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Depositional fluxes and concentrations of 7 Be and 210 Pb in bulk precipitation and aerosols at the interface of Atlantic and Mediterranean coasts in Spain

2011; American Geophysical Union; Volume: 116; Issue: D18 Linguagem: Inglês

10.1029/2011jd015675

2156-2202

R.L. Lozano, E.G. San Miguel, J.P. Bolı́var, M. Baskaran,

Groundwater and Isotope Geochemistry

Journal of Geophysical Research: AtmospheresVolume 116, Issue D18 Aerosol and CloudsFree Access Depositional fluxes and concentrations of 7Be and 210Pb in bulk precipitation and aerosols at the interface of Atlantic and Mediterranean coasts in Spain R. L. Lozano, R. L. Lozano Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorE. G. San Miguel, E. G. San Miguel Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorJ. P. Bolívar, J. P. Bolívar Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorM. Baskaran, M. Baskaran baskaran@wayne.edu Department of Geology, Wayne State University, Detroit, Michigan, USASearch for more papers by this author R. L. Lozano, R. L. Lozano Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorE. G. San Miguel, E. G. San Miguel Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorJ. P. Bolívar, J. P. Bolívar Department of Applied Physics, University of Huelva, Huelva, SpainSearch for more papers by this authorM. Baskaran, M. Baskaran baskaran@wayne.edu Department of Geology, Wayne State University, Detroit, Michigan, USASearch for more papers by this author First published: 30 September 2011 https://doi.org/10.1029/2011JD015675Citations: 49AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] Bulk depositional fluxes of 7Be and 210Pb in precipitation measured over a period of 16 months (April 2009–July 2010) in Huelva, Spain varied between 5.6 and 186 Bq m−2 month−1 (annual mean: 834 Bq m−2 year−1) and 0.8 and 8.1 Bq m−2 month−1 (annual mean: 59 Bq m−2 year−1), respectively, with the lowest depositional fluxes occurring during dry summer months. Quantitative evaluation of the precipitation-normalized seasonal depositional fluxes of 7Be and 210Pb indicates that the enrichment factor in winter is < 1.0 while in 2010 spring, it is significantly higher than 1, possibly indicating input of air from the stratosphere-troposphere exchange (for 7Be). The specific activities of 7Be and 210Pb varied from 0.03 to 7.42 Bq L−1 (mean = 2.5 Bq L−1) and 0.005 to 1.07 BqL−1 (mean = 0.23 Bq L−1), respectively, with the highest values corresponding to the spring season. The spatial and temporal variations of 7Be and 210Pb in aerosols from three stations are evaluated and compared to their monthly depositional fluxes. The mean depositional velocity of aerosols using 7Be and 210Pb are similar, ∼0.5 cm s−1 and are compared to other published values. This is the first time the fractional amounts of 7Be and 210Pb in monthly bulk precipitation are compared to the fractional amount of precipitation and provides insight on how the amount of precipitation plays a key role on the scavenging of these nuclides. The importance of dry fallout is evaluated for the study site which has direct implications for other areas in the Mediterranean Climate Zone. Key Points Quantitative evaluation of Be-7 and Pb-210 with precipitation is done Effects of air masses on the depositonal flux are evaluated Global data are synthesized 1. Introduction [2] Utility of 7Be (half-life = 53.3 days) and 210Pb (half-life = 22.1 yrs) as tracers for quantifying sediment dynamics in estuarine, coastal, lacustrine and riverine systems, soil erosion studies and in other environmental studies require the depositional fluxes of these nuclides [Robbins, 1978; Krishnaswami et al., 1980; Dominik et al., 1989; Baskaran et al., 1997; Sommerfield et al., 1999; García-Orellana et al., 2006; Church and Sarin, 2008; Jweda et al., 2008; Kaste and Baskaran, 2011; Matisoff and Whiting, 2011]. The variations in the concentrations of 7Be and 210Pb in aerosols have been modeled using modified versions of the global circulation models to determine the vertical transport and residence times of aerosols [Turekian et al., 1977; Brost et al., 1991; Balkanski et al., 1993; Koch et al., 1996]. These two radionuclides have distinctly different source functions. 