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Radiological characterisation and radon equilibrium factor in the outdoor air of a post-industrial urban area
Journal of Environmental Radioactivity 151 (2016) 126e135
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Review
Radiological characterisation and radon equilibrium factor in the outdoor air of a post-industrial urban area S. Rozas*, R. Idoeta, N. Alegría, M. Herranz Department of Nuclear Engineering and Fluid Mechanics, University of the Basque Country (UPV/EHU), Alameda Urquijo s/n, Bilbao 48013, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 24 September 2015 Accepted 27 September 2015 Available online xxx
The radiological characterisation of outdoor air is always a complicated task due to the several radioactive emissions coming from the different radionuclides and also because of the very short half-lives of radionuclides in the natural radioactive series. In some places, this characterisation could result in unusual values because the natural presence of radionuclides with terrestrial origin can be modified by manmade activities. Nonetheless, this characterisation is useful not only for air quality control purposes but also because radon and its progeny in the outdoor air are the main contributors to human exposure from natural sources. In this study, we have carried out air particle sampling, followed by gamma-ray spectrometry, alpha spectrometry and beta counting determinations for this purpose. Subsequently, the outdoor air has been radiologically characterised through the obtained data and using a pre-existing analytical method to take into account the radioactive decays of short half-life radionuclides during sampling, sample preparation and measuring times. Bilbao was chosen to carry out this work. It is a medium-sized town located in northern Spain, close to the Atlantic Ocean and at sea level. This city has a recent industrial past as there were numerous steel mills and other heavy industries, including some quarries, and some open pit mines close to it, which concluded in a remediation program. So, it is a place where the air is potentially modified by manmade activities. The obtained results show that activity concentration values for long-lived radionuclides that precede radon and thoron are in the order of 106 Bq m3 and long-lived ones after radon are around 104 Bq m3. Thoron progeny are around 2 102 Bq m3 and radon progeny are around 1.8 Bq m3. The mean radon equilibrium factor was 0.18. All of these values are close to the minimum UNSCEAR values, but show some variability, which highlights the importance of determining activity concentrations for each naturally occurring radionuclide and the equilibrium factor in the outdoor air. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Air quality control Radiological characterisation Natural radioactivity Radon equilibrium factor
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.1. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.2. Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.3. Activity determination in filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 2.4. Activity concentration determination in outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 2.5. Radon equilibrium factor determination in outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 3.1. Activity concentrations in the outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 3.2. Radon equilibrium factor in the outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
* Corresponding author. E-mail address: [email protected] (S. Rozas). http://dx.doi.org/10.1016/j.jenvrad.2015.09.023 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
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4.
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3.3. Influence of environmental parameters on radon equilibrium factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
1. Introduction Radionuclides existing in the outdoor air have cosmogenic or terrestrial origins. The cosmogenic isotopes are mainly 7Be, 14C and 3 H (UNSCEAR, 2000), and the terrestrials are 137Cs, 40K and other radionuclides from the natural radioactive series. All of them are usually present in sampled aerosols, albeit to a different extent. The concentration of terrestrial radionuclides in the lower layers of the atmosphere is heavily dependent on their removal from soils and, in the case of Rn and its progeny, also on the emanation of Rn from soil surfaces. Dilution factors, such as radioactive decay, alter these concentrations, which can also be affected by atmospheric mixing phenomena, season, time, height and location, as many studies have revealed (Arnold et al., 2009; Baciu, 2005; Florea and Duliu, 2012; Kulwant et al., 2005; Veleva et al., 2010). Additionally, these concentrations could also be modified by manmade activities, such as mining, quarries and other types of industrial facilities, including NORM/TENORM industries. In this case, as the existence of radioactive equilibrium between radionuclides from the uranium and the thorium-series cannot be assumed, the possible presence of an activity-supported and an unsupported component for the progeny should be taken into account. The concentration of cosmogenic radionuclides also depends on atmospheric mixing phenomena, season, time, height and location, being lower at sea level. But as they do not belong