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Atmospheric Environment 47 (2012) 84e91

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Atmospheric Environment

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7Be behaviour in the atmosphere of the city of Granada January 2005to December 2009

F. Piñero García a, M.A. Ferro García a,*, M. Azahra b

aRadiochemistry and Environmental Radiology Laboratory, Inorganic Chemical Department, Faculty of Sciences, University of Granada, Av. Fuentenueva, 18077 Granada, Spainb Physics Department, Faculty of Sciences, Abdelmalek Essaadi University, 93002 Tétouan, Morocco

a r t i c l e i n f o

Article history:Received 29 August 2011Received in revised form24 October 2011Accepted 14 November 2011

Keywords:7BeGround-level airSeasonal variationsMeteorological factorsAtmospheric deposition

* Corresponding author. Tel.: þ34 958 243325; faxE-mail address: ferro@ugr.es (M.A. Ferro García).

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.11.034

a b s t r a c t

This paper focuses on 7Be concentration in the atmospheric surface aerosols of the Spanish city ofGranada, in the southern Iberian Peninsula. We analyze the effects of meteorological conditions in thegeographical zone over the period 2005e2009. A prediction model is suggested to describe thebehaviour of atmospheric aerosols containing 7Be. The model was applied from January to May 2010 andshows that temperature, relative humidity and precipitation are the meteorological factors responsiblefor the behaviour of atmospheric aerosols. The statistical significance of the results was 99%. This studyhelps understanding of the atmosphere of Granada as well as the climate variables responsible fortransportation and deposition of atmospheric aerosols. Moreover, in case of a massive release ofradionuclides into the atmosphere, the results of this study will help predict behaviour in this city andother continental areas with a similar climate.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

7Be is a natural cosmogenic radionuclide, approximately 70% ofwhich is produced in the stratosphere (Kathren, 1984), with theremaining w30% being produced in the high troposphere (Lal andPeters, 1967). Concentration of 7Be reaches is highest at 15e20 kmand decreases exponentially with altitude. 7Be concentration in thetroposphere and the stratosphere depends on the flow of cosmicrays at altitude, whose density has an inverse relation with solaractivity, undergoing cyclical changes every 11 years (Hötzl et al.,1991; Azahra et al., 2003; Papastefanou and Ioannidou, 2004;Cannizzaro et al., 2004).

The cosmogenic radionuclide 7Be (half-life: 53.3 days) isproduced by spallation reactions between the protons of theprimary components and the secondary neutrons of cosmic rayswith nuclei of light elements found in the atmosphere, such ascarbon, oxygen and nitrogen (Papastefanou and Ioannidou, 1995).After production, 7Be reacts with air constituents and turns intoatoms or 7BeO and/or 7Be(OH)2 molecules, which diffuse in theatmosphere until becoming attached to available aerosol particles(0.07e2 mm diameter) drifting in the atmosphere (Papastefanouand Ioannidou, 1995). These small particles and with attached 7Be

: þ34 958 248526.

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are distributed over the earth’s surface and inserted into differentecosystems through dry and/or wet deposition (Papastefanou andIoannidou, 1995; Wallbrink and Murray, 1994; Dibb, 1990). There-fore, 7Be has been proposed as a natural atmospheric tracer (Lal andPeters, 1962; Brost et al., 1991; Vecchi and Valli, 1997).

7Be is widely used as a natural radiotracer in many environ-mental processes due to its characteristics: it has high specificactivity as a result of its short period of half-disintegration, it iseasily detected by gamma emission, it is a radionuclide with lowchemical reactivity and its concentration has not been altered byanthropogenic inputs. Moreover, 7Be and fission products (137Cs)usually behave in the same way despite having a different originand distribution throughout the atmosphere, (Ioannidou andPapastefanou, 2006).

