Journal of Environmental Radioactivity 138 (2014) 149e155
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Atmospheric deposition patterns of
210
Pb and 7Be in Cienfuegos, Cuba
ndez*, Yasser Morera-Go mez, He ctor Cartas-Aguila, Carlos M. Alonso-Herna n-Arruebarrena Aniel Guille Centro de Estudios Ambientales de Cienfuegos, AP 5, Ciudad Nuclear, Cienfuegos, Cuba
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2012 Received in revised form 4 July 2014 Accepted 29 August 2014 Available online
The radiometric composition of bulk deposition samples, collected monthly for one year, February 2010 until January 2011, at a site located in Cienfuegos (22 030 N, 80 290 W) (Cuba), are analysed in this paper. Measurement of 7Be and 210Pb activity concentrations were carried out in 12 bulk deposition samples. The atmospheric deposition fluxes of 7Be and 210Pb are in the range of 13.2e132 and 1.24e8.29 Bq m2, and their mean values are: 56.6 and 3.97 Bq m2, respectively. The time variations of the different radionuclide have been discussed in relation with meteorological factors and the mean values have been compared to those published in recent literature from other sites located at different latitudes. The annual average flux of 210Pb and 7Be were 47 and 700 Bq m2 y1, respectively. Observed seasonal variations of deposition data are explained in terms of different environmental features. The atmospheric deposition fluxes of 7Be and 210Pb were moderately well correlated with precipitation and well correlated with one another. The 210Pb/7Be ratios in the monthly depositions samples varied in the range of 0.05 e0.10 and showed a strong correlation with the number of rainy days. © 2014 Elsevier Ltd. All rights reserved.
Keywords: 7 Be 210 Pb Atmospheric deposition Cienfuegos Cuba
1. Introduction Naturally occurring beryllium-7 and the daughter-products of Rn, such as 210Pb and 210Po, have proven useful as tracers and have been utilized to gain insight into rates of sediment mixing, sedimentation, dynamics of particle transport, fate of particlereactive contaminants as well as meteorological information, especially on the origin of air masses and residence times of ndez et al., 2004, 2006; Baskaran and Shaw, aerosols (Alonso-Herna 2001; Blake et al., 2002; Ciffroy et al., 2003; Díaz-Asencio et al., 2009; Fitzgerald et al., 2001; Kadko and Olson, 1996; Sommerfield et al., 1999; Steinmann et al., 1999). The source terms of these radionuclides are relatively well known and are removed from the atmosphere by precipitation, dry fallout and radioactive decay. Knowledge of the behaviour of 7Be and 210Pb in the atmosphere will yield insight on the behaviour of other similar chemical species in the atmosphere. For example, Alonso-Hernandez et al. (2004) used the ratio of 7Be in surface air of Cienfuegos, in surface air of Miami FL (United States), as well as 137 Cs concentrations measured in surface air in Miami, to 222
* Corresponding author. Tel./fax: þ53 43965146. E-mail addresses: [email protected], [email protected] ndez). (C.M. Alonso-Herna http://dx.doi.org/10.1016/j.jenvrad.2014.08.023 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
reconstruct the monthly 137Cs signal in surface air of Cienfuegos due to global fallout between 1957 and 1994. Beryllium-7 (T1/2 ¼ 53.3 days) is a cosmogenic radionuclide produced in the stratosphere when cosmic rays bombard the nucleus of oxygen, and nitrogen atoms in the atmosphere (Lal et al., 1958), and its flux to the Earth's surface has a latitudinal dependency (Lal and Peters, 1967) similar to other cosmogenic nuclides. The concentration of 7Be in the atmosphere increases with altitude and its production rate for a given latitude (as well as the flux) on the Earth's surface is independent of longitude. Due to its short half-life, most of the 7Be produced in the stratosphere do not reach the troposphere except during spring when seasonal thinning of the tropopause takes place in mid-latitudes, resulting in air exchange between the stratosphere and troposphere. In most atmospheric studies, the 7Be concentration in aerosols does not remain constant with time and show seasonal variations in surface air. A high concentration of 7Be in surface air aerosols would indicate significant input from the upper troposphere-lower stratosphere regions (Dibb and Rice, 1989; Feely et al., 1989). At midlatitude, seasonal variations of air concentration of 7Be in spring have been observed, due to air masses with higher concentrations of 7Be in the stratosphere being injected into the troposphere at that period. In addition, rates of 7Be production are dependent upon solar activity and can therefore be influenced by latitude with increased production occurring at the poles owing to cosmic ray deflection towards polar regions. Production is also affected by
150
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
altitude with higher levels of production occurring in the stratosphere than in the troposphere (Kikuchi et al., 2009). Radon-222 (T1/2 ¼ 3.8 days), one of the daughter-products in the 238 U decay chain, predominantly emanates from the Earth's continental crust. The 222Rn flux from soils ranges from 42 to 267 Bq cm2 yr1, with a global average value of 54 Bq cm2 yr1 and is about 100 times higher than the oceanic flux (Turekian et al., 1977). Radon-222 undergoes radioactive decay in the atmosphere producing several daughter-products, including 210Pb. These nuclides are scavenged by aerosols and subsequently removed from the atmosphere, primarily by precipitation. Extensive studies on atmospherics depositions of 210Pb and 7Be were carried out in various regions of the world (Baskaran et al., 1993; Beks et al., 1998; Yamamoto et al., 2006). However, measurements of 210Pb and 7Be in atmospherics depositions, in Cuba, are very limited; in addition, the data on the concentrations in bulk
depositional fluxes of 7Be and 210Pb in the Caribbean countries and tropical regions are scarce. In consideration of the above, the objective of this work was to determine the depositional fluxes of 7Be and 210Pb in the bulk atmospheric deposition and discuss the meteorological factors controlling the depositions of these nuclides in this tropical area. 2. Sampling and measurements Sampling of bulk atmospheric deposition (wet þ dry) was carried out, between February 2010 and January 2011, at the Centre for Environmental Studies (CEAC) (22 030 N, 80 290 W), Cienfuegos province, Cuba. The sampling location is show in Fig. 1. The sampling site is one of the air monitoring stations in the Environmental Radiological Surveillance Network (RNVRA, spanish acronym), established in 1990 with the support of the International Atomic
Fig. 1. Map showing sampling location (red circle) in CEAC at Cienfuegos, Cuba. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
Energy Agency (IAEA). This site is situated 1.5 km south of the Caribbean Sea and 3 km north of the Cienfuegos's Bay. The most frequent wind direction is north-east. The standard meteorological data: precipitation, number of dry days and number of wet days were obtained by the Meteorological Institute located near from the sampling site. The chemical processing of the collected samples is similar to that described by Alonso-Hernandez et al. (2006). Atmospheric deposition samples were collected using five bulk rain collectors (50 L polyethylene) with a total surface area of 1.25 m2. The collectors were installed on a small building roof (about 5 m high off the ground) in the campus of the CEAC. To prevent any potential adsorption of 7Be and 210Pb onto the walls of the collectors, they were acidified prior to deployment with concentrated HCl. On a monthly basis, the total atmospherics depositions (wet & dry) were evaporated to dryness in order to obtain some residual samples, which in turn were weighed after drying, and transferred to a calibrated geometry. The activity of F0E3-emitting nuclides [210Pb (46.5 keV) and 7Be (477.6 keV)] in the residue samples was directly determined in a low level gamma spectrometric system with HPGe well detector (CAMBERRA, Type EGPC100 P-15), 11.1% relative efficiency, resolution of 1.86 keV at 1332 keV of 60Co and a 2048 channel MCA. The monthly 7Be depositions were corrected for radioactive decay from mid-months until the mid-counting times. The typical measurement time was 100,000 s. The activity concentration and its uncertainty were calculated using equations provided in the literature (IAEA, 1989). The uncertainty was calculated as the result of the uncertainty propagation process using the net peak area, the detector efficiency and the mass uncertainties, and in all calculations, resulted to be lower than 20%. This procedure was accredited by the National Office for Normalization for ISO-NC-17,025 and is recognized by the International Atomic Energy Agency through the ARCAL XXVI IAEA Regional Project. Since 2005, the CEAC is part of the IAEA-Network “Analytical Laboratories for the Measurement of Environmental Radioactivity” (ALMERA), and participates in the intercomparison exercises organized by the IAEA. The total depositional fluxes (F) for 7Be and 210Pb were calculated as follows:
F¼
AðBe=PbÞ ST
Table 1 Monthly amount of precipitation, number of rainy days, depositional fluxes of210Pb and 7Be and210Pb/7Be activity ratio. Month
Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10 Jan-11
18 193 21.2 37.4 198 231 220 411 13.6 30.5 14.8 38.5
Number of rainy days
9 5 5 9 19 23 18 19 13 6 5 3
210
7
Pb
210
Be
(Bq m2 month1)
(Bq m2 month1)
1.88 ± 4.77 ± 5.59 ± 4.52 ± N.M* 8.29 ± 5.36 ± 5.53 ± 2.79 ± 1.99 ± 1.24 ± 1.77 ±
24.60 47.00 40.20 54.90 N.M* 132 98.3 110 47.5 22.2 13.2 32.9
0.10 0.23 0.29 0.24 0.43 0.29 0.29 0.14 0.10 0.06 0.09
± ± ± ±
1.09 1.73 1.70 2.30
± ± ± ± ± ± ±
5.43 4.18 4.69 1.98 0.94 0.53 1.47
Pb/7 Be activity ratio 0.077 0.102 0.098 0.082 N.M* 0.063 0.055 0.050 0.059 0.090 0.093 0.065
± ± ± ±
0.005 0.006 0.006 0.005
± ± ± ± ± ± ±
0.004 0.003 0.003 0.003 0.005 0.006 0.004
*N.M: not measurement.