to radioactive series and their half-lives are relatively long, it is easy to measure them and only in some cases, like in the case of nuclear industries, can their concentration in the outdoor air be affected by manmade activities. Bilbao is a town located in northern Spain, close to the Atlantic Ocean (2 560 W, 43 150 N) and at sea level (Fig. 1). The city is situated at the bottom of a valley that is open to the sea, and which has a recent industrial past. Close to the city there were numerous steel mills and other heavy industries, including some limestone quarries, and some, now closed, open pit mines. Therefore, this city, which suffered high levels of pollution until the 1990s, could have singular air radiological characteristics. In the late 90's and the early 00's most of these industrial activities were stopped, industries were abandoned, industrial wastes remained on land and mines and quarries stayed in the open air. In this context, the Spanish Government and the regional Basque Government started a big remediation program to recover soil to use it in other secondary activities (residential and small industries), which is almost finished nowadays. So, the radionuclides released during the development of these industrial and remediation activities might have undergone the processes of deposition, resuspension and dispersion and hence, they might be modifying the activity concentration of radionuclides with terrestrial origin in the air. These reasons have motivated us to obtain the radiological characterisation of Bilbao's air. As no nuclear facilities e which would have been able to modify the inventory of cosmogenic radionuclides e exist around this city, only terrestrial radionuclides have been considered in this work. Nevertheless, measurements carried out on cosmogenic radionuclides show that their activity concentrations are around 103 Bq m3 for 7Be, 103 Bq m3 for 3H
and 102 Bq m3 for 14C. From a dosimetric point of view, the 14C and 3H contents are negligible. The remaining radionuclide, 7Be, has been previously analysed by this group (Alegría et al., 2010). To carry out this characterisation, aerosols and particles in the air were sampled by means of a filter and measured by gamma spectrometry. Therefore, only the attached fraction of radionuclides has been collected. In any case, as some authors suggest, most of the activity in the outdoor air is in the attached fraction and this frac€rfer et al., 1999). tion ranges from 88% to 96% (Porstendo In these filters, gamma emitter radionuclides were determined. However, to obtain activity concentrations in the sampled air, the obtained activity concentration of the short-lived radionuclides must be time-corrected, taking into account radioactive decays and ingrowths during sampling, sample preparation and measuring times. To make these time corrections, as the presence in the air of short-lived radon progeny depends also on the presence of radon in the air, Rn in air has been obtained by direct measurements. With these Rn and gamma spectrometry data and the use of a mathematical treatment developed by us and used for dose assessment in NORM industries (Herranz et al., 2014), the activity concentration of gamma emitters in sampled aerosols and particles was finally obtained. This characterisation, repeated weekly for a year, also allowed us to analyse the variations in radionuclide concentration. Additionally, for the most contaminated filters this process was completed carrying out radiochemical isolations in the filters, followed by alpha spectrometry and beta counting measurements, in order to obtain the activity concentration of alpha and beta emitters. The data obtained provided us with the isotopic ratios and relationships between radionuclides. Finally, it should be taken into consideration that, although their concentrations in the outdoor air are very low, radon and its progeny are the most important contributors to human exposure from natural sources. As the equivalent dose is mainly due to the short-lived progeny of radon, which is not always in radioactive equilibrium to its parent, effective dose assessment is usually carried out by measuring radon concentration and applying an equilibrium factor. Therefore, the most accurate estimation of this equilibrium factor is the guarantee of an accurate dose assessment (Forkapic et al., 2013; Chen, 2005, Kojima, 1996; UNSCEAR, 2000). Using the data obtained in the radiological characterisation of the outdoor air, equilibrium factors for radon and its progeny can be assessed and the values obtained can be compared with those typically used. In summary, the main objective of this work is the radiological characterisation of the outdoor air in a post-industrial urban area like Bilbao and, also, the determination of the equilibrium factor, considering in detail all its contributors. The laboratory where this work was carried out is accredited by ENAC (Spanish National Accreditation Body) since 2003, under the ISO/IEC standard 17,025, for the determinations carried out in this work. Thus, the procedures are validated and used routinely in national and international intercomparisons and proficiency test exercises. The materials and methods used to achieve the stated objectives and the obtained results are presented below.
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Fig. 1. Bilbao's location, indicated by an A on a Google map, 2015.
