7Be, 210Pb and 40K depositions over 11 years in Málaga

Journal of Environmental Radioactivity 178-179 (2017) 325e334

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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

7

Be,

210

Pb and

40


laga K depositions over 11 years in Ma

~ as a, *, E. Gordo a, E. Liger b, M. Cabello a, S. Can ~ ete a, M. Pe
rez c, C. Duen a P. de la Torre-Luque
laga, Ma
laga, Spain Department of Applied Physics I, Faculty of Sciences, University of Ma
laga, Ma
laga, Spain Department of Applied Physics II, Technical College Informatic Engineering, University of Ma c
laga, Ma
laga, Spain Department of Radiology and Health Physics, Ophthalmology and OTI, Faculty of Medicine, University of Ma a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2017 Received in revised form 15 September 2017 Accepted 15 September 2017 Available online 28 September 2017

The monthly bulk depositional fluxes of three natural radionuclides (7Be, 210Pb and 40K) were measured
laga) over an 11-year period from 2005 to 2015. The mean annual at a Mediterranean coastal station (Ma depositional fluxes of 7Be, 210Pb and 40K were 1215, 144 and 67 Bq m
2 year
1 respectively, showing a clear seasonal trend with minimum values recorded during summer and maximum values in winter. The rainfall regime with dry summers allows estimating the dry deposition. Assuming constant dry deposition through each year, 7Be, 210Pb and 40K would account for 12.5, 26.5 and 33% of the bulk fallout respectively which indicates that deposition for 210Pb and 40K are significantly higher than 7Be. The precipitation-normalized enrichment factor alpha used to explain seasonal variations in the depositional fluxes of radionuclides with respect the rainfall, indicates higher depositional fluxes during spring and summer than expected from the amount of rainfall. Despite their different origin, 210Pb and 7Be monthly depositional fluxes have strong correlation. The atmospheric deposition fluxes of 7Be, 210Pb and 40K were controlled mainly by the amount of rainfall (r ¼ 0.89, 0.91 and 0.66 respectively). Moreover, principal component analysis was applied to the datasets and deposition of radionuclides and rainfall in the same component highlighting the importance of the washout mechanism. The mean depositional velocity of aerosols evaluated using 7Be and 210Pb are similar and are compared to other published values. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Deposition Radionuclides Deposition velocity Meteorological parameters

1. Introduction This study involves the assessment of three fallout radionuclides: 7Be, 210Pb and 40K. These three radionuclides are useful markers of particles arising from their respective sources. 7Be is a cosmogenic radionuclide originated by spallation reactions of cosmic rays with light atmospheric nuclei, such as nitrogen and oxygen. The production of 7Be is dependent on altitude, latitude and solar cycle but has negligible dependence on longitude. The amount of 7Be that reaches the Earth's surface is controlled by processes including cosmic-ray intensity, precipitation scavenging, vertical and horizontal atmospheric transport. The atmospheric flux of 7Be observed worldwide in previous studies ranged approximately from 1000 to 6000 Bq m
2 year
1 (Yamamoto et al., 2006; Pham et al., 2013). 210 Pb is continuously produced in the atmosphere by radioactive

* Corresponding author. ~ as). E-mail address: [email protected] (C. Duen https://doi.org/10.1016/j.jenvrad.2017.09.010 0265-931X/© 2017 Elsevier Ltd. All rights reserved.

decay from its gaseous progenitor, 222Rn. The global 222Rn flux over continents has been estimated to range from 1300 to 1800 Bq m
2 d
1, while over oceanic areas is it around 17 mBq m
2 d
1. Therefore, the land-sea distribution is the main factor rather than longitude condition, beside atmospheric 210Pb concentration decreases with increasing altitude (Preiss et al., 1996; Garcia-Orellana et al., 2006). Atmospheric 40K has a crustal origin as it is found in most kinds of soil, and can be easily lifted and transported by wind erosion. This nuclide has been previously associated with the arrival of coarse re-suspended material (PM10, particulate matter with an aerodynamic diameter below 10 mm) from the African continent ~ as et al., 2011). (Karlsson et al., 2008; Duen In order to obtain information of atmospheric deposition of 7Be, 210 Pb and 40K more rigorously, long-term data set is required. Though the database of 7Be and 210Pb atmospheric deposition is continuously growing around the world, mainly due to routine monitoring programs of radioactive fallout, the factors influencing temporal variability of atmospheric fluxes at a given site are not totally well known. In addition, the lack of availability of long-term