otherwise specified) of 56.6 Bq m2 (SD ¼ 39.2 Bq m2); these values are in agreement with those reported from other locations (Beks et al., 1998; Ioannidou et al., 2005). Fig. 3 shows the seasonal increase of 7Be during spring and summer seasons, while there is a significant depletion during winter months. The highest 7Be fluxes were found in Jul 2010. During spring, due to troposphereestratosphere exchange of air masses in mid-latitude, a certain amount of air is exchanged between the troposphere and stratosphere. Since the production of 7 Be in the stratosphere is significantly higher than those in the troposphere (Lal and Peters, 1967), this exchange will result in a significant increase in the air concentration and increased inventory of 7Be in the troposphere. This phenomenon has also been corroborated by Alonso-Hernandez et al. (2004) for Cienfuegos. A study of the concentration of 7Be in the air carry out in Cienfuegos, between 1994 and 1998, concluded that the maximum values of 7 Be were observed in AprileJune. Similar trends have been reported by Feely et al. (1988, 1989) and Larsen et al. (Larsen et al., 1995) for Miami (25 4900 N, 80 1700 W), San Juan, Puerto Rico (8 5800 N, 79 3400 (18 2600 N, 66 0000 W), and Balboa, Panama W). In addition, the monthly variations in the depositional fluxes of 7 Be may be due to several factors such as cloud height, the amount
500
400
Precipitation (mm)
3. Results and discussion
The amount of precipitation, number of rainy days, depositional fluxes of 210Pb and 7Be, and 210Pb/7Be activity ratio results are given in Table 1. One bulk deposition sample (June 2010) was lost during sample handling. The trend and the amount of precipitation in the sample period are similar to the rainfall regime obtained for the last fifteen years in the area, being the sampling period studied representative for the area (see Fig. 2). More detail of rainfall regime in the study area has been described by Alonso-Hernandez et al. The monthly depositional fluxes of 7Be between February 2010 and January 2011 in Cienfuegos are shown in Fig. 3, together with monthly amounts of precipitation. The monthly depositional fluxes of 7Be varied between 13.2 and 132 Bq m2, with an average over the 11 months (‘average’ always represent arithmetic mean unless
Rain (mm)
where: ABe/Pb is the total activity (in Bq) deposited in the rain collector, T is the duration of deployment of the rain collector (in year) and S is the total surface area of the collector (in m2).
3.1. Monthly and seasonal variations of bulk depositional fluxes of 7 Be and 210Pb
151
300
200
100
0 Jan
Feb
Mar
Apr
May Jun
Jul
Aug Sep
Oct
Nov Dec
Fig. 2. Monthly precipitation amount in Cienfuegos, Cuba. Black line represents the precipitation from February 2010 until January 2011. The Box shows the statistical data for fifteen year (Average, Median, Standard deviation and Extreme values).
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
152
450
140 120
360
270
80 60
180
40
Precipitation (mm)
7
-2
Be Flux (Bq m )
100
90 20 0
0 Feb10 Mar10 Apr10 May10 Jul10 Aug10 Sep10 Oct10 Nov10 Dec10 Jan11
Month/Year 7
Fig. 3. Monthly Be depositional flux and precipitation amount (February 2010 to January 2011) in Cienfuegos, Cuba.
and duration of precipitation, the time elapsed between successive rain events and the vertical mixing of air masses at the sampling ~ as et al., 2011; McNeary and Baskaran, 2003). In fact, our site (Duen results appears to be seasonality in the 7Be flux with about 54.6% of the total annual deposition occurring during the summer months (July to September 2010), when 63.9% of the total annual precipitation occurred and thus, the lowest deposition occurring during the winter months (November to February) when precipitations are scarce. The depositional flux during those months is principally due to dry deposition. The minimal depositional flux from November to February was 13.2 Bq m2. If we assume this value to be the dry depositional flux for Cienfuegos which has remained constant throughout the year, the dry deposition would account for 15% of the bulk fallout. It is likely to be an overestimate, as the precipitation will efficiently strip-off the aerosols, and hence the dry fallout in rainy months is likely to be much lower than the dry months. The value obtained in this study (15%) is in agreement with values obtained in others studies (Baskaran et al., 2003; Lozano, 2011; McNeary and Baskaran, 2003; Olsen et al., 1985; Todd et al., 1989). The monthly depositional fluxes of 210Pb between February 2010 and January 2011 in Cienfuegos are graphically shown in Fig. 4, together with monthly amounts of precipitation. The 210Pb depositional monthly flux varied between 1.24 and 8.29 Bq m2
with an average of 3.97 Bq m2 (SD ¼ 2.20). The higher values were found in the summer months, while July 2010 had the highest flux during this study period. The average depositional flux of 210Pb during dry months (November to February) was 1.72 Bq m2. If we assume this value as the dry depositional flux which has remained constant throughout the year, the dry deposition would account for 35% of the bulk fallout. From our results, the dry deposition of 210Pb is considerably higher than that of 7Be. This variation can be attributed to the differences in their sources and their half-lives. The expected inventories of 210Pb in the upper 1e2 cm of the soil layer are expected to be much higher than that of 7Be. When soil dust from the upper 1e2 cm layer is resuspended, the dust is expected to have a much higher specific activity (Bq per gram of resuspended dust) for 210Pb than 7Be, and thus we expect higher dry fallout of 210Pb compared to 7Be. The temporal variations of the monthly depositional fluxes of 7 Be, 210Pb and the amount of precipitation indicate that generally, the amount of precipitation seems to be controlling the magnitude of the depositional fluxes of 7Be and 210Pb (Figs. 3 and 4). The deposition of 7Be and 210Pb on the Earth's surface varies with seasons, latitudes (for Be), longitudes (for Pb) and local meteorological conditions. The activity fluxes of 210Pb and 7Be show a similar trend in the studied months (Figs. 3 and 4). The maximum and minimum flux of 210 Pb and 7Be occurred both in July and December 2010, respectively. For both radionuclides, the lowest deposition was found during the months when there is little rain. In fact, a strong correlation (r ¼ 0.85) between monthly depositional flux of 210Pb and monthly depositional flux of 7Be was calculated (n ¼ 11, p < 0.0005) and shown in Fig. 5. Lead-210 and 7Be in the atmosphere have distinct sources due to their different modes of production. This high correlation between these two radionuclides suggests that both radionuclides cannot be used as independent atmospheric tracers. Similar strong correlation between these nuclides was found in Bermuda by Kim et al. (1999). The depositional monthly flux of 210Pb varied in the study period by a factor of 7, while 7Be varied by a factor of 10. The variation factor corresponding to the 210Pb in our study is particularly lower than that of other Caribbean Sea coast sites, such as Tampa Bay, FL (United States), where a value of 40 was reported (Baskaran and
9 10
450
8
360
6
270
210
7
Pb = 1.229 + 0.0484 * Be r = 0.85 p < 0.0005
8
2
90
210
-2
-1
Pb flux (Bq m month )
180
Precipitation (mm)
4
210
-2
Pb Flux (Bq m )
7
0
0 Feb10 Mar10 Apr10 May10 Jul10 Aug10 Sep10 Oct10 Nov10 Dec10 Jan11
Month/Year 210
Fig. 4. Monthly Pb depositional flux and precipitation amount (February 2010 to January 2011) in Cienfuegos, Cuba.