2. Materials and methods 2.1. Sampling The aerosol sampling station ASS 500 was used to collect aerosols from the outdoor air. For a week, a nominal air flow rate of 0.1389 m3 s1 was forced to pass through a filter by an air pump. All the air filtered throughout a week was monitored by means of a calibrated air flow totalizer. The filter used was a PTI type G3 polypropylene, with an active surface of 0.41 0.41 m2. Some experiments have been carried out to obtain the retention efficiency of the filter in the particular conditions of the sampling; a retention efficiency of 95.5% has been obtained for particles size larger than pez-Pe rez et al., 0.1 mm. This value matches those from reference (Lo 2013) in which the collection efficiency is analysed using the same filter, same equipment and same time collection as that of this work. This collection efficiency has been applied to assess the radionuclides activity concentration. This radiological control has been done since the late 90's in the framework of the Dense and Sparse Networks of the European Community. In Spain these networks depend on the Spanish Nuclear Safety Council (CSN) and our laboratory was selected by the CSN to run both networks in Bilbao, where water, soil and also air are controlled in a routine basis. The location of the sampling points and hence the location of aerosol sampling station were adopted in accordance with the CSN as representative places of the quality of the different environmental constituents. At the same place as the aerosol sampling station, radon activity
concentration values were obtained by the moving filter particulate monitor BAI 9100D, manufactured by Berthold, which continuously measures alpha and beta particulate activity in the presence of radon and applies the Alpha Beta Pseudo coincidence Difference (ABPD) method (Berthold, 2010) to provide radon activity concentration values every 10 min. This monitor belongs to the Automatic Stations Network to provide the radon reference values for the environmental radioactivity monitoring to the Spanish Nuclear Safety Council (CSN, 2013). These values complete, in a continuous way, the data obtained in the Sparse and Dense Networks. 2.2. Measurement Once a week, the filter was removed from the aerosol sampling station ASS 500, always at the same time, dried in an oven at 90 C for 10 min, folded, wrapped in a plastic bag and put in a polypropylene box. This preparation prevents losses of ingrowing radon, as measurements carried out weeks after sampling proved. Thereupon, the sample was analysed at three different times by gamma-ray spectrometry. As the objective is to determine activity concentrations in outdoor air, the first measurement was carried out immediately after sampling (early measurement) and establishing a short counting time (1800 s), in order to obtain a picture of the gamma emitters present in the air and to prevent the complete decay of the unsupported part of short-lived radionuclides in the filter. The two following measurements were carried out a week later and some weeks after the early measurement (delayed measurements) so as to obtain a picture of other radionuclides, whose
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concentration determinations are presented in Table 1. These intensities and other nuclear data have been taken from the database of Recommended Data from Laboratoire National Henri Becquerel (2015). In order to assess the activity values of 228Ra, 228Th, 224Ra and 226 Ra in filters from their progeny, the values were measured one week and once again some weeks after the early gamma-ray spectrometry, when the unsupported part of short-lived radionuclides had disappeared (Herranz et al., 2006). It was done as follows: From the measurements done a week after sampling, the activity values of 228Ra and 224Ra in filters were obtained. The values of 228Ra were determined through its daughter 228Ac, both radionuclides having reached secular equilibrium. Those of 224Ra were determined through its daughters 212Pb and 212Bi, to avoid the interference with 241.997 keV from 214Pb. For this measurement, it was considered that unsupported 212Pb and 212Bi had disappeared and 224Ra, 212Pb and 212Bi had reached a transient equilibrium with an activity ratio of 1.14. Results obtained for 224Ra were also corrected for its one-week ingrowth from 228Th. That 228Th was obtained from the second delayed measurement, when it was in equilibrium with 212Pb and 212Bi. Finally, second delayed measurements also provided the activity of 226Ra from its daughters 214Pb and 214Bi. These delayed measurements were also used to check the ability of the box to prevent radon losses by assessing the levels of 226Ra obtained by radioactive equilibrium with its progeny and 226Ra obtained by radiochemical methods. These results are shown in Table 4. In the cases of 234Th and 228Ac, the comparison of their activity values obtained through the three measurements allowed us to get conclusions on the existence of equilibrium between 234Th and 238 U and between 228Ac and 228Ra in the sampled air and, hence, to obtain the activities for 238U and 228Ra. Uranium (238U) and thorium (232Th) decay schemes are presented in Figs. 2 and 3, respectively, together with the method used for the determination of each radionuclide in the series. These delayed measurements also allowed us to determine long-lived radionuclides obtained directly from their respective gamma photopeak, like 137Cs, 40K and 210Pb, with better statistics and lower detection limits than in early measurements.
