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ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 2  |  Page : 73-78  

One year of 222Rn concentration at a typical rural site in South India


Department of Physics, Atmospheric and Space Science Research Laboratory, Bangalore University, Bengaluru, Karnataka, India

Date of Submission09-Jun-2021
Date of Decision20-Jul-2021
Date of Acceptance20-Jul-2021
Date of Web Publication23-Oct-2021

Correspondence Address:
Nagaraja Kamsali
Department of Physics, Bangalore University, Bengaluru - 560 056, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_21_21

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  Abstract 


The simultaneous measurements of atmospheric radon, ambient gamma radiations dose, and relevant meteorological parameters were carried out at the National Atmospheric Research Laboratory (NARL), Gadanki, India (13.459° N, 79.175° E) during June 2013–May 2014 are analyzed and presented. The results show that radon strongly correlates with temperature, relative humidity, and a weak correlation with air pressure, ambient gamma dose during fair weather days. Radon's well-defined monthly variability is observed, with the highest during winter and lowest during monsoon season. The fast Fourier transform analysis revealed a hidden memory in variations in radon activity with prominent peaks at 24 h and 12 h, indicating the influence of atmospheric stability on the abundance of radon in air. About 99% of radon activity lies below 70 Bq/m3 with a mean value of 11.81 ± 4.83 Bq/m3, and about 99% ambient gamma dose levels range from 140 to 240 nSv/h at NARL with a mean value of 192.17 ± 17.43 nSv/h. The ambient gamma dose levels are well within limits prescribed by the UNSCEAR.

Keywords: Ambient gamma dose, atmospheric stability, gadanki, radon


How to cite this article:
Kumar K C, Kamsali N. One year of 222Rn concentration at a typical rural site in South India. Radiat Prot Environ 2021;44:73-8

How to cite this URL:
Kumar K C, Kamsali N. One year of 222Rn concentration at a typical rural site in South India. Radiat Prot Environ [serial online] 2021 [cited 2021 Dec 7];44:73-8. Available from: https://www.rpe.org.in/text.asp?2021/44/2/73/329134




  Introduction Top


The exposure of humans to the radiations emitted by the nuclear species present in Earth's crust and the atmosphere is inevitable and inescapable. Among all the radionuclides, progenies of the primordial nuclides 238U and 232Th with half-lives of 4.5 × 109, and 1.4 × 1010 years, respectively, are given particular importance. These actinides exist in the lithosphere, and several daughter nuclides are formed during decay, finally end up in activity when they reach different isotopes of lead.[1] It is interesting to observe that, in both decay chains of 238U and 232Th; one intermediate nuclide is of significance in the field of atmospheric radioactivty studies in the lower atmosphere,[2],[3] namely 222Rn and 220Rn, respectively, but due to its lower half-life, 220Rn is given less importance over 222Rn. Studies on radon (222Rn, λ1/2~3.82 days) have attracted several researchers worldwide because of its radioactive, chemically inert, and gaseous nature. When radium decays in soil, due to its gaseous nature, daughter nuclei radon starts to exhale from voids and pores of the surface, released into the atmosphere. Being a radioactive gas, radon emits α-particle of energy 5.67 MeV and this energy ionizes the surrounding atmospheric air. In general, an oxygen and nitrogen-rich environment, an α-particle produced by radon decay can produce ~1.77 × 10+5 ion pairs/m3/s.[4] Several reports are available on the multidisciplinary application of radon measurements in atmospheric radioactivity, atmospheric electricity, earthquake prediction, and lower atmospheric stability applications.[5],[6],[7],[8] It is interesting to note that very few reports are available on the studies of radon and its progenies at the National Atmospheric Research Laboratory, Gadanki, India (NARL-13.459° N, 79.175° E), which is an autonomous institute for advanced atmospheric research in India.[9],[10] The measurements of radon, gamma radiation dose, and relevant meteorological parameters were carried out at NARL during June 2013–May 2014, and the detailed results are presented.

Site description

The simultaneous measurements of radon, ambient gamma levels, and relevant meteorological parameters were carried out at NARL, Gadanki, India. NARL is an autonomous research institute to carry out applied research in atmospheric and space science. It is equipped with MST radar, Ray Leigh Doppler Lidar, Ionosonde, and Doppler Sodar.