Furthermore, the connection between air mass transport and7Be activity at ground level has also been identified, such asseasonal variations caused by: 1) air mass exchange betweenstratosphere and troposphere; 2) vertical transfer of aerosols in thetroposphere coming from the tropopause at cloud level or below;3) northesouth mixing due to the important difference inproduction rates between higher and lower latitudes; 4) eastewestcirculation, where the differences between geographical andgeomagnetic latitudes play a key role; 5) heavy shower frequency(Feely et al., 1989). Variations in 7Be concentration are thereforea complex function of all these factors, as each process has its ownseasonal cycle and they do not happen in a controlled time or place.

Table 1Atmospheric 7Be specific activity reported at different locations.

Location Period Range (med. � s) (mBq m�3) Experimental conditions of aerosol sampling References

Málaga (Spain) 36.73N; 4.47W 2000e2006 2.47e14.9 (4.8 � 1.6) Samples collected to 12 m a.g.l. Flow rate: 40 L min�1.Cellulose nitrate filters (15.90 cm2, øpore ¼ 0.8 mm).Samples collected weekly and measured monthly.

Dueñas et al., 2009

Granada (Spain) 670 m a.s.l 37.18N; 3.59W 2005e2009 0.37e9.33 (4.51 � 0.23) Samples collected to 1.5 m a.g.l. Flow rate: 75 L min�1.Cellulose nitrate filters (15.90 cm2, øpore ¼ 0.8 mm).Samples collected weekly and measured monthly.

This work

Sevilla (Spain) 37.36N; 5.97W 2004e2005 3.1 Samples collected to 30 m a.s.l. Flow rate: 950 m3 h�1.Polypropylene filters 44 � 44 cm2.Samples collected and measured weekly.

Gonzalez et al. (2004,2005)

Tracy (California) 392 m a.s.l 37.65N; 121.53W 1977e1981 3.29e6.70 (5.14 � 0.7) Flow rate: 58.34 m3 h�1. Samples collected usingMicrosorban air filter medium 99/97- 4200, of 20 cm.

Feely et al., 1989

Palermo (Italy) 38.12N; 13.35E 1982e2002 2.16e10.57 (5.06) Samples collected to 20 m a.g.l. Flow rate: 900 m3 h�1.Cellulose filters 45 � 45 cm2.Samples collected and measured daily.

Cannizzaro et al., 2004

Cáceres (Spain). 405 m a.s.l 39.51N; 6.34W 1992e1994 11.3e0.88 (4.4 � 1.8) Samples collected to 1.5 m a.g.l. Flow rate: 60 L min�1.Cellulose nitrate filters (15.90 cm2, øpore ¼ 0.8 mm).Samples collected weekly and measured monthly.

Baeza et al., 1996

Barcelona (Spain) 41.38N; 2.12E 2001e2005 2.70e4.4 (3.5 � 0.4) Flow rate: 800 m3 h�1. Polypropylene filters (44 � 44 cm2).Samples collected and measured weekly.

Vallés et al., 2009

Bilbao (Spain) 43.27N; 2.94W 2000e2009 (3.06 � 1.02) Samples collected to 15 m a.g.l. Flow rate: 500 m3 h�1.Polypropylene filter (44 � 44 cm2, øpore ¼ 0.8 mm).Samples collected and measured weekly.

Alegría et al., 2010

Dome (France) 1465 m a.s.l 45.77N; 2.97E 2006e2008 3.64e10.7 Samples collected to 2 m a.g.l. Flow rate: 700 m3 h�1.Polypropylene fibres. Collection twice aweek from 10-2005 to 06-2007 and once a week since 07-2007.

Bourcier et al., 2011

Dome (France) 660 m. a.s.l. 45.72N; 3.09E 2004e2008 1.73e7.73 Samples collected to 2 m a.g.l. Flow rate 300 m3 h�1.Popolypropylene fibres. Collection twice a weekfrom 10-2005 to 06-2007 and once a week since 07-2007.

Bourcier et al., 2011

F.PiñeroGarcía

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Fig. 1. Monthly evolution of 7Be specific activity in the aerosols of Granada during theperiod January 2005eDecember 2009.

Fig. 2. Frequency histograms for 7Be specific activity and normal distribution, line.