activities can be determined through the equilibrium with their progeny, establishing a longer counting time (250,000 s) to get better statistics and lower detection limits. The detector used was an extended range coaxial germanium detector with a carbon composite window (Canberra, XtRa model) that permits an energy measuring range from 3 keV to 3 MeV with a 90% relative efficiency and a resolution of 2.2 keV (FWHM) at 1.33 MeV peak. Obtained spectra were processed by EG&G ORTEC Gamma Vision 6.01 commercial software and calibrations in energy and efficiency were made using a calibration source with the same geometry and composition as the sample, spiked with a certified multi gamma-ray emitter standard, from 47 to 1836 keV. Summing corrections were applied by means of the software EFFTRAN (Vidmar, 2015). Finally, after the gamma-ray spectrometry measurements in the filters with the highest activity, 210Pb, 210Po, 226Ra and isotopic U and Th were determined by radiochemical methods. To achieve these determinations, aliquots of the most contaminated filters were treated by cold lixiviation with aqua regia (for Po determination) and by acid digestion with a microwave system (for U, Th, Ra and Pb determinations). U and Th were isolated from the resulting solution by means of Dowex AG1 X8 anion exchange resins, Ra and Pb by precipitation and Po by direct deposition. This way, 238U, 235U, 234U, 232Th, 230Th, 228Th and 210Po could be measured by alpha spectrometry and 226Ra and 210Pb by alpha/beta counting. Detectors used for alpha spectrometry were of the passivated implanted silicon (PIPS) type, manufactured by Canberra, with a nominal background of 0.1389 s1m2. For alpha/beta counting we used a gas flow proportional counter equipped with 7 cm-diameter detectors with a nominal background of 0.00167 s1 for alpha counting and 0.0167 s1 for beta counting. For each measurement, uncertainty and detection limits were obtained. Combined uncertainties were obtained by means of the GUM (ISO/IEC Guide 98, 1995), considering all sources of uncertainty: calibration (standard certificate, preparation, counting and calibration fitting for gamma spectrometry), sample and background counting, sample preparation and a 3% for sampling. Decision thresholds and detection limits were obtained following ISO 11929 standard (ISO 11929, 2010); specific formulae used were presented in a paper (Herranz et al., 2008). 2.3. Activity determination in filters
2.4. Activity concentration determination in outdoor air The activity of radon progeny in the filters was obtained from the early gamma-ray spectrometry measurements. To do this, the possible ingrowth of 214Pb and 214Bi from 226Ra and 222Rn, and of 212 Pb and 212Bi from 228Th, 224Ra and 220Rn in the filters was considered negligible due to the short time between the end of sampling and the measurement (less than 1 h). This short time implies that the supported activity of 214Pb/214Bi and 212Pb/212Bi in the filters would never be more than 0.7% of 226Ra and 3% of 224Ra activity at the start of the measurement. Other short-lived radionuclides, like 228Ac and 234Th were also analysed in this early measurement. Photopeak efficiency and photon emission intensities of the energy peaks of 214Pb, 214Bi, 212Pb and 212Bi used for activity
Table 1 Photopeak efficiency and photon emission intensity of energy peak of
214
Pb,
214
Bi,
The activity concentration of radionuclides in the outdoor air could be assessed by the knowledge of the total air volume filtered and the activity values obtained in the filter. The activity content of short-lived Rn progeny radionuclides in filters was obtained at the measuring time from the early gammaray measurements. From these values, the activity concentration of these radionuclides in the outdoor air could be correctly obtained. To do this, we used a time correction based on Bateman equations, taking into account decay and ingrowth corrections during sampling time, measurement and the time lapse between them. This analytical method was developed for NORM industries dose assessment (Herranz et al., 2014).
212
Pb and
212
Bi used for activity concentration determination.
Radionuclide
Energy peak [keV]
εpi , photopeak efficiency [%]
Igi , photon emission intensity [%]
214
351.932 609.312 238.632 727.330
7.5 5.1 9.9 4.5
37.6 46.1 43.3 6.6
Pb 214 Bi 212 Pb 212 Bi
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Fig. 2. Simplified uranium decay scheme, with determined radionuclides and measurement techniques used.