7Be is produced in the upper and middle atmosphere (5–30 km) as a product of spallation of oxygen and nitrogen nuclide by energetic cosmic rays [Lal et al., 1958]. Due to its short mean life (77 days) and longer residence time of aerosols in the stratosphere (about 1–2 years), most of the 7Be produced in the stratosphere does not readily reach the troposphere, except when there is a seasonal thinning of the tropopause taking place at midlatitudes, resulting in transport of 7Be-containing aerosols into the troposphere. Like any other cosmogenic-nuclide, its flux to the Earth's surface is latitude-dependent. Due to its source, its concentration is expected to increase with height above the planetary boundary layer (PBL). During summer months, the air mass at the PBL gets heated and the lower-density air mass depleted in 7Be starts rising while it is replaced by sinking colder air mass enriched in 7Be from aloft. The flux of 7Be is expected to be independent of longitude. [3] 210Pb is produced in the atmosphere by radioactive decay from its progenitor, 222Rn [e.g., Poet et al., 1972; Tokieda et al., 1996]. It has been estimated that about 1 to 10% of the 222Rn produced from the decay of 226Ra in the upper one meter of the soil is released into the atmosphere. The average concentration of 226Ra in the upper crust is 31 Bq kg−1 while the concentration in surface ocean water is ∼1.3 mBq kg−1. It has been estimated that the global 222Rn flux from continent ranges from 1300 to 1800 Bq m−2 d−1, but ∼17 mBq m−2 d−1 from oceanic areas [Samuelsson et al., 1986; Nazaroff, 1992; Baskaran, 2010]. As a consequence, the standing crop of 210Pb in the atmosphere strongly depends on the longitude, depending on whether it is above the ocean or a continent. After its production in the atmosphere, it is removed primarily by precipitation (both rain and snow) and dry deposition processes. In coastal areas, the depositional fluxes of 210Pb are expected to depend on the relative portion of the continental and maritime air masses. Furthermore, the 222Rn emanation depends on several other factors, including concentration and distribution of 226Ra in the mineral grains, the porosity of the soils, the water content, among others. Combining these two radionuclides that have two different source functions, one can obtain information on the mechanisms and rates of removal of aerosols in the atmosphere, including chemical species injected in to the troposphere, elevation at which cloud condensation takes place, and source(s) of air masses. [4] The location of Huelva province at the edge of European Continent makes it as a suitable station for the measurement of 210Pb and 7Be in surface air under the influence of air masses of different origins. In particular, the present study intend to quantify the role of different air masses, in this case continental versus maritime (Atlantic and Mediterranean), on the temporal variability of the concentrations in the air and depositional fluxes of these nuclides in this area. Furthermore, scanty rainfall in the summer months will aid in quantifying the role of dry deposition of aerosols at the study site, and by analogy other sites in the Mediterranean climate zones. 2. Materials and Methods 2.1. Area of Study [5] The province of Huelva is located in the Iberian Peninsula, at halfway along the southwestern coast of Spain (Figure 1). The Tinto and Odiel Rivers cross part of the province and flow from the north to the south. In the north of the province (80 km far from the coastline) are the Sierra Morena Mountains, with an altitude of 940 m. Furthermore, there are three chemical industrial complexes in the surroundings of the city of Huelva (Figure 1) which started operating after the extensive industrialization process that began in the year 1960. Due to the wide spectrum of types of emission sources near the industrial complex of the city of Huelva and the coastal location, a study of air pollution is of great interest in this region. Figure 