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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 3  |  Page : 148-151  

Determination of atmospheric concentration of beryllium-7 at ground level


Radiological Protection Department, Bariloche Atomic Centre, National Atomic Energy Commission, Bariloche, Argentina

Date of Submission26-May-2018
Date of Decision21-Jun-2018
Date of Acceptance03-Aug-2018
Date of Web Publication19-Nov-2018

Correspondence Address:
Pablo A Andres
Av. E. Bustillo Km. 9,5; R8402AGP San Carlos de Bariloche
Argentina
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_35_18

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  Abstract 


Atmospheric concentration of beryllium-7 (7Be) at ground level was measured from September 2012 to October 2013 in San Carlos de Bariloche, Argentina, using a HPV-5300AFC brushless, automatic flow control outdoor high volume air sampler for continuous use, and 8”×10” 0.8 microns fiberglass filters. The activity of 7Be was determined on a high-purity germanium (HPGe) detector (relative efficiency 12.3%) by standard gamma spectrometry. The average value found was 4.1 ± 0.1 mBq/m3. Seasonal changes in the atmospheric concentration of 7Be were compared with other environmental variables (temperature, humidity, precipitations, and air kerma rate), and correlation, both positive and negative, was observed as expected. Measurements were carried out following the Radiological Environmental Monitoring Programme at Bariloche Atomic Centre. The aim of the study was to determine atmospheric concentration of 7Be at ground level and to correlate its seasonal changes with environmental variables. Air samplers and environmental dosemeter locations were far away from buildings and any other thing that could influence or alter the measurements. Pearson's correlation coefficient was calculated to evaluate the relation between atmospheric 7Be concentration and environmental variables of interest. These changes in 7Be concentration can be attributed to the variations and dynamic of air exchange between the stratosphere and the troposphere and to the variations of the tropopause height. To fully understand and correctly interpret the meaning of these seasonal variations in the concentrations of atmospheric 7Be, active atmospheric processes must be taken into account. According to the measurements carried out, contribution of 7Be concentration to the natural background radiation dose is negligible.

Keywords: Atmosphere, Bariloche, beryllium, environment, natural radioactivity


How to cite this article:
Andres PA. Determination of atmospheric concentration of beryllium-7 at ground level. Radiat Prot Environ 2018;41:148-51

How to cite this URL:
Andres PA. Determination of atmospheric concentration of beryllium-7 at ground level. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 10];41:148-51. Available from: http://www.rpe.org.in/text.asp?2018/41/3/148/245795




  Introduction Top


Beryllium-7 (7Be) is a naturally occurring radionuclide (Eγ= 477.6 keV; T½=53.2 days) produced as a result of spallation reactions with light elements such as nitrogen, oxygen, and carbon in high atmosphere, throughout a 7Be + e7 Li + v electronic capture process.[1],[2] This radionuclide enters the environment by wet deposition mainly (rain and snow), while dried deposition mechanism (gravity action) is below 10%.[3]

Atmospheric concentration of 7Be is not uniform; it depends on the geographical location, solar activity, exchange of air masses in the atmosphere, and the removal efficiency from the troposphere. After 7Be is formed, it rapidly associates with submicron-sized aerosol particles.[4] The world concentration of 7Be is altitude and latitude dependent and is equal to 12.5 mBq/m3, on average.[5] Several authors have reported changes in 7Be content in rain related to the magnitude and intensity of precipitations, length of the event, and time elapsed between events.[6],[7],[8],[9]

When 7Be falls down on the earth surface, it does not scatter in a homogeneous way, but it tends to affix itself in superficial and aquatic plants and on superficial soils to a lesser degree. The deposition velocity of aerosols can be affected by precipitation and seasonal factors. However, the deposition velocity reflects the effectiveness of aerosol scavenging by precipitation; thus, both the amount of precipitation and duration of rainfall must be considered. Rainfall reduces the air concentration of 7Be[10],[11] and increases the deposition velocity, which provides important information on the behavior of atmospheric particulates in the environment.[12],[13]

The production rate of 7Be in the atmosphere increases with a decrease in latitude, while the maximum deposition flux occurs in the middle latitudes due to vertical/horizontal air mass transport and the frequency of rainfall events.[14] This radionuclide is often used in climate change studies in several countries nowadays.[15]

This paper focuses on the determination of concentration of 7Be during a 12-month period in San Carlos de Bariloche, Argentina (41° 09' S; 71° 18' W; 893 m above sea level). Its relationship with other environmental variables was analyzed.