The activity concentration of long-lived radionuclides in the outdoor air can be directly assessed from their activity in filters, the air volume crossing the filter during the sampling time e mean value of 96,070 m3 e and the collection efficiency of the filter.
2.5. Radon equilibrium factor determination in outdoor air Following UNSCEAR formulae (2000), the equilibrium equivalent radon concentration for 222Rn (EEC222Rn) is determined by Eq. (2):
EEC222Rn ¼ 0:105C1 þ 0:515C2 þ 0:380C3
(2)
222
Fig. 3. Simplified thorium decay scheme, with determined radionuclides and measurement techniques used.
Then, by means of EEC222Rn and the Rn concentration mean value, the radon equilibrium factor (F) is assessed in the following manner:
We assumed that the activity of short-lived Rn progeny, during early gamma-ray measurements in filters, was unsupported by Ra and Rn. This is based on the low Ra content in filters obtained by delayed measurements (Table 2), the short time needed for the whole process and the fact that any potential Rn in filters was removed in the sample heating process. Nevertheless, we assumed that some of these short-lived Rn progeny were partially supported by the other short-lived Rn progeny. The analytical method used provides theoretical counts, obtained during the measurement time. These theoretical counts equal the counts obtained from the early gamma spectrometry for a measurement time tm, letting us determine the activity concentration in the outdoor air for radionuclide i, by Eq. (1):
F ¼ EEC222Rn =Cm
Z Cpi ¼ εpi Igi
tm 0
li Ni ðQi ; tÞdt
(1)
where Cpi is the experimental peak net area of radionuclide i (counts); εpi is the full energy peak efficiency of radionuclide i, summing corrections included; Igi is the photon emission intensity of radionuclide i; tm is the measurement time (s); Ni(Qi,t) is the amount of nuclei of radionuclide i at any time during measurement in the filter, which depends on Qi, the concentration of radionuclide i in the sampled air.
(3)
where F is the equilibrium factor; Cm is the radon concentration mean value (Bq m3); C1, C2 and C3 are the activity concentrations in the outdoor air for 218Po, 214Pb and 214Bi, respectively (Bq m3). Once the activity concentrations of 214Pb and 214Bi were determined, to apply these formulae, we needed the values for the activity concentration of 218Po in the outdoor air. We assumed that the activity concentration of 218Po was two times higher than that of 214Pb (Kulwant et al., 2005; Stajic and Nikezic, 2015; Tsuneo Kobayashi, 2002) with the same relative uncertainty. To assess the mean activity concentration of 222Rn in the air, the definition of the lapse time of interest is key. 214Pb and 214Bi activity values used to obtain EEC222Rn have negligible supported components regarding the unsupported ones (Table 2). Therefore, since the week-long sampling time is longer than the half-lives of 214Pb and 214Bi, the concentration values for both radionuclides come from the air sampled at the end of the sampling time and we assumed that these values remained constant over this time. As the radon content in the atmosphere is very variable, it is more precise to take 222Rn mean values corresponding also to the end of the sampling time. The last time value was chosen to be equal to 2 h, because the unsupported 214Pb and 214Bi sampled before that moment would have decayed by the measuring time to values
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135
the situation in each week. Compared to the ranges of activity concentration published by UNSCEAR (2000), which are based on activity concentrations from different regions and countries from all over the world, the obtained mean activity concentrations are close to the reported minimum values. Therefore, it can be concluded that the possible radionuclides released during the development of industrial and remediation activities, described in the Introduction section, have not produced a significant impact on the activity concentration of radionuclides in the air. Furthermore, it can be seen that in the uranium-series, 226Ra has an activity concentration of around 106 Bq m3, 210Pb is two orders of magnitude higher than 226Ra, and 222Rn and its progeny are, at least, six orders of magnitude higher than 226Ra. This behaviour is also found in the thorium-series, where the activity concentrations of 212Pb and 212Bi are around four orders of magnitude higher than the activity concentration of 224Ra determined analytically. These values agree with the ones from UNSCEAR (2000). 40K is around 105 Bq m3 and 137Cs is around 107 Bq m3, found only in 6 samples. Finally, 212Pb and 212Bi are found to be at complete equilibrium in 57% of samples; close to the equilibrium in 16% of them, taking into account associated relative uncertainties, which are around 5%. In 27% of samples, 212Bi was observed below detection limits. 214Pb and 214Bi were found close to the equilibrium in 80% of samples and in disequilibrium in 10% of them, considering also their uncertainties. 214Bi was also found below detection limits in 10% of cases.
