Applied Radiation and Isotopes 99 (2015) 179–185
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Air ions as indicators of short-term indoor radon variations P. Kolarž a,n, Z. Ćurguz b a b
Institute of Physics, University of Belgrade, 11080 Belgrade, Serbia Faculty of Transport and Traffic Engineering, University of East Sarajevo, 74000 Doboj, Bosnia and Herzegovina
H I G H L I G H T S
We studied indoor correlation between cluster air ions and radon concentrations. Air ions are proved to be a good indicator of rapid radon changes. Relation is slightly changing from linear to square root when concentration of both increasing. Methods of theoretical approach are presented.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2014 Received in revised form 18 February 2015 Accepted 2 March 2015 Available online 3 March 2015
Diurnal variations in the air ion concentration are subject to changes in the radon concentration. In this experiment, the air ion and radon concentrations were simultaneously measured using two air ion detectors and two continuous radon detectors. The results of the indoor measurements revealed a strong correlation between the concentrations of positive air ions and radon (with a correlation coefficient greater than 0.9). The radon-to-ion concentration ratio changes with an increase in the radon concentration from a linear to a square-root relation. This correlation provides a means of using air ion measurements as a high-confidence indicator of changes in the radon concentration, especially for shortterm measurements on the order of seconds or minutes, which is too short a measurement interval for conventional radon monitors. The use of air ions as an indicator of changes in radon concentration allows for investigation of the behavior of indoor radon and also allows radon to be used as a tracer gas for air mass exchange. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Air ions Indoor radon Thoron Natural radioactivity Tracer Recombination
1. Introduction Air ions are airborne charged particles with electrical mobility. The existence of air ions in the lower atmosphere is a consequence of various ionization processes: the radioactive decay of airborne radionuclides (radon and thoron), the decay of radioactive minerals from the earth's crust and ionization induced by cosmic radiation (Israël, 1970; Hirsikko et al., 2007). The ionization rate of the last two sources is almost constant on daily basis, whereas the radon concentration is variable over short periods of time. These variations are attributed to the level of radon exhalation from the soil and to local atmospheric conditions and are of considerable importance in indoor environments. Continuous real-time measurements of changes in radon activity suffer from limited time resolution, inertia of the measuring process and high n
Corresponding author. E-mail address: [email protected] (P. Kolarž).
http://dx.doi.org/10.1016/j.apradiso.2015.03.001 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
measurement uncertainty. To measure rapid or short-term changes in the radon concentrations in indoor spaces, an indirect method of measuring radon by measuring the air ion concentration can be utilized (Kolarž et al., 2009). In this way, the intensity and effectiveness of radon removal can be monitored during remediation, and radon can also be used as a tracer gas during air ventilation. 1.1. Radon Radon (222Rn) Is well-known radioactive noble gas that Is 7.5 times heavier than air. it Is produced in the decay chain of the primordial elements uranium and thorium, which are found in various concentrations in the soil worldwide. 222Rn α decays with a half-life of 3.82 days, and this decay Is followed by a series of four further decays (2 α and 2 β) with relatively short half-lives. when inhaled, short-lived radon progeny in the respiratory tract result in the deposition of α energy in the cells of the bronchial epithelium (Hopke et al., 1995).
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When generated in the Earth's crust, radon penetrates the pores in the ground and moves upward through diffusion and convection toward the surface and into the air (Nazaroff, 1992). The radon flux, i.e., exhalation rate, at the surface of the soil is strongly influenced by weather conditions such as precipitation, air pressure, and the temperature of the air as well as the permeability, thermal gradient and humidity of the soil (Dueñas et al., 1997). The transport of radon through the atmosphere is determined by thermal and other meteorological processes. The dynamics of changes in the radon concentration is driven by the temperature ratio between the surface soil and the air, which results in a radon maximum arising with the temperature inversion that occurs during the night and a minimum in the afternoon. When it is exhaled in an indoor space, Rn is prone to accumulation. The indoor radon concentration is a consequence of radon exhalation from the soil and from the materials of the building. The ratio between the contributions of these two sources depends on the underlying soil, the building materials and the construction quality. The primary mechanism for the entry of radon into buildings is a pressure-driven flow of soil gas through openings in the floor. Many epidemiological studies and pooled analysis indicate a strong correlation between lung cancer and radon concentration, which is especially high for the smoker population (Bochicchio, 2005; Darby et al., 2005). Radon is a harmful radioactive gas and does not combine chemically with other gasses. It is relatively easy to detect, making it suitable for use as a tracer gas for studying a number of processes in both indoor and outdoor atmospheres (Wilkening, 1990). Many experiments and calculations have been performed concerning the long-term natural ventilation rates in houses and caves using models and experimental results: see Wilkening and Watkins (1976), Capra and Silibello (1994), Nazaroff et al. (1981), and Perrier and Richon (2010). 1.2. Air ions Air ions are continually created by natural sources such as cosmic rays, the radioactive decay of the noble gas radon in the air and that of radioactive minerals in the ground. The average ionization rate in continental areas is approximately 10 ion pairs cm 3 s 1 at height of 1 m above the ground. The contribution of cosmic radiation to the air ion pair generation rate is approximately 2 ion pairs cm 3 s 1, and remainder is attributed to the decay of radioactive minerals. Shortly after the ionization process (within microseconds), primary ions evolve through the process of hydration and form small cluster ions, also known as small air ions or nano-air ions. The central ion of a cluster may contain one inorganic molecule and may be surrounded by one layer of water molecules (Mohnen, 1977). This class of air ions can survive up to 100 s, depending predominantly on air pollution and air density. The transformation from the gas phase to the aerosol phase proceeds through the process of spontaneous nucleation. Air ions are classified into small cluster ions (0.36–0.85 nm), large cluster ions (0.85–1.6 nm), intermediate ions (1.6–7.4 nm) and large ions (7.4–79 nm) (Hõrrak et al., 1994; Hõrrak, 2001). Only the smallest class of ions, with the highest mean natural mobilities of 1.36 (positive) and 1.53 cm2 V 1 s 1 (negative), is considered in this work. The air ion concentrations in natural surroundings and over soil typically lie in the range of 102–104 ions s cm 3, primarily depending on the radon exhalation potential and accumulation. The lower mobility of positive air ions grants them a lower rate of attachment (s) to aerosol particles (Z) and, consequently, a longer lifetime, resulting in higher concentrations of these air ions. The ratio of positive to negative ion concentrations (n) in the natural outdoor environment is n þ /n ¼1.12 (Hõrrak, 2001). Temporal