1Open in figure viewerPowerPoint Station locations. [6] Monitoring network sampling formed by three stations was utilized for aerosol sampling: 1) “El Arenosillo,” 2) “El Carmen” in Huelva town, and 3) “La Rábida” (Figure 1). El Arenosillo station (37°05′58″N, 6°44′15″W) is located in a fairly flat area away from industrial activities and is considered to be a reference (background) site. This site is located inside a pine forest in the Atmospheric Sounding Station, which belongs to the National Institute of Aerospatiale Technology, and it is at ∼1 km distance from the coastline and 35 km southeast of the city of Huelva, and 5 km from the National Park of Doñana. Air samples were collected from about 6 m above ground. The second sampling station was chosen at an urban site, at the border of the city of Huelva (population ∼150,000). Aerosol samples were collected from ∼10 m above the ground, at the El Carmen campus of the University of Huelva (37°16′07″ N, 6°55′27″ W). In addition, on the roof of Huelva station four drums were deployed for the collection of the bulk deposition samples. The third sampling station is sited at “La Rábida” at the campus of the University of Huelva, (37°11′60″N, 6°55′6″W), and close to the chemical industrial complex. Aerosol samples were collected in this station at ∼10 m above the ground. 2.2. Samplings and Radionuclide Measurements [7] Aerosol samples (PM10, < 10 μm) were collected onto quartz fiber filters (QF20 Schleicher & Schuell, 25.4 cm × 20.3 cm) with high volume samplers at a flow rate of 40 cfm (68 m3 h−1) in each of the three stations. Samples were collected for 48 h every fifteen days, from July 2004 to April 2010. The filters were weighed before aerosol collection (precision 0.1 mg) several times until mass remained constant, inside a chamber with controlled humidity. In order to evaluate the collection efficiency of the filters, we put two filters on the inlet of the sampler. The 7Be and 210Pb concentrations in the second filter were below the minimum detectable activity (MDA), which are 0.10 mBq m−3 and 0.14 mBq m−3, respectively. Taking these detection limits into account, a retention efficiency of > 96% is estimated for these filters. This is in contrast with the earlier studies where collection efficiency of Whatman 41 filters for 210Pb was found in the range of 70–80%, with ∼25% of the 210Pb in the first filter was found in the second filter (back-up filter) [Stafford and Ettinger, 1972; Turekian and Cochran, 1981; Turekian et al., 1989]. The reasons of these discrepancies are not clear, but could be due to differences in the sizes of aerosols (PM10 versus PM1 by earlier groups) or the differences in the composition of the filters used to collect aerosols. [8] Total deposition samples (also called bulk) were collected using 4 drums with 200 L of total capacity, and a total surface area of 4120 ± 30 cm2 (1030 cm2 each), from April 2009 up to July 2010. The precipitation record was obtained from a meteorological station close to the sampling site. The drums were cleaned three times by repeated washings with 10% HNO3 at the beginning of the bulk deposition collection. Prior to each deployment, 1 mg of stable Be and Pb in 100 mL of 10% HNO3 were added into the drums. The addition of acid is to prevent adsorption of particle-reactive radionuclides onto the polyethylene drum surfaces. At the end of the collection period, the collectors were emptied into a container; and their walls were rinsed twice with 50 mL of 8 M HNO3 (each), which were then added to the sample. In the summer months, the solution completely evaporated and we rinsed the rain collector in the same way as described above. Following this thorough acid cleaning and a distilled water rinsing, the collectors were deployed again after the addition of dilute acid and stable Pb and Be carriers. The sample is evaporated to about 50 mL and then to dryness. Then, it is dissolved with a mixture of strong acids (5 mL of 65% HNO3, 2 mL of 37% HCl, 15 mL of 40% HF), and finally the residue was re-dissolved in dilute 5% HNO3 (5 mL) and the solution was quantitatively transferred