6 5 4 3 2 1 0 0
20
40
60 7
80 -2
100
120
140
-1
Be flux (Bq m month )
Fig. 5. Monthly depositional flux of Cienfuegos, Cuba.
210
Pb versus monthly depositional flux of 7Be in
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
153
450
0,14
400
0,12 0,10 0,08
200
0,06
7
250
Pb/ Be
300
210
Monthly rain (mm)
350
150 0,04 100 0,02
50
Ja n1 1
ov 10
10 ct
ec 10 D
N
O
Se p1 0
Ju l1 0 Au g1 0
Ap r1 0 M ay 10
0,00 M ar 10
Fe b1 0
0
Month/Year Fig. 6. Relationship between the monthly deposition of 7Be and tion at Cienfuegos.
210
Pb and precipitaFig. 8. Relationships between
Swarzenski, 2007). The relatively low variation in the depositional flux of 210Pb at our study site is likely due to the source of air masses. Due to the low exhalation rates of 222Rn over the ocean as compared to continental areas, oceanic air masses are typically depleted in 222Rn and its daughter-products, including 210Pb; as a consequence, the standing crop of 210Pb in the atmosphere has a strong longitudinal dependency (Turekian et al., 1977). However, 7 Be is of cosmogenic origin, and its flux on the Earth's surface has a latitudinal dependency, and hence its atmospheric flux on the Earth's surface should be independent of geography, at any given latitude. Baskaran and Swarzenski (2007) reported for Florida a variation factor of 14 for 7Be, similar to the 10 obtained in this study. 3.2. Relationship between rain and fluxes of Cienfuegos
210
Pb and 7Be in
The monthly 210Pb and 7Be deposition fluxes versus amount of rain are plotted in Fig. 6. In the full study period, there is a significant correlation between 210Pb, 7Be fluxes and precipitations (r ¼ 0.62 and r ¼ 0.83, respectively). Similar values have been reported by others authors in coastal stations (Baskaran et al., 1993; Kim et al., 2000). If data were processed at shorter temporal scale rather than at annual level, it would show a better correlation between activity flux and precipitations (Fig. 7). It was found that the
Fig. 7. The correlation between flux of
210
210
Pb/7Be ratio and amount of precipitations.
correlations coefficients between precipitation and flux were 0.30 and 0.98 for 210Pb, 0.79 and 0.88 for 7Be in the months ranging from April to October and November to March, respectively. In the months, between November and March, there is a high correlation between activity flux of both nuclides and precipitation. Also a significant correlation is found between 7Be flux and precipitations in the months between April and October (0.79). These results indicate that the scavenge process of two nuclides in the low atmosphere have similar mechanism. The dominant factor of scavenge is precipitation for 210Pb and 7Be in the months between November and March, while in the months ranging from April to October 2010 the influence of precipitation is relevant only for the scavenge process of 7Be. However, there is a clear need of getting data on longer period (several years) to really evaluate the role of precipitation regime. 3.3. The ratio of activity flux of
210
Pb and 7Be
The ratios of 210Pb/7Be deposition fluxes and amount of precipitations in the sampling period are shown in Fig. 8. The variation range for the 210Pb/7Be ratio was 0.05e0.10 with an average of 0.08 (SD ¼ 0.02). This value is similar to the one reported by Baskaran et al. (1993) and Saari et al. (2010) and the 0.08 reported as world average (Du et al., 2008). In general, the 210Pb/7Be ratio decreases in
Pb and 7Be and precipitation at Cienfuegos for Apr.-Oct. and Nov.-Mar periods.
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
154 25
0,14
0,08 0,06
10
7
15
Pb/ Be
0,10
210
Number of rainy days
0,12 20
0,04 5 0,02
Ja n1 1
ec 10 D
ov 10
10 ct
N
O
Se p1 0
Ju l1 0 Au g1 0
Ap r1 0 M ay 10
0,00 M ar 10
Fe b1 0
0
Month/Year Fig. 9. Relationships between
210
Pb/7Be ratio and number of rainy days per months.
the months of highest precipitation and increases in the months of lowest precipitation. However, in January, February and March this behaviour was not observed and a poor correlation (r ¼ 0.58) between 210Pb/7Be ratio and precipitation was found. If we plotted the 210Pb/7Be ratio versus the number of rainy days, as seen in Fig. 9, a significant correlation (r ¼ 0.71) between the 210Pb/7Be ratio and precipitation would be seen. We attribute this result to the fact that in the months with more rainy days, the emanation of 222Rn from land surface decreases due to the saturation effect therefore decreases the 210Pb concentration in the atmosphere. The effect of moisture content on radon emanation has been reported on extensively (Barton and Ziemer, 1986; Menetrez et al., 1997; Schumann and Gundersen, 1997; Sun and Furbish, 1995). The 7Be concentration in the air is not affected by this factor; therefore the 210Pb/7Be ratio decreases during these months. 3.4. Annual bulk depositional fluxes of
210
Locations
(Ugur et al., 2011) (Beks et al., 1998) (Du et al., 2008) (Ioannidou et al., 2005)
Acknowledgements
210
Pb and 7Be
Pb and 7Be on various coastal stations in the world. Period
210
7
Pb
Be
2
1
Reference
(Bq m
y
Odawa, Japan Tsukuba, Japan Geneva, Switzerland Bordeaux, France Tampa, USA
2000e2003 1990 1991 2006 1990 1991 1997e1998 2001 1997e1998 2006e2007 2003e2004
48* 47 57 479 e e 73 182 150 103 123
e e e 2070 733 712 e 918 2087 1256 1468
Cienfuegos, Cuba
2010e2011
47
700
Izmir, Turkey Groningen, Holland Shanghai, China Thessaloniki, Greece
*In rainwater.
4. Conclusions We have evaluated for first time in Cuba the atmospheric deposition fluxes for 7Be and 210Pb obtained over a 11 months period (February. 2010 to January 2011) in Cienfuegos. This preliminary study provides a baseline data and will be helpful in further investigations at this site. The annual average flux of 210Pb and 7Be were 47 and 700 Bq m2 y1, respectively. The depositional fluxes of 7Be and 210Pb are strongly correlated; indicating that removal behaviour from the atmosphere is relatively similar. The depositional fluxes of radionuclides on this coastal site (Cienfuegos) have relatively low values, in particular 210Pb, probably reflecting significant input of marine air. A positive correlation has been found between the deposition fluxes, the amount of precipitation and the number of wet days. There is a statistical relationship between 7Be and 210Pb deposition fluxes and the amount of precipitation. This study suggests that continuous monitoring of radioelement fluxes is necessary to examine both episodic and long-term changes in annual and seasonal atmospheric fluxes. The option of having some additional permanent coastal sampling stations, on the Caribbean Sea, is desirable.