below the detection limit. After determining 212Pb and 212Bi activity concentrations, the equilibrium equivalent radon concentration for 220Rn (EEC220Rn) could also be calculated following Eq. (4):
EEC220Rn ¼ 0:913C1 þ 0:087C2
(4)
where C1 and C2 are the activity concentrations in the outdoor air for 212Pb and 212Bi, respectively (Bq m3). 3. Results and discussion 3.1. Activity concentrations in the outdoor air From 28 June 2013 to 27 June 2014, the outdoor air of Bilbao was characterised every week, following the previously described methodology. Obtained results are shown in Fig. 4, which illustrates activity concentration variation along time, in Bq m3, for 226Ra, 222Rn, 214 Pb, 214Bi, 210Pb, 212Pb, 212Bi, 40K and 137Cs. 234Th, 137Cs, 228Ac and 224 Ra were found below detection limits almost every time. For all radionuclides, their corresponding mean value along with standard deviation (SD) and values reported by UNSCEAR (2000) are presented in Table 2, where it can be seen that the obtained results fall within the range of these reference values. As it can be seen in Fig. 4, the activity concentration of each radionuclide varies smoothly within an order of magnitude over time, from sample to sample. Therefore, the mean value represents
Fig. 4. Activity concentrations (Bq m3) in the outdoor air for
226
Ra,
222
Rn,
214
Pb,
131
214
Bi,
210
Pb,
212
Pb,
212
Bi,
40
K and
137
Cs from sampling between 28/06/2013 and 27/06/2014.
Table 2 Mean values obtained, with their standard deviation (SD), and range of values (UNSCEAR, 2000) for activity concentrations of 212Pb, 212Bi, 226Ra, 222Rn, 218Po, 214Pb, 214Bi, 210Pb, 40 K and 137Cs in outdoor air from 28/06/2013 to 27/06/2014. Radionuclide
Mean value and SD of activity concentration [Bq m3]
Activity concentration range from UNSCEAR [Bq m3]
212
0.025 ± 0.015 0.019 ± 0.013 1.0 106 ± 8.2 13.2 ± 10.8 3.9 ± 3.2 2.0 ± 1.6 1.6 ± 1.4 2.6 104 ± 1.5 1.5 105 ± 8.5 2.7 107 ± 1.2
0.02e1 0.01e0.7 1 106 1e100 1e5 1e5 1e5 2 104e1 103 e 5 107e5 106
Pb Bi Ra 222 Rn 218 Po 214 Pb 214 Bi 210 Pb 40 K 137 Cs 212 226
10
7
104 106 107
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Following the method described in Section 2.2, filters with high natural radioactive content were analysed by alpha spectrometry and alpha/beta counting to determine activity concentrations for other alpha and beta emitters. This way, a complete radiological characterisation was carried out for some filters. A complete set of values for the dissolved filter with the highest content is shown in Table 3. These results can be compared with those reported by UNSCEAR (2000). Results from Table 3 confirm that all the radionuclides before radon are in the same order of activity concentration, 106 Bq m3, and so are long-lived radionuclides after radon, at 104 Bq m3. However, activity concentration mean values for radon and its short-lived progeny are around 13 and 1.8 Bq m3, respectively. Regarding the thorium-series, and as expected, radionuclides before thoron are in the same order of activity concentration, 106 Bq m3, but activity concentration mean values for the shortlived progeny of thoron are around 0.02 Bq m3. If attention is paid to alpha spectrometry results, there is an activity concentration ratio of 1.00 ± 0.15 between 232Th and 228Th, 1.11 ± 0.12 between 238U and 234U and 1.34 ± 0.42 between 228Ra and 224Ra; hence, each pair of radionuclides is in equilibrium and close to the corresponding values reported by UNSCEAR (2000). Between 234Th and 230Th the activity concentration ratio is 0.87 ± 0.08, and therefore, they are close to the equilibrium. The activity concentration found for 210Po is two times lower than that for 210Pb, probably due to the fact that some 210Po could have been removed when the filter was being dried in an oven at 90 C for 10 min (about 50% of it is vaporised at 50 C within 45 h (Person, 2014)), and to the different chemical behaviour of Pb and Po in the atmosphere. This filter heating does not affect lead, because although this radionuclide also presents some volatility, it is at a higher temperature (1750 C). On the other hand, as shown in Table 4, the activity concentration values for 226Ra obtained by alpha/beta counting and by gamma secular equilibrium with its progeny match fairly well, which demonstrates that the sample preparation for gamma-ray spectrometry prevents radon losses.