into a 5-mL gamma spectrometry vials. We assumed that there is no loss of 7Be and 210Pb (due to repeated rinsing with strong acids as given above) during the collection, evaporation and in the transfer of the solution to the counting vials. The vials are counted on a high-purity Ge well detector coupled to a multichannel analyzer. The MDA for 7Be and 210Pb are 0.06 Bq and 0.09 Bq, respectively. [9] On the other hand, the aerosol filters were cut in two halves, and both were weighed (0.1 mg precision). One half is wrapped, pressed in a plastic bag and measured by gamma spectrometry. The other half is totally dissolved with a mixture of strong acids (65% HNO3, 37% HCl, 40% HF), and the final solution was used for the determination of 210Po and other alpha emitting U and Th-isotopes (the results are not discussed in this paper). [10] Typically, the samples were counted by gamma spectrometry for about 48 h, depending on the activity of 2l0Pb in the sample, since all of the samples have relatively high activities of 7Be. The detectors with its shielding are located in a room with walls and ceiling made of 75 cm thick concrete at the basement of a four-story building. [11] Beryllium-7 and 210Pb concentrations in bulk deposition samples were quantified using a well Ge detector (Canberra), that has a full-width at half-maximum (FWHM) of 1.33 keV at 122 keV (57Co) and 2.04 keV at 1332 keV (60Co), and a peak/Compton ratio of 56.2/1. The detector was coupled to a multichannel analyzer and the detector was shielded with 10 cm lead shield. In order to avoid interferences from X-rays from the Pb in the shield, a layer of 2 mm thick of Cu layer is placed as a shield between the Pb shield and the detector. The full energy peak efficiency calibration for the peaks of interest was conducted using the solid RGU-1 Standard Reference Material, obtained from IAEA, with a known amount of 238U (4940 ± 30 Bq kg−1, and with all its daughter products in secular equilibrium). Absorption corrections were made for the measurements of 210Pb or 7Be, following the method of Appleby et al. [1992]. Absolute efficiencies for the standard geometry of the samples used in this study for 210Pb and 7Be were calculated to be about 46% and 13%, respectively. [12] Lead-210 and 7Be measurements in atmospheric filters were performed using an XtRa coaxial Ge detector (Canberra), with 38% relative efficiency and FWHM of 0.95 keV at the 122 keV (57Co) and 1.9 keV at the 1333 keV (60Co). The detector is coupled to a set of standard electronics components, including a multichannel analyzer, and was shielded with 15 cm thick Fe shield. The peak analysis of 7Be (branching ratio (BR) = 10.3%, Eγ = 477.7 keV) and 210Pb (BR) = 4.05%, Eγ = 46.5 keV) was conducted. Details on the efficiency calibration for 210Pb (counting efficiency 14.3%) and 7Be (counting efficiency 3.7%) determination in air filters are given elsewhere [Martínez-Ruiz et al., 2007]. [13] The atmospheric flux of 7Be and 210Pb are calculated using the expression where A is the activity in the sample (Bq), S is the surface area of the collector and t is the duration of deployment (months). The activity of the sample is obtained from the gamma spectra. 3. Results and Discussion 3.1. Variations of Monthly Precipitation and Bulk Depositional Fluxes of 7Be and 210Pb [14] The amount of rainfall, collection interval and depositional fluxes of 7Be and 210Pb for the 16 monthly bulk deposition (wet + dry) samples collected between April 2009 and July 2010 are given in Table 1. The monthly amount of precipitation is plotted in Figure 2. The year 2009 was the wettest in the last 40 years. The historic annual average rainfall in Huelva is about 550 L m−2 (=550 mm/yr), while for 12 months (April 2009 to March 2010), the total rainfall was ∼103 L m−2 (1000 mm). About 80–90% of the precipitation is confined to October–April, with very little rainfall in the remaining months. During this sampling period, the highest amount of precipitation was observed in December, January and February (∼200 mm/month, with 69.3% of the total in three months), although this amount may vary from October to