Despite it is necessary to obtain more data (at least 10 years), we try to estimate values of annual bulk depositional fluxes of 210Pb and 7Be in Cienfuegos between February 2010 and January 2011, calculated on 347 days of sampling (taking June out), were 47 and 700 Bq m2 y1, respectively. The preliminary annual 210Pb and 7Be atmospherics fluxes in Cienfuegos are compared in Table 2 with
Table 2 Depositional fluxes of
other coastal stations worldwide. The annual bulk depositional flux for 210Pb in Cienfuegos (47 Bq m2 y1) is at a relatively low value, which indicates a significant oceanic influence. This value is distinctly lower than other coastal stations, such as Shanghai, China, where a value of 479 Bq m2 y1 was reported (Du et al., 2008). In this case, a major component of the air masses above Shanghai is derived from the adjacent Asian continent. In contrast, Beks et al. (1998)(Beks et al., 1998) reported for the Netherlands similar values (47 and 57 Bq m2 y1) to the ones obtained for Cienfuegos in this study, indicating a major oceanic influence. Besides, in recent study others authors have been reporting similar values for 210Pb fluxes. In Izmir, Turkey, Ugur et al. (2010) reported a value of 48 Bq m2 y1. The annual 7Be depositional flux (700 Bq m2 y1) obtained in Cienfuegos is in the range of the inter-annual variation of the 7Be annual deposition in the mid-latitude region (458e3833 Bq m2 y1), in the 1987 to 2006 period (Baskaran et al., 1993; Caillet S. et al., 2001; Du et al., 2008, 2010; Hirose et al., 2004; Ioannidou et al., 2005; Saari et al., 2010; Yamamoto et al., 2006). However, the 7Be depositional flux in Cienfuegos is one of the lowest values. This result is likely due to the dilution effect, as explained by Baskaran and Swarzenski (2007) for a geographical area close to the sea. During the summer months, the source of water vapour for afternoon thunderstorms is generally from the ocean and the subsequent precipitation may occur within a few hours. The residence time of water vapour in this region during summer months is very short. Due to the occurrence of almost daily summer thunderstorms, the atmosphere is effectively flushed and thus the 7Be activities cannot build up. Lateral movement of marine air masses coupled with the vertical movement of water vapour towards the cloud condensation could result in the relatively lower activities of 7 Be in the air.
)
(Tateda and Iwao, 2008) (Hirose et al., 2004) (Caillet et al., 2001) (Saari et al., 2010) (Baskaran and Swarzenski, 2007) This work
This research work was undertaken in the framework of the IAEA TC Project CUB/7/008 ‘‘Strengthening the National System for Analysis of the Risks and Vulnerability of Cuba's Coastal Zone through the Application of Nuclear and Isotopic Techniques'’. Special gratitude goes to Jones Catherine for her great help during the elaboration of the manuscript and the two anonymous reviewers for their comments and suggestions.
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
References ndez, C.M., Cartas-Aguila, ~ oz-Caravaca, A., Alonso-Herna H., Díaz-Asencio, M., Mun 2004. Reconstruction of 137Cs signal in Cuba using 7Be as tracer of vertical transport processes in the atmosphere. J. Environ. Radioact. 75, 133e142. ndez, C.M., Díaz-Asencio, M., Mun ~ oz-Caravaca, A., Delfanti, R., Alonso-Herna Papucci, C., Ferretti, O., Crovato, C., 2006. Recent changes in sedimentation regime in Cienfuegos Bay, Cuba, as inferred from 210Pb and 137Cs vertical profiles. Cont. Shelf Res. 26, 153e167. Barton, T.P., Ziemer, P.L., 1986. The effects of particle size and moisture content on the emanation of Rn from coal ash. Health Phys. 50, 581e588. Baskaran, M., Coleman, C.H., Santschi, P.H., 1993. Atmospheric depositional fluxes of Be-7 and Pb-210 at Galveston and College-Station, Texas. J. Geophys. Res. Atmos. 98, 20555e20571. Baskaran, M., Hong, G.H., Dayton, S., Bodkin, J.L., Kelley, J.J., 2003. Temporal variations of natural and anthropogenic radionuclides in sea otter skull tissue in the North Pacific Ocean. J. Environ. Radioact. 64, 1e18. Baskaran, M., Shaw, G.E., 2001. Residence time of arctic haze aerosols using the concentrations and activity ratios of Po-210, Pb-210 and Be-7. J. Aerosol Sci. 32, 443e452. Baskaran, M., Swarzenski, P.W., 2007. Seasonal variations on the residence times and partitioning of short-lived radionuclides (234Th, 7Be and 210Pb) and depositional fluxes of 7Be and 210Pb in Tampa Bay, Florida. Mar. Chem. 104, 27e42. Beks, J.P., Eisma, D., van der Plicht, J., 1998. A record of atmospheric Pb-210 deposition in The Netherlands. Sci. Total Environ. 222, 35e44. Blake, W.H., Walling, D.E., He, Q., 2002. Using cosmogenic beryllium-7 as a tracer in sediment budget investigations. Geogr. Ann. Ser. A Phys. Geogr. 84A, 89e102. Caillet, S., Arpagaus, P., Monna, F., Dominik, J., 2001. Factor controlling 7Be and 210Pb atmospheric depositions as revealed by sampling individual rain events in the region of Genova, Switzerland. J. Environ. Radioact. 53, 241e256. Ciffroy, P., Reyss, J.L., Siclet, F., 2003. Determination of the residence time of suspended particles in the turbidity maximum of the Loire estuary by Be-7 analysis. Estuar. Coast. Shelf Sci. 57, 553e568. ndez, C.M., Bolan ~ os-Alvarez, Y., Go mez-Batista, M., Díaz-Asencio, M., Alonso-Herna ndez-Alberba, M., Eriksson, M., SanchezPinto, V., Morabito, R., Herna Cabeza, J.A., 2009. One century sedimentary record of Hg and Pb pollution in the Sagua estuary (Cuba) derived from 210Pb and 137Cs chronology. Mar. Pollut. Bull. 59, 108e115. Dibb, J.E., Rice, D.L., 1989. Temporal and spatial-distribution of beryllium-7 in the sediments of Chesapeake Bay. Estuar. Coast. Shelf Sci. 28, 395e406. Du, J., Wu, Y., Huang, D., Zhang, J., 2010. Use of 7Be, 210Pb and 137Cs tracers to the transport of surface sediments of the Changjiang Estuary, China. J. Mar. Syst. 82, 286e294. Du, J., Zhang, J., Zhang, J., Wu, Y., 2008. Deposition patterns of atmospheric 7Be and 210 Pb in coast of East China Sea, Shanghai, China. Atmos. Environ. 42, 5101e5109. ~ as, C., Orza, J.A.G., Cabello, M., Fernandez, M.C., Can ~ ete, S., Pe rez, M., Gordo, E., Duen 2011. Air mass origin and its influence on radionuclide activities (7Be and 210Pb) in aerosol particles at a coastal site in the western Mediterranean. Atmos. Res. 101, 205e214. Feely, H.W., Larsen, R.J., Sanderson, C.G., 1989. Factors that cause seasonal variations in beryllium-7 concentrations in surface air. J. Environ. Radioact. 9, 223e249. Feely, H.W., Lasen, R.J., Sanderson, C.G., 1988. Annual report of the surface air sampling programmes. EML-447. In: Department of Energy Report, New York. Fitzgerald, S.A., Klump, J.V., Swarzenski, P.W., Mackenzie, R.A., Richards, K.D., 2001. Beryllium-7 as a tracer of short-term sediment deposition and resuspension in the Fox River, Wisconsin. Environ. Sci. Technol. 35, 300e305. Hirose, K., Honda, T., Yagishita, S., Igarashi, Y., Aoyama, M., 2004. Deposition behaviors of Pb-210, Be-7 and thorium isotopes observed in Tsukuba and Nagasaki, Japan. Atmos. Environ. 38, 6601e6608.
