhanced resuspension. Some of the slides captured &&hot particles'' (see Wagenpfeil and Tschiersch, 1998) and were excluded from the data sets used to calculate the resuspension factor. Some slides were unsuitable for microscopic analysis because of the high number concentration of particles. However, the slides were suitable for determining the Cs concentration in air. Such slides have been included in the resuspension assessment.

3.1. Wind resuspension The Cs concentration in air at the site PripyatBeach varied by at least an order of magnitude during the measurement period (Fig. 4). In general, the Cs concentration declined with height: the mean concentration ($one standard deviation) was 1.1$0.8 mBq m\ at the height of 1.7 m and 0.6$0.5 mBq m\ at the height of 3.8 m. The variability in the concentration was similar at each of the two measurement heights (Table 3). The concentration of Cs at Zapolie and Kopachi was approximately one-third of that at the PripyatBeach site and declined with increasing height. The decline in concentration with height at both sites was smaller than at the Pripyat-Beach site. The Cs concentration was below the limit of detection at Novozybkov, except during a release of contaminated pine pollens (Wagenpfeil, 1996). The mean resuspension factor of Cs were calculated from the results of the RAI and the Cs surface contamination given in Table 1. For comparison, the mean resuspension factors for particles with d (10 lm were  determined from the concentration measurements of the EPA-type sampler &&Typhoon' which collects particles by drawing air through a "lter (Garger et al., 1997b; HollaK nder and Garger, 1996) and the surface contaminations in Table 1. The level of resuspension at each site appear to have been determined by the surface type of the soil. Resuspension was lowest at the site Pripyat-Beach which has the highest surface contamination. Resuspension at this site is likely to have been lower, in comparison to the other sites, since the sandy soil surface had a relatively large proportion of larger particles which would have been harder to resuspend (Besnus et al., 1997). This

Fig. 4. Atmospheric Cs concentration of particles '10 lm in aerodynamic diameter at the heights of 3.8 and 1.7 m and the Cs concentration ratio (A (3.8 m)/A (1.7 m)), measured during wind resuspension at the site Pripyat-Beach, July and August 1992. T T

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

3319

Table 3 Mean activity concentrations A and resuspension parameters at the measurement sites during wind-driven resuspension. For the  de"nition of the resuspension factor RF, the resuspension rate RR and the vertical #ux Q see Eqs. (5)}(7). The soil contaminations given in Table 1 were used. The resuspension factor RF was determined from concentration measurements in 1.2 m height with 2B l a high volume sampler (Typhoon) sampling particles (10 lm in aerodynamic diameter (HollaK nder and Garger, 1996) Parameter

Pripyat-Beach

Zapolie

Kopachy

Novozybkov

Height (m) A (mBq m\) 4 RF (m\) 0'B l RF (m\) 2B l Q (mBq m\ s\) RR (s\)

1.7 3.8 1.1$0.8 0.6$0.5 1;10\ 0.6;10\ 0.1;10\ *

1.7 3.8 0.3$0.1 0.2$0.1 6;10\ 4;10\ 6.3;10\ *

1.6 6.4 0.30$0.19 0.24$0.13 1.4;10\ 1.1;10\ 2.7;10\  6.8;10\ 
5.1;10\  3.1;10\ 
2.3;10\

2.2 (c) *

*

*

6.4 (c) * 14;10\ *

*

Notes   Sampling time: 30 July, 12:20}15:20 (error about 70%). 
 Sampling time: 30 July, 16:20}17:20 (error about 70%).  Below detection limit.

conclusion is supported by the fact that resuspension of coarse particles has been ten times greater than of particles with d (10 lm. The activity concentration in the  uppermost few millimeters of the sand surface, which would normally represent the depth of material which might resuspend, is likely to have been much smaller than the concentration at greater depths because of the rapid penetration of Cs in this soil. This would further reduce the atmospheric concentration of Cs. The highest resuspension factor was measured at Novozybkov. The surface at this site consists of bare soil, which is relatively easy resuspendable. Both Zapolie and Kopachy had a covering of grass above the soil surface, and the resuspension factors at these sites lie between those recorded at the other sites. Few data from the measurement periods were suitable to use in the resuspension #ux calculation since either the Cs concentration was below the limit of detection, or there were no meteorological data. 3.1.1. Inyuence of wind speed The wind speed during the measurement periods at Novozybkov and Zapolie was often equal. Therefore, there were only a few measurements which could be used to study the in#uence of the friction velocity on the resuspension process. However, during one set of measurement at Kopachy, there was a large variation in the wind speed. The ratio between the number concentration of particles measured in 6.4 m height and the number concentration of particles measured in 1.6 m height seemed to be independent of particle size at higher wind velocities. Only at low friction velocity a decrease of the number concentration ratio of particles with increasing particle diameter was observed (Fig. 5a). This means that at high wind speeds more coarse particles are resuspended and