326

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

data from temperate zones with a dry climate in the European continent, especially on the Mediterranean coast, makes it necessary to carry out more measurements and analyses. As a result, we
laga since have monitored the fluxes of fallout radionuclides in Ma ~ as et al. 2011). 2005, and reported some results (Duen Following the deposition of 7Be, 210Pb and 40K on the Earth's surface it is possible to assess erosion, transportation and deposition of soils and sediments from episodic to long-term timescales with more details. It is well known that African dust affects large areas around the world, such as the North Atlantic, America and Europe. The Mediterranean area is also frequently affected by these events that increase the atmospheric particulate matter concentration. The African mineral dust plumes may also significantly influence the atmospheric fluxes of 7Be, 210Pb and 40K by scavenging the mentioned radionuclides from the atmosphere during the transport process. In fact, Appleby et al. (2001) stated that these fallout events are relatively frequent in the Mediterranean region. In this work we have evaluated 11-years of atmospheric fluxes of 7
laga discussing their temporal and seasonal Be, 210Pb and 40K in Ma variations with the aim to better understand the influence of local meteorological conditions. Moreover, from simple methods of correlation analyses, elements of multifactorial analysis were used for more precise investigation of their influence. Principal component analysis was applied to the data sets in order to determine possible association among the variables tested. Furthermore, the months without rainfall at the study site (mainly during summertime), allows evaluating the dry deposition. 2. Material and methods 2.1. Study area The study was carried out in the city of M
alaga (4 280 800 W; 36 430 4000 N), in the south-east of the Iberian Peninsula, on the shores of the Mediterranean Sea and one of the provinces of the Andalusian Region, the southern-most region of Spain (Fig. S1). The orography and climate of M
alaga play an important role in the interpretation of the concentrations of aerosols. Prevailing winds are controlled by local topography with a SE and NW component, corresponding to the sea-land and land-sea breezes, respectively. The study site is affected by a coastal Mediterranean climate with dry summers and mild winters consequence of the influence of the sea, the high insolation due to the south facing coast, and protected from northern air masses by Betica mountains. Besides, M
alaga is frequently affected by African dust plumes mainly between May and September due to its geographical proximity to the African continent. The meteorological scenarios which cause this African dust transport through Spain were identified by Escudero et al. (2005). 2.2. Methodology From January 2005 to December 2015, 132 samples of bulk deposition were collected on a monthly basis. As the sampler was continuously exposed to the atmosphere, the sum of the wet and dry fallout (bulk deposition) was collected. The sampling point is situated 14 m above the ground, on the flat roof of the SCAI (Centralized centre for investigation) building (University of
laga) in an open area. The method and processing procedures Ma ~ as et al., 2011). Briefly, a volume of were described previously (Duen 6 L of the bulk deposition was reduced via evaporation to approximately 1 L and transferred to a Marinelli geometry container for gamma counting. Additionally, aerosol samples were collected weekly in cellulose membrane filters of 0.8 mm pore size and 47 mm diameter with an

air sampler lodged in an all-weather sampling station, situated on the roof near the bulk rain collector. A monthly composite sample containing 4e5 filters was formed (average air volume 1600 m3) for the calculation of 7Be and 210Pb. Detailed descriptions of the aerosol sampling method have been given in previous publications ~ as et al., 1999, 2009). (Duen Gamma counting of the aerosols and bulk deposition samples were performed using an intrinsic germanium coaxial detector, ReGe-type (CANBERRA), to determine radionuclide activities. The counting time was in the order of 90,000-172,0000 s. The concentrations were corrected for decay to the mid-collection period using (T1/2 ¼ 53 day) and 477.6 keV gamma-ray of 7Be, (T1/2 ¼ 22.3 year) and 46.5 keV line of 210Pb, and (T1/2 ¼ 1.3$109 year) and 1460.81 keV line for 40K. Further details of the efficiency and the low-background gamma-ray detection system have been previ~ as et al. (1999, 2004). ously described by Duen The atmospheric fluxes were calculated using the expression: F ¼ A/St (Bq m
2 month
1)

(1)

where A is the activity in the sample obtained from the gamma spectra, S is the surface area of the collector and t is the duration of sampling time. The meteorological data such as atmospheric pressure, air temperature, relative humidity, monthly wind distance, amount and duration of rainfall were obtained from the nearest weather station belonging to the network of the Spanish Meteorological Agency located 500 m away from the sampling site. Days affected by African dust outbreaks have been obtained from the data reported by CALIMA project (www.calima.es). Measurements of PM10 levels have been recorded during the same 12-year period in an air quality monitoring station belonging to the network of surveillance and control of the atmospheric pollution of Andalusia which is located near the study site. Details on the instrumentation and sampling procedures can be found in De la Rosa et al. (2010). 3. Results and discussion 3.1. Variations of monthly bulk depositional fluxes of 7Be, 210Pb and K