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IAEA. Measurement of Radionuclides in Food and the Environment. A Guidebook. (1989), Technical Reports Series. 295. Vienna, International Atomic Energy Agency. Ioannidou, A., Manolopoulou, M., Papastefanou, C., 2005. Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (40¦N). Appl. Radiat. Isot. 63, 277. Kadko, D., Olson, D., 1996. Beryllium-7 as a tracer of surface water subduction and mixed-layer history. Deep Sea Res. Part Oceanogr. Res. Pap. 43, 89e116. Kikuchi, S., Sakurai, H., Gunji, S., Tokanai, F., 2009. Temporal variation of 7Be concentrations in atmosphere for 8y from 2000 at Yamagata, Japan: solar influence on the 7Be time series. J. Environ. Radioact. 100, 515e521. Kim, G., Alleman, L.Y., Church, T.M., 1999. Atmospheric depositional fluxes of trace elements, 210Pb and 7Be to the Sargasso sea. Glob. Biogeochem. Cycles 13, 1183e1192. Kim, G., Hussain, N., Scudlark, J.R., Church, T.M., 2000. Factors influencing the atmospheric depositional fluxes of stable Pb, Pb-210, and Be-7 into Chesapeake Bay. J. Atmos. Chem. 36, 65e79. Lal, D., Malhotra, P.K., Peters, B., 1958. On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology. J. Atmos. Terr. Phys. 12, 306e328. Lal, D., Peters, B., 1967. Cosmic ray produced radioactivity on the earth. Handb. Phys. 46, 551e612. Larsen, R.J., Haagenson, P.L., Reiss, N.M., 1995. EML surface air sampling program 1990e1993 data. EML-572. In: USDOE, New York. Lozano, R.L., San Miguel, E.G., Bolivar, J.P., Baskaran, M., 2011. Depositional fluxes and concentrations of 7Be and 210Pb in bulk precipitation and aerosols at the interface of Atlantic and Mediterranean coasts in Spain. J. Geophys. Res. D. Atmos. 116. McNeary, D., Baskaran, M., 2003. Depositional characteristics of 7Be and 210Pb in southeastern Michigan. J. Geophys. Res. D. Atmos. 108, AAC-1. Menetrez, M.Y., Mosley, R.B., Snoddy, R., Brubaker, J., 1997. Evaluation of radon emanation from soil with varying moisture content in a soil chamber. Environ. Int. 22, S447eS453. Olsen, C.R., Larsen, I.L., Lowry, P.D., Cutshall, N.H., Todd, J.F., Wong, G.T.F., Casey, W.H., 1985. Atmospheric fluxes and marsh-soil inventories of Be-7 and Pb-210. J. Geophys. Res. Atmos. 90, 487e495. Saari, H.K., Schmidt, S., Castaing, P., Black, A.R., Sautour, B., Masson, O., Cochran, J.K. (Eds.), 2010. The Particulate 7Be/210Pbxs and 234Th/210Pbxs Activity Ratios as Tracers for Tidal-to-seasonal Particle Dynamics in the Gironde Estuary (France): Implications for the Budget of Particle-associated Contaminants, 408 ed., pp. 4784e4794. Schumann, R.R., Gundersen, L.C.S., 1997. Geologic and climatic controls on the radon emanation coefficient. Environ. Int. 22, S439eS446. Sommerfield, C.K., Nittrouer, C.A., Alexander, C.R., 1999. Be-7 as a tracer of flood sedimentation on the northern California continental margin. Cont. Shelf Res. 19, 335e361. Steinmann, P., Billen, T., Loizeau, J.I., Dominik, J., 1999. Beryllium-7 as a tracer to study mechanisms and rates of metal scavenging from lake surface waters. Geochim. Cosmochim. Acta 63, 1621e1633. Sun, H., Furbish, D.J., 1995. Moisture content effect on radon emanation in porous media. J. Contam. Hydrol. 18, 239e255. Tateda, Y., Iwao, K., 2008. High 210Po atmospheric deposition flux in the subtropical coastal area of Japan. J. Environ. Radioact. 99, 98e108. Todd, J.F., Wong, G.T.F., Olsen, C.R., Larsen, I.L., 1989. Atmospheric depositional characteristics of Beryllium-7 and Pb-210 along the Southeastern Virginia coast. J. Geophys. Res. Atmos. 94, 11106e11116. Turekian, K.K., Nozaki, Y., Benninger, L.K., 1977. Geochemistry of atmospheric radon and radon products. Annu. Rev. Earth Planet. Sci. 5, 227e255. Ugur, A., Ozden, B., Filizok, I., 2011. Determination of 210Po and 210Pb concentrations in atmospheric deposition in Izmir (Aegean sea-Turkey). Atmos. Environ. 45, 4809e4813. Yamamoto, M., Sakaguchi, A., Sasaki, K., Hirose, K., Igarashi, Y., Kim, C.K., 2006. Seasonal and spatial variation of atmospheric 210Pb and 7Be deposition: features of the Japan Sea side of Japan. J. Environ. Radioact. 86, 110e131.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Atmospheric deposition patterns of
210
Pb and 7Be in Cienfuegos, Cuba
ndez*, Yasser Morera-Go mez, He ctor Cartas-Aguila, Carlos M. Alonso-Herna n-Arruebarrena Aniel Guille Centro de Estudios Ambientales de Cienfuegos, AP 5, Ciudad Nuclear, Cienfuegos, Cuba
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2012 Received in revised form 4 July 2014 Accepted 29 August 2014 Available online
The radiometric composition of bulk deposition samples, collected monthly for one year, February 2010 until January 2011, at a site located in Cienfuegos (22 030 N, 80 290 W) (Cuba), are analysed in this paper. Measurement of 7Be and 210Pb activity concentrations were carried out in 12 bulk deposition samples. The atmospheric deposition fluxes of 7Be and 210Pb are in the range of 13.2e132 and 1.24e8.29 Bq m2, and their mean values are: 56.6 and 3.97 Bq m2, respectively. The time variations of the different radionuclide have been discussed in relation with meteorological factors and the mean values have been compared to those published in recent literature from other sites located at different latitudes. The annual average flux of 210Pb and 7Be were 47 and 700 Bq m2 y1, respectively. Observed seasonal variations of deposition data are explained in terms of different environmental features. The atmospheric deposition fluxes of 7Be and 210Pb were moderately well correlated with precipitation and well correlated with one another. The 210Pb/7Be ratios in the monthly depositions samples varied in the range of 0.05 e0.10 and showed a strong correlation with the number of rainy days. © 2014 Elsevier Ltd. All rights reserved.