F. Piñero García et al. / Atmospheric Environment 47 (2012) 84e9186

Furthermore, knowledge of the vertical distribution of natural andartificial radionuclides in the atmosphere is a key parameter formodelling studies of dry and wet deposition, which are importantprocesses to be taken into account in the case of a nuclear accident.These features imply that 7Be activity at ground level containsmixed information on both production and transport (Leppänenet al., 2010).

For all these reasons, this excellent atmospheric radiotracer ismeasured in many countries around the world. Table 1 presents theconcentrations of 7Be at different geographical points, as well assome experimental conditions of aerosol sampling. The differentvalues may be attributed to the difference in geographical coordi-nates and the local climate conditions. Therefore, 7Be surface airactivity shows a strong dependence on the specific circumstancesof the sampling point.

The city of Granada is located in south-east Spain in a naturalbasin surrounded by mountains of around 1000 m, while thehighest range (Sierra Nevada) is located towards the east of thebasin and contains the highest peak in the Iberian Peninsula. It isa non-industrialized medium-sized city, with a population of300,000. The near-continental conditions prevailing at this site areresponsible for large seasonal temperature differences, providingcold winters and hot summers. The area also experiences periods ofvery low relative humidity especially in summer. Moreover, theSierra Nevada Mountains isolate the city from sea winds and thereare few periods of strong wind, long hours of sunshine, and hightraffic flow with frequent morning thermal inversions.

The aim of this paper is to study the temporal evolution of 7Beconcentration in the surface atmosphere of the city of Granada. Wealso analyze the meteorological factors controlling the seasonalvariations found in 7Be specific activity in the aerosol samples overthe period 2005e2009 and propose a model to explain 7Bebehaviour that can be extrapolated to describe other radionuclidebehaviour in similar circumstances.

2. Materials and methods

Samples of atmospheric aerosols were collected weekly in theFaculty of Sciences in the city of Granada (Spain) over fiveconsecutive years. The air sampler is located 1.5 m above groundlevel. The city of Granada lies 687 a.s.l., latitude 37� 100 5000 Northand longitude 3� 350 4400 West.

Aerosol particles were collected on 47 mm cellulose filter with100% efficiency and 0.08 mm pore size. An air sampling pump witha flow rate of aproximateley 75e100 L min�1 was used and theaverage volume ranged from 750e850 m3 per week.

The corresponding filters for every month (4e5 filters) wereplaced inside a Petri box for measurement of the samples. 7Beestimation was carried out by gamma spectrometry using a coaxialreverse-type germanium detector with vertical configuration.Details of the low-background gamma-ray detection system usedwere previously described by Azahra et al. (2003).

The detector’s efficiency was determined by a calibrated gammacocktail supplied by the CIEMAT (241Am, 109Cd, 57Co, 139Ce, 113Sn,137Cs, 88Yand 60Co), placed on the same cellulose filter paper as thatused for collection of the atmospheric aerosols.

Radioactivity in the filters wasmeasuredwith a counting time of90000 s and 7Be concentration was calculated by a photopeakgenerated in the spectrum measure at 477.6 KeV energy with 3.17410�2 counting efficiency. Genie 2000 software by Canberra wasused to control the data acquisition as well as sample analysis andcalibration. Specific activity was modified by radioactive decay, asthe half-life of 7Be is 53.3 days.

The meteorological measurements for the study were providedby the ArmillaMilitary Air Academy, byway of the Spanish National

Institute of Meteorology (37� 080 1300 N, 03� 370 5300 W, and 687 m).Sunspot data were collected by the Sunspot Index Data Centre(SIDC) in Brussels, Belgium, (http://www.swpc.noaa.gov/).

3. Results and discussion

3.1. Behaviour of 7Be concentrations over time at ground level

A total of 60 samples of aerosols were analyzed over fiveconsecutive years and specific clear activity was detected. Fig. 1shows the monthly average values of 7Be specific activity overtime. Depending on the moment of sample collection, 7Be can beseen to change regularly over one year periods so we find a cyclicaland seasonal structure similar to the behaviour already observedduring the 1993e2001 research period (Azahra et al., 2003). Everyyear, the highest values correspond to summer months and thelowest ones to thewinter. This cyclical and seasonal variation is dueto the cosmogenic origin of this radionuclide and to the fact that7Be activity in surface air shows a high dependency on the samplingpoint (latitude and local climate). This kind of behaviour is typicalof middle latitude areas such as Granada (Dutkiewicz and Husain,1985; Baeza et al., 1996; Dueñas et al., 1999). Fig. 1 also showsthat this cyclical structure was suspended in 2007, as explainedbelow.