variations in the air ion concentration (neglecting the asymmetry between positive and negative ions and the electrostatic deposition of air ions) can be described using a simplified air ion balance equation (Israël, 1970):
dn = q − αn2 − sn dt
(1)
where q is the cluster ion production rate, α is the recombination rate of small ions, α ¼ 1.6 10 6 cm3 s 1 (Tammet and Kulmala, 2005) and s is the total rate of attachment of cluster ions to aerosol particles (in units of cm3 s 1). Similar to radon, the gaseous isotope thoron (220Rn) is an αparticle emitter, although it has a much shorter half-life (55 s). Consequently, it decays only in the near vicinity of its source (at distances of up to a few tenths of centimeters), which, in the case of buildings, is the construction materials. When it exists in higher concentrations, thoron is an important source of air ions. The presence of thoron can negatively influence the efficacy of air ions as indicators of short-term indoor radon variations, but thoron is rarely observed in significant concentrations. Following radon and thoron decay, all their short-lived decay products, such as polonium (218Po and 216Po) and lead (214Pb and 210 Pb), exist in the solid state and are predominantly α emitters. These products may be suspended in air in attached or unattached fractions or may be deposited on solid surfaces. In the air, these particles contributing to the air ion pair production rate (both the attached and unattached fractions). Their contribution to the air ionization is proportional to the radon concentration. If radon and thoron progenies are deposited on surfaces, then their ionization contribution is significantly reduced because of the limited angle of the free path for the emitted α particles to travel through the air and the electrostatic deposition of ions on insulating surfaces. In calculations, the air ion production rate (qRn) is related to the contribution of radon and its progeny. Because the half-lives of free air ions in the air are very short (on the order of a few tens of seconds) compared with the halflives of ventilation in rooms (on the order of hours), the assumption of a quasi-steady state can be applied, and the differential equation reduces to an algebraic quadratic equation. The air ion production rate q consists of at least two contributions, one (qRn) related to radon and its progeny and the other (qγ) related to gamma and cosmic radiation. At very low radon levels, the linear term of Eq. (1) cannot be neglected, resulting in a generally more complicated solution. Therefore, with increasing radon concentration, the dependence of the ion concentration on the radon concentration changes continuously from a constant value to a square-root relation. Given the approximations that the air ion and radon concentrations exist in a steady state and that the positive and negative ion concentrations are equal, the balance equation for small air ions is as follows:
αn2 + sn = qRn + qγ
(2)
where qRn is the rate of ion production by radon and its progeny and qγ is the rate of ion production by gamma and cosmic radiation (in units of m 3 s 1). The concentration of positive ions is often significantly different from the concentration of negative ions as a result of increased aerosol concentration, ion deposition on electrostatic surfaces and the atmospheric electrode effect in the near-ground layer (Hoppel et al., 1986). Because positive ion clusters have a lower mobility and are not affected by the electrode effect, their concentration is less susceptible to these influences, resulting in a higher and more stable concentration. Consequently, only the positive ion concentration is considered here.
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2.1. Air ion detectors
The solution of the balance equation is
n=
⎞ s ⎛ 4α ⎜ 1+ (qRn + qγ ) − 1⎟ 2 2α ⎝ s ⎠
(3)
and can also be written in the form of Eq. (4) in Graeffe and Keskinen (1989). The ion production rates must be related to the gamma dose rate (Ḋ ) , defined by qγ ¼d Ḋ , and the radon concentration a (in units of Bq m 3), defined by qRn ¼ ka, where d and k are the coefficients of ion production by the γ background and radon, respectively. The production rate arising from low Linear Energy Transfer (LET) radiation can be treated, under indoor conditions, as a constant value corresponding to the average indoor gamma dose rate, Ḋ ¼80 nGy h 1 (UNSCEAR, 2000). Inverting relation (3) yields the relation a(n):
a (n ) =
s α 2 d n + n − Ḋ , k k k
181
(4)
which is a parabola with its vertex in the fourth quadrant. To obtain a better approximation of the radon-to-air-ion ratio, simultaneous measurements of both parameters are needed. This is because of the influence of increased radon, thoron and aerosol concentrations as well as the existence of electrostatic fields on insulating surfaces (Graeffe and Keskinen, 1989).
Air ion detectors operate on the aspirated Gerdien condenser principle (Gerdien, 1905). All detectors have an autonomous power supply, a real-time clock, and sensors for temperature, pressure and relative humidity. Functions of the instrument such as dynamic zeroing, data acquisition, data averaging, sampling and polarity change are programmable, whereas acquisition is performed internally or by a PC (Kolarž et al., 2009; Kolarž et al., 2012). Three coaxial cylindrical electrodes are used, i.e., a central measuring electrode, a polarizing electrode and a shielding electrode (Fig. 1). Air passes through the electrode system, where the polarizing electrode deflects ions of the same charge onto the surface of the central electrode. The accumulated charge is measured by an electrometer and reported in units of ions cm 3. The size and mobility of the measured ions are determined by the polarizing voltage and the air flow. If both polarities are being measured, zeroing of the system is performed upon every alteration in polarity. It is necessary to compensate for the noise induced by changes in T and RH as well as external electromagnetic waves and the high-voltage peaks induced by polarity changes and fan stopping/starting (Kolarž et al., 2012). Digitalization of the CDI-06 output signal enables the programming of auto-zeroing and automatic long-term measurements without external control. The measuring uncertainty is within 5%.
2. Instrumentation and measurements
3. Experimental methods
Experiments were performed using 2 identical and previously calibrated CDI-06 air ion detectors (manufactured at the Institute of Physics, Belgrade) and two Rad-7 instruments (Durridge Company, USA). Measurements were performed simultaneously in the urban area of Belgrade city.