carried to higher elevations. Therefore, when the wind speed is high, the di!erence between the number concentration of coarse particles at the height of 6.4 m and at the height of 1.6 m becomes smaller. 3.2. Anthropogenic enhanced resuspension Anthropogenic enhanced resuspension was measured at Zapolie, Kopachy and Novozybkov. At Zapolie and Kopachy, the measurements were made during various simulated agricultural activities. Di!erent tractors were driven on prepared soil surfaces (free of vegetation) in the "eld to simulate the dust emission that would occur during agricultural work. The soil surface represented a "xed line source. In a typical experiment, the tractor started at one end of the "eld, passed the sampler (distance between the tractor and the sampler was at a minimum 50}100 m), drove to the other end of the "eld and returned. The tractor speed varied from 6 to 30 km h\. The duration of the sampling periods was 10 min to 2 h. At the "eld site in Novozybkov, the samplers were mounted in the middle of a "eld (area 500 m;500 m). Samples were taken during real soil management (harrowing, fertilizing and ploughing the land and planting of potatoes). The source of resuspended material represented a mobile source with a variable distance from the sampler. The tractor traversed the "eld in straight lines, but the distance between the sampler and the tractor varied according to the tractor's position in the "eld. The comparison of the results obtained by the di!erent samplers is most evident from the measurements made at Novozybkov because the time periods of sampling were nearly equal and only one measurement was made during one experimental period. During anthropogenic enhanced resuspension the resuspension factor varied from 10\ to 10\ m\ and was

3320

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

In Zapolie and Kopachy, in average over all the experiments, there was no signi"cant change in the Cs concentration as a function of height. This di!erence, compared to the measurements at Novozybkov, may be explained by the di!erent agricultural activities at the "eld sites. In Novozybkov, resuspension during real agricultural operations was measured and the tractor speed was lower ((10 km h\) than in Zapolie and Kopachy (15}30 km h\). Additionally, the distance of the samplers from the tractor was temporarily longer (up to 250 m) compared to Zapolie and Kopachy ((100 m). Therefore, it is likely, that coarse particles already settled down before they reached the sampler, especially at the height of 6.4 m.

Fig. 5. The particle number concentration ratio as function of the geometric (projected area) diameter during simultaneous measurements at two heights at friction velocity u ; (a) wind* driven resuspension at the site Kopachy (N (6.4 m)/N (1.6 m), T T symbols refer to di!erent experiments); (b) anthropogenic enhanced resuspension during soil management at the site Novozybkov (N (6.4 m)/N (2.2 m), symbols refer to di!erent exT T periments).

strongly dependent on the soil moisture content. The resuspension factor measured during anthropogenic activities was up to three orders of magnitude higher than that of wind resuspension (Fig. 6). 3.2.1. Dependence of airborne Cs concentration on height At Novozybkov, the concentration of Cs declined as a function of height. Also, the particle number concentration was lower in the height of 6.4 m than in the height of 2.2 m and decreased with increasing particle diameter (Fig. 5b). The lack of coarse particles in the height of 6.4 m led to a decrease of the activity concentration. There was a linear correlation between the activity concentration and the number concentration of particles with d '20 lm: the mean activity per particle was  0.75$0.15 lBq.

