seasonal variability in 7be depositional fluxes at granada, spain
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Applied Radiation and Isotopes 64 (2006) 228–234
www.elsevier.com/locate/apradiso
Seasonal variability in 7Be depositional fluxesat Granada, Spain
C. Gonzalez-Gomeza,b, M. Azahraa,d,�, J.J. Lopez-Penalvera,A. Camacho-Garcıac, T.El. Bardounid, H. Boukhald
aFacultad de Ciencias, Laboratorio de Radioquımica y Radiologıa Ambiental, Universidad de Granada, Av. Fuentenueva,
E-18071 Granada, SpainbInstituto Andaluz de Ciencias de la Tierra, CSIC, Facultad de Ciencias, E-18071 Granada, Spain
cInstituto de Tecnicas Energeticas, Universidad Politecnica de Cataluna, Avda. Diagonal, 647, E-08028 Barcelona, SpaindUniversite Abdelmalek Essaadi, Faculte des Sciences, Departement de Physique, BP 2121, M’hanech II, Tetouan, Maroc
Received 2 May 2003; received in revised form 29 April 2005; accepted 6 May 2005
Abstract
Measurement of 7Be depositional fluxes at Granada, Spain (3711005000N–313504400W, altitude 670m) in the period
1995 through 1998 indicates substantial variations between the four seasons and also between corresponding seasons in
different years, ranging from 23.6 to 242Bqm�2 per season. A strongly positive correlation with precipitation is shown,
which explains about 70% of the variations in the 7Be depositional fluxes over the 16 seasons studied. The
depositional 7Be flux is on the average highest in the fall and lowest in the summer. The study shows that precipitation
primarily controls the 7Be depositional flux and plays a dominant role in the removal of 7Be from the troposphere.
The average annual 7Be depositional flux at Granada amounts to 469+145Bqm�2.
r 2005 Elsevier Ltd. All rights reserved.
1. Introduction
The accident in the Chernobyl nuclear power station
on 26 April 1986 demonstrated the need for different
countries, through official institutions, to establish and
to maintain an environmental radiological surveillance
network. In our laboratory we are continuously
measuring the radioactivity in air, water and soil with
the aim of an environmental radiological surveillance
e front matter r 2005 Elsevier Ltd. All rights reserve
radiso.2005.05.049
ing author. Facultad de Ciencias, Laboratorio
ca y Radiologıa Ambiental, Universidad de
uentenueva, E-18071 Granada, Spain.
48526.
resses: [email protected], [email protected]
in the region of Granada, Spain. This surveillance is
integrated in the network of Environmental Radiologi-
cal Surveillance stations (REVIRA), which can be found
throughout Spain. A program for monitoring 7Be
depositional fluxes was implemented to provide infor-
mation on the input of this radionuclide to Granada.7Be is formed by spallation reactions, disintegration
of nuclei of nitrogen and oxygen atoms that have been
hit by cosmic ray neutrons (Bhandari et al., 1966).
Production of 7Be occurs mainly in the stratosphere
(Bhandari et al., 1970; Bleichrodt, 1978; Dutkiewicz and
Husain, 1985). However, since the residence time of
aerosols in the stratosphere is of the order of a year
(Thomas et al., 1970; Reiter, 1975), and the 7Be half life
is only 53.4 days, most of the 7Be found in precipitation
is normally of tropospheric origin (Bleichrodt, 1978).
d.
ARTICLE IN PRESSC. Gonzalez-Gomez et al. / Applied Radiation and Isotopes 64 (2006) 228–234 229
The stratospheric source can become a significant—or
even a dominant—source during the seasonal thinning
of the tropopause when exchange between the strato-
sphere and troposphere is enhanced (Reiter, 1975;
Dutkiewicz and Husain, 1985).
