Empirical scavenging coefficients of radioactive substances released from chernobyl

Atmospheric Em'ironment Vol. 25A. No. 2, pp. 263 270, 1991 0004 6981/91 $3.00+0,00 Printed in Great Britain, Pergamon Press pie

EMPIRICAL SCAVENGING COEFFICIENTS OF RADIOACTIVE SUBSTANCES RELEASED FROM

CHERNOBYL

K1RSTI JYLH~. Department of Meteorology, University of Helsinki, Hallituskatu 11-13, SF-00100 Helsinki, Finland

(First received 21 February 1990 and in final form 3 July 1990)

Abstract After the accident at the Chernobyl power plant on 26 April 1986, most parts of Europe were affected by the associated radiation pollution. In this paper the dependence of the precipitation scavenging coefficient A (s- 1 ) on the rainfall rate R (mm h ~ ) is studied on the basis of radioactivity and radar rainfall measurements in Southern Finland after the accident. The average scavenging coefficient weighted by the high-altitude concentrations of radionuclides involved was found to be A= 10-4s -1 R °64, in good agreement with earlier investigations. The results also suggest that weather radar may form an important and effective part of a real-time radiation monitoring and warning system.

Key word index: ChernobyL radioactive fallout, wet scavenging coefficient, weather radar, Finland.

I. I N T R O D U C T I O N

An explosive accident, leading to a widespread dispersion of radioactive pollutants, took place at the Cher- nobyl nuclear power plant (51 °17'N, 30°15'E) at 2123 UTC on 25 April 1986. The explosion during the accident and the associated fire released large amounts of radioactive substances into the atmosphere. A part of the radioactive cloud reached South- ern Finland within two days, on the afternoon of 27 April. However, consistent increases of radioactivity at the ground were not detected until 29 April when a rainfall area crossed Southern Finland. This was an indication of the importance of wet deposition, observed also, e.g. in Sweden (Persson et al., 1986), Great Britain (ApSimon et al., 1988; Clark and Smith, 1988; Smith, 1988; Wheeler, 1988), F.R.G. (Horn et al., 1987), Austria (Kolb et al., 1986), Switzerland (Czarnecki et al., 1986) and Monaco (Ballestra et al., 1987).

The term precipitation scavenging is commonly used to mean the attachment of gaseous and particulate pollutants to cloud droplets, ice crystals or raindrops followed by droplet removal from the atmosphere to the earth's surface by rain or snow. The types and efficiencies of wet removal mechanisms depend on meterological conditions and the substances involved. In-cloud scavenging (rainout) involves processes occuring within the cloud, e.g. nucleation of hydrometeors, whereas below-cloud scavenging (washout) refers to the incorporation of material into precipitation due to processes taking place below the cloud base, e.g. by inertial impaction and interception.

For particles and gases that are irreversibly captured by hydrometeors, wet deposition can be con- sidered as an exponential decay process. In this case the rate of change in concentration C (gin-3 or

Bqm -3) in the air is:

dC/d t = - AC, (1)

where the proportionality factor A is known as the scavenging coefficient (s- ~ ). It is affected by the prop- erties of the pollutants, such as physical state, solubil- ity, chemical reactivity and size, and by meteorological factors such as cloud formation and growth mechanisms, and the size distribution and terminal velocities of raindrops (see e.g. Slinn, 1977; Levine and Schwartz, 1982). According to theoretical studies by, e.g. Scott (1982) and Chang (1984) it can be parameterized in terms of the precipitation rate R (mm h - 1 ):

A = a R b (2)

where parameters a and b depend on the types of pollutants and precipitation.

In this paper the A-R relationship (2) for different radionuclides will be studied empirically on the basis of radioactivity and radar rainfall measurements in Finland after the Chernobyl accident. At first, the weather situation and the three-dimensional distribution of pollutants are outlined. A more comprehensive description of the measurement and analysis methods as well as of the synoptic and mesoscale aspects of the transport of pollutants to Finland can be found in Puhakka et al. (1990), which also contains preliminary results of the dependence between the radioactive fallout and precipitation in Finland.

