Atmospheric concentrations of 7Be and 210Pb in Granada, Spain

Journal of Radioanalytical and Nuclear Chemistry, Vol. 261, No. 2 (2004) 401–405

0236–5731/2004/USD 20.00 Akadémiai Kiadó, Budapest© 2004 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

Atmospheric concentrations of 7Be and 210Pb in Granada, SpainM. Azahra,1,2* J. J. López-Peñalver,1 M. A. Camacho-García,3 C. González-Gómez,1,2

T. El Bardouni,4 H. Boukhal41 Laboratorio de Radioquímica y Radiología Ambiental, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain

2 Instituto Andaluz de Ciencias de la Tierra, CSCI. Facultad de Ciencias, E-18071 Granada, Spain3 Instituto de Técnicas Energéticas, Universidad Politécnica da Cataluña, Avda, Diagonal, 647, E-08028 Barcelona, Spain4 Université Abdelmalek Essaadi, Faculté des Sciences, Département de Physique, BP 2121, M’hanech II, Tétouan, Maroc

(Received January 23, 2004)

Aerosols samples in near-surface air of Granada (Spain) were collected on a weekly basis. The seasonal 210Pb and 7Be concentrations weredetermined during the five-year period, from October 1993 to September 1997. The elements, despite their different origin and their differentdistribution throughout the atmosphere, present the same seasonal variation. There was a tendency for a maximum during the summer season and aminimum during fall and/or winter. In this work, the concentration of 7Be and 210Pb and meteorological data have been used in order to determinethe periods of the potential radioactive pollution. This study, also, shows that the deposition of 7Be occurs primarily by precipitation except duringthe investigation periods where precipitation was scarce and irregular.

Introduction

The long term monitoring of radioactivity in the airprovides useful information about the radiation in theenvironment due to natural and man-made sources andhelps to evaluate their impact on man. Inhalation is oneof the pathways by which radioactive nuclides areincorporated into the human body. Atmosphericradionuclide monitoring is only a means to makedecisions and take provisions for mitigating the risk ofdiseases reaching the general public. It is not thepurpose of this paper to predict the impact of specificincremental radiation exposure arising from humanactivities, but with the information base provided,national and international bodies may be able to selectappropriate criteria for the radiological protection ofpopulation and natural ecosystem. In our laboratory, wehave been continuously measuring the radioactivity inthe surrounding as part of a general project, undertakenby many laboratories in Spain in collaboration with theSpanish authorities with the aim of starting anenvironmental radiological surveillance plan in theautonomous region of Granada. In this project, 210Pband 7Be activities are routinely measured in near-surfaceair of Granada.

210Pb (T1/2 = 22 y) is produced by radioactive decayfrom its progenitor, 222Rn. Radon, which is a noble gas,an isotope in the 238U decay chain, emanates primarilyfrom land surfaces. The concentration of 210Pb in the airdecreases with the altitude due to the decrease of 222Rnconcentration.1 In contrast, 7Be (T1/2 = 53.3 d) isproduced throughout the atmosphere as a product of thefission of oxygen and nitrogen nuclei by energeticcosmic rays.2

* E-mail: [email protected]

210Pb and 7Be provide a useful tool for studying thebehavior of aerosol because soon after their formationthe 210Pb and 7Be atoms becomes irreversibly attachedto submicron aerosol particles.3,4 Beryllium-7 and 210Pbin the form of this fine aerosol fraction maysubsequently enter the marine as well as the terrestrialvegetation environment via wet or dry deposition. It hasbeen shown in many places around the globe that thereare seasonal variations on 7Be and 210Pb concentrations.7Be concentrations, in ground level air at middlelatitudes have been up to 7 mBq/m3 in the last decades,with a general variation in annual production rates5–7due to variations in galactic cosmic rays. In this paper,we present the five-year study of 7Be and 210Pbconcentrations in near-surface air of Granada, for theperiod from October 1993 to September 1997. The mainobjective of the present work was to show that theseasonal variation of the 7Be and 210Pb is a function ofat least two meteorological parameters, the temperatureand the rainfall. The correlation observed is shown to benot uniform from season to season, but depends onwhether the investigation period was dry or wet.