In order to use 7Be effectively as tracer and to
decipher whether it can be used to trace different
processes, its depositional characteristics must be
known. The rate at which 7Be is delivered to the earth’s
surface has been estimated, indirectly by measuring 7Be
inventories in sediment cores (Mc Caffrey and Thom-
son, 1980; Olsen et al., 1985) and by directly by
measuring 7Be depositional fluxes in precipitation
(Cruikshank et al., 1956; Lal et al., 1979; Turekian
et al., 1983; Olsen et al., 1985; Bascaran, 1995; Bascaran
et al., 1993; Duenas et al., 2002). However, the 7Be
surface air concentration shows a strong dependence on
location of the sample collection site (which is coupled
to the latitude and the local climate) and the seasonal
cycle. Therefore, different annual cycles of 7Be in the
ambient air of the earth’s surface are observed at
different geographical locations, depending on the
contribution of each individual parameter. Thus, 7Be
depositional fluxes need to be locally determined in
order to understand the contribution of the various
parameters. In this study 7Be depositional fluxes at
Granada in the period 1995 through 1998 are presented
and evaluated.
2. Experimental
Total-deposition samples were collected on a monthly
basis, on the roof of the building of the Faculty of
Sciences of the University of Granada, situated at
3711005000N and 013504400W, at about 670m above sea
level. Granada is in the southeast zone of the Iberian
Peninsula at about 50 km from the Mediterranean coast.
The climate in the region is characterised by average
monthly temperatures ranging between 7.5 and 26.5 1C
and by a low rainfall (annual average: 450mmy�1). The
amount of precipitation was determined at the Faculty
of Science of the University of Granada and the
temperature data were supplied by the ‘‘Centro Meter-
ologico de Andalucia Oriental’’.
The sum of wet and dry fallout was collected with an
inclined stainless steel pluviometer situated at 20m
above the ground level. The pluviometer had a cross
section of 1m2 and polyethylene vessels of 52-L capacity
acted as reservoirs for the rain samples. At the end of
each month the pluviometer was cleaned with 5L of
distilled water to obtain the dry fallout. The long
exposure times were necessary because of the low 7Be
activities. At the end of each collection, the samples were
acidified with nitric acid to pH 1, preventing adsorption
of 7Be onto the surface of the polyethylene vessels.
To obtain seasonally integrated samples, after homo-
genising, a quantity from each of the monthly samples
was taken for a three-month mixed sample. From that a
volume of 2L was taken and evaporated on a flexible
plastic sheet under infrared heating at about 80 1C. The
residue was transferred into a Petri capsule of 47mm
diameter. Measurement of 7Be in each sample was
carried out by non-destructive g-ray spectrometry by
means of its 477.7 keV g-ray using a reversed-electrode
coaxial intrinsic germanium detector, with a resolution
of 2 keV at 1332 keV. The detector is placed inside a lead
shield with the dimensions of 47 cm� 47 cm� 48.5 cm
and with a thickness of 10.5 cm. The inside of the lead
shield was covered with a 7-mm iron layer to reduce the
lead X-ray peaks.
The detector efficiency was determined using a standard
with known quantities of dissolved 133Ba, 60Co, 137Cs,152Eu and 241Am. After dissolution in distilled water to
form a volume of 2L, it was evaporated on a flexible
plastic sheet and then packed in a 47-mm diameter Petri
capsule to provide the same geometry. The detection
efficiency for the 477.7-keV photopeak was found to be
4.12%. Counting time for each sample was 1500min and
the overall experimental error was estimated to be 10% (at
2s) for most measurements. The spectral data for winter
1996 and winter1997 were near the detection limit LOD,
and thus the associated error is higher. Peak analysis of7Be (I ¼ 10:42%, 477.7 keV) was done using the SPEC-
TRAN AT peak-analysis software (Canberra Company).
The final 7Be spectral data were corrected for decay
during the collection period, using the correction factor:
lt
1� expð1� ltÞ,
where l is the decay constant and t the collection period.
3. Results and discussion
3.1. Seasonal data for 7Be depositional flux,
precipitation and temperature
Table 1 gives the seasonal 7Be depositional flux
(Bqm�2 per season), the seasonal amount of precipita-
tion (mm per season), the seasonal rainfall duration
(min per season) and the average seasonal temperature
(1C) during the period 1995 through 1998.