To evaluate the transport properties of the underlying soil and the concentration of 238U, the 222Rn exhalation rate (flux density, ϕ) from the ground surface was measured nearby the measuring location. The radon activity was measured in 15-min sniff mode as described by the Durridge (2000) owner's manual:
Fig. 1. Block diagram of the cylindrical air ion detector (CDI-06).
182
ϕ = B V /S
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(5)
where B is the slope of the linear fit to the increasing radon concentration in the emission chamber; V is the volume of the air loop, 0.025 m3 (air volume of the Rad-7 unit plus the laboratory drying unit and the internal volume of the emission chamber); and S is the base surface area, 0.071 m2. Measurements were performed in a single room of an old house with wooden floors and no concrete foundations in the center of Belgrade. The floor surface of the measured room was 5 m 3 m, and it was 3.2 m high (volume 48 m3). One radon detector and one air ion detector were placed in the middle of the room, whereas the other pair of instruments was placed outside, 2 m from the outer wall of the room at the location where the radon flux was measured. All instruments were placed 1 m above the ground. During the long-term measurements, the radon concentration sampling was set to one-hour intervals, whereas the air ion detectors were set for 2-min intervals of sampling and zeroing of a single polarity.
4. Results and discussion The average daily value of the radon exhalation flux was 30 mBq m 2 s 1, slightly higher than the world average, which lies in the range between 16 and 26 mBq s 1 m 2 (UNSCEAR, 1993). Thoron was measured using the Rad-7 instrument in the house with the inlet leaning against the wall. The thoron concentration was found to be close to zero; thus, its influence on the air ion concentration was neglected. The temperature of the room during the measurements was approximately (23 72) °C, and the relative humidity was (517 3)%, whereas the outside temperature was in the diurnal range between 16 and 25 °C. The diurnal changes in the concentrations of positive and negative air ions and of radon, with maxima in the early morning and minima in the afternoon, are shown in Fig. 2. Because radon is a gas and is prone to accumulate in indoor spaces, its maximum concentration over the entire span of the 2-day measurements (Fig. 2) was 624 Bq m 3, whereas the average was
(398755) Bq m 3. The average concentration of positive air ions was 5064 ions cm 3, whereas that of negative air ions was 4120 ions cm 3. The unipolarity factor of positive and negative ions was n þ /n ¼1.23, meaning that the surrounding air was relatively unpolluted, i.e., aerosol free. Despite the care that was taken when entering the room, with minimal instances of door opening during the measurement, an apparent decrease in the air ion concentration was measured (Fig. 2), indicating that air ions could be used as a tracer for the radon ventilation exchange rate. This possibility was analyzed in separate measurements, in which the indoor radon and air ion concentrations were lowered to the outdoor background level by opening the window, as shown in Figs. 3 and 4. After only one window (1.2 m2) was opened without a draft in the room where the measurements were performed, the air ion concentration decreased by 50% during the first 3 min and reached the outdoor concentration value within 9 minutes (E650 positive ions cm 3), meaning that a balance was established between the outdoor and indoor radon (air ion) concentrations. The reverse process was much slower, primarily because of diurnal changes in radon exhalation and the longer time required for accumulation. The gray zone in Fig. 2 represents an interval during which one of Rad-7 detectors was set to 5-min measuring periods. The purpose of applying this instrument setting was to demonstrate that with a broad dispersal of the data and an average measuring uncertainty of 227 Bq m 3, the accuracy of the tracking of changes in the radon concentration is lost. During the collection of the data shown in Fig. 3, the window was opened two times, once at the beginning of the measurement and again after the early-morning radon concentration maximum had been reached. Both times, the room was completely ventilated, as indicated by the equalization of the outdoor and indoor radon and air ion concentrations. After the first opening of the window, the increase in the radon concentration was slow, and the radon and air ion measurements were in relatively good agreement. After second opening of the window, the radon measurements showed signs of considerable integration behavior, obscuring the fine structure of the change in the ion concentration
Fig. 2. Concentrations of positive (upper graph) and negative (lower graph) air ions and of radon during 2-day measurements performed on the ground floor of a house in Belgrade. Radon was measured in 1-h intervals and in 5-min intervals (gray zone to the right) to emphasize the effect of data dispersal.
P. Kolarž, Z. Ćurguz / Applied Radiation and Isotopes 99 (2015) 179–185
183
Fig. 3. Air ion and radon measurements during two short periods of window opening (half-day measurements).
Fig. 4. Magnified view of a 50-min period during the second window opening showing the indoor and outdoor air ion concentrations.
(i.e., radon concentration) during the indoor-outdoor air exchange. This observation was a consequence of the higher measuring interval (15 min is the shortest possible time required to obtain results for relatively low data dispersion) but also of the presence of residual radon in the Rad-7 measuring chamber and drying unit (2.5 L) and the relatively low air flow (2 L min 1), which also caused significant time integration of the measurements. 4.1. Correlation coefficients (a) The results of the 2-day measurements are presented in Fig. 2. Approximately 32 one-hour averages during the 2-day measurements of the concentrations of positive and negative cluster ions and of radon were calculated and are shown on a linear scale (Figs. 5 and 6). The correlation coefficients of the air ion and radon concentrations were 0.79 for positive ions and 0.80 for negative ions (Fig. 5), whereas the correlation coefficients calculated on a logarithmic scale were 0.64 and 0.73, respectively. Excluding the outlying point, the slopes (b 7 ) of the n(a) relations were found to be b þ ¼0.5467 0.069 and b ¼0.63 7 0.08 for 32 h
Fig. 5. Correlation coefficients of air ion and radon concentrations on a linear scale.
and b þ ¼ 0.587 0.07 and b ¼ 0.637 0.082 for the first 12 h (Figs. 7 and 8). Using only the first-12-h averages from the measurements presented in Fig. 2, the correlation coefficient is 0.97 for both negative and positive ions. This value is much higher than those for the 32-h averages, which are 0.78 and 0.79 for positive and negative air ion concentrations, respectively (Fig. 6). The slope for the negative-ion case (b ) is significantly higher than that for the positive-ion case (b þ ) (Fig. 8), and the difference b b þ is significantly higher than zero in both cases. (b) Half-day measuring results are presented in Fig. 3, and Fig. 4 presents only a short subset of these data. A power-law fit to 11 pairs of results for air ions and radon concentration obtained during the “window open” regime yielded a value of 0.899, which is higher than the value of 0.580 obtained for the 2-day measurement. This difference was most likely caused by rapid changes in the radon concentration and the lag time of the Rad-7 instrument.