3.2.2. Dependence of activity on the size of particles As expected the highest activity was found in the samples with the lowest cut-o! diameter (d '10 lm).  A signi"cant proportion of the Cs activity was associated with particles d '20 lm (Table 4). The activity  of Cs as a function of particle size varied only a little from site to site during agricultural activities. The data obtained from the measurements made at Kopachy show that there is a relatively small scatter in the relative activity concentration during the duration of the experimental period. The contribution of coarse particles to the resuspended activity during agricultural activities has been assessed by comparing the atmospheric concentration of Cs detected by the RAI to that detected by the IAS (Table 5). Approximately one-third of the Cs activity at Novozybkov and Zapolie was associated with particles' 10 lm in aerodynamic diameter. The ratio of the activity represented by each size fraction and the total activity declined with increasing particle cut-o! diameter. In Zapolie particles with d '10 lm contained 36$15%  of the total resuspended activity. This result is supported by comparison of the RAI-results with the results obtained by a high-volume sampler &&Grad'' (Garger et al., 1997b). The mean activity concentration ratio between the RAI and the Grad sampler during enhanced resuspension due to anthropogenic activities was 0.39$0.15. Agricultural activities change the size distribution of particles and it was demonstrated by measurements with an Aerodynamic Particle Sizer (APS 3310, TSI Inc.) in Garger et al. (1998): the increase of airborne particle concentration was largest in the large and coarse particle range (3 lm(d (30 lm).  3.2.3. Inyuence of the soil moisture content During the measurement campaigns in Zapolie, May 1993, and Novozybkov, May 1994, soil samples were taken just before the tractor experiments to measure the soil moisture content (given as the soil moisture percentage, r ). From these data, it was possible to study the  in#uence of the soil moisture content on the resuspension

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

3321

Fig. 6. Range of measured resuspension factors (depicted with lines) during wind-driven and anthropogenic enhanced resuspension at di!erent heights for particles '10 lm in aerodynamic diameter at the sites Pripyat-Beach, Zapolie and Kopachy and for particles '20 lm in aerodynamic diameter at the site Novozybkov. At the site Pripyat-Beach there was no experiment with agricultural activity, at the site Novozybkov the values obtained from measurements during wind-driven resuspension were below detection limit. Table 4 Percentage of the activity concentration of Cs, A , as meas4 ured with the RAI, for particles '20 lm and '28 lm in aerodynamic diameters in relation to the activity concentration for all particles '10 lm in aerodynamic diameter at di!erent sites Field site

Zapolie Kopachy Novozybkov Mean

A (d*20 lm) 4

A (d*28 lm) 4

A (d*10 lm) 4 (%)

A (d*10 lm) 4 (%)

51$12 58$4 59$9 56

49$8 40$7 48$13 46

process almost independently of the e!ect of atmospheric turbulence in the boundary layer since the friction velocity varied only over a limited range during the experiments: u "0.32$0.07 m s\ at Zapolie and * u "0.2$0.1 m s\ at Novozybkov. * The measured decline in the resuspension factor with increasing soil moisture content was modelled using an

Table 5 Percentage of the activity concentration of Cs, A , as meas4 ured with the RAI, for particles '10 lm, '20 lm and '28 lm in aerodynamic diameters in relation to the total activity concentration of Cs as measured by the IAS at di!erent sites (C no synchronous sampling by RAI and IAS) Field site

Zapolie Novozybkov

A (d*10 lm) A (d*20 lm) A (d*28 lm) 4 4 4 A (total) 4 (%)

A (total) 4 (%)

A (total) 4 (%)

36$15 33$9

14$2 22$9

C 16$7

exponential function (characterized by j) of the resuspension factor, RF, with soil moisture content (Fig. 7): RF"RF e\H P 

(8)

from which the resuspension factor for dry soil, RF ,  can be calculated. Values of RF (r "0%) and RF    (r "5%) are given in Table 6 according to particle size  range, measurement site and vehicle used in agricultural experiments.

3322

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

These data show that soil with a moisture content of 5% is substantially less resuspendable than dry soil. At Novozybkov an increase of the soil moisture content from 0% (dry soil) to 5% reduced the resuspension factor by a factor of 7.3. There is evidence from the measurements made at Novozybkov to suggest that particles with d '20 lm are more resuspendable than particles  from a wider size range with increasing soil moisture content. An increase of the relative soil moisture content from 0 to 5% reduces the resuspension factor by 23. These data imply that it would be easier to limit the resuspension of smaller particles, compared to particles with d '20 lm, by wetting the soil. In the case of moist  soil (r "5%) particles'20 lm in aerodynamic dia meter comprise about 30% of the total resuspended material, for dry soil (r "0%) they comprise only 10%.  Clausnitzer and Singer (1996) studied the dependence of

Fig. 7. Resuspension factor, RF, as function of the relative soil moisture content, r , for the total particle size range (data from  the IAS) and the particle fraction '20 lm in aerodynamic diameter (obtained from the RAI) during agricultural activities at the site Novozybkov.

the dust concentration in the respirable particle range from the soil humidity. They observed an exponential decrease of the dust concentration with increasing soil moisture content. The determined exponent j"0.54 (%)\ is close to results reported here.