Keywords: 7 Be 210 Pb Atmospheric deposition Cienfuegos Cuba
1. Introduction Naturally occurring beryllium-7 and the daughter-products of Rn, such as 210Pb and 210Po, have proven useful as tracers and have been utilized to gain insight into rates of sediment mixing, sedimentation, dynamics of particle transport, fate of particlereactive contaminants as well as meteorological information, especially on the origin of air masses and residence times of ndez et al., 2004, 2006; Baskaran and Shaw, aerosols (Alonso-Herna 2001; Blake et al., 2002; Ciffroy et al., 2003; Díaz-Asencio et al., 2009; Fitzgerald et al., 2001; Kadko and Olson, 1996; Sommerfield et al., 1999; Steinmann et al., 1999). The source terms of these radionuclides are relatively well known and are removed from the atmosphere by precipitation, dry fallout and radioactive decay. Knowledge of the behaviour of 7Be and 210Pb in the atmosphere will yield insight on the behaviour of other similar chemical species in the atmosphere. For example, Alonso-Hernandez et al. (2004) used the ratio of 7Be in surface air of Cienfuegos, in surface air of Miami FL (United States), as well as 137 Cs concentrations measured in surface air in Miami, to 222
* Corresponding author. Tel./fax: þ53 43965146. E-mail addresses: [email protected], [email protected] ndez). (C.M. Alonso-Herna http://dx.doi.org/10.1016/j.jenvrad.2014.08.023 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
reconstruct the monthly 137Cs signal in surface air of Cienfuegos due to global fallout between 1957 and 1994. Beryllium-7 (T1/2 ¼ 53.3 days) is a cosmogenic radionuclide produced in the stratosphere when cosmic rays bombard the nucleus of oxygen, and nitrogen atoms in the atmosphere (Lal et al., 1958), and its flux to the Earth's surface has a latitudinal dependency (Lal and Peters, 1967) similar to other cosmogenic nuclides. The concentration of 7Be in the atmosphere increases with altitude and its production rate for a given latitude (as well as the flux) on the Earth's surface is independent of longitude. Due to its short half-life, most of the 7Be produced in the stratosphere do not reach the troposphere except during spring when seasonal thinning of the tropopause takes place in mid-latitudes, resulting in air exchange between the stratosphere and troposphere. In most atmospheric studies, the 7Be concentration in aerosols does not remain constant with time and show seasonal variations in surface air. A high concentration of 7Be in surface air aerosols would indicate significant input from the upper troposphere-lower stratosphere regions (Dibb and Rice, 1989; Feely et al., 1989). At midlatitude, seasonal variations of air concentration of 7Be in spring have been observed, due to air masses with higher concentrations of 7Be in the stratosphere being injected into the troposphere at that period. In addition, rates of 7Be production are dependent upon solar activity and can therefore be influenced by latitude with increased production occurring at the poles owing to cosmic ray deflection towards polar regions. Production is also affected by
150
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
altitude with higher levels of production occurring in the stratosphere than in the troposphere (Kikuchi et al., 2009). Radon-222 (T1/2 ¼ 3.8 days), one of the daughter-products in the 238 U decay chain, predominantly emanates from the Earth's continental crust. The 222Rn flux from soils ranges from 42 to 267 Bq cm2 yr1, with a global average value of 54 Bq cm2 yr1 and is about 100 times higher than the oceanic flux (Turekian et al., 1977). Radon-222 undergoes radioactive decay in the atmosphere producing several daughter-products, including 210Pb. These nuclides are scavenged by aerosols and subsequently removed from the atmosphere, primarily by precipitation. Extensive studies on atmospherics depositions of 210Pb and 7Be were carried out in various regions of the world (Baskaran et al., 1993; Beks et al., 1998; Yamamoto et al., 2006). However, measurements of 210Pb and 7Be in atmospherics depositions, in Cuba, are very limited; in addition, the data on the concentrations in bulk
depositional fluxes of 7Be and 210Pb in the Caribbean countries and tropical regions are scarce. In consideration of the above, the objective of this work was to determine the depositional fluxes of 7Be and 210Pb in the bulk atmospheric deposition and discuss the meteorological factors controlling the depositions of these nuclides in this tropical area. 2. Sampling and measurements Sampling of bulk atmospheric deposition (wet þ dry) was carried out, between February 2010 and January 2011, at the Centre for Environmental Studies (CEAC) (22 030 N, 80 290 W), Cienfuegos province, Cuba. The sampling location is show in Fig. 1. The sampling site is one of the air monitoring stations in the Environmental Radiological Surveillance Network (RNVRA, spanish acronym), established in 1990 with the support of the International Atomic
Fig. 1. Map showing sampling location (red circle) in CEAC at Cienfuegos, Cuba. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
Energy Agency (IAEA). This site is situated 1.5 km south of the Caribbean Sea and 3 km north of the Cienfuegos's Bay. The most frequent wind direction is north-east. The standard meteorological data: precipitation, number of dry days and number of wet days were obtained by the Meteorological Institute located near from the sampling site. The chemical processing of the collected samples is similar to that described by Alonso-Hernandez et al. (2006). Atmospheric deposition samples were collected using five bulk rain collectors (50 L polyethylene) with a total surface area of 1.25 m2. The collectors were installed on a small building roof (about 5 m high off the ground) in the campus of the CEAC. To prevent any potential adsorption of 7Be and 210Pb onto the walls of the collectors, they were acidified prior to deployment with concentrated HCl. On a monthly basis, the total atmospherics depositions (wet & dry) were evaporated to dryness in order to obtain some residual samples, which in turn were weighed after drying, and transferred to a calibrated geometry. The activity of F0E3-emitting nuclides [210Pb (46.5 keV) and 7Be (477.6 keV)] in the residue samples was directly determined in a low level gamma spectrometric system with HPGe well detector (CAMBERRA, Type EGPC100 P-15), 11.1% relative efficiency, resolution of 1.86 keV at 1332 keV of 60Co and a 2048 channel MCA. The monthly 7Be depositions were corrected for radioactive decay from mid-months until the mid-counting times. The typical measurement time was 100,000 s. The activity concentration and its uncertainty were calculated using equations provided in the literature (IAEA, 1989). The uncertainty was calculated as the result of the uncertainty propagation process using the net peak area, the detector efficiency and the mass uncertainties, and in all calculations, resulted to be lower than 20%. This procedure was accredited by the National Office for Normalization for ISO-NC-17,025 and is recognized by the International Atomic Energy Agency through the ARCAL XXVI IAEA Regional Project. Since 2005, the CEAC is part of the IAEA-Network “Analytical Laboratories for the Measurement of Environmental Radioactivity” (ALMERA), and participates in the intercomparison exercises organized by the IAEA. The total depositional fluxes (F) for 7Be and 210Pb were calculated as follows:
F¼
AðBe=PbÞ ST
Table 1 Monthly amount of precipitation, number of rainy days, depositional fluxes of210Pb and 7Be and210Pb/7Be activity ratio. Month
Feb-10 Mar-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10 Dec-10 Jan-11
18 193 21.2 37.4 198 231 220 411 13.6 30.5 14.8 38.5
Number of rainy days
9 5 5 9 19 23 18 19 13 6 5 3
210
7
Pb
210
Be
(Bq m2 month1)
(Bq m2 month1)
1.88 ± 4.77 ± 5.59 ± 4.52 ± N.M* 8.29 ± 5.36 ± 5.53 ± 2.79 ± 1.99 ± 1.24 ± 1.77 ±
24.60 47.00 40.20 54.90 N.M* 132 98.3 110 47.5 22.2 13.2 32.9
0.10 0.23 0.29 0.24 0.43 0.29 0.29 0.14 0.10 0.06 0.09
± ± ± ±
1.09 1.73 1.70 2.30
± ± ± ± ± ± ±
5.43 4.18 4.69 1.98 0.94 0.53 1.47
Pb/7 Be activity ratio 0.077 0.102 0.098 0.082 N.M* 0.063 0.055 0.050 0.059 0.090 0.093 0.065
± ± ± ±
0.005 0.006 0.006 0.005
± ± ± ± ± ± ±
0.004 0.003 0.003 0.003 0.005 0.006 0.004
*N.M: not measurement.