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184
Table 1 Calibration values for the radon and air ion concentrations. Dose rate, nGy h 1 1 84 2 84
Radon concentration (ai), Bq m 3
Ion concentration (ni), cm 3
150 6000
3100 22,600
4.2. Relation between air ions and radon (n(a)) One of the purposes of the measurements was to fit the relation between the air ion and radon concentrations, n(a) or a(n), to the experimental results. To this end, two distinct pairs of measurement results were chosen, e.g., one point at which the radon concentration was low (and the influence of gamma radiation high) and the second point at a higher radon concentration (Table 1). Choosing such values for calibration (fitting) and substituting these data into relation (2) yields two linear equations for ̇ ) and k as follows: the calibration constants d′ (d′ = Dd Fig. 6. Radon and air ion correlation coefficients calculated for the first 12 hours and for 32 hours until the first entry into the room and the opening of the windows.
positive ions negative ions Power (positive ions) Power (negative ions)
7000
Air ion concentrations, ions cm-3
d‵ + ka2 = αn22 + sn2
(6) 3
6000
1
The value s¼ 0.004 cm s provides the best fit to the experimental data for both the theoretical functional form of n(a) and that of the inverted a(n) relation (Eqs. (3) and (4)), as illustrated in Fig. 9.
8000 y = 209.35x0.5457 R² = 0.9096
5000
5. Conclusion
4000 3000
y = 102.64x0.6294 R² = 0.9019
2000 1000 0 0
100
200
300
400
500
600
Radon concentration, Bq m-3 Fig. 7. First-32-hour fits and the parameters of the power-law fits. 8000
7000
Air ion concentrations, ions cm-3
d‵ + ka1 = αn12 + sn1
Simultaneous measurements of radon and air ion clusters were performed indoors in a single room with an elevated radon concentration. The room was intentionally ventilated to measure the rate of radon removal and to correlate the measured values. Radon is a noble gas and thus is very useful as a tracer gas, i.e., as an indicator of air exchange dynamics. The indoor measurement results obtained in this experiment confirmed a strong correlation between the concentrations of positive (and negative) air ions and radon. This relation changes slightly from a linear relation to a square-root relation as the concentrations of both increases. Nevertheless, the determination of the exponent describing the relation between the air ion and radon concentrations depends on many parameters, such as the level of radon and the concentrations of its progeny, the thoron concentration, the concentration and size distribution of aerosol particles, the rate of attachment of
y = 182.78x0.5799 R² = 0.9923
6000
5000
4000
positive ions 3000
Negative ions 2000
Power (positive ions) y = 110.3x0.6309 R² = 0.9869
1000
Power (Negative ions)
0 0
100
200
300
400
500
600
Radon concentration, Bq cm-3 Fig. 8. First-12-hour fits and the parameters of the power-law fits. Fig. 9. Comparison of the experimental data with the theoretical relation n(a).
P. Kolarž, Z. Ćurguz / Applied Radiation and Isotopes 99 (2015) 179–185
ions to aerosol particles, the gamma dose rate, and the deposition of ions on electrostatic surfaces. Therefore, there is no universal calibration factor, and the exponent (b) for the conversion of the air ion concentration to the radon concentration varies over time, thus requiring an in situ calibration of both parameters. When measurements of changes in radon concentration over very short time periods (on the order of seconds or minutes) are needed, the air ion concentrations can serve as high-confidence indicators of those changes. In other words, the fine structure of the dynamics of radon removal during house ventilation can be precisely determined using this method. In our experiment, we demonstrated that after a window was opened without a draft, the air ion concentration decreased by 50% during the first 3 min and reached the level of the outdoor background after 9 min. At the same time, a continuous radon-monitoring instrument measured only an overall decrease in the radon concentration, without registering the fine structure of the air exchange behavior.
Acknowledgments The Ministry of Education and Science of the Republic of Serbia supported this work under Project nos. 171020 and 45003. The authors offer many thanks to Dr. Jozef Thomas from NRPI Praha for his many useful comments and calculations.
References Bochicchio, F., 2005. Radon epidemiology and nuclear track detectors: Methods, results and perspectives. Radiat. Meas. 40, 177–190. Capra, D., Silibello, C., 1994. Influence of ventilation rate on indoor radon concentration: theoretical evaluation and experimental data in a test chamber. J. Environ. Radioact. 24, 205–2015. Darby, S., Hill, D., Auvinen, A., Barros-Dios, J.M., Baysson, H., Bochicchio, F., 2005. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. Br. Med. J. 330, 223–226. Dueñas, C., Fernandez, M.C., Carretero, J., Liger, E., Perez, M., 1997. Release of 222Rn from some soils. Ann. Geophys. 15, 124–133. Durridge Company Inc., 2000. Rad-7 radon detector. Owner's Manual, Bedford.