The geographical surrounding of NARL is complex, with dense vegetation, and rocky hills, as shown in [Figure 1]. Both Southwest and Northeast monsoons influences are perceived at NARL. The general direction of the wind changes from season to season. The wind is southerly and southeasterly throughout April, westerly from May to September, and northeasterly during October and November.[9],[11]
Figure 1: Geological location of national atmospheric research laboratory

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  Materials and Methods Top


The measurements of radon activity were carried out using Genitron made AlphaGUARD PQ 2000 PRO (AG). AG works on the basic principle of the design optimized pulse ionization chamber (alpha spectroscopy), and in regular operation, the measuring gas gets in diffusion mode through a large surface glass fiber filter into the ionization chamber. In alpha guard, the type of radon detector used is pulse optimized ionization chamber with a potential difference of +750 VDC between electrodes. The mode of operation is 3D-alpha spectroscopy and current mode. The total detector volume is 0.62 L, and the active detector volume is 0.56 L. The type of radon-filter paper (detector entry window) is a glass fiber filter paper with a retention coefficient >99.9%. The fast digital signal sampling network using three separate ADC channels is used for detector signal acquisition. The detector's sensitivity is around 99% for radon and 1-count/min at 20 Bq/m3. The background signal due to internal detector contamination is <1 Bq/m3. The maximum activity of radon that this instrument can detect is 20,00,000 Bq/m3. The measurement cycle time is 10-min and 60-min in diffusion mode and 1-min and 10-min for flow-through mode.

The data holding capacity is 3 days for 1-min, 1-month for 10-min, and 6 months for 60-min measuring cycle. It can run up to 10 days from an internal battery. The calibration factors are obtained experimentally and certified by the National Institute of Standard and Technology (USA), National Physical Laboratory (UK), and Physikalisch-Technische Bundesanstalt (Germany) US-Environmental protection agency conducted tests and certified that only ±2% deviations in alpha guard measurement. Therefore, the alpha guard can be used as a reference device to calibrate other active and passive radon detectors.[12],[13] The continuous ambient gamma dose measurements were carried out using an integrated Geiger-Muller (GM) tube inside the AG. The GM tube inside AG is completely protected, and the measuring range is from 20 nSv/h to 10 mSv/h (Sv) with a resolution of 1 nSv/h. AG also consists of temperature, atmospheric pressure, and relative humidity sensors, making it an ideal instrument to monitor radon and ambient gamma dose with maintenance-free operation.[14] AG was installed inside the Stevenson screen to avoid the direct effect of solar radiation, rainfall, and wind at the height of 1 m from Earth's surface.


  Results and Discussions Top


The continuous measurement of the activity of radon with ambient temperature and relative humidity from December 6, 2013 to December 11, 2013 is shown in [Figure 2] and [Figure 3], respectively. It is observed that the meteorological parameters significantly affect the local radon activity at NARL.[9] It is observed that a Pearson correlation coefficient of ‒0.75 exists between radon and ambient temperature, and this kind of behavior is observed for most of the fair-weather days at NARL. This kind of behavior may be attributed to the formation of temperature inversion near Earth's surface before the sunrise when the temperature is lowest, and this inversion layer traps most of the atmospheric constituents a few meters near the surface of Earth, including radon.[15] In general, during this period, the wind is also at its lowest; hence, atmospheric constituents' advection is minimal. After sunrise, the temperature inversion is destroyed by the thermal energy, and increased vertical mixing of atmospheric constituents occurs. This mixing process is maximal at afternoon hours when the temperature is highest, and the wind is also at its highest. Hence, both vertical mixing and horizontal advection significantly reduce the concentration of radon during afternoon hours. As air temperature controls the water evaporation and air saturation, the lower temperature leads to the highest relative humidity values at NARL and hence a positive Pearson correlation coefficient of 0.66 was observed between radon and relative humidity.
Figure 2: Variation of radon with ambient temperature at national atmospheric research laboratory

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Figure 3: Variation of radon with relative humidity at national atmospheric research laboratory

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However, it was observed that a very weak correlation exists between radon and air pressure and ambient gamma dose levels. It is also observed that this kind of relationship between radon and meteorological parameters also exists for diurnal averaged monthly means, as shown in [Figure 4].
Figure 4: Scatter plots between diurnal averaged monthly means of radon and measured parameters at national atmospheric research laboratory

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It's worth noting that the association between radon activity and ambient gamma dosage is poor (‒0.013), indicating that background gamma radiation levels at NARL are unaffected by meteorological conditions. At NARL, the diurnal variations in radon activity for most of the fair-weather days were the same, i.e., higher values during early morning hours before sunrise, sharp decline after sunrise till afternoon, and minima till late afternoon hours. However, the magnitude of its variation depends on the month in which measurements were carried out. One such measurement at NARL is shown in [Figure 5], and observations were on 10 August 2013 and 10 January 2014. It was observed that over NARL, on January 10, 2014, the radon (max-min) was 42 Bq/m3. On April 10, 2014, it was found to be 13 Bq/m3, and on August 10, 2013, it was found to be 8 Bq/m3.
Figure 5: Variation of Radon over national atmospheric research laboratory at different days