Experimental7Be and 210Pb concentrations, in near-surface air,

were measured at the University of Granada (Spain)situated at a latitude 37°10'50" North and a longitude3°35'44" West, at an altitude about 670 m above the sealevel. The aerosol particles were collected using an air-sampling pump with a 30 l/min constant flow rate. Thepump was placed at a height of 10 m above the groundand the particles were retained on cellulose filters of0.8-µm pore size and 47-mm diameter (collectionefficiency was 99.9%). The samples were collected for

M. AZAHRA et al.: ATMOSPHERIC CONCENTRATIONS OF 7Be AND 210Pb IN GRANADA, SPAIN

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one week, which corresponds to a sampled volume ofabout 300 m3. Seasonal 7Be and 210Pb concentrationswere obtained by measuring 13 accumulated filters(collected during 13 weeks of the season) in the samePetri dishes. The measurements were carried out by non-destructive gamma-ray spectrometry by means of their477.6 keV (I = 10.3%) and 46.5 keV (I = 4.5%) γ-rays,respectively. The detector was a reversed electrodecoaxial intrinsic germanium detector (ReHPGe) with aresolution of 2 keV at 1332 keV. Careful efficiencycalibration was carried out using a gamma solutioncocktail, containing 133Ba, 60Co, 137Cs, 152Eu and241Am. The solution was put into Petri dishes to providethe same geometry for the measurements and countedfor 1500 minutes by a gamma-ray spectrometer. Thegamma-ray spectrum of the solution was exploited bythe SPECTRAN-AT V.4.0 software developed byCanberra Industries Inc., USA, to give the efficiencycurve. The detection efficiency was found to be 8.2%and 13.3% for the 477.6 keV and 46.5 keV γ-rays,respectively. The program was used to control the dataacquisition and storage as well as the sample analysisand calibration. Samples were counted for 1500 minutes.

The concentrations were corrected for decay to the mid-collection period.

The temperature parameters were supplied by theCentro Meterologico de Andalucia Oriental, and theseasonal rainfall were obtained with help of an inclinedstainless steel pluviometer, situated at the roof of theFaculty of Sciences of the University of Granada.

Results and discussion

The analysis performed on 20 atmospheric samplesfrom October 1993 to September 1997 highlighted thepresence of 7Be and 210Pb in all the samples. In Figs 1and 2, we have plotted the seasonal 210Pb and 7Beconcentrations, respectively. The results obtained showthat both elements present in the near-surface air ofGranada exhibit a maximum during summer, while theminimum was obtained during fall and/or winter. Suchseasonal variations had been observed in other sites aswell.8–10 It can be seen from the data that the levels of210Pb and 7Be varied by the same manner, and presentthe same seasonal variations, despite their differentorigin and their different distribution throughout theatmosphere.

Fig. 1. Seasonal concentrations of 7Be in air for 1993–1997


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Fig. 2. Seasonal concentrations of 210Pb in air for 1993–1997

During the investigation period the 210Pb activityvaried by more than a factor of 5, between 0.2 mBq/m3in the winter 1996 and 1.1 mBq/m3 in summer 1995with an arithmetic mean of 0.57 mBq/m3. This value isconsistent with the concentration of 210Pb in groundlevel air that has been reported by others at sites with alatitude similar to that of Granada.11 PREIS et al.11indicated that at northern midlatitudes (60°N–30°N), thesurface concentration values of 210Pb in air were highover continental areas, in relation to the rest of theworld. However, significant differences appear insidethe continental areas of this latitudinal band. Theyconcern the central versus coastal sites where oceanicinfluence prevails and the western versus eastern coastsare distinguished by zonal circulation. In Europe, forsites situated at longitude between 0 and 5, 210Pbconcentrations were about 0.55 mBq/m3,11 which isfairly comparable with our value obtained.

Concerning 7Be, the activities, in general, werehigher than those for 210Pb during the observationperiod. The activity measured varied by a factor of 2,between 3.4 mBq/m3 obtained in winter 1993 and7.6 mBq/m3 obtained in summer with an arithmeticmean of 5 mBq/m3. This latter is in good agreement withthe concentrations of 7Be in ground-level air reported byothers for different countries with a latitude similar tothat of Granada.12,13 Seasonal variations show minimumconcentrations in fall and winter and maximum during

the summer, which probably reflect the seasonalvariations in the transport rate of the air from thestratosphere to the troposphere.

Correlation of the seasonal 7Be and 210Pbconcentrations with temperature and rainfall

Table 1 shows the linear correlation coefficient andthe p-values for the 7Be and 210Pb concentrations in air(mBq/m3), the seasonal amount of rainfall (LL in l/m2),and the average seasonal temperature (Tav in °C).Concerning 210Pb, the p-values in this table are in allcases less than 0.01, which means that there is astatistically significant relationship between 210Pbconcentration and the studied parameters at a 99%confidence level. 7Be concentration revealed apronounced positive correlation with Tav. Weakcorrelation is observed between 7Be concentration andLL. This is because Granada is situated in a climaticzone where there are no strong seasonal variations inrainfall. Literature data suggest that several sites situatedin climatic zones with strong seasonal variations inrainfall rate experience an inverse relationship betweenthe rainfall and 7Be concentrations, especially indicatingthe importance of the washout of the atmospheric aerosolcarrying 7Be.12,14,15 Another factor that should beconsidered is the variation in precipitation at Granada.