40

Samples from 132 months were continuously collected from January 2005 to December 2015. The amount of rainfall as well as the total (wet and dry) bulk deposition of 7Be, 210Pb and 40K expressed in Bq m
2 deposited monthly are reported in Table S1.
laga is about 530 mm, The historic annual average rainfall in Ma however, for the study period the annual mean rainfall was 417 mm. The year 2010 was the wettest with 911 mm, while 2008 was the driest with 214 mm. About 80e90% of the rainfall is confined within SeptembereApril, with very little rainfall in the remaining months. The dry period mainly covers the months of June, July and August. Fig. 1 exhibits the monthly amount of precipitation during the study period. There are several outliers in winter and autumn highlighting the irregularity of rainfall in the study site. The monthly depositional fluxes of 7Be and 210Pb range from 3 to 1284 Bq m
2 month
1 (annual average ¼ 1215 Bq m
2 year
1) and from 1 to 102 Bq m
2 month
1 (annual average ¼ 144 Bq m
2 year
1) respectively. The lowest depositions of both radionuclides principally come about during the summer months (June, July and August) when there is usually no precipitation, (Table S1). The highest flux of 7Be occurred in December 2010 coinciding with the highest amount of precipitation (247.4 mm), while for 210Pb the highest flux was measured in November 2012 with the amount of

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

327

250

Precipitation (mm)

200

150

100

50 Jan

0 Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec


laga during 2005e2015. The horizontal line inside the box indicates the median, the box covers the 25e75% percentiles and the cross Fig. 1. Monthly precipitation data in Ma represents the mean value. The length of the whiskers is 1.5 times the interquartile range. The outliers are anomalous values.

precipitation (232.5 mm). The lowest fluxes of 7Be and 210Pb were registered in August 2014. Similar yearly values of 7Be and 210Pb deposition fluxes were recorded over western (Gonzalez Gomez et al., 2006; Garcia-Orellana et al., 2006; Pham et al. 2013) and eastern (Ioannidou and Papastefanou, 2006) Mediterranean regions. The influence of continental air masses generally produce higher fluxes for the inland sites (Winkler and Rosner, 2000; Mietelski et al., 2016) especially for 210Pb. The monthly depositional flux of 40K varied between 0.5 and 81 Bq m
2 month
1 (annual average ¼ 67 Bq m
2 year
1). The lowest flux of 40K occurred in May 2013 while the highest one occurred in October 2008 due to an intense African dust outbreak (Cabello et al., 2012). Moreover 20% of the study period (26 months) corresponds to months without precipitation and hence, the depositional flux during these months is totally due to dry deposition. These dry periods mostly appear during the month of July (11 months), the month of June (8 months) and finally August (7 months). The dry depositional fluxes for these dry months vary from 2.9 to 45.2 (mean 12.6 Bq m
2 month
1), from 0.9 to 6.6 (mean 3.2 Bq m
2 month
1) and from 0.5 to 3.4 (mean 1.85 Bq m
2 month
1) for 7Be, 210 Pb and 40K respectively. Assuming that the dry deposition has remained constant throughout the study period, the dry deposition for 7Be, 210Pb and 40K would account for 12.5, 26.5 and 33% respectively of the bulk fallout. In our study, the dry depositions for 210 Pb and 40K are significantly higher than 7Be, and this may be attributed to the differences in their sources, their half-lives (210Pb has a mean life of 32 years, 7Be is 77 days and 40K is 1.88$109 years) and therefore the contribution from resuspension of dust particles is proportionally higher for 210Pb and 40K than it is for 7Be (Huh et al., 2006). Both percentages of dry fallout are similar to the ones obtained by (Lozano et al., 2011). Fig. S2 reflects a strong correlation between both monthly depositional fluxes of 210Pb and 7Be (r ¼ 0.91), although 210Pb and 7 Be in the atmosphere have distinct sources and different modes of production. This high correlation between these two radionuclides suggests that both radionuclides have similar deposition patterns, and so, they cannot be used as independent atmospheric tracers. This is also a fairly common observation obtained in different stations around the world, both continental and oceanic stations (Baskaran et al., 1993; Kim et al., 2000; Caillet et al., 2001; McNeary and Baskaran, 2003) and may demonstrate that deposition of both nuclides is governed by the same processes.

3.2. Relationships between seasonal variations of the depositional fluxes and amount of precipitation To explain the seasonal variations in the depositional fluxes of Be, 210Pb and 40K, we have calculated the precipitation-normalized enrichment factor alpha defined as follows (Baskaran, 1995): 7

a ¼ (Fs/Fy)/(Rs/Ry)