2. OBSERVATIONS

2.1. Precipitation area measured by radar

A considerable part of the fallout in Finland after the Chernobyl accident was in the form of wet depos-

263

264 KIRSTI JYLHA

ition caused by a cyclone, which crossed southern and central parts of Finland on 29 April. The time evolu- tion of the precipitation area related to the cyclone was measured by the Doppler weather radar of the University of Helsinki (60°10'N, 24°57'E). Figure 1 shows the rainfall distributions for 1200 and 1800 UTC. During the rainfall cumulonimbus, stratocumu- lus and altocumulus clouds were observed, and thun- derstorms with hail occurred, indicating that the rainfall area had a convective nature. The clouds extended from about 1-2 km up to 7 km, while the 0°C level was at a height of 2.1 km.

The quantitative analysis of precipitation in this study is based on video recordings of the logarithmic equivalent radar reflectivity factor d B Z e at 15 min intervals. The values of reflectivity factor were conver- ted into values of the rainfall rate R, which in turn were adjusted using raingauge measurements by a method described by Koistinen and Puhakka (1981). As a result, the final rainfall analysis fits the rain gauge values at gauge locations, preserving at the same time all the details of the radar measurements between gauges.

Figure 2 shows the the accumulated rainfall totals

a ~ 29 April 1986 1200 UTC

P /

b A0P0i,U 6 P / . / . /

Fig. 1. The precipitation area over Southern Finland at (a) 1200 UTC and (b) 1800 UTC on 29 April 1986. The outer and inner circles show the radar measurement areas of radius 224 km until 1315 UTC and of radius 168 km after that, respectively. Isopleths of the equivalent reflectivity factor are solid lines for - 5 and 25 dBZ c and dashed lines for 15 dBZ, . The locations of some places are also shown (after Puhakka et al., 1990.) HEL: Helsinki. NUR: Nurmij/irvi. LOV: Loviisa. OLK: Olkiluoto. TAM:

Tampere. JOK: Jokioinen. P: Pori.

Scavenging coefficients of radioactive substances 265

101 , , , r , , , , ~ , , , - ,

100 r ~

/

OLK

§ 10 "1 / E /

4_N_O_L_ re I '

10-2 I I ~ Ir J HEL

,!/ lO-3 / , , , , , , , , , / / 1 , , , ,

6 9 12 15 18 21 Time (UTC)

Fig. 2. Accumulated rainfall amounts as a function of time at HEL, NUR, LOV and OLK on 29 April 1986.

for 29 April as a function of time at four places, Helsinki (HEL), Nurmij/irvi (NUR), Loviisa (LOV) and Olkiluoto (OLK), which will be used in this study to estimate the relationship between the scavenging coefficient and rainfall rate. For the geographical positions of the places see Fig. 1. The rain at HEL and N U R was very weak and the accumulated totals were less than 0.1 mm. At LOV and O L K the rainfall rates were somewhat higher, on average 0.2-0.3 mm h - ~, but even there the amounts did not exceed 1 mm. Since the rainfall rates in Helsinki were so very low, one may ask whether the radar echoes were in fact received from clouds and not at all from falling raindrops. However, there were visual observations of precipitation in the centre of Helsinki and at its airport. A weak shower at the airport was indeed observed to have had a strong effect on atmospheric electricity at ground level (Tuomi, 1988).

2.2. Radioactivity in the air and on the ground

Radioactive substances mainly arrived over Fin- land at a height of about 1500 m (Sinkko et al., 1987). This can be seen from Fig. 3, which shows vertical profiles of the gamma radiation values above some locations along the track of a research aircraft near midday on 29 April. A distinct increase in external gamma radiation values was observed from a height of about 1000 m to slightly above 2000 m near HEL (Fig. 3a) and to above 3000 m near TAM (Fig. 3b), the maximum being at a height of about 1500m. The highest values of radioactivity, near the town of Pori on the west coast of Finland (see Fig. 1), were also found at that altitude (Fig. 3c), but the vertical structure had been modified as it was raining there at the time, and pollutants were already being deposited.