F. Piñero García et al. / Atmospheric Environment 47 (2012) 84e91 87

The average specific activity was 4.56 � 0.22 mBq m�3 what iscomparable with the concentration of 7Be measured by differentauthors in their respective papers studied (see Table 1). The activityof 7Be in August, 9.334 mBq m�3, is the highest found in Granada(Spain) since 1993, whereas the lowest was in January 2007,0.534 mBq m�3. The mean annual activity of 7Be (mBq m�3) duringthis period was in 2005 (4.06), in 2006 (4.31), in 2007 (3.26), in2008 (5.37), and in 2009 (5.81). During the year 2009 the meanannual of 7Be was 29% higher than the typical mean value of4.5 mBq m�3 of Granada during 17 years of sampling. It was due tothe particular minimum of solar activity of that year and was alsoobserved in the measurements in Thessaloniki, Greek (Ioannidouet al., 2011).

3.2. Statistical study

The overall results of 7Be specific activity were examinedstatistically in order to determine fit as defined by average andtypical deviation. The frequency histogram of 7Be specific activity(Fig. 2) shows asymmetry on the right-hand side (Asymmetrycoefficient As ¼ 0.379 > 0) and is higher than normal distribution(Curtosis coefficient G2 ¼ 0.395 > 0).

The KolmogoroveSmirnov test was applied to decide whichtheoretical distribution (Normal, Normal-log, Uniform and Expo-nential) fits best to our data. The test confirms that normal andnormal-log distribution are statistically significant for a 95%confidence interval. The PP plot, Fig. 3, was used in order to decidewhich the best distribution was. The Fig. 3 shows that normaldistribution is better than normal-log one, because points areclosed to the diagonal line. Once fitted into a normal distribution,the values defining distribution were calculated (average value4.56 mBq m�3 and typical deviation 1.72 mBq m�3).

3.3. Standardization of data

The variables shown in this paper were normalized in order tocompare the results obtained as they fluctuate constantly duringthe period of time studied. The standardization criteria were usedto convert the data into their normal form N(0, 1):

Z ¼ ðX � XÞ=s (1)

Fig. 3. PP-plot. Left: Normal Distributio

where: Z is the resulting normalization value; X is the real value, Xis the arithmetic average and s is the standard deviation.

3.4. Factors controlling the atmospheric activity of 7Be

3.4.1. Variables studiedWe examined the effects of sunspots, atmospheric pressure,

precipitation, temperature, relative humidity on 7Be activity in theaerosol samples. 7Be specific activity was statistically correlatedwith the normalized monthly averages of the variables studied.

There is 99% statistically significant correlation with tempera-ture, precipitation and relative humidity, but 7Be behaviour is littleaffected by atmospheric pressure and sunspots.

Regarding atmospheric pressure, the correlation found is notstatistically significant, (r ¼ �0.193, p-value ¼ 0.139). On the onehand, this is due to the low influence, 5%, of this parameter on theearth’s cosmic radiation (Rankama, 1963), while, on the other,several authors (Dueñas et al., 1999) consider that the influence ofthis variable is still unknown.

A statistically significant correlation between the solar activityand 7Be specific activity in aerosols over the sampling period(2005e2009) has not been found. The Pearson correlation coeffi-cients obtained are very weak (r ¼ �0.024, p-value ¼ 0.854).Throughout the investigation studied we cannot observe anevident relation between two variables as it is shown in Fig. 4.Although during some periods when solar activity is high, cosmicrays are deflected away from the solar system and 7Be productiondecreases. In this sense, many studies have found correlationbetween solar activity and 7Be concentration in near-surface air(Hötzl et al., 1991; Azahra et al., 2003; Cannizzaro et al., 2004;Aldahan et al., 2008; Kikuchi et al., 2009; Alegría et al., 2010).

