2013 International Nuclear Atlantic Conference - INAC 2013
Recife, PE, Brazil, November 24-29, 2013
ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN
ISBN: 978-85-99141-05-2
Be-7 MEASURED AT GROUND AIR LEVEL AND RAINFALL IN THE
CITY OF SÃO PAULO
Sandra R. Damatto, Joseilton M. Souza, Jonatan V. Frujuele, Marcelo F. Máduar,
Lucio Leonardo, Brigitte R. S.Pecequilo
Laboratório de Radiometria Ambiental
Instituto de Pesquisas Energéticas e Nucleares (IPEN / CNEN - SP)
Av. Professor Lineu Prestes 2242
05508-000 São Paulo, SP
[email protected]
ABSTRACT
The cosmogenic radionuclide 7Be (T1/2 = 53.3 d), produced in the upper atmosphere by cosmic ray spallation of
oxygen and nitrogen, is one of the cosmogenic radionuclides that can be used as tracer for heavy metals and
pollutants in the environment, tracer of soil erosion and sedimentation in lakes, among other examples. Their
subsequent deposition to the land surface occurs as both wet and dry fallout, although it has been demonstrated
that 7Be fallout is primarily associated with precipitation. There is limited data on the concentration of 7Be in
rainfall and in particulate in the Southern Hemisphere and in Brazil, compared with data from the Northern
Hemisphere. This paper presents the results obtained of 7Be concentrations measured from April 2011 to June
2013, in samples of air at ground level, each fifteen days, and rainfall in all the rainy events that occurred at
Instituto de Pesquisas Energéticas e Nucleares (IPEN), which has its campus located in the city of São Paulo,
state of São Paulo, Brazil. The concentrations of 7Be were measured by non-destructive gamma-ray
spectrometry using an extended range closed-end coaxial Be-layer HPGe detector with 25% relative efficiency
and associated electronic devices and live counting time varying from 100,000 s to 300,000 s. The results
obtained of 7Be in particulate and in rainfall were correlated to seasons, precipitation, temperature and sunspot
number. The higher values obtained for the concentrations were in spring and summer time presenting good
correlations with the amount of precipitation and sunspot number and a clear seasonal variation.
1. INTRODUCTION
7
Be (T1/2= 53.3 days), a natural radionuclide of cosmogenic origin, is continuously produced
in the upper atmosphere by the spallation process of light atmospheric nuclei such as nitrogen
and oxygen when they interact with protons and neutrons of cosmic rays. These atoms readily
react to produce BeO or Be(OH)2 molecules, which in turn diffuse through the atmosphere
until they attach to an atmospheric aerosol. Subsequent deposition to the land surface
therefore occurs as both wet and dry fallout, although it has been demonstrated that 7Be
fallout is primarily associated with precipitation [1].
This cosmogenic radionuclide is used as tracer for heavy metals and pollutants in the
environment [2], for tracing soil erosion [3], transport processes in watershed and
chronometer in aquatic and atmospheric systems lakes [1, 4-5]. The study of these processes
requires a good knowledge of its input from the atmosphere, and the 7Be behavior study in
the amosphere will give insight on the behavior of similar chemical species in atmosphere
aerosols.
About 70% of the 7Be is produced in the stratosphere and the remaining 30% in the
troposphere. In general, most of the 7Be do not reach the troposphere except for spring when
season thinning of the troposphere occurs at mid-latitudes, resulting in air exchange between
stratosphere and troposphere [5]. The 7Be produced in the stratosphere usually stays there for
about a year before entering the troposphere, where it remains for about 6 weeks. Its transfer
to Earth’s surface is achieved by gravitational settling and precipitation. The study of possible
seasonal changes in 7Be concentrations due to an increase of its migration from the
stratosphere into the troposphere gives useful information about its distribution and behavior
[6].