(2)

where R represents the amount of rainfall, F the depositional fluxes and the subscripts s and y indicate the recording period in one particular season and year respectively. When this parameter takes values greater than 1, it indicates that the depositional fluxes were higher than expected according to the amount of rainfall, whereas values lower than 1 mean a decrease of depositional fluxes. The fractional amount of precipitation and depositional fluxes of 7 Be, 210Pb and 40K for 44 seasons from January 2005 until December 2015 are given in Table S2. We have observed high alpha values mainly during spring and summer periods due to the rain regime in Malaga whereas during autumn and winter the alpha values are generally less than 1. The alpha values for 7Be, 210Pb and 40K varied within 0.40e3, 0.51e3.72 and 0.17e8.35 respectively. The highest alpha value for both 7Be and 210Pb was recorded in the spring of 2005, and for 40K during the summer of 2012. The minimum alpha values were observed in autumn 2013 for 7Be and for 210Pb in winter 2006. The values of alpha for 40K were not usually detected in winter and autumn, and as a result its values were null. Similar alpha values for 7Be and 210Pb, indicate that both radionuclides have predominantly the same mechanism of deposition (wet fallout). The wet deposition in M
alaga usually appears in winter, autumn and spring. From Table S2 it can be observed that alpha values are similar to those mentioned seasons for both radionuclides, whereas the main discrepancy remains in that the alpha value for summer is higher for 210Pb than for 7Be. This fact is consistent with higher dry deposition for 210Pb (26.5%) compared to 7Be (12.5%). 3.3. Statistical analyses Correlation analysis is a useful technique to identify potential association between variables. Table S3 reports correlation coefficient values between gamma radionuclides, meteorological variables such as air temperature T (ºC), relative humidity RH (%),

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

328

Flux of 7Be (Bq m-2 month-1)

pressure P (hPa), monthly distance travelled by wind Wd (Km), amount of rainfall R (mm), duration of rainfall DR (min), and other variables such as the number of monthly days with African dust outbreaks (Nº int.) and PM10 (mg m
3). Table S3 exhibits that the depositional flux of 7Be has a statistically significant relationship with depositional flux of 210Pb, amount and duration of rainfall, depositional flux of 40K, relative humidity and air temperature. The depositional flux 210Pb is also significantly correlated with amount and duration of rainfall, flux of 40K, relative humidity and air temperature. Finally, the depositional flux of 40K is

significantly correlated with amount and duration of rainfall relative humidity of air and PM10. The depositional fluxes of the mentioned radionuclides show a poor correlation with other meteorological parameters such as pressure, monthly distance travelled by wind and the number of monthly days with African dust outbreaks. The fluxes of 7Be, 210Pb and 40K are mainly controlled by the amount of rainfall. Fig. 2 shows the fluxes of 7Be, 210Pb and 40K versus the amount of rainfall as well as the equations which correlate these variables. According to these equations, when there is no rainfall, the dry fallout would be 6.3, 1.4 and 0.34 Bq m
2

1500

FluxBe= 6.33+ 2.6 rainfall

1200

r=0.88

900

600

300

0 0

50

100

150

200

250

Amount of rainfall

Flux of Pb-210

Flux of 210Pb (Bq m-2 month-1)

Amount of rainfall (mm) 120

FluxPb =1.43 + 0.31 rainfall

100

80

r=0.90

60

40

20

0 0

50

100

150

200

250

200

250

Amount of rainfall

Flux of 40K (Bq m-2 month-1)

Amount of rainfall (mm) 100

80

FluxK =0.34+ 0.20 rainfall

60

r=0.66

40

20

0 0

50

100

150

Amount of rainfall (mm) Fig. 2. Scatter plot of fluxes of Be-7, Pb-210 and K-40 versus amount of rainfall with the fitted regression line (blue line). The prediction intervals (grey lines) and the confidence intervals for the regression lines with a 95% of confidence (green lines) are show. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen Table 1 Results from factor analysis applied on fluxes of 7Be, parameters, PM10 and African dust outbreaks.

210

Pb and

40

PC1 (55%)

PC2 (21%)

Deposition 7Be Deposition 210Pb Deposition 40K Amount of Rainfall PM10 Air temperature Air relative humidity Number of Intrusions Duration of rainfall

0.885 0.941 0.749 0.930 0.197
0.407 0.724
0.166 0.894


0.078
0.048 0.250
0.146 0.785 0.779
0.291 0.798
0.309


1

7

210

and presents significant loading for the number of African dust outbreaks, PM10 and air temperature. This high correlation between these three parameters is because most of the African dust outbreaks occur during the summer (Fig. 3). This figure shows clear differences among the months with maximum values in summer in agreement with (Prospero et al. 2002) who concluded that the maximum dust transport occurs in summer, coinciding with the maximum temperature.