This can be explained by the fact that the period described inthis paper is not a complete solar cycle (11 years). In addition,a change in the solar cycle occurred between 23 and 24, with theend of the 23rd solar cycle being more extended than usual. Asa result, there was no significant change in solar activity over thelast two years (Fig. 4).

For this reason, the flow density of cosmic rays reaching earth’satmosphere was approximately the same during these two years.Consequently, the amount of 7Be produced in the higher layers ofthe atmosphere varied only slightly. The variations detected in the

n. Right: Log-Normal Distribution.

Fig. 4. Comparison of monthly average values of 7Be specific activity and sunspots inthe sampling period 2005e2009. 7Be e Histograms and Sunspot e Line.

F. Piñero García et al. / Atmospheric Environment 47 (2012) 84e9188

surface atmosphere of the city of Granada were mainly due to localmeteorological variables, which are the critical parametersresponsible for the sampled aerosols’ behaviour.

3.4.2. Air temperatureAir temperature is one of the most influential variables in 7Be

seasonal behaviour in the city of Granada. As shown by the resultsobtained, there is a very strong positive correlation between airtemperature and 7Be specific activity (r ¼ 0.816, p-value ¼ 0). Fig. 5shows distribution of monthly average values of radiotracer activitytogether with mean monthly air temperature. The highest valuesappear during summer months and lower values during wintermonths.

This seasonal pattern is reinforced by several hypotheses ofauthors such as Feely et al. (1989); Al-Azmi et al. (2001); Baeza et al.(1996); Gerasopoulos et al. (2001); Leppänen et al. (2010), amongmany others, who believe the maximum 7Be values found in thehottest months are caused by the intrusion of stratospheric airricher in 7Be into the troposphere as a result of the upwardmovement of the tropopause level in these months and by an

Fig. 5. Comparison of monthly average values of 7Be specific activity and air temper-ature during the period 2005e2009. 7Be e Histograms and Temperature e Line.

increased rate of vertical transport of 7Be from the upper tropo-sphere to the middle and lower troposphere, especially importantin mid-latitude zones (30� Ne40� N).

The usually high air temperatures during summer monthsaccentuate the mixing of vertical air masses in the tropospherewith subsequent transport of cosmogenic radionuclides from thehigh to the middle and lower troposphere. In the summer months,the troposphere is less stable and more turbulent, producinga higher mixing of air masses due to a convective circulation isproduced, carrying surface air upward and bringing downward airfrom higher levels. This vertical transport carries down to thesurface layer the 7Be that has been produced within the uppertroposphere, as well as that which has entered the tropospherefrom the stratosphere, (Feely et al., 1989).

In this process, the increase of cosmogenic radionuclidesassumes the importance in medium latitudes such as Granada,where the daily temperature variations are high throughout theyear, often reaching up to 20 �C. In addition, this effect is amplifiedby the lower scavenging due to the lower precipitation in thesummer months compared to the winter ones. This coincides withthe results of Lee et al. (2002) and Cannizzaro et al. (2004) in theirrespective areas studied.

We think that vertical transport from the mid troposphere intothe low troposphere is the mechanism responsible for the highervalues of 7Be activity reported in the present study, and highlightedby previous studies such as Hernandez et al. (2008). Following ourobservations, we conclude that in this period the vertical mixing ofair masses at surface level is accentuated by the particulargeographical location and climate conditions of the city of Granada,with low precipitation and wind during the above mentionedwarmer months. On the other hand, during the winter months theactivity concentrations of 7Be in surface air are low due to the lowerair temperatures, the higher rainfall, and the lower height of thetropopause.

With the aim of better understanding the influence of thisparameter on the results, we studied the variation of meanmonthlymaximum and minimum temperatures (DT ¼ Tmax � Tmin, �C) withmonthly average values of 7Be specific activity (see Fig. 6). On theone hand, we do strongly believe that high DT makes verticalaerosol transport easier within the troposphere. This could explainhigh values of 7Be found during the summer months. Likewise, thisvariation could explain the higher concentration values of 7Be

Fig. 6. Comparison of monthly average values of 7Be specific activity and the variationof mean monthly maximum and minimum temperatures, DT ¼ Tmax � Tmin, �C, duringthe period 2005e2009. 7Be e Histograms and DT e Line.