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Gerdien, H., 1905. Demonstration eines Apparates zur absoluten Messung der elektrischen Leitfahigheit der Luft. Phys. Zeitung 6, 800–801. Graeffe, G., Keskinen, J., 1989. Small ion concentration in houses with enhanced radon concentration. Environ. Intern. 15, 309–313. Hirsikko, A., Paatero, J., Hatakka, J., Kulmala, M., 2007. The 222Rn activity concentration, external radiation dose and air ion production rates in a boreal forests in Finland between March 2000 and June 2006. Boreal Environ. Res. 12, 265–278. Hoppel, W.A., Anderson, R.V., Willett, J.C., 1986. The Earth's Electrical Environment. National Academy Press, Washington, USA, pp. 149–165. Hopke, P., Jensen, B., Li, C.S., Montassier, N., Wasiolek, P., Cavallo, A.J., Gatsby, K., Socolow, R., James, A., 1995. Assessment of the exposure to and dose from radon decay products in normally occupied homes. Environ. Sci. Technol. 29, 1359–1364. Hõrrak, U., Iher, H., Luts, A., Salm, J., Tammet, H., 1994. Mobility spectrum of air ions at Tahkuse observatory. J. Geophys. Res. 99, 10697–10700. U. Hõrrak, 2001. Air Ion Mobility Spectrum at a Rural Area (Ph.D. thesis), Tartu, Estonia. Israël, H., 1970. Atmospheric electricity. Jerus.: Israel Progr. Sci. Transl. Kolarž, P., Marinković, B.P., Filipović, D.M., 2009. Daily variations of indoor air-ion and radon concentrations. Appl. Radiat. Isot. 67, 2062–2067. Kolarž, P., Miljković, B., Ćurguz, Z., 2012. Air-ion counter and mobility spectrometer. Nucl. Instrum. Methods B 279, 219–222. Mohnen, V.A., 1977. Formation, nature and mobility of ions of atmospheric importance In: Dolezalek, H., Reiter, R. (Eds.), Electrical Processes in Atmospheres. Dr. Dietrich Steinkopff Verlag, Darmstadt, Germany, pp. 1–17. Nazaroff, W.W., Boegel, M.L., Hollowell, C.D., Roseme, G.D., 1981. The use of mechanical ventilation with heat recovery for controlling radon and radon daughter concentrations in houses. Atmos. Environ. 15, 263–270. Nazaroff, W.W., 1992. Radon transport from soil to air. Rev. Geophys. 30, 137–160. Perrier, F., Richon, P., 2010. Spatiotemporal variation of radon and carbon dioxide concentrations in an underground quarry: coupled processes of natural ventilation, barometric pumping and internal mixing. J. Environ. Radioact. 101, 279–296. Tammet, H., Kulmala, M., 2005. Simulation tool for atmospheric aerosol nucleation bursts. J. Aerosol Sci. 36, 173–196. UNSCEAR, 2001. Exposures from Natural Radiation Sources, Report to the General Assembly with Scientific Annexes. United Nations Scientific Committee on Effects of Atomic Radiation, United Nations, New York.. UNSCEAR, 1993. Sources and Effects of Ionizing Radiation. Report to the General Assembly with Scientific Annexes. United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York. Wilkening, M.H., Watkins, D.E., 1976. Air Exchange and 222Rn Concentrations in the Carlsbad Caverns. Health Phys 31, 139. Wilkening, M., 1990. Radon in the Environment, Studies in Environmental Science, 40. Elsevier, Amsterdam.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Air ions as indicators of short-term indoor radon variations P. Kolarž a,n, Z. Ćurguz b a b
Institute of Physics, University of Belgrade, 11080 Belgrade, Serbia Faculty of Transport and Traffic Engineering, University of East Sarajevo, 74000 Doboj, Bosnia and Herzegovina
H I G H L I G H T S
We studied indoor correlation between cluster air ions and radon concentrations. Air ions are proved to be a good indicator of rapid radon changes. Relation is slightly changing from linear to square root when concentration of both increasing. Methods of theoretical approach are presented.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2014 Received in revised form 18 February 2015 Accepted 2 March 2015 Available online 3 March 2015
Diurnal variations in the air ion concentration are subject to changes in the radon concentration. In this experiment, the air ion and radon concentrations were simultaneously measured using two air ion detectors and two continuous radon detectors. The results of the indoor measurements revealed a strong correlation between the concentrations of positive air ions and radon (with a correlation coefficient greater than 0.9). The radon-to-ion concentration ratio changes with an increase in the radon concentration from a linear to a square-root relation. This correlation provides a means of using air ion measurements as a high-confidence indicator of changes in the radon concentration, especially for shortterm measurements on the order of seconds or minutes, which is too short a measurement interval for conventional radon monitors. The use of air ions as an indicator of changes in radon concentration allows for investigation of the behavior of indoor radon and also allows radon to be used as a tracer gas for air mass exchange. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Air ions Indoor radon Thoron Natural radioactivity Tracer Recombination
1. Introduction Air ions are airborne charged particles with electrical mobility. The existence of air ions in the lower atmosphere is a consequence of various ionization processes: the radioactive decay of airborne radionuclides (radon and thoron), the decay of radioactive minerals from the earth's crust and ionization induced by cosmic radiation (Israël, 1970; Hirsikko et al., 2007). The ionization rate of the last two sources is almost constant on daily basis, whereas the radon concentration is variable over short periods of time. These variations are attributed to the level of radon exhalation from the soil and to local atmospheric conditions and are of considerable importance in indoor environments. Continuous real-time measurements of changes in radon activity suffer from limited time resolution, inertia of the measuring process and high n
Corresponding author. E-mail address: [email protected] (P. Kolarž).
http://dx.doi.org/10.1016/j.apradiso.2015.03.001 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
measurement uncertainty. To measure rapid or short-term changes in the radon concentrations in indoor spaces, an indirect method of measuring radon by measuring the air ion concentration can be utilized (Kolarž et al., 2009). In this way, the intensity and effectiveness of radon removal can be monitored during remediation, and radon can also be used as a tracer gas during air ventilation. 1.1. Radon Radon (222Rn) Is well-known radioactive noble gas that Is 7.5 times heavier than air. it Is produced in the decay chain of the primordial elements uranium and thorium, which are found in various concentrations in the soil worldwide. 222Rn α decays with a half-life of 3.82 days, and this decay Is followed by a series of four further decays (2 α and 2 β) with relatively short half-lives. when inhaled, short-lived radon progeny in the respiratory tract result in the deposition of α energy in the cells of the bronchial epithelium (Hopke et al., 1995).