4. Conclusions During wind resuspension the mean resuspension factor for particles with d '10 lm varied between  1;10\ and 6;10\ m\. This is in the same order of magnitude as the resuspension factor for smaller particles as parallel measurements showed. During anthropogenic enhanced resuspension an increase of the resuspension factor up to a factor 1000 was observed. The resuspension factor ranged between 10\ and 10\ m\. The activity concentration of Cs in the range d '10 lm was not uniform distributed: in the  size range 10}20 lm 44%, in the range 20}28 lm 10% and in the range'28 lm 46% of the activity is found. Microscopic analysis of the samples (d '20 lm) pro ved a linear relation between particle number and activity, e.g. 0.75$0.15 lBq per particle was measured at Novozybkov. Comparison with an isokinetic air sampler collecting total resuspended material showed that about one-third of the resuspended radionuclide Cs was attached on particles'10 lm and about 20% on particles'20 lm in aerodynamic diameter. Increasing soil moisture content was decreasing the resuspension factor exponentially. The decrease was even larger for the total resuspended material. This "nding means that the relative contribution of coarse particles to the total resuspended material changes with soil moisture, too. Arti"cial wetting of the soil surface will be an e!ective countermeasure for reducing resuspension of deposited radionuclides or other potential harmful substances. The airborne concentration will be reduced as

Table 6 Reduction factor F of the resuspension factor RF, characterized by k, after increasing the relative soil  moisture content from 0% (RF ) to 5% (RF ) at di!erent sampling sites for the coarse particle range and   total suspended particles Site

Particle size range

RF (m\) 

RF (m\) 

j ((%)\)

F 

Zapolie: small tractor Zapolie: lorry Novozybkov

d*10 lm All particles
d*20 lm All particles
d*20 lm All particles


5.5;10\ 4.7;10\ 9.0;10\ 7.2;10\ 1.9;10\ 2.0;10\

5.5;10\ 1.7;10\ 7.5;10\ 4.4;10\ 2.6;10\ 8.7;10\

0.92 0.66 0.49 0.54 0.40 0.63

100 28 12 16 7.3 23

Results calculated from data obtained by the RAI.
Results calculated from data obtained by the IAS.

F. Wagenpfeil et al. / Atmospheric Environment 33 (1999) 3313}3323

well as the size distribution will change to a smaller portion of inhalable "ne particles.

Acknowledgements The authors thank Dr. E.K. Garger and Mr. V. Kashpur, Institute of Radioecology UAAS, Kiev and their team for helpful scienti"c discussions and experimental assistance at the sampling sites Pripyat-Beach, Zapolie and Kopachy, Dr. S.K. Gordeev, Russian Scienti"c-Practical and Expert-Analytical Center, Moscow and his team for logistical help at the sampling site Novozybkov, Dr. J. Watterson, AEA Technology, Culham, U.K. for providing the meteorological data to calculate the friction velocity and for helpful comments to the manuscript. The authors are grateful to the help by Dr. S. Matthias-Maser, Mr. R. Kaltenbach from University Mainz for the construction of the rotating arm impactors. The experiments were performed in the framework of &&The agreement for International Collaboration on the Consequences of the Chernobyl Accident between the European Communities and Russian Federation, the Belarus and the Ukraine''. The programme was Experimental Collaboration Project No 1 (ECP1), which was supported by the CEC under the contracts COSU-CT92-0015, COSU-CT93-0039 and COSO-CT94-0077).