Fig. 8. Comparison of monthly mean variation of 7Be specific activity and relativehumidity during the period 2005e2009. 7Be e Histograms and Relative Humidity e Line.

F. Piñero García et al. / Atmospheric Environment 47 (2012) 84e91 89

activity found in October, November and December 2007 duringthe study. It seems reasonable to think that, in light of the results,the influence of vertical transport of air within the troposphereincreases with higher DT. On the other hand, we consider that thelower DT could be related to persistent heat inversions produced bythe lack of strong winds in Granada, making 7Be transport tosurface levels of the atmosphere more difficult and explaining thelow activity values found in April 2007 and May 2008.

3.4.3. Precipitation and relative humidityFig. 7 shows the evolution in time of monthly average 7Be

activity values in the aerosol samples analyzed, as well as theamounts of precipitation during the sampling period. A negativecorrelation can be observed between both parameters, with 7Be inthe aerosols collected being lower when precipitation volume rises.This inverse correlation is confirmed by the negative Pearson’coefficients between 7Be activity and rainfall (r ¼ �0.594,p-value ¼ 0), and relative humidity (r ¼ �0.783, p-value ¼ 0). Thismeans that both parameters produce a scavenging effect onatmospheric aerosols, reducing 7Be concentration in the surfaceatmosphere. The cause of this behavior is strong contrast betweenwet and dry seasons in Granada. Similar observations have beenreported by numerous authors at different locations (Tremblay andServrankx, 1993; Baeza et al., 1996; Dueñas et al., 2009).

An increase in precipitation was observed in April, May andSeptember 2007, with September being the month of highestprecipitation in the sampling period. The presence of 7Be fell duringthese months, due to the washing effect of the low troposphere,which carried away the aerosols suspended in surface levels of theatmosphere. Another reason for the low 7Be activity levels foundcould also be the effect of the dust particles from the Saharabetween 3rd and 7th September 2007. This event was detected andanalyzed by the Atmospheric Physics Group of the University ofGranada (Guerrero-Rascado et al., 2009). The Sahara desert is themost important source of mineral dust (Liu et al., 2008) and thesemineral dust particles may have acted as scavengers of 7Be at thistime. This behaviour agrees with the observations by Hernandezet al. (2008), who pointed out that when these episodes occur insummer there is a decrease of 7Be-aerosol residence time in thesurface atmosphere due to the presence of Saharan mineral dustparticles.

However, the opposite effect took place in November andDecember 2007 (Fig. 7). The facts explained above were the key to

Fig. 7. Comparison of monthly average values of precipitation and 7Be specific activityduring the period 2005e2009. 7Be e Histograms and Precipitation e Line.

understanding what happened in 2007 and the reasons why thecyclical structure typical of middle latitudes was not fulfilled as inother years (Fig. 1).

The relative humidity is other influential factor in 7Be seasonalbehaviour in the city of Granada. It is characterized by very lowrelative humidity periods, especially in the summer. Namely, themean monthly values of relative humidity throughout the yearsvaried from 32% (summer) to 87% (winter). Fig. 8 shows themonthly variation of 7Be concentration together with meanmonthly relative humidity values. This figure indicates that for theperiod studied high values of relative humidity facilitate thegravimetric deposition processes of the aerosols with a fall inradioactivity of the aerosols sampled. That coincides with thestrong correlation found between the mean monthly concentrationof 7Be and the relative humidity indicated previously (�0.783). Theexplanation for this behaviour is that the aerosols behave as nucleifor water vapour condensation in the atmosphere; that is animportant growth mechanism for submicrometre aerosol particles.When humidity is higher, their size increases and their residencetime in the atmosphere decreases under the influence of gravity,resulting in a lower 7Be content in the surface air.

