|Year : 2011 | Volume
| Issue : 4 | Page : 235-239
Study of radon exhalation rate from soil, Bangalore, South India
GV Ashok1, N Nagaiah1, NG Shiva Prasad2, MR Ambika2
1 Department of Physics, Government College (Autonomous), Mandya, India
2 Department of Physics, Government First Grade College, Srirangapatna, India
|Date of Web Publication||17-Jan-2013|
Department of Physics, Government College (Autonomous), Mandya
Source of Support: None, Conflict of Interest: None
The 222 Rn exhalation rate measurements are useful in identifying the locations of high radon exposure. On the other hand, atmospheric 222 Rn concentrations have been observed as an effective tracer for terrestrial air mass movements and atmospheric mixing mass movements. In view of this, year-long measurements of 222 Rn and its exhalation rates were carried out at J B Campus of Bangalore city, and the diurnal and seasonal variations have also been studied. Diurnally, exhalation rate was found to be high during night and early morning hours and low during afternoon hours. The diurnal variability of temperature difference between the lower and upper layers of the soil was found to be responsible for the observed diurnal variations. The monthly average of 222 Rn exhalation rate ranges from 10.63 ± 1.75 to 4.95 ± 0.65 mBq m -2 s -1 . The annual mean value of radon exhalation rate was found to be 8.27 ± 1.83 mBq m -2 s -1 .
Keywords: Bangalore, diurnal variation, radon exhalation rate, LLRDS, soil
|How to cite this article:|
Ashok G V, Nagaiah N, Shiva Prasad N G, Ambika M R. Study of radon exhalation rate from soil, Bangalore, South India. Radiat Prot Environ 2011;34:235-9
|How to cite this URL:|
Ashok G V, Nagaiah N, Shiva Prasad N G, Ambika M R. Study of radon exhalation rate from soil, Bangalore, South India. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 6];34:235-9. Available from: http://www.rpe.org.in/text.asp?2011/34/4/235/106093
| 1. Introduction|| |
The mean annual effective dose to the human beings from natural sources of radiation is about 2.4 mSv.  The inhaled radon and its progeny account for more than 50% of the total radiation dose from these natural sources.  Although indoor environment is more important for the exposure of the public than outdoors, the release of 222 Rn from the soil and the building materials made of raw earth have been identified as the main factors influencing the indoor 222 Rn levels in most of the buildings. , Therefore, information on the regional distribution of 222 Rn exhalation from the earth's surface is considered as useful for identifying areas with a risk of high radon exposure to public. Since radon is chemically inert and its source term is relatively well-known ( 226 Ra in soil), the exhalation studies provide information on the transfer component of gaseous exchange between the soil and the atmosphere. Therefore, radon is used as a tracer for atmospheric transportations and as precursors for natural phenomena such as volcanic eruptions and earthquakes.
The radon released from the decay of 226 Ra in soil escapes from the soil grains due to alpha recoil and reaches the atmosphere through the soil pores. The concentration of radon in the lower atmosphere depends on the exhalation rates prevailing during that time, which in turn depends on the meteorological and geophysical parameters. Atmospheric 222 Rn concentrations have been observed as an effective tracer for terrestrial air mass movements and atmospheric mixing phenomenon. , Its characteristics of chemical inertness and the optimum half-life of 3.82 d have made 222 Rn a useful tool for the evaluation of transport and mixing schemes in chemical transport models.  In view of these, measurements of radon exhalation rate and radon concentration in air near the ground level were carried out for a period of one year in Bangalore (J B campus). This study happens to be the first of its kind in this area.