February. There are many other climate regions similar to coastal Mediterranean climate region (e.g., San Francisco, Los Angeles in USA). The amount of precipitation in these places also is negligible in the warmer months (June, July, August, and September) (Figure 2). Taking into account Figure 2 and the specific activities (Bq L−1) shown in Table 1, it can be seen the dilution effect of the rainfall: when a high amount of precipitation occurs, the specific activities are lower than in periods with less precipitation. Figure 2Open in figure viewerPowerPoint Monthly precipitation in Huelva during the sampling period (April 2009 to July 2010). Table 1. Sampling Time, Amount of Precipitation, Specific Concentrations and Bulk Depositional Fluxes of 7Be and 210Pb in Huelva (Spain) From April 2009 to August 2010aa Flux and washout ratios for 7Be and 210Pb are also shown. Date In Date End Days in Collectionbb Number of rain days given in parentheses. Rainfall (mm) 7Be (Bq L−1) 210Pb (Bq L−1) 7Be/210Pb 7Be Flux (Bq m−2 month−1) 210Pb Flux (Bq m−2 month−1) 210Pb Velocity (cm s−1) 7Be Velocity cm (s−1) 210Pb Washout Ratio 7Be Washout Ratio DA 1 18/04/2009 27/04/2009 9 (3) 39 0.03 0.005 5.14 27.5 ± 1.4 5.3 ± 0.4 0.91 0.25 28 8 DA 2 27/04/2009 15/05/2009 18 (2) 2.4 0.82 0.065 12.50 25.9 ± 1.1 2.1 ± 0.2 0.09 0.14 90 139 DA 3 15/05/2009 25/06/2009 41 (4) 1.2 7.42 1.068 6.95 13.8 ± 0.9 2.0 ± 0.3 0.20 0.07 3285 1203 DA 4 25/06/2009 31/07/2009 36 (0) 0 — — 5.46 5.6 ± 0.6 1.0 ± 0.1 0.25 0.05 DA 5 31/07/2009 01/09/2009 32 (0) 0 — — 6.79 9.6 ± 0.3 1.4 ± 0.1 0.06 0.06 DA 6 01/09/2009 01/10/2009 30 (3) 10.2 3.44 0.259 13.28 75 ± 5 5.6 ± 0.5 0.28 0.41 401 583 DA 7 01/10/2009 04/11/2009 34 (12) 25.1 4.52 0.318 14.21 114 ± 4 8.0 ± 0.3 0.63 1.30 775 1604 DA 8 04/11/2009 01/12/2009 27 (16) 28.8 1.33 0.093 14.40 42.7 ± 1.3 3.0 ± 0.1 0.11 0.36 107 351 DA 9 01/12/2009 22/12/2009 21 (14) 202 0.13 0.007 19.04 50.4 ± 2.0 2.6 ± 0.2 0.64 0.54 52 44 DA 10 03/01/2010 01/02/2010 29 (16) 196 0.40 0.034 11.99 97 ± 3 8.1 ± 0.3 1.33 0.52 171 67 DA 11 01/02/2010 01/03/2010 28 (21) 227 0.04 0.003 13.19 11.1 ± 0.4 0.8 ± 0.1 0.26 0.12 30 13 DA 12 01/03/2010 01/04/2010 31 (15) 109 1.59 0.099 16.04 174 ± 5 10.9 ± 0.4 0.78 0.97 221 277 DA 13 01/04/2010 01/05/2010 30 (8) 42.3 3.93 0.169 23.31 186 ± 6 8.0 ± 0.4 1.09 1.77 719 1162 DA 14 01/05/2010 01/06/2010 31 (5) 3.6 7.42 0.630 11.78 39.1 ± 1.8 3.3 ± 0.3 — — DA 15 01/06/2010 01/07/2010 30 (3) 15 2.04 0.183 11.17 30.7 ± 0.9 2.7 ± 0.2 — — DA 16 01/07/2010 01/08/2010 31 (0) 0 — — 5.36 20.4 ± 0.6 3.8 ± 0.2 — — Range (0.06–1.33) (0.05–1.77) 28–3285 8–1604 Mean 0.51 ± 0.12 0.50 ± 0.15 534 ± 300 496 ± 180 a Flux and washout ratios for 7Be and 210Pb are also shown. b Number of rain days given in parentheses. [15] The monthly depositional flux of 7Be ranges from 5.6 to 186 Bq m−2 month−1 (Table 1) with the lowest deposition occurring during the summer months (June and July) when there is no precipitation. The depositional flux during these two months is completely due to dry deposition. The average dry depositional flux for these months is 7.5 Bq m−2 month−1. If we assume that the dry deposition has remained constant throughout the year, the dry deposition would account for 11% of the bulk fallout. It is likely that this is an overestimate, because the precipitation will efficiently strip-off the aerosols and hence the dry fallout in rainy months is likely much less than the dry months. The dry depositional flux is spatially and temporally variable. For example, the dry fallout in Galveston, Texas varied between 3.0 and 7.4% of the bulk depositional flux in October 1990, but it accounted for ∼87% during October 1991 and this was attributed to very low rainfall [Baskaran et al., 1993]. This value of 11% is in agreement with values obtained in others studies at similar latitudes [Olsen et al., 1985; Todd et al., 1989; McNeary and Baskaran, 2003]. [16] The monthly depositional flux of 210Pb varied between 0.8 and 10.9 Bq m−2 month−1. The average dry depositional flux of 210Pb during dry months is 1.2 Bq m−2 month−1. If we assume that the dry deposition has remained constant throughout the