ALT(m)

300O

2O0O

1000

(pSv) h 1 0.2 0.4 0 6 0 8

I I I I I t I

, I I

I I

, N •

I - -

A

0'2' 0:4' 016' da ' 110' 112 REL CONC

ALT (m) ~ _ 02 t I

3000 ~ .

2000 '

1000 --0

(pSv) h -1 0.4 0.6 O.8

B

- a " "1 • ! I '

• 1

o 1 2 o16 . . . . . . o . 08 lo 12 REL CONC

ALT (m)

3OOO

2OOO

1000

(pSv) h 1 0.2 0.4 0.6 0.8

1 I 1 i I I I

P '012 . . . . . . . 0.4 0.6 0.8

REL CONC.

C

' ' 112 1.0

Fig. 3. Vertical distributions of measured gamma dose rates (dots) and calculated concentrations of the radioactivity (solid lines) near (a) HEL, (b) TAM and (c) Pori according to the research flight over Southern Finland at 1000-1200 UTC on 29 April 1986. Concentra- tions are relative to the radionuclide concentrations at a height of 1.5 km between HEL

and LOV (after Sinkko et al., 1987).

In addition to high-altitude gamma radiation measurements, an air filter sample of particulate radioactivity was made during the flight at a height of 1500 m between H E L and LOV (Sinkko et al., 1987). The calculated distributions of the radioactivity concentrations in Fig. 3 were scaled to correspond to the gamma flux of that air sample. An analysis of the sample indicated that the most dominant radionuclides were 13tI, 132Te and 137Cs (column 3 in Table 1).

266 KIRSTI JYLH.~

Table 1. Concentrations, Co(1.5), ofthe most dominant radionuclides at a height of 1.5 km between HEL and LOV between 1000 and 1200 UTC on 29 April 1986, and in samples of ground-level air, C,, together with total deposition, Dt, collected at HEL, NUR, LOV and OLK during measurement times shown for each case. The values are decay-corrected to the midpoint of a sampling period. Half-lives and dry deposition velocities, Vd, at HEL on 30 April 1986 are also shown (based on Sinkko

et al., 1987; Anttila et al., 1987; Sax6n et al., 1987)

Co(1.5) C~ (Bq m -3 ) D, (kBq m -2) Vd Nuclide Half-life (Bqm -3) HEL~" NUR:~ L O V § OLK[I HELt NUR:~ L O V ¶ OLX** (cms -1)

I°3Ru 39.4 d 28.5 0.18 0.080 0.14 0.2 0.68 1.7 7.5 17.0 0.45 1°6Ru 367 d 27.0 0.091 0.040 0.045 0.047 0.15 1.3 1.7 3.5 129mTe 33.5 d 52.0 0.17 0.088 0.11 0.12 1.5 1.8 8,0 16.0 132Te 78 h 420 1.3 0.60 1.20 1.10 5.2 13.0 55.0 120 0.46 131I* 8.04 d 690 2.2 i 't :~ ~:~ 3.6 7.5 93.0 76 0.20 133I* 20.9 h 77 0.19 i 't ~ :~:~ 0.28 0.4 §§ §§ §§ 134Cs 2.06 y 97 0.15 0.078 0.29 0.23 0.57 0.90 3.3 5.6 §§ 136Cs 13.1 d 35 0.059 0.030 §§ §§ 0.23 0.31 1A 1.8 §§ 137Cs 30.2 y 167 0.25 0.13 0.52 0.42 0.91 0.7 6.0 10.0 0.42 14°Ba 12.8 d 65 0.19 0.11 0.31 0.43 1.0 1.1 1.5 7.1 0.36