K, meteorological

Variable

329

3.4. Variations in the specific activities of 7Be, Be/210Pb in bulk deposition samples

210

Pb,

40

K and ratio

7

The results from measurements of specific activities of7Be, 210Pb, K and ratio 7Be/210Pb from January 2005 to December 2015 were analysed. Table S4 shows some statistical variables such as arithmetic mean (AM), geometric mean (GM), standard deviation (SD), dispersion factor of geometric mean (DF), maximum and minimum values as well as the coefficient of variation (CV) for 7Be, 210Pb and 40 K in Bq L
1 as well as the ratio7Be/210Pb in bulk deposition. The radionuclides 7Be and 210Pb were detected in all samples (N ¼ 132), while 40K only appeared in approximately 50% of them (N ¼ 67). The activity concentration of 7Be ranged from 0.29 to 8.3 Bq L
1 (mean ¼ 2.6 Bq L
1). The corresponding values for 210Pb varied between 0.05 and 1.3 Bq L
1 (mean ¼ 0.40 Bq L
1) and from 0.02 to 0.84 Bq L
1 (mean ¼ 0.20 Bq L
1) for 40K. The ratio7Be/210Pb ranges from 1.28 to 30.0 (mean ¼ 7.8). To gain a better understanding of the differences in the deposition behaviour of the gamma radionuclides, the temporal variations in the activity concentrations of radionuclides 7Be, 210Pb, 40K and ratio 7Be/210Pb were depicted with the mean value indicated by horizontal solid lines in Fig. 4. The variations in the activity concentrations of the mentioned ~ as et al. radionuclides depend on multiple factors as stated by (Duen 2003, 2011, 2016; McNeary and Baskaran, 2003). Activity concentrations of 7Be exhibit their maximum values in the spring and summer seasons probably due to the depletion of the tropopause and higher convection favouring the stratosphere-troposphere exchange (Baskaran et al.,1993; Steinmann et al., 2013) as well as the low amounts of rainfall (dry period). Thus, the highest activity concentration of 7Be occurred in May (2009) while the lowest value was recorded in December (2013). Likewise, the activity concentrations of 210Pb and 40K exhibited minimum values in cold months and maximum values in warm months. The highest activity concentrations of 210Pb occurred in July (2006) and the lowest value in December (2013). High ratios of 7Be/210Pb are mainly observed during the wet period (winter, spring and autumn) and may be 40

40

month for Be, Pb and K respectively. Comparing these data with the fluxes measured in dry fallout, which were 12.6 for 7Be, 3.2 for 210Pb and 1.85 for 40K Bq m
2 month
1 (Section 3.1) we could conclude that the dry fallout taking place in rainy months is much less than that during dry months probably due to the fact that precipitation removes the aerosols more efficiently. Thus, the percentages of dry fallout obtained in section 3.1 (12.4% for 7Be 26.7% for 210Pb and 33% for 40K) would be overestimated. Principal component analysis is a multivariate statistical technique which aims to reduce the dimensionality of a data set to highlight the similarities and differences of the analysed variables. Principles of this statistical technique and different applications can ~ ero be found in literature (Rosner, 2000; Viana et al., 2006; Pin Garcia et al., 2012; Gordo et al., 2015). These statistical analyses were carried out using SPSS V-12 software. Principal component analysis was applied to the monthly depositional fluxes of 7Be, 210Pb and 40K, PM10, meteorological parameters (rainfall, air temperature and relative humidity) and number of monthly African dust outbreaks. Results obtained for the Varimax rotated factor loadings for each variable are presented in Table 1 and loadings >0.6 are marked in bold. The first principal component (PC1) explained 55% of the total variance of the data set and included fluxes of the three radionuclides 7 Be, 210Pb and 40K, rainfall (amount and duration) and relative humidity of air. The high loading values for fluxes of three radionuclides for PC1 suggests that despite their different origins, the deposition of radionuclides seems to be governed by the same scavenging mechanisms. Moreover, the PC1 includes rainfall (both amount and duration), as well as relative humidity which would indicate rainfall is the main parameter to control the radionuclide fluxes. The second component (PC2) explained 21% of the total variance

30

Number of African outbreaks

25

20

15

10

5

0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Fig. 3. Number of monthly days affected by African dust outbreaks observed in M
alaga during 2005e2015. Meaning of marks are analogous to the ones of Fig. 1.

330

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

Fig. 4. Temporal evolution of specific activities of Be-7, K-40, Pb-210 and ratio Be-7/Pb-210 in bulk deposition samples.

elucidated by the arrival of humid maritime air masses which lead to intense precipitation. In contrast, low ratios observed during the dry period (summertime) could be attributed to both the arrival of enriched 210Pb continental air masses (Huh et al., 2006) including African dust outbreaks (Fig. 3) and the scarce rainfall (Fig. 1). To reduce the dimension of the data set, principal component analysis was also applied to the monthly specific activities of 7Be, 210 Pb and 40K, PM10, meteorological parameters (rainfall, air

temperature and relative humidity) as well as the number of monthly African dust outbreaks. Table 2 exhibits the varimax rotated factor matrix. The first principal component (PC1) explained 34% of the total variance of the data set. PC1 illustrates a positive correlation with 40K activity (0.728), PM10 (0.785), number of African dust outbreaks (0.657) and air temperature (0.625). The number of African dust outbreaks is highly correlated with PM10 (Table S3) and furthermore most of the intrusions come about in