Cosmic radionuclide concentration in air at ground level can vary greatly according to
latitude and altitude, season of the year and local meteorological conditions. 7Be
concentration in surface air depends on wet scavenging, stratosphere to troposphere
exchange, downward transfer in the troposphere and horizontal transfer from middle and
subtropical latitudes to higher and lower latitudes [7, 8]. Generally the values measured at
middle latitudes are higher than those at northern latitude [6, 9-10]. The production rate of
7
Be changes according to variations in flux of cosmic rays caused by the 11-year sunspot
cycle. This phenomenon is one of the major parameters that cause the variation in 7Be
concentration in surface air [11, 12].
The aim of this paper is to present preliminary results of the 7Be concentration in ground
level air and rainfall in the city of São Paulo, SP, Brazil. From April 2011 to June 2013,
samples of particulate and rainfall have been measured by gamma spectrometry and the
resulting seasonal 7Be data were correlated to seasons, rainfall, temperature and sunspot
number.
2. MATERIALS AND METHODS
2.1. Site Description
The sampling site is located at IPEN – Instituto de Pesquisas Energéticas e Nucleares, which
is approximately 10 km west from downtown São Paulo. The city of São Paulo is situated on
a plateau in southeastern Brazil, at latitude 23°33’58.27”S and longitude 46 44’14.82”W,
with an average altitude of 760 m above sea level. Climate in the area is temperate tropical
with dry period in winter and rainy in summer. The annual average temperature for the
studied period was 21.4oC, showing minimum and maximum average temperature of 16.9oC
and 25.8oC, respectively. The annual rainfall in IPEN campus, for the studied period, ranged
from 1590 mm to 2081 mm and the winds are predominantly from the SSE sector.
2.2. Sampling
In the framework of an environmental radiological monitoring program carried out at IPEN
facilities, due to their routine gaseous effluents releases, atmospheric air is sampled at 1.0 m
above the ground. The air is continuously pumped by a low volume sampler (Staplex model
LVC-50) and forced to pass through cellulose filters with 47 mm diameter with retention
efficiency near 100%, for the collection of particulates. The filters are replaced every 15
days, which correspond to a filtered air volume of about 3,000 m3.
INAC 2013, Recife, PE, Brazil.
Rainfall was collected in each rainy event from April 2011 to June 2013, in a polyethylene
container of 0,132 m2, totaling 123 rainy events. At the end of each rainy event and when the
sample was collected, the container was rinsed with 300 mL of 1M HNO3 and 200 mL super
pure water, which were added to the sample. It was only considered precipitation values
grater than 5 mm, because this volume is used to characterize different radionuclides
deposition process [4, 13].The amount of precipitation in mm was obtained from a rain gauge
installed at the sampling point. Since the rainfall container was exposed to the atmosphere
continuously, it was collected the total deposition, that is, the wet and dry fallout. All the
collected samples were acidified with HNO3 at pH lower than 2, in order to avoid losses due
to adsorption on the concentration process, and then concentrated to a final volume of
100 mL.
2.3. Be-7 determination
Measurement of 7Be, in both samples, particulate and rainfall, was carried out by nondestructive γ-ray spectrometry using the 477.61 keV γ-ray. An extended range closed-end
coaxial Be-layer HPGe detector with 25% relative efficiency, 2.09 keV resolution at
1.33 MeV and associated electronic devices was used, with live counting time ranged from
100,000 to 300,000 s. The spectra were acquired by multichannel analyser Ethernim and, for
the analysis, WinnerGamma software was used. The activity was corrected for decay of 7Be
to the date of collection. The associated uncertainty for one sigma confidence ranged from
10% to 30% from the obtained results.
3. RESULTS AND DISCUSSION
3.1. 7Be in particulate
Fig. 1 presents the 7Be results, in mBq m-3, plotted in function of the sampling date for the
study period (April 2011 to June 2013); the values ranged from 0.31 ± 0.07 mBq m-3 in
September 2012 to 2.44 ± 0.15 mBq m-3 in March 2013, winter and summer time,
respectively.