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Table 1. Correlation matrix for 7Be concentration, 210Pb concentration, seasonalamount of rainfall, and the average seasonal temperature

Parameter 210Pb 7Be LL Tav210Pb 1 0.535 (0.008) –0.553 (0.008) 0.766 (0.001)7Be 1 –0.393 (0.15) 0.733 (0.001)LL 1 –0.445 (0.025)Tav 1

The total annual rainfall during the 1993–1997period ranged between 224 and 751 l/m2 which aredistributed throughout the year although summers couldbe drier than the rest of the year.

To illustrate the influence of the rainfall over 7Beand 210Pb concentrations, two separate periods wereinvestigated: the dry period, between 1993 and 1995 (12samples), where the amount of precipitation did notexceed 312 mm, and the wet period (characterized byabundant and regular rainfall), between 1996 and 1997,where the amount of precipitation was between 452–623 mm (8 samples). Tables 2 and 3 present the linearcorrelation coefficients obtained between 7Be and 210Pbconcentrations, LL and Tav, during the dry and wetperiod, respectively.

It can be seen from Table 2 that, during the dryperiod, the two elements correlate significantly with Tav.Using the backward regression method, and according tothe selected criteria chosen by the program, the modelequations for 7Be and 210Pb concentrations during thedry period are given by:

7Be (mBq/m3) = (0.161±0.026)Tav+(2.096±0.442) (1)210Pb (mBq/m3) = (0.034±0.009)Tav+(0.097±0.015)(2)

In Eq. (1) the R-square is 0.79 and the p-value is lessthan 0.01, which means that during the dry period, thereis statistically a significant relationship between 7Beconcentrations and the seasonal average of thetemperature. In Eq. (2) the R-square is 0.59 and the p-value is less than 0.01, which imply also that there is astatistically significant relationship between the 210Pbconcentrations and the seasonal average of thetemperature.

During this investigation period, a correlationbetween the 7Be concentrations and the rainfall couldnot been observed i.e., no washout was produced byrain.16,17 The positive correlation between temperatureand 7Be indicated that the 7Be concentrations increasedwith rising temperature, i.e., the highest 7Be quantityappeared in summer. This phenomenon is mainlyattributed to an admixture of stratospheric-produced 7Bein the near-surface air, or to the vertical transport withinthe troposphere. This study shows that the highest 7Beconcentrations were registered during the summermonths. The result seems to indicate a considerablecontribution of a stratospheric 7Be source at Granada.

The increase of the 210Pb concentrations in summer,compared to the rest seasons, can be explained11 by thefact that when the air mass is stable, 222Rn and itsdaughters accumulate in the boundary layer leading tohigh surface concentration of 210Pb. The consequence isthat when seasonal anticyclones occur over a continentarea (as in the case of Granada), the air masses becameenriched in 210Pb.

From Table 3, we can appreciate the great inverserelationship, during the wet period, between 7Beconcentrations and LL, indicating the washout of theatmospheric aerosol that carries 7Be. This is consistentwith the explanation given by others.12,14,15 210Pbconcentrations correlate both with LL and Tav.

Using the backward regression method, andaccording to the selected criteria chosen by the program,the equations of the model for 7Be and 210Pbconcentrations during the wet period are given by: 7Be (mBq/m3) = –(0.015±0.001)LL+(8.126±0.226) (3) 210Pb (mBq/m3) = (0.044±0.01)Tav+(0.183±0.168) (4)

Table 2. Linear correlation coefficients between parameters for the dry periodParameter 7Be 210Pb LL Tav

7Be 1 0.73 (0.01) –0.393 (0.22) 0.89 (0.000)210Pb 1 0.333 (0.29) 0.731 (0.009)

Table 3. Linear correlation coefficients between parameters for the wet periodParameter 7Be 210Pb LL Tav

7Be 1 0.7 (0.03) –0.981 (0.000) 0.85 (0.001)210Pb 1 –0.76 (0.008) 0.878 (0.001)


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The R-square, in Eq. (3) is 98.1%, and the p-value isless than 0.01, so there is statistically a significantrelationship between 7Be concentrations and LL at 99%confidence level. Equation (4) shows a significantrelationship between 210Pb concentrations and Tav.