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen Table 2 Results from factor analysis applied on 7Be, 210Pb and 40K in bulk deposition samples and some variables in Malaga ifrom the period from January 2005 until December 2015. Variable

PC1 (34%)

PC2 (22%)

PC3 (15%)

7

0.138 0.080 0.728 0.785 0.625
0.014 0.657 0.106

0.001
0.182 0.084 0.213
0.498 0.880
0.208 0.820

0,887 0,866 0,172
0,148 0,145 0,041 0,393
0,235

Be (Bq/L) 210 Pb (Bq/L) 40 K (Bq/L) PM10 Temperature Relative humidity Number of intrusions Rainfall amount

331

Table 3 Results from factor analysis applied on 7Be, 210Pb in aerosol samples and some variables in Malaga in the period from January 2005 until December 2015.

7

Be (mBq/m3) 210 Pb (mBq/m3) Temperature Relative humidity Rainfall amount PM10 Number of intrusions

PC1 (26%)

PC2 (26%)

PC3 (25%)

0.872 0.923 0.285 0.001
0.143
0.153 0.341

0.108 0.064 0.704
0.173
0.050 0.869 0.706


0.258 0.064
0.422 0.868 0.832 0.094
0.272

Factor loadings greater than 0.5 have been highlighted in bold.

Factor loadings greater than 0.5 have been highlighted in bold.

summer, explaining the correlation with air temperature. The second component (PC2) explains 22% of total variance and is determined by high loadings with rainfall amount and relative humidity. The third factor (PC3) explains 15% of the total variance

and presents high loading values for 7Be and 210Pb activities which reinforces that despite their different origins, both radionuclides seem to be governed by the same scavenging mechanism in this sampling station. Fig. S3 shows the monthly specific activity of 40K in deposition samples and PM10 during the measurement period, highlighting

Fig. 5. Temporal evolution of concentrations (Bq/m3) of Be-7, Pb-210 and ratio Be-7/Pb-210 in aerosol samples.

332

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

the similar behaviour of 40K and PM10. Straight lines show the annual average specific activity of 40K and PM10. Regarding PM10, three different average values were evaluated because during 2008, 2009 and 2010, works on excavating a tunnel for M
alaga Metro began next to the sampling point increasing the aerosol back
ndez et al. (2005); Karlsson et al. (2008) and Duen ~ as ground. Herna et al. (2011) suggested using the 40K as a tracer of African dust outbreaks. 3.5. Temporal variations of the concentrations of 7Be and 210Pb and 7 Be/210Pb activity ratio in aerosol samples
laga there is a monitoring atmospheric radioactivity proIn Ma gram financed by the Spanish Nuclear Security Council since 1992. The results from the measurements of 7Be, 210Pb and 40K concentrations, and ratio 7Be/210Pb in aerosol samples from January 2005 to December 2015 were analysed in this section to derive the mainly statistic parameters. Table S5 presents basic statistics results obtained from the mentioned radionuclides and ratio such as arithmetic mean (AM), geometric mean (GM), standard deviation (SD), dispersion factor of geometric mean (DF), maximum and minimum values as well as the coefficient of variation (CV). These values are given in mBq m
3 for 7Be, 210Pb and 40K. Fig. 5 shows the surface air activity concentration of 7Be, 210Pb as well as 7Be/210Pb ratio during the 11-year period of measurement. 7 Be activity ranged from 0.65 to 14.9 mBq m
3 with a mean value of 4.21 mBq m
3. This value is comparable with concentrations of 7Be on ground level air measured in other sites such as Thessalonica (4.8 mBq m
3; Papastefanou and Ioannidou, 1991); Detroit (4.8 mBq m
3; McNeary and Baskaran, 2003); Huelva (5.10 mBq ~ ero-Garcia m
3; Lozano et al., 2011); Granada (4.56 mBq m
3; Pin et al., 2012); Lisbon (4.0 mBq m
3; Carvalho et al. 2013). Krajny et al. (2014) stated that the 7Be activity concentration in low troposphere depends on multiple factors, which may be classified as static (geographical location) and dynamic (e.g., solar activity, meteorological conditions, horizontal and vertical air masses transport). The concentration of 7Be in aerosols varied following a cyclical and seasonal pattern. The highest value was exhibited in August 2005 while the lowest occurred in February 2010. 210 Pb in surface air is in the range of 0.05e1.51 mBq m
3 with an average of 0.48 mBq m
3. This average concentration is also comparable to the values reported for other sites such as Tsukuba (0.45 mBq m
3) (Sato et al., 1994); Palermo (0.74 mBq m
3) (Cannizaro et al., 2004); Munich-Neuherberg (0.57 mBq m
3) (Winkler and Rosner, 2000); Granada, (0.62 mBq m
3) (Camacho García, 2000); Detroit (1.15 mBq m
3) (McNeary and Baskaran, 2003); Lisbon (0.40 mBq m
3) (Carvalho et al. 2013). The ratio 7Be/210Pb is commonly used as an indicator for the efficiency of mixing in the atmosphere (Krajny et al., 2014) and was in the range of 2.71e45.82. This range is similar to the values recorded in other sites such as Mount Waligan station in China (3e18) and Mount Cimone in Italy (3e20) (Lee et al., 2007; Belgrade (2e13) Todorovic et al., 2005; Bermuda (9e50) and Barbados (4e50) Arimoto et al., 1999; Poland (3e31) Krajny et al., 2014). The box and whiskers diagrams for 7Be, 210Pb and ratio 7Be/210Pb concentrations in air exhibit a clear seasonal pattern (Fig. S4). In relation to 7Be, this seasonal trend is typical of the middle latitudes and is due to the cosmogenic origin and the characteristics of the ~ as et al., 2009). sampling station as latitude and local climate (Duen The emanation rate of 222Rn from soil, and hence the availability of atmospheric 210Pb levels depend on several factors such as the moisture content of the soil, grain size, and in general diurnal or seasonal variations in meteorological conditions like atmospheric pressure, temperature inversions or precipitations accumulation (Lozano et al., 2011; Tositti et al., 2014; Gordo et al., 2015). In order