The seasonal precipitation shows a high variability,
not only between the four seasons in a year, but also
between corresponding seasons in different years. Using
the mean and associated standard deviation over the
four years, it can be said that the precipitation is highest
in the fall (1787113mm) and lowest in the summer
(37713mm), while the spring (99750mm) and the
winter (1397114mm) have an intermediate position.
The annual amount of precipitation in 1995 and 1998
ARTICLE IN PRESS
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
95-w
Sp
Sm
F 96-w
Sp
Sm
F 97-w
Sp
Sm
F 98-w
Sp
Sm
F
Period
Fra
ctio
nal
of R
ain
or D
epos
itio
nal
F
lux
of 7
Be
Be-7Rainfall
Fig. 1. Fractional amount of precipitation (rainfall during a
particular season divided by the annual rainfall) and fractional
depositional flux (depositional flux during a particular season
divided by the annual depositional flux) of 7Be, for Granada,
Spain, period 1995 through 1998. Data for winter 1996 and
winter1997 are near the detection limit. W corresponds to
winter (January+February+March), Sp to spring (April+
May+June), Sm to summer (July+August+September) and F
to fall (October+November+December).
Table 17Be depositional flux (Be-7, Bqm�2 per season), seasonal
amount of precipitation (Pp, mm per season), seasonal rainfall
duration (Pd, min per season) and average seasonal temperature
(T , in 1C, averaged over the season) for Granada, Spain, period
1995 through 1998
Period Be-7 Pp Pd T
95 W 23.6 41 223 10.3
Sp 24.5 28 156 19.3
Sm 80.2 26 104 23.5
F 157 131 584 14.0
96 W 189 297 1410 8.8
Sp 108 120 565 17.9
Sm 37.8 56 318 22.2
F 242 279 1370 11.9
97 W 94.4 144 697 11.2
Sp 197 144 620 17.8
Sm 80.2 31 129 22.6
F 220 262 1207 12.5
98 W 108 74 329 10.6
Sp 174 102 422 17.1
Sm 47.2 33 165 23.9
F 94.4 41 170 11.7
Data for winter 1996 and winter1997 are near the detection
limit. W corresponds to winter (January+February+March),
Sp to spring (April+May+June), Sm to summer (July+
August+September) and F to fall (October+November+
December).
C. Gonzalez-Gomez et al. / Applied Radiation and Isotopes 64 (2006) 228–234230
was respectively 225 and 250mm and these are about
50% lower than the average annual amount of
precipitation, being 450mm. In 1996 and 1997, the
amount of rainfall was 751 and 581mm, respectively,
being 40% and 22% higher than the annual amount of
rain.
The seasonal 7Be depositional fluxes vary by a factor
of 10, ranging from 23.6Bqm�2 per season (winter
1995) to 242Bqm�2 per season (fall 1998). Using the
mean and associated standard deviation over the four
years, it can be said that the depositional flux is highest
in the fall (178766Bqm�2) and lowest in the summer
(61722Bqm�2), while the spring (126777Bqm�2) and
the winter (104768Bqm�2) have also an intermediate
position here.
Seasonal variations in 7Be surface air concentrations
and 7Be depositional fluxes have been observed in many
places. It has been shown that the 7Be surface air
concentration depends on wet scavenging, stratospher-
e–troposphere exchange of air masses, downward
transfer of aerosols in troposphere and horizontal
movement of aerosols from subtropical and lower and
higher latitudes (Feely et al., 1981, 1989; Brost et al.,
1991). The seasonal variation in the depositional fluxes
as well as in the surface air is a complicated function of
all four processes since each of these may have its own
seasonal cycle (Brost et al., 1991). According to
Bascaran (1995), the seasonal variations in depositional
flux, which are related to surface air concentrations, are
caused by at least three factors: (1) seasonal variations
in amount of precipitation, (2) increased stratosphere–
troposphere exchange during the winter and early spring
and (3) increased vertical transport of 7Be from the
upper troposphere to the middle and lower troposphere
due to decreased stability of the troposphere during the
summer months.