References Atiemo, M.A., Yoshida, K., Zoerb, G.C., 1980. Dust measurements in tractor and combine cabs. Transactions of the ASAE, pp. 571}576. Besnus, F., Peres, J.M., Guillou, P., Kashparov, V., Gordeev, S, Mironov, V., Knatko, V., Bondar, J., Kudrjashov, V., Sokolik, G., Leynova, S., Aragon, A., Espinosa, A., 1997. Contamination characteristics of podzols from districts of Ukraine, Belarus and Russia strongly a!ected by the Chernobyl accident. Report EUR 16912 EN, O$ce for O$cial Publications of the European Communities, Luxembourg. Bunzl, K., HoK tzl, H., Rosner, G., Winkler, R., 1995. Unexpectedly slow decrease of Chernobyl-derived radiocaesium in air and deposition in Bavaria/Germany. Naturwissenschaften 82, 417}420. Clausnitzer, H., Singer, M.J., 1996. Respirable dust production from agricultural operations in the Sacramento valley, California. Journal of Environmental Quality 25, 877}884. Garger, E.K., Zhukov, G.P., Sedunov, Yu, S., 1990. Estimating parameters of wind lift of radionuclides in the zone of the Chernobyl Nuclear Power Plant. Soviet Meteorology and Hydrology, 1, 5}10 (in Russian).

3323

Garger, E.K., Ho!man, F.O., Thiessen, K.M., 1997a. Uncertainty of the long-term resuspension factor. Atmospheric Environment 31, 1647}1656. Garger, E.K., Kashpur, V., Belov, G., Demchuk, V., Tschiersch, J., Wagenpfeil, F., Paretzke, H.G., Besnus, F., HollaK nder, W., Martinez-Serrano, J., Vintersved, I., 1997b. Measurement of resuspended aerosol in the Chernobyl area. Part I: discussion of instrumentation and estimation of measurement uncertainty. Radiation Environment Biophysics 36, 139}148. Garger, E.K., Paretzke, H.G., Tschiersch, J. 1998. Measurement of resuspended aerosol in the Chernobyl area. Part III: size distribution and dry deposition velocity of radioactive particles during anthropogenic enhanced resuspension. Radiation Environment Biophysics 37, 201}208. HollaK nder, W., Garger, E.K. Eds., 1996. Contamination of surfaces by resuspended material. Report EUR 16527 EN, O$ce for O$cial Publications of the European Communities, Luxembourg. IAEA, 1986. Summary Report on the Post-Accident Review Meeting on the Chernobyl Accident, Vienna. Int. At. Energy Agency Saf. Ser. No. 75-INSAG-1. IAEA, 1991. The international Chernobyl project &Surface contamination maps'. Report by an international advisory committee, Vienna, ISBN 92-0-129291-0. ICRP, 1994. Human respiratory tract model, ICRP Publication No. 66. Annals of the ICRP 24 (1}4). Jaenicke, R., Junge, C., 1967. Studien zur oberen GrenzgroK {e des natuK rlichen Aerosols. Beitra( ge zur Physik der Atmospha( re 40, 129}143 (in German). Kownacka, L., Jaworowski, Z., 1994. Nuclear weapon and Chernobyl debris in the troposphere and lower stratosphere. The Science of the Total Environment 144, 201}215. NoreH n, O., 1985. Dust concentrations during operations with farm machines. ASAE Paper No. 85}1055, pp. 1}9. Paretzke, H. G., Garland, J. A., 1992. Assessment of the radiological signi"cance of surface contamination by entrained radioactivity. CEC-Report, Luxembourg. Pires do Rio, M. A., Amaral, E. C. S., Paretzke, H. G., 1993. The resuspension of Cs in an urban area: the experience after the Goiania accident. Journal of Aerosol Science 25, 821}831. Roedel, W., 1992. Physik unserer Umwelt: Die Atmospha( re. Springer, Berlin (in German). Sehmel, G. A., 1973. Particle resuspension from an asphalt road caused by car and truck tra$c, Atmospheric Environment 7, 291}301. Tschiersch, J., Georgi, B., 1987. Chernobyl Fallout size distribution in urban areas. Journal of Aerosol Science 18, 689}692. Wagenpfeil, F., 1996. Messung der Resuspension von Riesen-Aerosolpartikeln in der Region von Tschernobyl. Dissertation der FakultaK t fuK r Physik der Ludwig-Maximilians-UniversitaK t MuK nchen (in German). Wagenpfeil, F., Tschiersch, J., 1998. Resuspension of fuel hot particles in the Chernobyl area. Journal of Environmental Radioactivity (submitted).