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When generated in the Earth's crust, radon penetrates the pores in the ground and moves upward through diffusion and convection toward the surface and into the air (Nazaroff, 1992). The radon flux, i.e., exhalation rate, at the surface of the soil is strongly influenced by weather conditions such as precipitation, air pressure, and the temperature of the air as well as the permeability, thermal gradient and humidity of the soil (Dueñas et al., 1997). The transport of radon through the atmosphere is determined by thermal and other meteorological processes. The dynamics of changes in the radon concentration is driven by the temperature ratio between the surface soil and the air, which results in a radon maximum arising with the temperature inversion that occurs during the night and a minimum in the afternoon. When it is exhaled in an indoor space, Rn is prone to accumulation. The indoor radon concentration is a consequence of radon exhalation from the soil and from the materials of the building. The ratio between the contributions of these two sources depends on the underlying soil, the building materials and the construction quality. The primary mechanism for the entry of radon into buildings is a pressure-driven flow of soil gas through openings in the floor. Many epidemiological studies and pooled analysis indicate a strong correlation between lung cancer and radon concentration, which is especially high for the smoker population (Bochicchio, 2005; Darby et al., 2005). Radon is a harmful radioactive gas and does not combine chemically with other gasses. It is relatively easy to detect, making it suitable for use as a tracer gas for studying a number of processes in both indoor and outdoor atmospheres (Wilkening, 1990). Many experiments and calculations have been performed concerning the long-term natural ventilation rates in houses and caves using models and experimental results: see Wilkening and Watkins (1976), Capra and Silibello (1994), Nazaroff et al. (1981), and Perrier and Richon (2010). 1.2. Air ions Air ions are continually created by natural sources such as cosmic rays, the radioactive decay of the noble gas radon in the air and that of radioactive minerals in the ground. The average ionization rate in continental areas is approximately 10 ion pairs cm 3 s 1 at height of 1 m above the ground. The contribution of cosmic radiation to the air ion pair generation rate is approximately 2 ion pairs cm 3 s 1, and remainder is attributed to the decay of radioactive minerals. Shortly after the ionization process (within microseconds), primary ions evolve through the process of hydration and form small cluster ions, also known as small air ions or nano-air ions. The central ion of a cluster may contain one inorganic molecule and may be surrounded by one layer of water molecules (Mohnen, 1977). This class of air ions can survive up to 100 s, depending predominantly on air pollution and air density. The transformation from the gas phase to the aerosol phase proceeds through the process of spontaneous nucleation. Air ions are classified into small cluster ions (0.36–0.85 nm), large cluster ions (0.85–1.6 nm), intermediate ions (1.6–7.4 nm) and large ions (7.4–79 nm) (Hõrrak et al., 1994; Hõrrak, 2001). Only the smallest class of ions, with the highest mean natural mobilities of 1.36 (positive) and 1.53 cm2 V 1 s 1 (negative), is considered in this work. The air ion concentrations in natural surroundings and over soil typically lie in the range of 102–104 ions s cm 3, primarily depending on the radon exhalation potential and accumulation. The lower mobility of positive air ions grants them a lower rate of attachment (s) to aerosol particles (Z) and, consequently, a longer lifetime, resulting in higher concentrations of these air ions. The ratio of positive to negative ion concentrations (n) in the natural outdoor environment is n þ /n ¼1.12 (Hõrrak, 2001). Temporal
variations in the air ion concentration (neglecting the asymmetry between positive and negative ions and the electrostatic deposition of air ions) can be described using a simplified air ion balance equation (Israël, 1970):
dn = q − αn2 − sn dt
(1)
where q is the cluster ion production rate, α is the recombination rate of small ions, α ¼ 1.6 10 6 cm3 s 1 (Tammet and Kulmala, 2005) and s is the total rate of attachment of cluster ions to aerosol particles (in units of cm3 s 1). Similar to radon, the gaseous isotope thoron (220Rn) is an αparticle emitter, although it has a much shorter half-life (55 s). Consequently, it decays only in the near vicinity of its source (at distances of up to a few tenths of centimeters), which, in the case of buildings, is the construction materials. When it exists in higher concentrations, thoron is an important source of air ions. The presence of thoron can negatively influence the efficacy of air ions as indicators of short-term indoor radon variations, but thoron is rarely observed in significant concentrations. Following radon and thoron decay, all their short-lived decay products, such as polonium (218Po and 216Po) and lead (214Pb and 210 Pb), exist in the solid state and are predominantly α emitters. These products may be suspended in air in attached or unattached fractions or may be deposited on solid surfaces. In the air, these particles contributing to the air ion pair production rate (both the attached and unattached fractions). Their contribution to the air ionization is proportional to the radon concentration. If radon and thoron progenies are deposited on surfaces, then their ionization contribution is significantly reduced because of the limited angle of the free path for the emitted α particles to travel through the air and the electrostatic deposition of ions on insulating surfaces. In calculations, the air ion production rate (qRn) is related to the contribution of radon and its progeny. Because the half-lives of free air ions in the air are very short (on the order of a few tens of seconds) compared with the halflives of ventilation in rooms (on the order of hours), the assumption of a quasi-steady state can be applied, and the differential equation reduces to an algebraic quadratic equation. The air ion production rate q consists of at least two contributions, one (qRn) related to radon and its progeny and the other (qγ) related to gamma and cosmic radiation. At very low radon levels, the linear term of Eq. (1) cannot be neglected, resulting in a generally more complicated solution. Therefore, with increasing radon concentration, the dependence of the ion concentration on the radon concentration changes continuously from a constant value to a square-root relation. Given the approximations that the air ion and radon concentrations exist in a steady state and that the positive and negative ion concentrations are equal, the balance equation for small air ions is as follows:
αn2 + sn = qRn + qγ
(2)
where qRn is the rate of ion production by radon and its progeny and qγ is the rate of ion production by gamma and cosmic radiation (in units of m 3 s 1). The concentration of positive ions is often significantly different from the concentration of negative ions as a result of increased aerosol concentration, ion deposition on electrostatic surfaces and the atmospheric electrode effect in the near-ground layer (Hoppel et al., 1986). Because positive ion clusters have a lower mobility and are not affected by the electrode effect, their concentration is less susceptible to these influences, resulting in a higher and more stable concentration. Consequently, only the positive ion concentration is considered here.
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2.1. Air ion detectors
The solution of the balance equation is
n=
⎞ s ⎛ 4α ⎜ 1+ (qRn + qγ ) − 1⎟ 2 2α ⎝ s ⎠
(3)
and can also be written in the form of Eq. (4) in Graeffe and Keskinen (1989). The ion production rates must be related to the gamma dose rate (Ḋ ) , defined by qγ ¼d Ḋ , and the radon concentration a (in units of Bq m 3), defined by qRn ¼ ka, where d and k are the coefficients of ion production by the γ background and radon, respectively. The production rate arising from low Linear Energy Transfer (LET) radiation can be treated, under indoor conditions, as a constant value corresponding to the average indoor gamma dose rate, Ḋ ¼80 nGy h 1 (UNSCEAR, 2000). Inverting relation (3) yields the relation a(n):
a (n ) =
s α 2 d n + n − Ḋ , k k k
181
(4)
which is a parabola with its vertex in the fourth quadrant. To obtain a better approximation of the radon-to-air-ion ratio, simultaneous measurements of both parameters are needed. This is because of the influence of increased radon, thoron and aerosol concentrations as well as the existence of electrostatic fields on insulating surfaces (Graeffe and Keskinen, 1989).