* Concentrations and dry deposition velocities are for particle-bound iodine but deposition values for total iodine. t From 0700 UTC on 29 April to 0700 UTC on 30 April 1986. :~ From 1200 UTC on 29 April to 1200 UTC on 30 April 1986. §From 13 April to be exact, but in practice from about 1200 UTC on 27 April to 0700 UTC on 30 April 1986. II From 15 April to be exact, but in practice from about 1200 UTC on 27 April to 0930 UTC on 30 April 1986. ** From 1 April to be exact, but in practice from about 1200 UTC on 27 April to 0740 UTC on 30 April 1986. ~'t 85% of the measured concentration of total iodine. *:~ 50% of the measured concentration of total iodine. §§ Not analyzed.

Areal distributions of gamma radiation dose rates at a height of 1.5 m above the ground were measured by the radiation monitoring network in Finland at 1200 UTC on 29 April and at 1200 UTC on 3 May (Puhakka et al., 1990). At noon on 29 April the radiation values were still generally low compared with the noise level of the measuring instruments, about 0.5 #Sv h - t . There were, however, some con- siderably higher peaks of about 4.5 #Sv h- 1. On 3 May, which was the next day of available radiation analysis, the external gamma radiation level had increased almost everywhere. This can have resulted both from increased concentrations of airborne radionuclides and also from depositions of nuclides on the ground. The dominant gamma-emitting nuclides in ground-level air and on the ground at HEL, NUR, LOV and OLK are presented in columns 4-7 and 8 - t l of Table 1, respectively. For the collection periods of the samples see footnotes in Table 1.

It should be noticed that Table 1 gives the values of total deposition. However, in the following we are interested in wet deposition only. To exclude that part of the total deposition not caused by precipitation, the values of dry deposition velocity, defined as the ratio of dry deposition flux to ground-level airborne concentration, should be known. The last column in Table 1 contains dry deposition velocities which were measured in Helsinki on 30 April 1986 (Anttila et al., 1987).

2.3. Characteristics o f the radioactive substances

In order to understand the behaviour under conditions of precipitation of the airborne fission products released from the Chernobyl plant it is essential to consider their physical and chemical form. According

to ambient radioactive aerosol size distribution measurements in Helsinki during 7-14 May the geometric mean of aerodynamic diameters for 132Te, l°3Ru and 137Cs was in the range 0.65-0.93 #m and for particle- bound 13q in the range 0.33-0.57 gm (Kauppinen et al., 1986). A large part of 1311 was, however, in gaseous form or only weakly adsorbed on particles: at the end of April the percentage was 85% at NUR and about 50% at LOV and OLK (Sinkko et al., 1987; Ilus et al., 1987).

Characteristics of the radionuclides from Cherno- byl have also been studied, e.g. by Cuddihy et al. (1989), Baltensperger et al. (1987) and Tschiersch and Georgi (1987). According to them, 13~I was mainly transported in the gaseous phase and was adsorbed during the whole period of travel onto local particles. Thus local processes were dominant for the size distribution of particulate ~31I. Instead, nuclides ~°3Ru, ~32Te and 137Cs were probably attached at quite an early stage to aerosols, and grew by coagu- lation with other particles during transport. It was also observed that activity distributions of the radionuclides ~°3Ru, 132Te and ~37Cs in rain samples collected in Switzerland after the Chernobyl accident were in good agreement with concentration distributions of sulphate, nitrate and ammonium ions. On this basis, removal mechanisms were probably quite similar for both groups (Jost et al., 1986).

It should be noticed that a part of the radioactivity in Finland was caused by the occurrence of so-called hot particles, especially found in southwestern parts of Finland. They wire mostly fragments of uranium fuel enhanced with varying amounts of nonvolatile fission products 95Zr, 95Nb, l°3Ru, l°6Ru, 141Ce and t44Ce


and also volati le nucl ides 134Cs a n d ~ 37Cs ( R a u n e m a a

et al., 1987; S inkko et al., 1987). H o t part icles , w h o s e

m e a n geomet r i c a e r o d y n a m i c d i ame te r was measu red

to be 1 0 p m ( R a u n e m a a et al., 1987), were at least par t ly depos i t ed by dry remova l processes (Luok- k a n e n et al., 1988).