year, the dry deposition would account for 24% of the bulk fallout. Benitez-Nelson and Buesseler [1999] reported that dry deposition accounts for less than 1% of the 7Be flux, but accounts for 12% of the 210Pb flux. In our study, the dry deposition for 210Pb is significantly higher than that of 7Be and the differences are attributed to the differences in their sources and their half-lives. The expected inventories 210Pb in the upper 2–3 cm of the soil layer are expected to be much higher than that of 7Be. When soil dust from the upper 2–3 cm layer is resuspended, the dust is expected to have much higher specific activity (Bq/gram of resuspended dust) of 210Pb than 7Be, and thus we expect higher dry fallout of 210Pb compared to 7Be. [17] The lowest deposition was found during the summer months when there is little rain (or there is no precipitation). This is reflected in Figure 3, with a strong correlation between monthly depositional flux of 210Pb and monthly depositional flux of 7Be (r = 0.96). Lead-210 and 7Be in the atmosphere have distinct sources due to different modes of their production. This high correlation between these two radionuclides suggests that both radionuclides cannot be used as independent atmospheric tracers. Even in island locations such as Bermuda, strong correlation between these nuclides was found [Kim et al., 1999] and was attributed to major transport of 210Pb via the upper troposphere from continents. From all the earlier published data before 1993 from different regions around the world, it was shown that there was a strong correlation between the fluxes of 7Be and 210Pb and thus, 7Be and 210Pb could not be used as two – independent atmospheric tracers [Baskaran et al., 1993]. The possibility of fractionation between 7Be and 210Pb scavenging in the atmosphere depending on the intensity of washout with altitude does exist, although it is not proven yet [Kim et al., 2000; Church and Sarin, 2008]. Figure 3Open in figure viewerPowerPoint Monthly depositional flux of 7Be versus monthly depositional flux of 210Pb in Huelva, Spain. [18] The activity ratios of 7Be/210Pb (Table 1) ranged from 5.1 to 23.3, (mean = 11.9 ± 1.3), though a majority of the samples (13 out of 16) remains fairly constant and fall within two ranges, one 5 to 7 and other 11 to 15. It is interesting to note that the ratios tend to decrease in summer months, although there is no clear trend. Due to absence of precipitation during summer months, the fine soil particulate matter containing much higher specific activity of 210Pb compared to 7Be can be easily resuspended and as a consequence may result in lower 7Be/210Pb ratios. This observation is similar to results reported in other studies, although very low summer precipitation is unique in this study [McNeary and Baskaran, 2003; Hirose et al. 2004; Dueñas et al., 2005]. 3.2. Relationship Between Seasonal Variations in the Depositional Fluxes and Amount of Precipitation [19] In order to quantify the importance of the amount of precipitation on the depositional fluxes of these radionuclides, we have calculated the fractional amounts of precipitation and depositional fluxes of 7Be and 210Pb for the sixteen month sampling period (April 2009 – July 2010). The fractional amount of precipitation and depositional fluxes of 7Be and 210Pb for five seasons from April 2009 until July 2010 are given in Table 2. Table 2. Seasonal Amount of Precipitation, Depositional Fluxes of 7Be and 210Pb, Normalized Enrichment Factors for 7Be and 210Pb, and 7Be/210Pb Activity Ratios From April 2009 to July 2010 Parameter Spring 2009 Summer 2009 Autumn 2009 Winter 2009/10 Spring 2010 Summer 2010 Precipitation mm 61.4 1.2 64.1 626.4 155.4 15 7Be Fallout Bq cm−2 0.16 0.087 0.695 0.477 1.198 0.153 7Be (α) 0.87 24.13 3.61 0.25 2.57 3.40 210Pb Fallout Bq cm−2 0.022 0.013 0.050 0.035 0.062 0.005 210Pb (α) 1.79 54.61 3.84 0.28 1.97 1.55 7Be/210Pb 8.2 8.5 15.9 13.7 15.4 5.4 [20] From Table 2, it is possible to infer the following: i) The seasonal 7Be flux increased by a factor ∼7 from spring 2009 to spring 2010, although the amount of precipitation increased only by a factor ∼2.5; ii) lowest depositional fluxes of 7Be and 210Pb were consistently found in the summer months due to very low amount of precipitation; and iii) highest depositional fluxes of both 7Be and 210Pb were found in spring 2010, although the highest rainfall was found during winter 2009/2010. The increase in 7Be is likely due to input from stratosphere-troposphere exchange while the increase in 210Pb could be due to higher release rates of 222Rn from the continents due to warmer temperatures in spring compared to winter and higher inputs of land-derived air masses that are elevated in 210Pb concentrations. [21] Due to troposphere–stratosphere exchange of air masses in midlatitudes during spring, a certain amount of air is exchanged between the troposphere and stratosphere. In the case of 7Be, the two key factors that control the depositional fluxes of 7Be at any site are: (1) The amount and frequency of precipitation and (2) the amount of 7Be derived from stratosphere due to vertical mixing. During summer months, the atmosphere is quite dynamic and hence large scale atmospheric mixing takes place. During vertical mixing, the 210Pb is mixed upward to the middle and upper troposphere due to convective mixing where precipitation scavenging is much less, leading to a decrease in the depositional flux of 210Pb. [22] The precipitation-normalized enrichment factor (α) is defined as [Baskaran, 1995] where Rs and Rt are the amount of rainfall during a particular season (“s”) and in one year (“t” = total) respectively, and Fs and Ft are the corresponding depositional fluxes of 7Be and 210Pb in that particular season and year, respectively. Alpha values greater than 1 indicate that the depositional fluxes are higher than expected from the amount of rainfall, and values less than 1 indicate depletion of radionuclide fluxes. [23] In Figures 4a and 4b, it is observed that when rainfall is scarce there is enrichment for both 7Be and for 210Pb (higher concentration). This enrichment is observed in summer months, when the precipitation is nearly zero. Using equation (2), the alpha values can be calculated for each month. The range of alpha values for 7Be for each month varied between 0.05 and 12.5, while for 210Pb it varied between 0.06 and 25.4. In both cases the maximum alpha values are observed in June 2009 and the minimum in February 2010. This peak in June 2009 (maximum in summer, Table 2) is likely due to intrusion of air from the stratosphere to troposphere as well as the dominance of dry fallout during summer. The minimum in alpha values occur during February 2010, in which there are more rainy days (21), and amount of precipitation (227 mm) is maximum throughout the sampling period. Similar behavior for 7Be and 210Pb indicates that the main mechanism of deposition is the wet fallout. Furthermore, it can be seen that the ratio of maximum to minimum alpha for 210Pb (195) is about twice the ratio of maximum to minimum alpha for 7Be (95). This fact is consistent with higher dry deposition for 210Pb (24%) compared to 7Be (11%). Figure 4Open in figure viewerPowerPoint The precipitation-normalized enrichment factor (α, calculated using equation (1)) for monthly (a) 7Be and (b) 210Pb. 3.3. Variations in the Specific Activity of 7Be and 210Pb With Amounts of Rainfall [24] The variations in the activity concentrations (Bq L−1) of 7Be and 210Pb in the bulk deposition samples may be due to several factors such as cloud height, the amount and duration of rainfall, the time elapsed between successive rain events and the vertical mixing of air masses at the sampling site [McNeary and Baskaran, 2003; Dueñas et al. 2005]. Results show a range of values for 7Be activity concentrations between 0.03 Bq L−1 and 7.42 Bq L−1 (Table 1), with a mean value of 2.5 Bq L−1. The corresponding values for 210Pb varied between 0.003 Bq L−1 and 1.07 Bq L−1, with mean value of 0.23 Bq L−1. Values of the specific activity concentrations of 7Be and 210Pb are comparable to those reported in other places in the Mediterranean coast in Europe [Ródenas et al., 1997; Dueñas et al., 2005; García-Orellana et al., 2006; Baskaran, 2010]. Th