3.2. Fractional amount of precipitation and fractional7Be depositional flux
The fractional amount of precipitation (rainfall
during a particular season divided by the annual
rainfall) and the fractional depositional flux (deposi-
tional flux during a particular season divided by the
annual depositional flux) of 7Be in the period 1995
through 1998 are plotted in Fig. 1. The graph visualises
the above-mentioned variations, not only between the
seasons in a year, but also between corresponding
seasons in different years. The graph further indicates a
fair correspondence between the fractional 7Be deposi-
tional flux and the fractional amount of rain.
3.3. Precipitation-normalised depositional flux
Another way of examining the depositional flux is the
use of the precipitation-normalised enrichment factor a,defined as
a ¼ ðDs=DtÞ=ðRs=RtÞ,
ARTICLE IN PRESSC. Gonzalez-Gomez et al. / Applied Radiation and Isotopes 64 (2006) 228–234 231
where Ds and Dt are the depositional fluxes during a
particular season and the year involved, respectively,
and Rs and Rt are the corresponding amount of rainfall
in the same season and year. Values well above 1
indicate that the depositional fluxes are higher than
could be expected from the amount of rainfall, and well
below 1 indicate the reverse.
Table 2 presents a-factors for the 16 seasons in period
1995 through 1998. Although the seasons show con-
siderable variations in the a-factor over the years, the
following trends can be noted. The average a-factor is inthe winter considerably below 1 and in the fall and the
spring around the value 1. For the summer the average
a-factor is larger than 1. In other words, during the
summer season the depositional fluxes of 7Be relative to
the amount of precipitation are generally higher than
other seasons. The highest a value has been recorded in
summer 1995 and summer 1997. These high values are
mainly due to the small amount of rain in those seasons
compared with the total annual rainfall, so that dry
deposition may become relatively more important.
Table 2
Precipitation-normalised seasonal enrichment a-factor for
Granada, Spain, in the period 1995 through 1998
Period Winter Spring Summer Fall
1995 0.46 0.68 2.44 0.95
1996 0.83 1.17 0.88 1.13
1997 0.64 1.34 2.54 0.82
1998 0.86 1.01 0.86 1.36
Mean 0.70 1.04 1.83 1.05
Winter ¼ January+February+March, spring ¼ April+May+
June, Summer ¼ July+August+September and fall ¼ October+
November+December.
Table 3
Annual averages of 7Be depositional fluxes and rainfall at different l
Fallout
(Bqm�2 y�1)
Location Latitude
North
469 Granada, Spain 371
412 Malaga, Spain 361
528 Damascus, Syria 331
1030 Canberra, Australia 351
2133 New Hamsphire, USA 41.51
2767 Massachusetts, USA 431
2850 Bermuda 331
2968 Galvetson, Texas, USA 291
3780 New Haven, Connecticut,
USA,
411
Possibly also the increased vertical transport in the
troposphere in the summer months may play a role here.
Similar results are observed in some midlatitude zones
(361–511) such as [New haven, Connecticut, USA (411N;
72.21W), Turekian et al. (1983)]; [Norfolk, Virginia,
USA (361 530 N; 761 180 W), Olsen et al. (1985)] and
[Paris, France (48.81N; 2.31E), Thomas, 1988].
3.4. Annual 7Be depositional flux
The annual 7Be depositional flux ranged from 285 to
592Bqm�2 y�1 with a mean and standard deviation of
4697145Bqm�2 y�1. This mean value is of the same
order as 605Bqm�2 y�1 derived from the Brost (1991)
global model of 7Be deposition over land. Table 3 gives
some 7Be depositional fluxes and average annual
rainfall at different locations in the world with compar-
able latitudes. It can be noted that our value is lower
than those from other locations, but it is comparable
with those determined at Malaga and Damascus.
Nevertheless, this lower value agrees with the general
assumption that precipitation is the main factor
controlling the 7Be depositional flux. The precipitation
in the Granada region has been roughly 50% lower than
the average rainfall for two years out of four years of
measurement.