It should be noted that during the investigationperiod, the two radionuclides,7Be and 210Pb, correlatesignificantly also with each other. The correlationcoefficient obtained was 0.535. Since their modes ofproductions are different, 7Be and 210Pb have twodistinct sources. However, they are removed from theatmosphere by very similar mechanisms. Thus, theycould not be used as two independent atmospherictracers as it is suggested by TUREKIAN.18

Conclusions7Be and 210Pb concentrations have been determined

in near-surface air of Granada (Spain) at the Universityof Granada during the period of 1993–1997. 7Beconcentrations varied from 3.4 to 7.6 mBq/m3, and thoseof 210Pb from 0.2 to 1.1 mBq/m3. The annual averagevalues obtained for both radionuclides during theinvestigation period were in good agreement with thosemeasured for other countries.14,19,20,11

7Be and 210Pb concentrations exhibit the sameseasonal variation: the fallout minima were confined toeither autumn and/or winter, and, in all the investigatedyears, the maximum concentrations were observedduring summer. During all the periods themeteorological parameters such as temperature andrainfall, were the main factors responsible for thevariations. The inverse relationship between 7Beconcentrations and the seasonal amount of rainfall wasnot generally observed, and then, the washout of theatmospheric aerosol that carries 7Be was only observedin climatic zones having strong seasonal variations inrainfall.

Finally, a model was given to predict the periods ofpotential radioactive pollutions by the determination of7Be and 210Pb concentrations. In the future, thecorrelation might be extended to other meteorologicalparameters such as humidity, solar variation, pressureand wind speed in order to determine the impact of the

combined meteorological parameters on theconcentrations of the radionuclides in question. This willhelp to evaluate the radiation dose to which the publicmay be exposed to at these concentrations of pollutantsin the air.

References

1. H. E. MOORE, S. E. POET, E. A. MARTELL, J. Geophys. Res., 48(1973) 169.

2. E. BRUNINX, J. Report, CERN, Geneva, Switzerland, 61 (1964) 1.3. H. E. MOORE, S. E. POET, E. A. MARTELL, Natural. Radiat.

Environ., III., CONF-780422, 1980, p. 415.4. E. A. BONDIETTI, C. PAPASTEFANOU, C. RANGARAJAN, ACS

Symp. Ser, P. K. HOPKE (Ed.), American Chemical Society,Washington, 331 (1987) 377.

5. M. AZAHRA, A. CAMACHO-GARCÍA, C. GONZÁLEZ-GÓMEZ,J. J. LÓPEZ-PEÑALVER, T. EL BARDOUNI, Appl. RadiationIsotopes, 59 (2003) 159.

6. F. CANNIZZARO, G. GRECO, C. SPITALE, E. TOMARCHIO, Nucl.Geophys., 9 (1995) 597.

7. D. AL-AZMI, A. M. SAYED, H. A. YATIM, Appl. RadiationIsotopes, 55 (2001) 413.

8. T. MATSUNAMI, A. MIZOHATA, T. MAMURO, Ann. Rept. Radiat.Center Osaka Prefect, 20 (1979) 1.

9. C. R. OLSEN, I. L. LARSEN, P. D. LOWRY, N. H. CUTSHAL,J. F. TODD, G. T. F. WONG, W. H. CASEY, J. Geophys. Res., 90(1985) 10487.

10. J. F. TODD, G. T. F. WONG, C. R. OLSEN, I. L. LARSEN,J. Geophys. Res., 94 (1989) 11106.

11. N. PREISS, M.-A. MÉLIÈRES, M. POURCHET, J. Geophys. Res.,101 (1996) 28847.

12. H. W. FEELY, R. J. LARSEN, C. G. SANDERSON, J. Environ.Radioact., 9 (1989) 223.

13. C. DUEÑAS, M. C. FERNÁNDEZ, E. LIGER, CARRETERO, Atmos.Environ., 33 (1999) 3705.

14. P. J. WALLBRINK, A. S. MURRAY, J. Environ. Radioact., 25(1994) 213.

15. Y. ISHIKAWA, H. MURAKAMI, T. SEKINE, K. YOSHIHARA,J. Environ. Radioact., 26 (1995) 19.

16. J. TREMBLAY, R. SERVRANKX, J. Radioanal. Nucl. Chem., 172(1993) 49.

17. M. BETOLLI, L. CANTELLI, S. DEGETTO, L. TOSSITIO,O. TUBERTINI, S. VALCHERS, J. Radioanal. Nucl. Chem., 190(1995) 137.

18. K. K. TUREKIAN, L. K. BENNINGER, E. P. DION, J. Geophys. Res.,88 (1983) 5411.

19. L. DURANA, M. CHUDÝ, J. MASARIK, J. Radioanal. Nucl. Chem.,207 (1996) 345.

20. F. CANNIZZARO, G. GRECO, M. RANELI, C. SPITALE,E. TOMARCHIO, Appl. Radiation Isotopes, 51 (1999) 239.

Documents

Atmospheric concentrations of 7Be and 210Pb in Granada, Spain