to accentuate any seasonal trend, the Kruskal-Wallis test was performed to find significant differences for the medians of 7Be among groups (p < 0.05). Multiple range test indicates that values recorded during the warm seasons (summer and spring) were significantly different from those recorded during cold months (winter and autumn). The high concentrations found in the spring/summer months are mainly caused, among others factors by air masses transport from the stratosphere to troposphere, which is induced by heating of the earth surface, causing an upward movement of the tropopause. The rising temperature enhances the exchange of air masses with consequent transport of 7Be to the ground level air. On the contrary in cold months, these exchange process is reduced, and as a result, the transport of 7Be produced in the stratosphere declines. (Blazej and Mietelski, 2014). Likewise, the activity of 210Pb shows higher values in summer and lower values during the other seasons with significant differences between medians according to Kruskal-Wallis test (p < 0.05). According to multiple range tests, winter values were significantly different from summer and autumn, which at the same time shows significant differences with spring. Finally the ratio 7Be/210Pb shows particularly high values during spring and winter (Fig. S4). According to the Kruskal-Wallis test there are significant differences between medians (p < 0.05). High 7 Be/210Pb activity ratio values during spring months can be attributed to the increased rate of vertical transport of 7Be (from the upper troposphere to the middle and lower troposphere) and 210Pb (from the lower troposphere to upper troposphere) during these months. The multiple range test only indicates significant differences between spring and autumn. However these activity ratios exhibit large variations over the 11 years period at the study site, partly caused by factors that vary from year to year depending on local weather conditions. Principal component analysis was performed on the monthly radionuclide concentrations (7Be and 210Pb), PM10, meteorological parameters (rainfall, air temperature and relative humidity) as well as the number of monthly African dust outbreaks from 2005 to 2015, and reduces the data set into three principal components. Table 3 exhibits the factor loading of the three principal components at a 95% significance level accounting for 77% of the total variance. The first factor explains 26% of the total variance and shows high correlation between the radionuclides 7Be and 210Pb. The second factor accounts for 26% of the total variance and shows correlations between the number of days affected by African dust outbreaks, air temperature and PM10. The third factor explains 25% of the total variance and exhibits significant loading for rainfall and relative humidity of air. 3.6. Total deposition velocity of aerosols using 7Be and

210

Pb

Dry and wet depositions remove the atmospheric radionuclides and deposit the radionuclides on Earth's surface. The parameter “deposition velocity” usually characterizes the deposition rates. The total deposition velocity (Vd) for any radionuclide is determined using the following equation (McNeary and Baskaran, 2003): Vd ¼ F/Cs

(3)

where F is the total flux on a nuclide to the Earth's surface and Cs is the activity concentration of that nuclide in the superficial air. These velocities have only been evaluated for the radionuclides 7 Be and 210Pb, because the radionuclide 40K has not been detected on the aerosol filters. Using 7Be and 210Pb to determine the total deposition velocities present some advantages such as: (1) the facility to measure the concentration of 7Be and 210Pb; (2) the relatively constant production rate of these radionuclides at a given

~ as et al. / Journal of Environmental Radioactivity 178-179 (2017) 325e334 C. Duen