Regression analysis of the average annual rainfall at
the different locations in the world with 7Be fallout
gives a correlation coefficient of 0.57. This may suggest
that annual 7Be fallout world-wide may be constant at
any time of the year and that the surface inventory is
largely related to rainfall. Differences in data may also
result from differences in phase of the solar cycle. Earlier
a strong variation in production rate of 7Be has been
observed, due to variation in the flux of cosmic galactic
primary radiation caused by the 11-year sunspot cycle
(Azahra et al., 2003; Al-Azmi et al. 2001; Cannizzaro
et al., 1995).
ocations with comparable latitudes
Rainfall
(mm)
Reference Period of
collection
452 This study 1995–1998
308 Duenas et al. 1992–1999
153 Othman et al. 1995–1997
660 Walbrink and Murray —
— Benitez-Nelson 1996–1998
— Benitez-Nelson 1996–1998
1700 Turekian et al. 1977–1978
1390 Baskaran et al. 1989–1991
1240 Turekian et al. 1977–1978
ARTICLE IN PRESS
Table 4
Linear regressions between seasonal 7Be depositional flux (Be-7, Bqm�2 per season), seasonal amount of rain (Pp, mm per season),
seasonal rainfall duration (Pd, min per season) and/or seasonal average temperature (T, 1C per season)
Linear regression equation Eq. no r2 p-value
Be-7 ¼ ð0:650� 0:106Þ Pp þ ð43:7� 15:3Þ (1) 0.729 o 0.001
Be-7 ¼ ð0:131� 0:025Þ Pd þ ð47:7� 16:7Þ (2) 0.671 o 0.001
Be-7 ¼ ð�5:76� 3:28Þ T þ ð209� 55Þ (3) 0.181 0.098
Pp ¼ ð�10:24� 3:89Þ T þ ð276� 65Þ (4) 0.331 0.020
Pd ¼ ð�48:5� 18:5Þ T þ ð1304� 309Þ (5) 0.329 0.020
Pd ¼ ð0:210� 0:004ÞPd þ ð2:202� 3:00Þ (6) 0.994 o 0.001
Be-7 ¼ ð4:59� 0:87ÞPp þ ð�0:831� 0:183ÞPd þ ð38:5� 9:9Þa (7) 0.895 o 0.001
Be-7 ¼ ð0:693� 0:133ÞPp þ ð1:33� 2:36ÞT þ ð17:6� 48:9Þ (8) 0.736 o 0.001
Be-7 ¼ ð0:137� 0:031ÞPd þ ð0:91� 2:62ÞT þ ð30:0� 54:0Þ (9) 0.674 o 0.001
aDue to the strong correlation between Pp and Pd, regression equation (7) has become an artefact. Data for Granada, Spain, in the
period 1995 through 1998.
C. Gonzalez-Gomez et al. / Applied Radiation and Isotopes 64 (2006) 228–234232
3.5. Relationship of 7Be depositional flux with rainfall
and temperature
Single linear regression analysis was used to identify
which meteorological parameters are associated with the
seasonal 7Be depositional fluxes. The parameters
available are the amount of rainfall per season (Pp, in
mm), the rainfall duration per season (Pd in min) and
the average temperature per season (T in 1C). Table 4
gives the (squared) linear correlation coefficients be-
tween the 7Be depositional fluxes per season and each of
these three parameters as independent variable.
For correlation with Pp and Pd the p-values are
o0.001, so the correlation is statistically significant at
the level of 499.9%. Amount of rain and rain duration
explain 73% (¼ r2, Eq. (1)) and 67% (¼ r2, Eq. (2)),
respectively, of the variation in 7Be depositional fluxes.
The fact that these values have the same order of
magnitude is understandable, since amount of rain and
rain duration are highly correlated (r2 ¼ 0:994, Eq. (6)).The linear correlation of 7Be depositional flux with rain
has the underlying implicit basic assumption that the
rain-independent part of the 7Be depositional flux (dry
deposition) is constant and the rain-dependent part (wet
deposition) is fully linear in rain (expressed either in mm
or in min). Both assumptions may not be fully true,
especially the latter one. It is conceivable that repeated
or prolonged rain showers may not be so effective in
wet 7Be deposition, since the air was already depleted
in 7Be by earlier rain. This may lead mathematically to
a levelling off of the accumulated wet 7Be deposition at
increased accumulated rain values, which may lead to an
increase of the intercept (the constant in the equations).