3. EXPERIMENTAL METHODS

3.1. Determination of the A-R relationships

The main goal of this study is to empirically determine the dependence A=aR b between the rainfall rate R and the scavenging coefficient A for gamma-emitting nuclides. The method used is based on the fact that, knowing the scavenging coefficient and the vertical distribution of pollutants as a function of time, it is possible to evaluate the accumulated wet deposition. Actually, in this case the scavenging coefficient is not known, and to predict wet deposition it is necessary to assume a relationship between A and R, the latter being measured by radar. The desired values of the parameters a and b in the A-R relationship are those which produce agreement between observed wet deposition Dwo and predicted wet deposition Dwp. In other words, the ratio r between them should be equal to unity. Because there are two unknown variables, a and b, to be solved, the condition r = 1 should be fulfilled simultaneously at two places. However, owing to the possibility of measurement errors and invalid assumptions, four places will be used, denoted HEL, NUR, LOV and OLK (see Fig. 2 and Table 1).

In practice, the desired values of a and b in the A-R relationship were obtained graphically by determining intersection points of isopleths of r = 1, plotted as a function of a and b. As there are altogether four places, namely those mentioned above, from which deposition observations were available and for which the ratios r could be calculated, six intersection points of isopleths of r = 1 were found. The scattering of intersection points results partly from measurement errors and partly from the assumptions which had to be made. In an ideal situation they would all coincide. The uncertainty for each of the six values of parameters a and b obtained from the intersection points was estimated, after which a weighted average for a and b, respectively, could be determined. Before viewing the results, the assumptions which were made in order to calculate the ratios r will be discussed.

3.2. Assumptions concerning observed and predicted wet deposition

Actually, the part of the total deposition caused by precipitation was not measured, and by the observed wet deposition Dwo we mean the observed total deposition Dt after subtraction of the estimated dry deposition Da:

Dwo = Dt - Dd. (3)

The fallout of radioactive materials by dry deposition mechanisms can be parametrized in terms of the dry deposition velocity va and the time-average concentration of pollutants in ground-level air, C,:

Da = va Cs A T, (4)

where A T is the time period for the collection of a deposition sample.

On the other hand, if the vertical distribution of the concentration C and the scavenging coefficient A are known as a function of time, the accumulated wet deposition Dwp (Bq m - 2) at a point during a time interval from t I to t 2 can be calculated by integrating over the hydrometeor's path and over the time of precipitation:

t2 Z2

D w p = f f A C d z d t (5)

I i z l

where t is the time and z is the altitude and zl and z 2 are the lower and upper boundaries of the wetted plume, respectively (S]inn, 1977; Whelpdale, 1982).

To evaluate the observed wet deposition Dwo from (3) and (4) by using the values in Table I, the following assumptions were made.

(I) The dry deposition velocities presented in Table I were valid on 29-30 April at HEL and NUR and on 27--30 April at LOV and OLK. Moreover, a dry deposition velocity is the same for all isotopes of an element.

(2) The difference between the total deposition and the estimated dry deposition was due to the precipitation on 29 April only. This is quite probable, because according to synoptic weather observations on 30 April rain did not occur until late in the evening, several hours after the deposition collectors were emptied. There had been some very light showers in the western part of the study area on 27 and 28 April, but the possible effect of them on the total deposition measured at Olkiluoto was not taken into account.

(3) The removal of gaseous iodine by rain was ineffective and, in consequence, the activity of the wet deposition was mainly due to the particulate fraction of iodine. The assumption is required, as only particle-bound iodine was collected during the research flight. Furthermore, the percentage of particulate 133I in ground-level air was supposed to be the same as for 131| (see section 2.3).