3.2. Radon equilibrium factor in the outdoor air Following the methodology described in Section 2.5, equilibrium equivalent radon concentrations (EEC222Rn and EEC220Rn) were obtained and the radioactive equilibrium factor for F was assessed. Fig. 5 shows the equilibrium factor and EEC220Rn/EEC222Rn ratio obtained for each day of filter removal. Corresponding mean values, with their standard deviation (SD), and values reported by UNSCEAR (2000) are presented in Table 5. As it can be seen in Fig. 5, the value of the equilibrium factor, F, varies rapidly, with 0.51 and 0.06 being its maximum and minimum values, respectively. There are two abnormal values, 0.01 and 0.05, which were not taken into account. This proves the importance of determining the equilibrium factor in each specific place, as it varies considerably from day to day and sometimes, it is quite far from the typical outdoor equilibrium factor of 0.6 (UNSCEAR, 2000). EEC220Rn/EEC222Rn values are around 0.018, with maximum and minimum values being 0.077 and 0.002, respectively. Since this ratio is strongly affected by 212Pb and 214Pb activity concentrations, it can be roughly understood as the ratio of 212Pb to 214Pb. These mean values can be compared to the range proposed by UNSCEAR (2000) as shown in Table 5. In this Table it can be seen that both F and EEC220Rn/EEC222Rn obtained mean values are close to the minimum values reported by UNSCEAR (2000). 3.3. Influence of environmental parameters on radon equilibrium factor In order to explain the equilibrium factor variability from sample to sample, and taking into account that natural radionuclides are strongly affected by environmental parameters, a set of such parameters e temperature, relative humidity, precipitation, pressure, irradiation and wind speed and direction e were controlled by a weather station located close to the aerosol sampling station. See Table 6.
Table 3 Air activity concentrations of the filter with the highest content, with their uncertainties (coverage factor ¼ 1), and UNSCEAR air activity concentrations. Radionuclide 232
Th Ra 228 Th 224 Ra 212 Pb 212 Bi 235 U 238 U 234 Th 234 U 230 Th 226 Ra 214 Pb 214 Bi 210 Pb 210 Po 228
Determination method
a spectrometry g spectrometry a spectrometry g spectrometry g spectrometry g spectrometry a spectrometry a spectrometry g spectrometry a spectrometry a spectrometry g spectrometry g spectrometry g spectrometry g spectrometry a spectrometry
Activity concentration in air [Bq m3] 6
1.19 10 ± 1.26 1.85 106 ± 1.05 1.19 106 ± 1.27 2.48 106 ± 7.60 5.13 102 ± 2.99 3.62 102 ± 9.25
UNSCEAR air activity concentration [Bq m3]
7
5 107 1 106 1 106 e 2 102e1 1 102e7 101 5 108 1 106 e e 5 107 1 106 1e5 1e5 2 104e1 103 5 105
10 107 107 108 104 104 108 108 108 107 107
106 106
*DL ¼ Detection Limit.
Table 4 226 Ra activity concentrations (Bq m3), with their uncertainties (coverage factor ¼ 1), obtained by two methods. Radionuclide 226
Ra
Determination method
a/b counting g spectrometry
Activity concentration in air [Bq m3] 6
7
3.53 10 ± 2.84 10 3.49 106 ± 4.01 107
% Relative difference 2.40
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135
Fig. 5. Equilibrium factor between and 27/06/2014.
222
Rn and its progeny (F) and
220
Rn/222Rn equilibrium equivalent concentration ratio (EEC220Rn/EEC222Rn), from sampling between 28/06/2013
Table 5 Mean values, with their standard deviation (SD), for F and EEC220Rn/EEC222Rn and range proposed by UNSCEAR from 28/06/2013 to 27/06/2014.