Yu and Lee (2002) were also observed strong negative correla-tion between 7Be activity concentrations and the relative humidity.They had demonstrated that activity median aerodynamic diam-eter, AMAD, of 7Be-aerosols are high correlated with relativehumidity, while temperature does not affect the AMADs of the7Be-aerosol. This has reflection in our results shown in Fig. 8. We

Fig. 9. Comparison of the standardized activity of 7Be during the period January2010eMay 2010: Model vs. Measurements.

F. Piñero García et al. / Atmospheric Environment 47 (2012) 84e9190

indicate that relative humidity over 60% produces a suddendecrease in the specific activity values.

3.5. Prediction model

Having determined the climate parameters affecting 7Bebehaviour in the atmosphere of Granada and by using the statisticsprogram STATGRAPHICS-PLUS, we can draw up the most appro-priate model for the standardized experimental data obtained. Inview of the results, we can say that the best meteorological vari-ables by which to explain the behaviour of 7Be bearing aerosols inthe surface atmosphere of Granada city are air temperature (T),relative humidity (Hr) and precipitation (P).

A7Be¼ 1:125$10�5 � 0:284$Hr � 0:179$P þ 0:476$T (2)

The Pearson coefficient was 0.849. This means that our modelexplains 72.16% of the variation observed in 7Be results obtained inthis study. In order to validate the model, the standardized exper-imental values of 7Be from January 2010 to May 2010 have beencompared statistically with the results obtained by the model(Fig. 9).

The result of this comparison is highly significant (99%) whichconfirms the validity of the model.

4. Conclusions

1) The values of 7Be activity in air are fitted to the normaldistribution.

2) The trend over time shows a cyclical and seasonal behavior,with a maximum in the summer and a minimum in thewinter. The annual average of 7Be concentration was4.56 � 0.22 mBq m3.

3) The mean monthly of 7Be in August-09, 9.334 mBq m3, is thehighest value in Granada since 1993, due to the lowest solaractivity.

4) The combined effects of the elevation of the tropopause, thevertical transport of air mass in troposphere and the lowhumidity explain the high 7Be activity values found in summermonths.

5) In summer and spring months, when DT rises and rainfall isvery low, there is significant vertical air mass mixing with thesubsequent transport of cosmogenic radionuclides to groundlevel from upper troposphere.

6) The higher DT, the greater is the influence of vertical transportof air mass within the troposphere, especially the downwardtransport of 7Be from the middle troposphere to a ground level.

7) The frequent thermal inversions produced in the surface air ofGranada during the winter and autumn months, scarcity ofstrongwinds and lowDT explain the fall in the concentration of7Be during those months.

8) Scavenging of 7Be in the surface atmosphere is affected by wetdeposition, precipitation, and/or by gravimetric deposition,relative humidity and exogenous particles from the Sahara.

9) Relative humidity over 60% produces a sudden decrease in thespecific activity values.

10) The prediction model proposed and described explains the72.16% of the behavior of 7Be in the city of Granada consideringmeteorological variables such as: air temperature, precipitationand relative humidity. Moreover, the model has been validatedwith the results found in the period JanuaryeMay 2010.

In summary, this paper demonstrates that in periods of low solaractivity, the sun doesn’t modulate the cosmic ray flux and theconcentration of 7Be. For this reason, we conclude that the behavior

of this radiotracer is strongly affected by seasonal variation of localmeteorological variables, given the special characteristics of thiscity’s geographical location as indicated. These results suggest thatthe 7Be concentrations in surface air have unique behavior at eachindividual site that must be determined, because the factorscontrolling 7Be variations in the surface atmosphere can be appli-cable to the behavior that other radionuclides of anthropogenicorigin might present in the case of a nuclear accident.

Acknowledgements

We wish to thank the Spanish Nuclear Safety Council (CSN) forthe kind support given to the Radiochemistry and EnvironmentalRadiology Laboratory of the University of Granada since 1993 asa member of the laboratories of the Spanish Sparse Network forEnvironmental Surveillance. This study was also supported by theSpanish Ministry of Education (CGL2010-19849).

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