otherwise specified) of 56.6 Bq m2 (SD ¼ 39.2 Bq m2); these values are in agreement with those reported from other locations (Beks et al., 1998; Ioannidou et al., 2005). Fig. 3 shows the seasonal increase of 7Be during spring and summer seasons, while there is a significant depletion during winter months. The highest 7Be fluxes were found in Jul 2010. During spring, due to troposphereestratosphere exchange of air masses in mid-latitude, a certain amount of air is exchanged between the troposphere and stratosphere. Since the production of 7 Be in the stratosphere is significantly higher than those in the troposphere (Lal and Peters, 1967), this exchange will result in a significant increase in the air concentration and increased inventory of 7Be in the troposphere. This phenomenon has also been corroborated by Alonso-Hernandez et al. (2004) for Cienfuegos. A study of the concentration of 7Be in the air carry out in Cienfuegos, between 1994 and 1998, concluded that the maximum values of 7 Be were observed in AprileJune. Similar trends have been reported by Feely et al. (1988, 1989) and Larsen et al. (Larsen et al., 1995) for Miami (25 4900 N, 80 1700 W), San Juan, Puerto Rico (8 5800 N, 79 3400 (18 2600 N, 66 0000 W), and Balboa, Panama W). In addition, the monthly variations in the depositional fluxes of 7 Be may be due to several factors such as cloud height, the amount
500
400
Precipitation (mm)
3. Results and discussion
The amount of precipitation, number of rainy days, depositional fluxes of 210Pb and 7Be, and 210Pb/7Be activity ratio results are given in Table 1. One bulk deposition sample (June 2010) was lost during sample handling. The trend and the amount of precipitation in the sample period are similar to the rainfall regime obtained for the last fifteen years in the area, being the sampling period studied representative for the area (see Fig. 2). More detail of rainfall regime in the study area has been described by Alonso-Hernandez et al. The monthly depositional fluxes of 7Be between February 2010 and January 2011 in Cienfuegos are shown in Fig. 3, together with monthly amounts of precipitation. The monthly depositional fluxes of 7Be varied between 13.2 and 132 Bq m2, with an average over the 11 months (‘average’ always represent arithmetic mean unless
Rain (mm)
where: ABe/Pb is the total activity (in Bq) deposited in the rain collector, T is the duration of deployment of the rain collector (in year) and S is the total surface area of the collector (in m2).
3.1. Monthly and seasonal variations of bulk depositional fluxes of 7 Be and 210Pb
151
300
200
100
0 Jan
Feb
Mar
Apr
May Jun
Jul
Aug Sep
Oct
Nov Dec
Fig. 2. Monthly precipitation amount in Cienfuegos, Cuba. Black line represents the precipitation from February 2010 until January 2011. The Box shows the statistical data for fifteen year (Average, Median, Standard deviation and Extreme values).
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
152
450
140 120
360
270
80 60
180
40
Precipitation (mm)
7
-2
Be Flux (Bq m )
100
90 20 0
0 Feb10 Mar10 Apr10 May10 Jul10 Aug10 Sep10 Oct10 Nov10 Dec10 Jan11
Month/Year 7
Fig. 3. Monthly Be depositional flux and precipitation amount (February 2010 to January 2011) in Cienfuegos, Cuba.
and duration of precipitation, the time elapsed between successive rain events and the vertical mixing of air masses at the sampling ~ as et al., 2011; McNeary and Baskaran, 2003). In fact, our site (Duen results appears to be seasonality in the 7Be flux with about 54.6% of the total annual deposition occurring during the summer months (July to September 2010), when 63.9% of the total annual precipitation occurred and thus, the lowest deposition occurring during the winter months (November to February) when precipitations are scarce. The depositional flux during those months is principally due to dry deposition. The minimal depositional flux from November to February was 13.2 Bq m2. If we assume this value to be the dry depositional flux for Cienfuegos which has remained constant throughout the year, the dry deposition would account for 15% of the bulk fallout. It is likely to be an overestimate, as the precipitation will efficiently strip-off the aerosols, and hence the dry fallout in rainy months is likely to be much lower than the dry months. The value obtained in this study (15%) is in agreement with values obtained in others studies (Baskaran et al., 2003; Lozano, 2011; McNeary and Baskaran, 2003; Olsen et al., 1985; Todd et al., 1989). The monthly depositional fluxes of 210Pb between February 2010 and January 2011 in Cienfuegos are graphically shown in Fig. 4, together with monthly amounts of precipitation. The 210Pb depositional monthly flux varied between 1.24 and 8.29 Bq m2
with an average of 3.97 Bq m2 (SD ¼ 2.20). The higher values were found in the summer months, while July 2010 had the highest flux during this study period. The average depositional flux of 210Pb during dry months (November to February) was 1.72 Bq m2. If we assume this value as the dry depositional flux which has remained constant throughout the year, the dry deposition would account for 35% of the bulk fallout. From our results, the dry deposition of 210Pb is considerably higher than that of 7Be. This variation can be attributed to the differences in their sources and their half-lives. The expected inventories of 210Pb in the upper 1e2 cm of the soil layer are expected to be much higher than that of 7Be. When soil dust from the upper 1e2 cm layer is resuspended, the dust is expected to have a much higher specific activity (Bq per gram of resuspended dust) for 210Pb than 7Be, and thus we expect higher dry fallout of 210Pb compared to 7Be. The temporal variations of the monthly depositional fluxes of 7 Be, 210Pb and the amount of precipitation indicate that generally, the amount of precipitation seems to be controlling the magnitude of the depositional fluxes of 7Be and 210Pb (Figs. 3 and 4). The deposition of 7Be and 210Pb on the Earth's surface varies with seasons, latitudes (for Be), longitudes (for Pb) and local meteorological conditions. The activity fluxes of 210Pb and 7Be show a similar trend in the studied months (Figs. 3 and 4). The maximum and minimum flux of 210 Pb and 7Be occurred both in July and December 2010, respectively. For both radionuclides, the lowest deposition was found during the months when there is little rain. In fact, a strong correlation (r ¼ 0.85) between monthly depositional flux of 210Pb and monthly depositional flux of 7Be was calculated (n ¼ 11, p < 0.0005) and shown in Fig. 5. Lead-210 and 7Be in the atmosphere have distinct sources due to their different modes of production. This high correlation between these two radionuclides suggests that both radionuclides cannot be used as independent atmospheric tracers. Similar strong correlation between these nuclides was found in Bermuda by Kim et al. (1999). The depositional monthly flux of 210Pb varied in the study period by a factor of 7, while 7Be varied by a factor of 10. The variation factor corresponding to the 210Pb in our study is particularly lower than that of other Caribbean Sea coast sites, such as Tampa Bay, FL (United States), where a value of 40 was reported (Baskaran and
9 10
450
8
360
6
270
210
7
Pb = 1.229 + 0.0484 * Be r = 0.85 p < 0.0005
8
2
90
210
-2
-1
Pb flux (Bq m month )
180
Precipitation (mm)
4
210
-2
Pb Flux (Bq m )
7
0
0 Feb10 Mar10 Apr10 May10 Jul10 Aug10 Sep10 Oct10 Nov10 Dec10 Jan11
Month/Year 210
Fig. 4. Monthly Pb depositional flux and precipitation amount (February 2010 to January 2011) in Cienfuegos, Cuba.
6 5 4 3 2 1 0 0
20
40
60 7
80 -2
100
120
140
-1
Be flux (Bq m month )
Fig. 5. Monthly depositional flux of Cienfuegos, Cuba.
210
Pb versus monthly depositional flux of 7Be in
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
153
450
0,14
400
0,12 0,10 0,08
200
0,06
7
250
Pb/ Be
300
210
Monthly rain (mm)
350
150 0,04 100 0,02
50
Ja n1 1
ov 10
10 ct
ec 10 D
N
O
Se p1 0
Ju l1 0 Au g1 0
Ap r1 0 M ay 10
0,00 M ar 10
Fe b1 0
0
Month/Year Fig. 6. Relationship between the monthly deposition of 7Be and tion at Cienfuegos.