7
mBq m
Be
2011 - 2012 - 2013
-3
2,5
2
1,5
1
0,5
2011
2012
06/27
05/24
04/12
03/15
02/15
01/18
12/28
11/30
11/01
10/05
09/11
08/03
07/03
06/01
05/04
04/05
03/09
02/10
01/13
12/20
11/18
10/27
09/23
08/26
07/29
07/01
06/03
05/06
04/08
0
2013
Figure 1. 7Be in mBq m-3 in particulate for the study period (April 2011 to June 2013).
INAC 2013, Recife, PE, Brazil.
Fig. 2 presents the correlation of the average monthly 7Be concentration, in mBq m-3, in
particulate with sunspot number, for the study period. A good correlation, inverse
relationship, but not totally perfect was observed. This inverse correlation is attributed to the
cyclic pattern resulting from about 11-year temporal variations associated with the solar
activity, which has a direct relation with a sunspot number. The present work started in the
maximum of the cycle 24, which began in 2008 and had a peak in November 2011, with the
highest sunspot number. From this month forth, the sunspot number presented a trend of
decrease, what can be observed in Fig. 2. The sunspot numbers were obtained from the
Sunspot Index Data Center (SIDC) in Brussels [14].
mBq m -3
7
mBq m-3
Be X Sunspot
Sunspot
Sunspot
100
2,00
6/13
5/13
4/13
3/13
2/13
1/13
12/12
11/12
9/12
10/12
8/12
7/12
6/12
5/12
4/12
3/12
2/12
0
1/12
10
0,00
12/11
20
0,20
11/11
30
0,40
10/11
40
0,60
9/11
50
0,80
8/11
60
1,00
7/11
70
1,20
6/11
80
1,40
5/11
90
1,60
4/11
1,80
Figure 2. Correlation between 7Be, mBq m-3 in particulate and sunspot number.
Fig. 3 presents the correlation of the average monthly 7Be concentration, in mBq m-3, in
particulate with average temperature in the city of São Paulo, in °C, for the study period. It
can be observed that, in general, there is a good correlation between 7Be and temperature,
with increase of the concentration in warmer months and decrease in colder months. The
temperature data were obtained from CIIAGRO [15].
mBq m -3
7
mBq m-3
Be X °C
°C
°C
30
2,00
1,80
25
1,60
1,40
20
1,20
1,00
15
0,80
10
0,60
0,40
5
0,20
Figure 3. Correlation between 7Be, mBq m-3, in particulate and temperature
INAC 2013, Recife, PE, Brazil.
6/13
5/13
4/13
3/13
2/13
1/13
12/12
11/12
10/12
9/12
8/12
7/12
6/12
5/12
4/12
3/12
2/12
1/12
12/11
11/11
10/11
9/11
8/11
7/11
6/11
5/11
0
4/11
0,00
Fig. 4 presents the correlation of the average monthly 7Be concentration, in mBq m-3, in
particulate with average precipitation, in mm, measured inside of IPEN campus for the study
period. For the whole period studied it was found that the highest concentrations of 7Be were
obtained in months with high indexes of rainfall due to the washout of the atmosphere by the
precipitation.
7
mBq m -3
mBq m-3
Be X mm
mm
mm
2,00
300
1,75
250
1,50
200
1,25
1,00
150
0,75
100
0,50
50
0,25
0
6/13
5/13
4/13
3/13
2/13
1/13
12/12
11/12
9/12
10/12
8/12
7/12
6/12
5/12
4/12
3/12
2/12
1/12
12/11
11/11
9/11
10/11
8/11
7/11
6/11
5/11
4/11
0,00
Figure 4. Correlation between 7Be, mBq m-3, in particulate and precipitation
3.2. 7Be in rainfall
The monthly amount of precipitation for the studied period is plotted in Fig. 5. The
precipitation for the study period ranged from 5 to 160 mm of each rainy event and in
September of 2011 and August of 2012 there was no precipitation.