| 2. Materials and Methods|| |
2.1. Radon concentration in the atmosphere
The concentration of 222 Rn in ambient air was determined using LLRDS (Low Level Radon Detection System). It consists of sample collection chamber of volume 5 liters. It is provided with a swage lock connector for an air inlet and outlet. It can be evacuated for collecting the samples by vacuum transfer technique. In this method, the LLRDS chamber was evacuated and the air was allowed inside, until it attains the pressure equilibrium with the atmosphere (2-3 min). A delay of 10 min. was allowed for the complete decay of thoron, which may be present in the chamber. A negative potential (~900 V DC) was then applied generally for about 90 min. for the saturation of radon daughter atoms on the collection plate. At the end, the plate was removed and alpha counted. From the counts obtained, the concentration of radon was calculated using the following expression, 
Where, C is the net counts observed during the counting period,
E is the efficiency of Alpha counting system (30%),
V is the volume of the chamber (5 Liter),
Z is the correction factor for radon daughters and is a function of charge collection time t, counting time (say from T 1 and T 2 ) and the decay constants of radon daughters,
F is the collection efficiency of 218 Po atoms, which is given by
F = 0.9[1- exp (0.039H-4.118)], Where, H is relative humidity (%).
The detection limit of LLRDS is 1.7 Bq m -3 when the relative humidity is 0% and it is 8.8 Bq m -3 when the latter is 100%. The LLRDS was calibrated with respect to both the applied negative potential and the relative humidity at EAD, BARC, Mumbai, India. 
The short-term fluctuations in radon concentrations and the typical diurnal measurements were carried out using an AlphaGUARD, a radon monitor. It is a continuous active radon sampling sensor. It has an ionization chamber and uses an alpha spectroscopy to detect radon.
2.2. Radon exhalation rate from soil
In the present work, the exhalation rate of 222 Rn was estimated by employing the accumulation chamber method. , The accumulation chamber used in the present work was fabricated using a metallic chamber of height 26 cm and diameter 28 cm with its open end buried to a depth of 4 cm at the sampling location. Radon exhaled from the soil was allowed to accumulate in the chamber for a period of 1 hour. The collected air containing the exhaled radon was then transferred into the pre-evacuated LLRDS chamber, and the radon concentration was estimated. The radon exhalation rate (Bq m -2 s -1 ) from the soil was calculated from the estimated radon concentration in the accumulation chamber using the formula,
Where, C R is the concentration of radon (Bq m -3 ), V 1 is the volume of the accumulation chamber (m 3 ), V 2 is the volume of the LLRDS chamber (m 3 ), λ is the decay constant of radon (s -1 ), A is the exhalation area (m 2 ), t is the duration of accumulation of radon gas in the collection chamber (s).
| 3. Results and Discussions|| |
A typical diurnal variation of 222 Rn exhalation rate measured along with ambient temperature and relative humidity in the month of May is presented in [Figure 1]. The exhalation rate ranges from 5.49 to 14.4 mBq m -2 s -1 with a mean value of 10.1 ± 2.82 mBq m -2 s -1 . Radon exhalation rate was found to be high during night and early morning hours, followed by the decrease after sunrise and reaches a minimum during afternoon hours. The ground surface gets heated due to sun during daytime and gets cooled during the night causing marked diurnal change in temperature near the earth's surface, whereas the soil remains relatively warmer at night. As a result, temperature gradient between the soil at a few centimeters depths and on the surface is established. This leads to significant diurnal variation of radon exhalation rates and its concentrations in atmosphere. , In addition to this, exhalation of radon from the soil depends on the emanation factor, which is known to increase with the moisture content of the soil. The temperature drop at night may causes condensation in the soil. The interstitial water due to condensation will absorb the radon atoms recoiling from the soil particle and due to the low solubility of radon in water, the radon emanation rate will increase, thereby giving rise to higher radon concentration in the morning. Therefore, the soil moisture in interstitial pores also directly affects radon emanation by capturing the radon recoiled from the soil matrix. , Hence, negative correlation of 0.72 and a positive correlation of 0.89 are found to exist between exhalation rate and ambient temperature and exhalation rate and relative humidity, respectively.