(4) The error in evaluation of Dwo is comprised of a relative error of 30% in the dry deposition velocities and 10% in the total depositions and concentrations near the ground.

In order to predict the values of wet deposition D~,p for different nuclides using (5), additional assumptions had to be made.

(5) The dependence of the scavenging coefficient on height can be ignored within the radioactive airmass. Thus, the scavenging coefficient for a nuclide can be parametrized in terms of the surface layer rainfall rate, as measured by radar with an estimated accuracy of + 50%.

(6) Though the nuclide concentrations in the air varied with space, the composition of the radioactive cloud before rain was uniform over the southern part of Finland.

(7) There was no advection of pollutants before precipitation, but after that polluted air flowed continuously into the area, totally compensating the deposited radioactivity. However, when estimating the uncertainty of the empirical A R relationships, results from the use of two other assumptions, (a) negligible advection and (b) advection proportional to the concentration, were also taken into account (Jylh/i, 1990).

(8) The vertical profiles of the nuclide concentrations over HEL and NUR were similar to that in Fig. 3a with an uncertainty of +20%. The concentrations over LOV were hall" the concentrations in Fig. 3a, based on the continuous gamma radiation measurements during the flight. For OLK, in turn, the integrated concentrations in the vertical can be evaluated from the observations presented in Fig. 3c. Here, in accordance with assumption 7, it was supposed that the total integrated concentration from the surface to the upper boundary of the radioactive airmass had remained constant, even though the shape of the distribution had been modified due to rain.

By taking into account assumptions 5-8, (5) for predicted wet deposition can be written in terms of the observed airborne concentration C O and the rainfall intensity R using (2), the A-R relationship:

t2 Z2

Dwp=faRbd t . fCodz . (6) t l z l

268 K1RSTI JYLH, ~.

In practice, the integrals were estimated numerically by the trapezium method. The second term on the right-hand side of (6), i.e. the observed nuclide concentration before rain, integrated with respect to height, was obtained by multiplying the concentrations at a height of 1.5 km to the east of HEL (Table 1) by one of the vertical profiles of normalized concentrations in Fig. 3, in accordance with assumptions 6 and 8.

4. RESULTS AND DISCUSSION

4.1. Empirical scaven#in# coefficients

The empirical relationships between the rainfall rate R and the scavenging coefficient A for radionuclides lO3Ru ' 106Ru ' 129mTe ' 132Te ' 134Cs ' 136Cs ' 137Cs '

l*°Ba and particulate 1311 and 133I are presented in Table 2. The parameter b is typically 0.5-0.7 whereas a ranges from 10 -s s-1 to nearly 10 -3 s-1. The results for l°3Ru and l°6Ru are the most inaccurate. This was not unexpected since ruthenium appeared in Finland in two different chemical forms, which behave differ- ently in wet conditions (Lang et al., 1988).

The average scavenging coefficient for all the nuclides in Table 2, weighted by the high altitude nuclide concentrations, is A = 10 -4 s- t R0.64. It corresponds to decreases in airborne concentrations of about 30% over LOV and OLK but less than 4% at HEL and NUR. According to aircraft observations on the morning of the following day, 30 April, only a few millesimals of radionuclides remained at high altitudes over Southern Finland (Sinkko et al., 1987). The decreases in upper-air concentrations near HEL and NUR during 29-30 April thus resulted partly from deposition, but mainly because the radioactive air was carried away by the horizontal flow on 30 April. At LOV and OLK and especially in those parts of Southern Finland, where rainfall amounts were higher, the wet deposition was most likely more important in cleaning the air.

The A-R relationships in Table 2 parametrize scavenging caused by hydrometeors mostly in the liquid phase, as the melting layer was at a height of about 2 km and the radioactive substances were mainly found at heights below that (Fig. 3). However, hail also occurred at places. The base of clouds was at about 1-2 km, which means that the wet removal may have been caused partly by in-cloud scavenging and partly by below-cloud scavenging. Thus the results in Table 2 represent the total effect of those two mechanisms.