Comparing the amount of precipitation among the 26 months of study, it is possible to verify
the climate of temperate tropical that is characteristic of São Paulo with dry periods in winter
and rainy in summer only in 2011, where the winter months presented the lowest values of
rainfall. In 2012, June and July presented high values, comparable to the summer months of
January and February. In 2013, July also presented high values, like in 2012.
Rainfall
mm
300
250
200
150
100
50
0
A M J J A S O N D
2011
J F M A M J J A S O N D
2012
J F M A M J
2013
Figure 5: Monthly precipitation during the sampling period (April 2011 to June 2013).
INAC 2013, Recife, PE, Brazil.
Fig. 6, 7 and 8 present the correlation between 7Be concentration, in Bq L-1, and precipitation
for 2011, 2012 and 2013, respectively. For the entire studied period it is possible to verify
that there is an evident correlation of 7Be concentration in rainfall and the amount of
precipitation. The highest values obtained were found in months where the precipitation was
low and inversely the lowest in months of high indexes of rainfall, due to the dillution effect
of the sample [16].
7
-1
Bq L
Be X mm
2011
Bq/L
mm
mm
8,00
140
7,00
120
6,00
100
5,00
80
4,00
60
3,00
30/12
24/12
18/12
15/12
14/12
8/12
10/12
7/12
1/12
29/11
22/11
12/11
27/10
26/10
8/10
11/10
2/10
31/8
20/8
30/6
9/6
26/6
7/6
15/5
1/5
27/4
0
21/4
20
0,00
9/4
1,00
12/4
40
2/4
2,00
Figure 6. Correlation between 7Be concentration, in Bq L-1, and precipitation for 2011
-1
7
Bq L
12,00
Be X mm
2012
Bq/L
mmmm
160
140
10,00
120
8,00
100
6,00
80
60
4,00
40
2,00
20
27/12
19/12
15/12
5/12
28/11
24/11
10/11
27/10
19/10
11/10
16/7
7/7
1/6
19/6
12/5
26/4
12/4
8/4
24/3
15/3
10/3
27/2
23/2
19/2
14/2
24/1
19/1
17/1
5/1
0
12/1
0,00
Figure 7. Correlation between 7Be concentration, in Bq L-1, and precipitation for 2012
7
-1
Bq L
16,00
Be X mm
2013
Bq/L
mm
mm
100
90
14,00
80
12,00
70
10,00
60
8,00
50
6,00
40
30
4,00
20
10
0,00
0
8/1
10/1
12/1
14/1
16/1
19/1
26/1
1/2
6/2
7/2
8/2
14/2
15/2
18/2
19/2
23/2
26/2
7/3
8/3
11/3
16/3
20/3
24/3
26/3
5/4
6/4
12/4
22/5
28/5
1/6
11/6
16/6
24/6
2,00
Figure 8. Correlation between 7Be concentration, in Bq L-1, and precipitation for 2013
INAC 2013, Recife, PE, Brazil.
4. CONCLUSIONS
This paper presented the preliminary results for 7Be concentration measured at ground air
level and in all the rainy events that occurred from April 2011 to June 2013, at IPEN, São
Paulo, SP, Brazil in order to provide results of its concentration in Brazil and to understand
the atmospheric behavior of this radionuclide in the Southern Hemisphere.
The obtained results for 7Be concentration in air surface showed a good agreement with data
from Northern hemisphere of similar latitude, in spite of the city of São Paulo being situated
in low latitudes of the Southern hemisphere. 7Be concentrations displyed clearly seasonal
variations, due to the increase of its concentration during spring time and variation in
production rate of 7Be; wich is attributed to variations in the flux of cosmic rays.
For rainfall the results showed that there is an evident correlation of 7Be concentration with
the amount of precipitation, therefore high values were obtained in dry months and low
values in rainy months, due to the dillution effect of the sample.
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