333

site; and (3) the similar size distributions of the atmospheric aerosol where 7Be and 210Pb are attached, and other particulate contaminants of interest, permits determining the fluxes of these contaminants on the Earth's surface, when the deposition velocities of these nuclides and the concentration of these contaminants in ~ as et al., 2004). air are known (McNeary and Baskaran, 2003; Duen The total deposition velocity for 7Be ranges from 0.026 (July 2005) to 1.6 (December 2010) cm s
1 with a mean value of 0.91 cm s
1. Values of the total deposition velocity for 210Pb vary between 0.06 (August 2008) and 2.5 (February 2010) cm s
1, with a mean value of 0.96 cm s
1. Table S6 shows the annual deposition velocity for the study period. These data suggest that 7Be and 210Pb attach onto the aerosols by similar mechanisms (Winkler and Rosner, 2000; Papastefanou and Ioannidou, 1991), and this fact could explain why deposition velocities are very similar. Our estimations of Vd for 7Be and 210Pb based on an 11 year study period do not totally agree with those reported in other studies (Table S7) where the deposition velocity of 7Be range from 0.4 to 2. 8 cm s
1, while the one obtained using 210Pb is in the range 0.5e1.9 cm s
1. The deposition velocity measurements of both radionuclides obtained during the study period are within the range of those reported by other authors in different countries (Table S7). These differences among researchers can be explained due to a large number of factors such as sampling methods, collection height or meteorological information and others. The potential effects of resuspension may also be considered.

month
1 respectively. The depositional flux of 7Be, 210Pb and 40K show seasonal trends with minimum values in summer and high values in winter and are mainly controlled by the amount precipitation indicating that their removal behaviour from the atmosphere is relatively similar. Principal component analysis applied to the datasets reveal that depositional fluxes behaviour is represented by two PCs which explain 76% of total variance. PC1 was related positively to fluxes of 7 Be, 210P, 40K, relative humidity and rainfall (amount and duration). PC2 was positively related to air temperature, PM10 and the number of African dust outbreaks. PC1 indicates that rainfall seems to be an important factor controlling the fluxes of radionuclides while PC2 informs us of the influence of African dust outbreaks at this station. The fraction of annual dry depositional flux for 7Be, 210Pb and 40 K to total flux is estimated to be 12.4 and 26.7 and 33% of total flux respectively. Higher values for 40K and 210Pb relative to 7Be are observed and could be attributed to resuspended soil dust that is enriched in 40K and 210Pb but not 7Be. The deposition velocities of aerosols based on 7Be and 210Pb varied between 0.026 and 1.6 cm s
1 (mean ¼ 0.91 cm s
1) and 0.06 to 2.5 (mean ¼ 0.96 cm s
1) respectively. The corresponding washout ratios varied between 75-5300 (mean ¼ 840) and 93e3800 (mean ¼ 1140) for 7Be and 210Pb respectively.

3.7. Washout ratios of 7Be and

Supplementary data related to this article can be found at https://doi.org/10.1016/j.jenvrad.2017.09.010.

210

Pb

The radioactive materials can be removed from the atmosphere by rainout and washout. Generally, both washout and rainout are incorporated into a dimensionless parameter, denomined “washout ratio”, WR. The WR, for 7Be and for 210Pb can be calculated from the following relationship (Papastefanou and Ioannidou, 1991; McNeary and Baskaran, 2003):

WR ¼ r

Crain Cair

(4)

where r is the density of air at standard conditions (1.2 kg m
3 at 20  C and 760 mm Hg) and Crain and Cair are the radionuclide (7Be or 210 Pb) concentrations in bulk deposition (in Bq kg
1) and surface air (in Bq m
3), respectively. Table S6 shows the annual washout ratios
laga and annual rainfall amount. for the sampling station in Ma Overall, the washout ratios at this coastal site varied between 75 (December 2013) and 5300 (February 2010) with a mean of 840 for 7 Be and between 93 (January 2006) and 3800 (January 2009) with a mean of 1140 for 210Pb. The highest values are obtained over the years with the highest precipitation, 2010, 2012 and 2006. These values can be compared to those recorded in other places: 370 and 215 for 7Be and 210Pb, respectively, for the Southeastern Virginia coast (Todd et al., 1989); 144 for 7Be at Tessaloniki (40 380 N; 22 580 E) (Papastefanou and Ioannidou, 1991); 30 for 210Pb, for the Western North Atlantic (Hussain et al., 1998); 948 and 637 for 7Be and 210Pb, respectively for the Southeastern Michigan, Detroit (McNeary and Baskaran, 2003). 4. Conclusions This work reports the results of a study focused on the temporal variations and statistical analyses for the depositional fluxes of three natural radionuclides 7Be, 210Pb, 40K collected over the years
laga (southern 2005e2015 at a Mediterranean coastal station in Ma Spain). The monthly depositional flux of 7Be, 210Pb and 40K varied between 3 and 1284, between 1 and 102, and 0.5 and 81 Bq m
2

Appendix A. Supplementary data

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