|Figure 1: Diurnal variation of 222Rn exhalation rate, ambient temperature, and relative humidity|
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In order to understand the variation of 222 Rn exhalation rate, and hence the 222 Rn concentration in the atmospheric air, the simultaneous measurements of 222 Rn concentration, its exhalation rate, soil temperature at a depth of about 10 cm, and air temperatures were carried out in the month of February. The results obtained are presented in the [Figure 2]. A good positive correlation is found to exist between radon and its exhalation rate. It can be seen from the Figure that the exhalation rate increases as the difference between soil and air temperature increases. Exhalation rate reaches its minimum in the afternoon when the temperature difference is low. Thus, the diurnal variability of temperature difference between the lower and upper layers of the soil was found to be responsible for the process of release of radon from the soil to the atmosphere. Cooling of the upper layer of the soil during night hours and the constant high temperature of the lower layers found to promote the upward convective radon flux and bring the radon to the soil surface. , Therefore, higher exhalation rate and temperature inversions in the night and early morning hours enhance the atmospheric radon concentration during that time. A good positive correlation is found to exist between radon and its exhalation rate (0.80). The radon concentration in air ranges from 3.7 to 31.5 Bq m -3 with a mean value of 10.8 Bq m -3 showing the diurnal variation of order 8, whereas the exhalation rate ranges from 5.2 to 12.3 mBq m -2 s -1 showing the diurnal variations of order 2.4. Thus, the order of variation of exhalation rate observed in a day is found to be much smaller compared to that of radon in the lower atmosphere.
|Figure 2: The diurnal variability of 222Rn and its exhalation rate with soil and air temperatures|
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To study the short-term fluctuations, 222 Rn concentrations were measured for every 10 minutes using an Alpha Guard in a fair weather day. The meteorological parameters such as ambient temperature, relative humidity, and atmospheric pressure were also measured simultaneously. In addition, simultaneous measurement of 222 Rn exhalation rates at a time interval of 2 hours was carried out. The results obtained are presented in the [Figure 3]. It can be seen from the Figure that in spite of the short time fluctuations, the same trend of diurnal variations was observed in 222 Rn concentrations and ambient temperature and relative humidity. The fluctuations observed in 222 Rn concentrations may be attributed to continuous variations in the meteorological parameters. The small variation in atmospheric pressure was observed throughout the day (914 to 918 mBar). This shows that, temperature and humidity have a more significant effect on outdoor radon concentration than atmospheric pressure. This study confirms the strong influence of diurnal variability of ambient temperature, relative humidity, and exhalation rate on radon concentration in the atmosphere.
The monthly average values obtained from the diurnal measurements of 222 Rn exhalation rates are presented in [Figure 4] and [Table 1]. Almost the same trend of diurnal variation was observed in all the months. It can be seen from the table that the mean exhalation rate was found to be high in the month of December (10.63 ± 1.75 mBq m -2 s -1 ) and low (4.95 ± 0.65 mBq m -2 s -1 ) in the month of July. The lower exhalation rates were observed in the months of July, August, September, and October due to enhanced water content in the surface soil, which blocks the pores, through which the radon exhales in to the atmosphere. The annual mean value of 222 Rn exhalation rate was found to be 8.27 ± 1.83 mBq m -2 s -1 , which is lower than the mean worldwide flux of 222 Rn of 26 mBq m -2 s -1 . 
|Figure 3: Short time fluctuations in 222Rn concentration and meteorological parameters|
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|Table 1: Average diurnal 222Rn exhalation rates (mBq m-2 s-1) in different months of a year|
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The seasonal variations in exhalation rates are presented in [Figure 5]. It can be seen from the Figure that exhalation rates are lower during rainy and summer compared to winter season. Seasonal averages of winter and summer were found to be almost same (~10 mBq m -2 s -1 ), whereas during rainy season, it was found to be 6.2 mBq m -2 s -1 . The reduction in the exhalation rate from the soil during rainy season may be attributed to filling up of soil pores by water molecules and is as reported by Wilkening, 1990. 
| 4. Conclusion|| |
Good correlation was found to exist between radon and its exhalation rate, both reaches maximum levels in the early morning hours and minimum in the afternoon. The diurnal variability of temperature difference between the lower and upper layers of the soil was found to be responsible for the process of release of radon from the soil to the atmosphere. The order of variation of exhalation rate observed in a day is found to be much smaller compared to that of radon in the lower atmosphere. The ambient temperature and humidity are found to have a more significant effect on outdoor radon concentration than atmospheric pressure. The exhalation rate was found to be high in the month of December and low in the month of July. Again, during rainy season, the exhalation rate was found to be lower compared to winter and summer. The annual mean value of radon exhalation rate was found to be 8.27 mBq m -2 s -1 .
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
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