Thus, possibly the constants in Eq. (1) and (2) (43.7 and
47.7, respectively) may be too high due to this effect. In
this respect it should be noted that for two seasons the
sum for wet and dry deposition is ca. 24Bqm�2 (see
Table 1), being half the value of the constant in Eq. (2).
As a preliminary conclusion it can be said that dry
deposition plays a rather minor role in the 7Be
depositional flux.
The correlation with temperature is negative, but the
p-value 0.098 implies that it is not significant at the 95%
level. The negative correlation stems from the fact that
in the summer with low rainfall the 7Be depositional
flux is generally lower than that in the winter with more
rainfall.
In addition, we have performed multiple linear
regression analysis using two independent variables to
see whether the correlation could be improved. Since the
variables Pp and Pd are highly correlated (r2 ¼ 0:994),multiple regression with these parameters will be
unsuccessful and will lead to artefacts (as may be seen
in Table 4, Eq. (7)); thus, this approach is not considered
any further. Insertion of T as an independent variable in
addition to the independent variable Pp or Pd leads only
to a marginally improved correlation, viz. r2 ¼ 0:736(Eq. (8)) versus r2 ¼ 0:729 (Eq. (1)) and r2 ¼ 0:674
ARTICLE IN PRESSC. Gonzalez-Gomez et al. / Applied Radiation and Isotopes 64 (2006) 228–234 233
(Eq. (9)) versus r2 ¼ 0:671 (Eq. (2)). The coefficients for
T in Eqs. (8) and (9) are positive, although not
significant, contrary to the negative sign in Eq. (3). This
result underlines the explanation that the negative
correlation in Eq. (3) between 7Be depositional flux
and temperature is due to lower precipitation in summer
indeed.
All these results together indicate the importance of
wet scavenging of carrier aerosols which may be the
main process controlling the 7Be at Granada. Other
investigators (Olsen et al., 1985; Dibb, 1989; Todd et al.,
1989; Shuler et al., 1991; Bascaran et al., 1993; Duenas
et al., 2002) have also indicated a linear relationship
between the amount of precipitation and 7Be deposi-
tional fluxes, showing that the precipitation plays a
dominant role in the removal of this radionuclide from
the troposphere.
4. Conclusion
Variations within a factor of 10 are found in 7Be
depositional fluxes between the seasons in a year and
also between corresponding seasons in different years
for Granada in the period 1995 through 1998. Together
with the fact that only four years out of the 11-year solar
cycle were studied, the results must primarily be taken as
indicative. Nevertheless, there is a clear trend in the
distribution of 7Be depositional fluxes, showing a
minimum during the summer and a maximum in the
fall. Regression analysis shows that about 70% of the
depositional variations over the 16 seasons studied is
explained by correlation with precipitation (expressed
either as mm rain or in min). Wet deposition plays a
dominant role in the 7Be depositional flux, while dry
deposition has only a rather minor role.
Acknowledgements
The authors thank Prof. Dr. J.J.M. de Goeij (Delft
University of Technology, Delft, The Netherlands) for
his suggestions and critical comments in preparing the
manuscript. One of us thanks the Spanish Agency of
International Cooperation for a predoctoral grant.
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Further reading
Benıtez-Nelson, C.R., Buessler, K.O., 1999. Phosphorous-32,
phosphorous-37, beryllium-7 and lead-210: Atmospheric
fluxes and utility in tracing stratosphere/troposphere
exchange. J. Geophys. Res. 104, 11745–11754.
Dutkiewicz, V.A., Husain, L., 1979. Determination of strato-
spheric ozone at ground level using 7Be/ozone ratios.
Geophys. Res. Lett., 6171–6174.