The data, on which the dependences in Table 2 are based, are fairly sparse: observations made at only four places were available. Moreover, precipitation at each of these places was light, the rainfall amount being less than 1 mm. Especially in Helsinki the radar echoes were very weak. Additional measurements of deposition and high-altitude air concentration at places with moderate or heavy rain would have been valuable.

In addition, many simplifying assumptions had to be made. For example, precipitation rate was taken as

Table 2. Empirical values of a and b with limits of error in the relationship A = a R b between scavenging coefficient A (s- 1) of different radionuclides, originated at Chernobyl, and rainfall rate R (mm h-~) in Southern Finland on 29 April

1986

Nuclide a (s- 1 ) b

t°3Ru (4+3)× 10 -4 0.72__+0.09 l°6Ru (2+2) x 10 -4 1.2+0.2 129raTe (1.3+1.2) x 10 * 0.4+0.2 132Te (1.8+ 1.2) x 10-'* 0.71+0.11 t31I(p) (7+5) x 10 -5 0.69+0.12 133I(p) (1.6+ 1.3) x 10 -s 0.5+0.2 134Cs (2.8+0.6) x 10 -s 0.51 +0.07 136Cs (2.4-t-0.5) x 10 -s 0.43__+0.08 13~Cs (3.4+0.9) x 10 -5 0.59+0.08 l*°Ba (3+2) x 10 -5 0.3+0.5 Weighted average 1.0 x 10 -4 0.64

(p): Particle-bound iodine.

independent of height. In fact, the precipitation rate often changes insignificantly with altitude below the melting layer, where the pollutants were in this case mainly concentrated, even though at higher altitudes the precipitation rate generally decreases with height (Herzegh and Hobbs, 1980; Passarelli, 1978; Stewart et al., 1984). On the other hand, if efficient evaporation occurs in the subcloud layer, rainfall intensity may be weaker near the ground than at higher altitudes or there may be no precipitation at all at the surface. The case of effective evaporation is rather complicated, because then, in addition to the dependence of rainfall rate on height, a portion of pollutants captured by hydrometeors would be transferred back to the gas phase or to aerosols. The neglect of evaporation can overestimate below-cloud wet removal of pollutants by 25-30% (Chang, 1986). Another question is the importance of the roles of afternoon dry convection and downdrafts due to falling rain in bringing pollutants down to the ground from upper levels. The influence of dry convection was possible and even probable in Southern Finland outside the coastal area (Puhakka et al., 1990), but according to Greenfield (1957) it is unlikely that any considerable amounts of pollutants can be deposited onto the ground as a direct result of downdrafts.

In spite of the arguments above, the results agree quite well with earlier studies. According to measurements made in the U.S.S.R. and in Belgium in 1959-1962, the scavenging coefficients of fission products varied from 2 x 10 _4 S- 1 to 2 X 10- 3 S- 1 in the case of rainout, and from about 5x 10-6s -1 to 4 x 10- 5 s- 1 in the case of washout (Makhon'ko, 1967).

Similar orders of magnitude resulted from other experimental investigations of wet removal coefficients for various particles and gases, made during the last three decades (McMahon and Denison, 1979; Sperber and Hameed, 1986). A more recent study by Horn et al. (1987) of radioactivity in the rainwater in the F.R.G. after the Chernobyi accident showed that the scavenging coefficients of l°3Ru, total 1311 and 137Cs


were proportional to R °'74 on 3 May and to R °'s6 on 5 May 1986. With regard to theoretical research, scavenging coefficients for soluble aerosol particles, de- rived by Scott (1982) and Chang (1986), were also not very different from those in Table 2.