210
Pb and precipitaFig. 8. Relationships between
Swarzenski, 2007). The relatively low variation in the depositional flux of 210Pb at our study site is likely due to the source of air masses. Due to the low exhalation rates of 222Rn over the ocean as compared to continental areas, oceanic air masses are typically depleted in 222Rn and its daughter-products, including 210Pb; as a consequence, the standing crop of 210Pb in the atmosphere has a strong longitudinal dependency (Turekian et al., 1977). However, 7 Be is of cosmogenic origin, and its flux on the Earth's surface has a latitudinal dependency, and hence its atmospheric flux on the Earth's surface should be independent of geography, at any given latitude. Baskaran and Swarzenski (2007) reported for Florida a variation factor of 14 for 7Be, similar to the 10 obtained in this study. 3.2. Relationship between rain and fluxes of Cienfuegos
210
Pb and 7Be in
The monthly 210Pb and 7Be deposition fluxes versus amount of rain are plotted in Fig. 6. In the full study period, there is a significant correlation between 210Pb, 7Be fluxes and precipitations (r ¼ 0.62 and r ¼ 0.83, respectively). Similar values have been reported by others authors in coastal stations (Baskaran et al., 1993; Kim et al., 2000). If data were processed at shorter temporal scale rather than at annual level, it would show a better correlation between activity flux and precipitations (Fig. 7). It was found that the
Fig. 7. The correlation between flux of
210
210
Pb/7Be ratio and amount of precipitations.
correlations coefficients between precipitation and flux were 0.30 and 0.98 for 210Pb, 0.79 and 0.88 for 7Be in the months ranging from April to October and November to March, respectively. In the months, between November and March, there is a high correlation between activity flux of both nuclides and precipitation. Also a significant correlation is found between 7Be flux and precipitations in the months between April and October (0.79). These results indicate that the scavenge process of two nuclides in the low atmosphere have similar mechanism. The dominant factor of scavenge is precipitation for 210Pb and 7Be in the months between November and March, while in the months ranging from April to October 2010 the influence of precipitation is relevant only for the scavenge process of 7Be. However, there is a clear need of getting data on longer period (several years) to really evaluate the role of precipitation regime. 3.3. The ratio of activity flux of
210
Pb and 7Be
The ratios of 210Pb/7Be deposition fluxes and amount of precipitations in the sampling period are shown in Fig. 8. The variation range for the 210Pb/7Be ratio was 0.05e0.10 with an average of 0.08 (SD ¼ 0.02). This value is similar to the one reported by Baskaran et al. (1993) and Saari et al. (2010) and the 0.08 reported as world average (Du et al., 2008). In general, the 210Pb/7Be ratio decreases in
Pb and 7Be and precipitation at Cienfuegos for Apr.-Oct. and Nov.-Mar periods.
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
154 25
0,14
0,08 0,06
10
7
15
Pb/ Be
0,10
210
Number of rainy days
0,12 20
0,04 5 0,02
Ja n1 1
ec 10 D
ov 10
10 ct
N
O
Se p1 0
Ju l1 0 Au g1 0
Ap r1 0 M ay 10
0,00 M ar 10
Fe b1 0
0
Month/Year Fig. 9. Relationships between
210
Pb/7Be ratio and number of rainy days per months.
the months of highest precipitation and increases in the months of lowest precipitation. However, in January, February and March this behaviour was not observed and a poor correlation (r ¼ 0.58) between 210Pb/7Be ratio and precipitation was found. If we plotted the 210Pb/7Be ratio versus the number of rainy days, as seen in Fig. 9, a significant correlation (r ¼ 0.71) between the 210Pb/7Be ratio and precipitation would be seen. We attribute this result to the fact that in the months with more rainy days, the emanation of 222Rn from land surface decreases due to the saturation effect therefore decreases the 210Pb concentration in the atmosphere. The effect of moisture content on radon emanation has been reported on extensively (Barton and Ziemer, 1986; Menetrez et al., 1997; Schumann and Gundersen, 1997; Sun and Furbish, 1995). The 7Be concentration in the air is not affected by this factor; therefore the 210Pb/7Be ratio decreases during these months. 3.4. Annual bulk depositional fluxes of
210
Locations
(Ugur et al., 2011) (Beks et al., 1998) (Du et al., 2008) (Ioannidou et al., 2005)
Acknowledgements
210
Pb and 7Be
Pb and 7Be on various coastal stations in the world. Period
210
7
Pb
Be
2
1
Reference
(Bq m
y
Odawa, Japan Tsukuba, Japan Geneva, Switzerland Bordeaux, France Tampa, USA
2000e2003 1990 1991 2006 1990 1991 1997e1998 2001 1997e1998 2006e2007 2003e2004
48* 47 57 479 e e 73 182 150 103 123
e e e 2070 733 712 e 918 2087 1256 1468
Cienfuegos, Cuba
2010e2011
47
700
Izmir, Turkey Groningen, Holland Shanghai, China Thessaloniki, Greece
*In rainwater.
4. Conclusions We have evaluated for first time in Cuba the atmospheric deposition fluxes for 7Be and 210Pb obtained over a 11 months period (February. 2010 to January 2011) in Cienfuegos. This preliminary study provides a baseline data and will be helpful in further investigations at this site. The annual average flux of 210Pb and 7Be were 47 and 700 Bq m2 y1, respectively. The depositional fluxes of 7Be and 210Pb are strongly correlated; indicating that removal behaviour from the atmosphere is relatively similar. The depositional fluxes of radionuclides on this coastal site (Cienfuegos) have relatively low values, in particular 210Pb, probably reflecting significant input of marine air. A positive correlation has been found between the deposition fluxes, the amount of precipitation and the number of wet days. There is a statistical relationship between 7Be and 210Pb deposition fluxes and the amount of precipitation. This study suggests that continuous monitoring of radioelement fluxes is necessary to examine both episodic and long-term changes in annual and seasonal atmospheric fluxes. The option of having some additional permanent coastal sampling stations, on the Caribbean Sea, is desirable.
Despite it is necessary to obtain more data (at least 10 years), we try to estimate values of annual bulk depositional fluxes of 210Pb and 7Be in Cienfuegos between February 2010 and January 2011, calculated on 347 days of sampling (taking June out), were 47 and 700 Bq m2 y1, respectively. The preliminary annual 210Pb and 7Be atmospherics fluxes in Cienfuegos are compared in Table 2 with
Table 2 Depositional fluxes of
other coastal stations worldwide. The annual bulk depositional flux for 210Pb in Cienfuegos (47 Bq m2 y1) is at a relatively low value, which indicates a significant oceanic influence. This value is distinctly lower than other coastal stations, such as Shanghai, China, where a value of 479 Bq m2 y1 was reported (Du et al., 2008). In this case, a major component of the air masses above Shanghai is derived from the adjacent Asian continent. In contrast, Beks et al. (1998)(Beks et al., 1998) reported for the Netherlands similar values (47 and 57 Bq m2 y1) to the ones obtained for Cienfuegos in this study, indicating a major oceanic influence. Besides, in recent study others authors have been reporting similar values for 210Pb fluxes. In Izmir, Turkey, Ugur et al. (2010) reported a value of 48 Bq m2 y1. The annual 7Be depositional flux (700 Bq m2 y1) obtained in Cienfuegos is in the range of the inter-annual variation of the 7Be annual deposition in the mid-latitude region (458e3833 Bq m2 y1), in the 1987 to 2006 period (Baskaran et al., 1993; Caillet S. et al., 2001; Du et al., 2008, 2010; Hirose et al., 2004; Ioannidou et al., 2005; Saari et al., 2010; Yamamoto et al., 2006). However, the 7Be depositional flux in Cienfuegos is one of the lowest values. This result is likely due to the dilution effect, as explained by Baskaran and Swarzenski (2007) for a geographical area close to the sea. During the summer months, the source of water vapour for afternoon thunderstorms is generally from the ocean and the subsequent precipitation may occur within a few hours. The residence time of water vapour in this region during summer months is very short. Due to the occurrence of almost daily summer thunderstorms, the atmosphere is effectively flushed and thus the 7Be activities cannot build up. Lateral movement of marine air masses coupled with the vertical movement of water vapour towards the cloud condensation could result in the relatively lower activities of 7 Be in the air.
)
(Tateda and Iwao, 2008) (Hirose et al., 2004) (Caillet et al., 2001) (Saari et al., 2010) (Baskaran and Swarzenski, 2007) This work
This research work was undertaken in the framework of the IAEA TC Project CUB/7/008 ‘‘Strengthening the National System for Analysis of the Risks and Vulnerability of Cuba's Coastal Zone through the Application of Nuclear and Isotopic Techniques'’. Special gratitude goes to Jones Catherine for her great help during the elaboration of the manuscript and the two anonymous reviewers for their comments and suggestions.
ndez et al. / Journal of Environmental Radioactivity 138 (2014) 149e155 C.M. Alonso-Herna
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