Air ion detectors operate on the aspirated Gerdien condenser principle (Gerdien, 1905). All detectors have an autonomous power supply, a real-time clock, and sensors for temperature, pressure and relative humidity. Functions of the instrument such as dynamic zeroing, data acquisition, data averaging, sampling and polarity change are programmable, whereas acquisition is performed internally or by a PC (Kolarž et al., 2009; Kolarž et al., 2012). Three coaxial cylindrical electrodes are used, i.e., a central measuring electrode, a polarizing electrode and a shielding electrode (Fig. 1). Air passes through the electrode system, where the polarizing electrode deflects ions of the same charge onto the surface of the central electrode. The accumulated charge is measured by an electrometer and reported in units of ions cm 3. The size and mobility of the measured ions are determined by the polarizing voltage and the air flow. If both polarities are being measured, zeroing of the system is performed upon every alteration in polarity. It is necessary to compensate for the noise induced by changes in T and RH as well as external electromagnetic waves and the high-voltage peaks induced by polarity changes and fan stopping/starting (Kolarž et al., 2012). Digitalization of the CDI-06 output signal enables the programming of auto-zeroing and automatic long-term measurements without external control. The measuring uncertainty is within 5%.
2. Instrumentation and measurements
3. Experimental methods
Experiments were performed using 2 identical and previously calibrated CDI-06 air ion detectors (manufactured at the Institute of Physics, Belgrade) and two Rad-7 instruments (Durridge Company, USA). Measurements were performed simultaneously in the urban area of Belgrade city.
To evaluate the transport properties of the underlying soil and the concentration of 238U, the 222Rn exhalation rate (flux density, ϕ) from the ground surface was measured nearby the measuring location. The radon activity was measured in 15-min sniff mode as described by the Durridge (2000) owner's manual:
Fig. 1. Block diagram of the cylindrical air ion detector (CDI-06).
182
ϕ = B V /S
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(5)
where B is the slope of the linear fit to the increasing radon concentration in the emission chamber; V is the volume of the air loop, 0.025 m3 (air volume of the Rad-7 unit plus the laboratory drying unit and the internal volume of the emission chamber); and S is the base surface area, 0.071 m2. Measurements were performed in a single room of an old house with wooden floors and no concrete foundations in the center of Belgrade. The floor surface of the measured room was 5 m 3 m, and it was 3.2 m high (volume 48 m3). One radon detector and one air ion detector were placed in the middle of the room, whereas the other pair of instruments was placed outside, 2 m from the outer wall of the room at the location where the radon flux was measured. All instruments were placed 1 m above the ground. During the long-term measurements, the radon concentration sampling was set to one-hour intervals, whereas the air ion detectors were set for 2-min intervals of sampling and zeroing of a single polarity.
4. Results and discussion The average daily value of the radon exhalation flux was 30 mBq m 2 s 1, slightly higher than the world average, which lies in the range between 16 and 26 mBq s 1 m 2 (UNSCEAR, 1993). Thoron was measured using the Rad-7 instrument in the house with the inlet leaning against the wall. The thoron concentration was found to be close to zero; thus, its influence on the air ion concentration was neglected. The temperature of the room during the measurements was approximately (23 72) °C, and the relative humidity was (517 3)%, whereas the outside temperature was in the diurnal range between 16 and 25 °C. The diurnal changes in the concentrations of positive and negative air ions and of radon, with maxima in the early morning and minima in the afternoon, are shown in Fig. 2. Because radon is a gas and is prone to accumulate in indoor spaces, its maximum concentration over the entire span of the 2-day measurements (Fig. 2) was 624 Bq m 3, whereas the average was
(398755) Bq m 3. The average concentration of positive air ions was 5064 ions cm 3, whereas that of negative air ions was 4120 ions cm 3. The unipolarity factor of positive and negative ions was n þ /n ¼1.23, meaning that the surrounding air was relatively unpolluted, i.e., aerosol free. Despite the care that was taken when entering the room, with minimal instances of door opening during the measurement, an apparent decrease in the air ion concentration was measured (Fig. 2), indicating that air ions could be used as a tracer for the radon ventilation exchange rate. This possibility was analyzed in separate measurements, in which the indoor radon and air ion concentrations were lowered to the outdoor background level by opening the window, as shown in Figs. 3 and 4. After only one window (1.2 m2) was opened without a draft in the room where the measurements were performed, the air ion concentration decreased by 50% during the first 3 min and reached the outdoor concentration value within 9 minutes (E650 positive ions cm 3), meaning that a balance was established between the outdoor and indoor radon (air ion) concentrations. The reverse process was much slower, primarily because of diurnal changes in radon exhalation and the longer time required for accumulation. The gray zone in Fig. 2 represents an interval during which one of Rad-7 detectors was set to 5-min measuring periods. The purpose of applying this instrument setting was to demonstrate that with a broad dispersal of the data and an average measuring uncertainty of 227 Bq m 3, the accuracy of the tracking of changes in the radon concentration is lost. During the collection of the data shown in Fig. 3, the window was opened two times, once at the beginning of the measurement and again after the early-morning radon concentration maximum had been reached. Both times, the room was completely ventilated, as indicated by the equalization of the outdoor and indoor radon and air ion concentrations. After the first opening of the window, the increase in the radon concentration was slow, and the radon and air ion measurements were in relatively good agreement. After second opening of the window, the radon measurements showed signs of considerable integration behavior, obscuring the fine structure of the change in the ion concentration
Fig. 2. Concentrations of positive (upper graph) and negative (lower graph) air ions and of radon during 2-day measurements performed on the ground floor of a house in Belgrade. Radon was measured in 1-h intervals and in 5-min intervals (gray zone to the right) to emphasize the effect of data dispersal.