4.2. Comparison of estimated and measured dose rates

After determining the empirical A-R relationships for various nuclides, they were used to evaluate gamma radiation dose rates in ground-level air on 29 April and 3 May 1986 for those radiation monitoring stations in Southern Finland, where only dose rates were measured but direct observations of deposition were not available (section 2.2). A description of the method used can be found in Jylh/i (1990). Several assumptions were made, the most crucial one being uniformity of the horizontal distribution of radioactive substances in the atmosphere. Calculated values were then compared with observed ones, in order to test out the A-R relationships.

The percentage of estimates that agreed with observations to within a factor of two was 62% for 29 April and 76% for 3 May 1986, and there was a significant positive correlation between measured and computed values. However, rather more than half of the model calculations underestimated dose rates (Jylh/i, 1990). A reason for the slight calculations underestimation by model calculations may be the fact that neither the wet deposition of gaseous iodine nor the occurrence of hot particles were taken into account when predicting dose rates, even though these may well have heightened radiation levels measured near the ground. In addition, the rain which occurred during the night of 30 April l May may have caused some additional fallout, but the question as to the importance of that rainfall cannot be reliably answered, as neither radar nor ground-level dose rate measurements were made during that period. One explanation for the inconsis- tences was a lack of detailed information on the areal distribution of the pollutants in the atmosphere. Fur- thermore, part of the differences between observed and calculated dose rates was most likely due to errors in radiation and rainfall measurements (Puhakka et al. 1990; Jylh~i, 1990).

On the whole, the observations and predictions agree surprisingly well with each other. This proves the importance of precipitation in depositing radioactive materials onto the ground, and consequently, the usefulness of weather radar in predicting the occurrence of wet deposition. It also supports the empirical relationships between the scavenging coefficient and the rainfall intensity, presented in section 4.1.

5. CONCLUSIONS

This paper demonstrated the use of weather radar in investigating the precipitation scavenging of pollutants. Radar measurements of the rainfall rate in Southern Finland after the Chernobyl accident were

combined with simultaneous radioactivity observations in the air and on the ground. As a result, the relationship between the scavenging coefficient A (s - i ) and the rainfall rate R (mm h 1) could be estimated for radionuclides l°3Ru, l°6Ru, 129roTe, 132Te, 134Cs, 136Cs, 137Cs, 14°Ba and particulate 131I

and 133I. The dependences parametrized the total effect of in-cloud and below-cloud wet removal caused by hydrometeors mostly in the liquid phase. The average scavenging coefficient weighted by the high altitude nuclide concentrations was A = 1 0 - 4 s 1 R TM. Even though several assumptions had to be made, the results were in good agreement with earlier studies. They were also supported by a comparison of measured and calculated gamma radiation dose rates.

In the case of an accidental release, an estimate of transport and deposition is needed rapidly. However, the composition and vertical distribution of the pollutants involved may be unknown at that time. Quantit- ative predictions of wet deposition, based on airborne concentrations combined with the relationships between the rainfall rate and the scavenging coefficient, are then difficult to make. However, weather radar can also be used to give qualitative estimates of the fallout. As is well known, it is capable of giving three-dimensional data on precipitation essentially in real time. A consequence of this is that, on the basis of the move- ment and development of precipitation areas, it en- ables short-term forecasts of the areas most likely to be significantly contaminated by wet deposition to be made.

To conclude, weather radar or a network of radars may form an important part of a real-time monitoring and warning system in case of emergency. On the other hand, digital records of radar measurements can be effectively used in experiments by which transport and deposition theories and models are developed and tested. Although this paper is concerned with radioactive fallout, it is obvious that the conclusions are valid for the wet deposition of any substances.

Acknowledgements--The author would like to express her thanks to Timo Puhakka for encouragement during the work and comments on the manuscript, and also for financial support granted from the Aili Nurminen, the Vilho, Yrj6 and Kalle Vhis/ilfi, the Jenny and Antti Wihuri and the Maj and Tor Nessling Foundations and from the Ministry of Trade and Industry.

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Empirical scavenging coefficients of radioactive substances released from chernobyl