  Materials and Methods Top


The city of San Carlos de Bariloche is located in Northern Patagonia, Argentina, close to the Andes mountain chain (41° 09' S; 71° 18' W; 893 m above sea level). Combination of altitude, latitude, and winds dominance from the West-Northeast quadrant and South generates a mild-cold climate with dry season, which shows a noticeable variation of rains from West to East. The precipitations' gradient fluctuates from 4000 mm/year in mountain points (Valdivian temperate forests) to only 600 mm/year in the Limay river area. The most urban population is located in the valleys where annual precipitations are in the 800–1000 mm/year range. About 70% of precipitations occur during autumn and winter months.

The area – according to Thornthwaite classification – is a climate region defined as follows:[16]

  • AC' 2ra': This stands for a microthermal high wet climate, with little or no water deficit and low thermal concentration in summer
  • B3C' 2sa'+: This stands for a microthermal wet climate, with moderate water deficit and low thermal concentration in summer.


Each aerosol sample was collected during 20 h on 8” × 10” 0.8 μ fiberglass filters by constant flow rate samplers (HPV-5300AFC brushless and automatic flow control outdoor high volume air sampler for continuous use). The activity of 7Be was determined on a high-purity germanium detector (relative efficiency 12.3%) by standard gamma spectrometry. Detector calibration was performed using gamma standard calibrated sources (241Am, 57Co, 22Na, 137Cs,54 Mn, 60Co, 133Ba, and 152Eu) following conventional acknowledged methods.[17],[18] The photopeak efficiency was 1.8% at 477 keV. Pulse height spectrum analysis was made with a computer-based multichannel analysis (4096 energy channels), and each filter was measured for at least 48 h. Samples were collected once a month from September 2012 to October 2013. The combined measurement uncertainty of the results was calculated at the 95% level of confidence (k = 2).


  Results Top


Concentration of 7Be at ground level is shown in [Table 1]. The average concentration of 7Be can be determined as 4.1 ± 0.1 mBq/m3.
Table 1: Atmospheric concentration of 7Be

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[Table 2] shows weather conditions during the sampling period. Data were obtained from the local aerodrome meteorological station. Values are indicated as an average of daily measurements followed by one standard deviation. Air kerma rate values were measured at the sampling point with thermoluminescent dosemeters (LiF 700H), and the combined measurement uncertainty of the results was calculated at the 95% level of confidence (k = 2).[19]
Table 2: Weather conditions during the sampling period

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From the analysis of the concentration changes of 7Be and its relationship with changes of other environmental variables over the year, a positive correlation can be observed between concentration of 7Be and temperature changes (Pearson's correlation coefficient, r = 0.70, average temperature; r = 0.61, average maximum temperature; and r = 0.72, average minimum temperature). These variations can be seen in [Figure 1]. According to the literature, an important factor in producing higher concentrations of 7Be during warmer months is the increase of vertical transport within the troposphere occurring during those months.[20] This vertical transport carries down to the surface layer of 7Be that has been produced within the upper troposphere, as well as that which has entered the troposphere from the stratosphere.[21] Moreover, no correlation between wind speed and air kerma rate could be found.
Figure 1: Relationship between monthly average temperature (middle full line), monthly average maximum temperature (upper full line), monthly average minimum temperature (lower full line), and atmospheric concentration of beryllium-7 (bars) at ground level over the year

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On the other hand, since the sampling site is located in a region where there are strong seasonal variations in the rainfall rate, a negative correlation was observed, as expected, between concentration of 7Be and variables such as relative humidity (r = −0.33) and precipitations (r = −0.11). This negative correlation could indicate the importance of washout of the atmospheric aerosol that carries the 7Be.[21] However, the correlation coefficients are not strong enough to confirm the previous hypotheses.


  Conclusions Top


Seasonal variations in the concentration of 7Be at ground level were observed as expected, according to the literature. These changes in 7Be concentration can be attributed to the variations and dynamic of air exchange between the stratosphere and the troposphere and to the variations of the tropopause height. To fully understand and correctly interpret the meaning of these seasonal variations in the concentrations of atmospheric 7Be, active atmospheric processes must be taken into account.