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183
Fig. 3. Air ion and radon measurements during two short periods of window opening (half-day measurements).
Fig. 4. Magnified view of a 50-min period during the second window opening showing the indoor and outdoor air ion concentrations.
(i.e., radon concentration) during the indoor-outdoor air exchange. This observation was a consequence of the higher measuring interval (15 min is the shortest possible time required to obtain results for relatively low data dispersion) but also of the presence of residual radon in the Rad-7 measuring chamber and drying unit (2.5 L) and the relatively low air flow (2 L min 1), which also caused significant time integration of the measurements. 4.1. Correlation coefficients (a) The results of the 2-day measurements are presented in Fig. 2. Approximately 32 one-hour averages during the 2-day measurements of the concentrations of positive and negative cluster ions and of radon were calculated and are shown on a linear scale (Figs. 5 and 6). The correlation coefficients of the air ion and radon concentrations were 0.79 for positive ions and 0.80 for negative ions (Fig. 5), whereas the correlation coefficients calculated on a logarithmic scale were 0.64 and 0.73, respectively. Excluding the outlying point, the slopes (b 7 ) of the n(a) relations were found to be b þ ¼0.5467 0.069 and b ¼0.63 7 0.08 for 32 h
Fig. 5. Correlation coefficients of air ion and radon concentrations on a linear scale.
and b þ ¼ 0.587 0.07 and b ¼ 0.637 0.082 for the first 12 h (Figs. 7 and 8). Using only the first-12-h averages from the measurements presented in Fig. 2, the correlation coefficient is 0.97 for both negative and positive ions. This value is much higher than those for the 32-h averages, which are 0.78 and 0.79 for positive and negative air ion concentrations, respectively (Fig. 6). The slope for the negative-ion case (b ) is significantly higher than that for the positive-ion case (b þ ) (Fig. 8), and the difference b b þ is significantly higher than zero in both cases. (b) Half-day measuring results are presented in Fig. 3, and Fig. 4 presents only a short subset of these data. A power-law fit to 11 pairs of results for air ions and radon concentration obtained during the “window open” regime yielded a value of 0.899, which is higher than the value of 0.580 obtained for the 2-day measurement. This difference was most likely caused by rapid changes in the radon concentration and the lag time of the Rad-7 instrument.
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184
Table 1 Calibration values for the radon and air ion concentrations. Dose rate, nGy h 1 1 84 2 84
Radon concentration (ai), Bq m 3
Ion concentration (ni), cm 3
150 6000
3100 22,600
4.2. Relation between air ions and radon (n(a)) One of the purposes of the measurements was to fit the relation between the air ion and radon concentrations, n(a) or a(n), to the experimental results. To this end, two distinct pairs of measurement results were chosen, e.g., one point at which the radon concentration was low (and the influence of gamma radiation high) and the second point at a higher radon concentration (Table 1). Choosing such values for calibration (fitting) and substituting these data into relation (2) yields two linear equations for ̇ ) and k as follows: the calibration constants d′ (d′ = Dd Fig. 6. Radon and air ion correlation coefficients calculated for the first 12 hours and for 32 hours until the first entry into the room and the opening of the windows.
positive ions negative ions Power (positive ions) Power (negative ions)
7000
Air ion concentrations, ions cm-3
d‵ + ka2 = αn22 + sn2
(6) 3
6000
1
The value s¼ 0.004 cm s provides the best fit to the experimental data for both the theoretical functional form of n(a) and that of the inverted a(n) relation (Eqs. (3) and (4)), as illustrated in Fig. 9.
8000 y = 209.35x0.5457 R² = 0.9096
5000
5. Conclusion
4000 3000
y = 102.64x0.6294 R² = 0.9019
2000 1000 0 0
100
200
300
400
500
600
Radon concentration, Bq m-3 Fig. 7. First-32-hour fits and the parameters of the power-law fits. 8000
7000
Air ion concentrations, ions cm-3
d‵ + ka1 = αn12 + sn1
Simultaneous measurements of radon and air ion clusters were performed indoors in a single room with an elevated radon concentration. The room was intentionally ventilated to measure the rate of radon removal and to correlate the measured values. Radon is a noble gas and thus is very useful as a tracer gas, i.e., as an indicator of air exchange dynamics. The indoor measurement results obtained in this experiment confirmed a strong correlation between the concentrations of positive (and negative) air ions and radon. This relation changes slightly from a linear relation to a square-root relation as the concentrations of both increases. Nevertheless, the determination of the exponent describing the relation between the air ion and radon concentrations depends on many parameters, such as the level of radon and the concentrations of its progeny, the thoron concentration, the concentration and size distribution of aerosol particles, the rate of attachment of
y = 182.78x0.5799 R² = 0.9923
6000
5000
4000
positive ions 3000
Negative ions 2000
Power (positive ions) y = 110.3x0.6309 R² = 0.9869
1000
Power (Negative ions)
0 0
100
200
300
400
500
600
Radon concentration, Bq cm-3 Fig. 8. First-12-hour fits and the parameters of the power-law fits. Fig. 9. Comparison of the experimental data with the theoretical relation n(a).
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ions to aerosol particles, the gamma dose rate, and the deposition of ions on electrostatic surfaces. Therefore, there is no universal calibration factor, and the exponent (b) for the conversion of the air ion concentration to the radon concentration varies over time, thus requiring an in situ calibration of both parameters. When measurements of changes in radon concentration over very short time periods (on the order of seconds or minutes) are needed, the air ion concentrations can serve as high-confidence indicators of those changes. In other words, the fine structure of the dynamics of radon removal during house ventilation can be precisely determined using this method. In our experiment, we demonstrated that after a window was opened without a draft, the air ion concentration decreased by 50% during the first 3 min and reached the level of the outdoor background after 9 min. At the same time, a continuous radon-monitoring instrument measured only an overall decrease in the radon concentration, without registering the fine structure of the air exchange behavior.
Acknowledgments The Ministry of Education and Science of the Republic of Serbia supported this work under Project nos. 171020 and 45003. The authors offer many thanks to Dr. Jozef Thomas from NRPI Praha for his many useful comments and calculations.
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