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Hence, it was found that the radon activity was higher during winter months as compared to monsoon and summer, and the same can be observed in [Figure 6], which represents the monthly mean values of radon at NARL from June 2013 to May 2014.[16],[17],[18]
Figure 6: Whisker's plot of the monthly mean of Radon activity at national atmospheric research laboratory

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In [Figure 6], the lower asterisk mark is the average minimum activity of radon, lower line in a rectangle is 10% percentile, square in a rectangle is median, middle line in a rectangle is mean, and upper line in a rectangle is 90% percentile. The upper asterisk is the average maximum activity of radon for each month. This kind of behavior of radon is well reported for a similar environment.[16],[19] Radon activity variations mainly depend on the source of its origin, i.e. from the soil and prevailing meteorological conditions. Variations are in the order of diurnal, monthly, seasonal, and yearly time scales are prominent.

Only the continuous measurements can give precise information about the possible memory hidden within the time series of a measured quantity. In the standard inspection of time series data, it is not accessible to identity such memory, if any. However, using special techniques, it is possible to identify any such information. One such technique is the fast Fourier transform (FFT) that analyses the amplitude versus frequency of the measured quantities. The dataset of radon was checked using the FFT technique in the frequency domain to see the periodicity hidden within the time series. Fourier transform technique makes tapering of the data window and averaging of segments is not considered. The uniform sampling of the required data to the sample is employed in this technique. The FFT spectrum of radon for the data of NARL is shown in [Figure 7]. It is interesting to observe that a well-defined peak is also present at a time period corresponding to 12 h apart from regular 24 h trend for the study period. The regular inspection of time series data reveals only the 24 h trend in all the measured quantities in general. The observed peak at 12 h may be generally attributed to the well-known frequencies of solar radiation tide, which is the main reason for the temporal variations in activity concentration of radon in atmosphere air.[20],[21] Moreover, the shortest time scales (12 h) could be due to the nighttime boundary layer formation/evolution and growth of radon activity.[22]
Figure 7: Fast Fourier transform spectra of radon activity over national atmospheric research laboratory

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In contrast, the 24 h cycle is due to the formation of temperature inversions during early morning hours, breaking of inversion after sunrise, and increase in vertical mixing till late afternoon hours.[23] It is interesting to observe that the frequency distribution of radon activity follows the exponential decay trend with adj R2 of 0.95, as shown in [Figure 8]. This indicates that 99% of radon activity over NARL is below 70 Bq/m3. However, the frequency distribution of ambient gamma dose levels at NARL follow the Gaussian curve with an adj R2 of 0.99, and the 99% of data range from 140 to 240 nSv/h, as shown in [Figure 9].
Figure 8: Frequency distribution curve of radon at national atmospheric research laboratory

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Figure 9: Frequency distribution curve of ambient gamma levels at national atmospheric research laboratory

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This indicates that the ambient gamma dose levels over NARL are constant over time, and this is the first baseline data of gamma radiation over this environment. It was also found that the measured ambient gamma dose values are well within the UNSCEAR limit of 2.4 mSv/year.[24] The detailed statistics of all the measured parameters are tabulated in [Table 1].
Table 1: Detailed statistics of all the measured quantities at National Atmospheric Research Laboratory

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It was found that the measured radon activity and ambient gamma dose levels at NARL are comparable with the values reported for the South Indian environment.[15],[16],[18],[19],[25],[26]


  Conclusion Top


For the first time, the simultaneous measurements of atmospheric radon, ambient gamma dose levels, and meteorological parameters were analyzed at NARL during June 2013–May 2014. A well-defined and similar diurnal trend was observed in radon activity for all fair-weather days with different magnitudes. A monthly variability exists in the activity of radon, with higher during winter months and lowest during monsoon months. A strong correlation exists between radon and temperature and relative humidity, but a weak correlation exists with air pressure. FFT analysis revealed another peak at around 12 h which may be attributed to the nighttime boundary layer formation/evolution and growth of radon activity. The frequency distribution of radon activity has followed exponential decay trend with 99% values laying within 70 Bq/m3 whereas for ambient gamma dose levels, it was found that the Gaussian fitting is the possible fit with adj R2 of 0.99 and 99% values lie within 140–240 nSv/h. The present research work has established a baseline dataset of radon and gamma dose levels over NARL, and with coordinated experiments, it is possible to utilize radon activity in understanding several research problems in the lower atmosphere at NARL.

Acknowledgments

We would like to thank the Indian Space Research Organization for the financial assistance through the RESPOND project (Grant Number: B 19012/72/2011-II dated 23-12-2011) and Director, NARL for the permission to carry out experiments.

Financial support and sponsorship

Indian Space Research Organizationthrough RESPOND project (Grant Number: B 19012/72/2011-II dated 23-12-2011).

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
    Tables

  [Table 1]



 

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