According to the measurements carried out, contribution of 7Be concentration to the natural background radiation dose is negligible.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Kaste JM, Norton SA, Hess C. Environmental chemistry of beryllium-7. Rev Mineral Geochem 2002;50:271-89.  Back to cited text no. 1
    
2.
Yoshimori M, Hirayama H, Mori S, Sasaki K, Sakurai H. Be-7 nuclei produced by galactic cosmic rays and solar energetic particles in the earth's atmosphere. Adv Space Res 2003;32:2691-6.  Back to cited text no. 2
    
3.
Benitez-Nelson CR, Buesseler KO. Phosphorus 32, phosphorus 37, beryllium 7 and lead 210. Atmospheric fluxes and utility in tracing stratosphere/troposphere exchange. J Geophys Res 1999;104:11745-54.  Back to cited text no. 3
    
4.
Bondietti EA, Hoffmann FO, Larsen IL. Air-to-vegetation transfer rates of natural submicron aerosols. J Environ Radioact 1984;1:5-27.  Back to cited text no. 4
    
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UNSCEAR. Sources and Effects of Ionizing Radiation. Vol. 1. United Nations, New York: United Nations Scientific Committee on the Effects of Atomic Radiation; 2000.  Back to cited text no. 5
    
6.
Wallbrink PJ, Murray AS. Fallout of 7Be in South Eastern Australia. J Environ Radioact 1994;25:213-28.  Back to cited text no. 6
    
7.
Caillet S, Arpagaus P, Monna F, Dominik J. Factors controlling 7Be and 210Pb atmospheric deposition as revealed by sampling individual rain events in the region of Geneva, Switzerland. J Environ Radioact 2001;53:241-56.  Back to cited text no. 7
    
8.
Ioannidou A, Papastefanou C. Precipitation scavenging of 7Be and 137Cs radionuclides in air. J Environ Radioact 2006;85:121-36.  Back to cited text no. 8
    
9.
Lohaiza F, Juri Ayub J, Velasco H, Rizzott M, Di Gregorio D, Huck H, et al. Depósito atmosférico de Berilio-7 en suelo (Atmospheric deposition of beryllium-7 at ground level). Anales AFA Montevideo 2011;23:157-60.  Back to cited text no. 9
    
10.
Arkian F, Meshkatee AH, Bidokhti AA. The effects of large-scale atmospheric flows on berylium-7 activity concentration in surface air. Environ Monit Assess 2010;168:429-39.  Back to cited text no. 10
    
11.
Kikuchi S, Sakurai H, Gunji S, Tokanai F. Temporal variation of (7)Be concentrations in atmosphere for 8y from 2000 at Yamagata, Japan: Solar influence on the (7)Be time series. J Environ Radioact 2009;100:515-21.  Back to cited text no. 11
    
12.
Garger EK. Air concentrations of radionuclides in the vicinity of chernobyl and the effects of resuspension. J Aerosol Sci 1994;25:745-53.  Back to cited text no. 12
    
13.
Young JA, Silker WB. Aerosol deposition velocities on the Pacific and Atlantic Oceans calculated from 7Be measurements. Earth Planet Sci Lett 1980;50:92-104.  Back to cited text no. 13
    
14.
Kulan A, Aldahan A, Possnert G, Vintersved I. Distribution of 7Be in surface air of Europe. Atmos Environ 2006;40:3855-68.  Back to cited text no. 14
    
15.
Liu J, Starovoitova VN, Wells DP. Long-term variations in the surface air 7Be concentration and climatic changes. J Environ Radioact 2013;116:42-7.  Back to cited text no. 15
    
16.
Muñoz E, Garay A. Caracterización Climática de la Provincia de Río Negro. Comunicación Técnica 20. Área Recursos Naturales-Agrometeorología (Climatic characterization of Rio Negro province. Technical Report 20. Natural Resources – Agrometeorology Department). INTA EEA Barilochep; 1985. p. 56.  Back to cited text no. 16
    
17.
Gilmore GR. Spectrometer cailbration. In: Practical Gamma-Ray Spectrometry. 2nd ed. Warrington, UK: John Wiley & Sons, Ltd.; 2008. p. 143-63.  Back to cited text no. 17
    
18.
Knoll GF. Lithium-drifted germanium detectors. In: Radiation Detection and Measurement. 1st ed. New York: John Wiley & Sons, Inc.; 1979. p. 414-70.  Back to cited text no. 18
    
19.
Scarnichia E, Levanon I, Andres P, Miani C, Ramírez S. Radiological Environmental Monitoring with LiF700H Dosemeters. XII International Symposium. XXII National Congress on Solid State Dosimetry. Available from: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/43/009/43009449.pdf. [Last updated on 2011 Sep 07; Last accessed on 2018 Jul 11].  Back to cited text no. 19
    
20.
Aegerter S, Bhandari N, Rama, Tamhane AS. Be7 and P32 in ground level air. Tellus 1996;18:212-5.  Back to cited text no. 20
    
21.
Feely HW, Larsen RJ, Sanderson CG. Factors that cause seasonal variations in beryllium-7 concentrations in surface air. J Environ Radioact 1989;9:223-49.  Back to cited text no. 21
    


    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2]



 

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