F EEC220Rn/EEC222Rn
133
Obtained mean and SD values
UNSCEAR
0.18 ± 0.07 0.018 ± 0.014
0.20e1 0.01e0.08
In an attempt to fit an exponential and linear function to the set of equilibrium factor values and environmental parameters by the method of least squares, no clear correlations were obtained. We have sorted the data from different environmental parameters into bins and the average value of the factor F within each bin can be seen in Fig. 6. From these results it can be seen a weak but significant correlation between both parameters, as other authors also suggest (Winkler et al., 2001). In the case of the relationship with wind speed, no clear correlation is found for factor F, although, EEC220Rn and EEC222Rn both show a negative exponential dependence on wind speed. As it can be seen in Fig. 6, F increased at this location if the wind was blowing from north-west direction, this fact being highly dependent on specially favoured atmospheric conditions of each location (Winkler et al., 2001). It is also observed in the same figure that the equilibrium factor presents a weak negative correlation with irradiation. No clear correlations are obtained when attempting to find any relationship with pressure and humidity.
4. Conclusions Combining Bateman equations customised for joining collection, ingrowth and decay of radionuclides during sampling with
early and delayed gamma-ray measurements after sampling, a yearlong full radiological picture of the air of a post-industrial urban area (Bilbao) is obtained. The radiological characterisation throughout a year in the outdoor air of Bilbao shows a low committed impact of past manmade activities on the radiological quality of the outdoor air. Therefore, as activities obtained are close to the minimum values collected by UNSCEAR, it can be concluded that the possible radionuclides released in the past in Bilbao during the development of the industrial and remediation activities do not have a significant impact in the activity concentration of radionuclides with terrestrial origin in the air. Despite the variability of the results from sample to sample, we observed that radionuclides before radon and thoron are in the same order of activity concentration, 106 Bq m3, as well as longlived radionuclides after radon, which are about 104 Bq m3. However, the activity concentration values for 222Rn short-lived progeny are around 1.8 Bq m3, and those for 212Pb and 212Bi are around 0.02 Bq m3. There is a remarkable difference in activity concentration between radionuclides before and after radon and thoron that can be related to radon and thoron emanation from soil. In any case, activity concentration values are close to the values presented in the literature (Arnold et al., 2009; Baciu, 2005; Florea and Duliu, 2012; Kulwant et al., 2005; UNSCEAR, 2000; Veleva et al., 2010). To obtain the specific value of the equilibrium factor between radon and its progeny, the activity concentration values for 222Rn were around 13 Bq m3. The calculated equilibrium factor values were around 0.18, close to the reference minimum values and below the suggested value of 0.6. These obtained factors showed some variability, which highlights the importance of determining the equilibrium factor in outdoor air in each specific place. Finally,
Table 6 Mean, maximum and minimum recorded values of environmental parameters during the last 2 h of weekly samplings from 28/06/2013 to 27/06/2014. Environmental parameter
Mean value
Maximum value
Minimum value
Temperature [ C] Relative humidity [%] Pressure [hPa] Irradiation [W m2] Wind speed [m s1] Wind direction [ ] Precipitation [mm], registered only 9 days
18.75 68.14 1012.24 185.75 0.43 208.60 0.32
31.23 90.65 1028.00 792.25 2.20 e 6.90
9.97 42.65 993.42 0.02 0.15 e 0.10
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S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135
Fig. 6. Correlations between environmental parameters (temperature, relative humidity, pressure, irradiation and wind speed and direction) and radon equilibrium factor (F). R2 corresponds to the linear least squares fitting.
the obtained EEC220Rn/EEC222Rn mean value was also close to the minimum value from the range provided by UNSCEAR (2000). We have tried to analyse the influence of a set of environmental parameters on radon equilibrium factor. In this sense, we have to remark that we have collected data for this factor F weekly throughout a year and that some environmental parameters are found to be within a narrow range of values, as the wind speed. Consequently, these facts have not allowed us to obtain any clear correlations for some parameters like wind speed, humidity and pressure. However, a negative correlation between the equilibrium factor and temperature as well as irradiation was observed and the north-west wind direction seems to increase this factor F.
Acknowledgement Authors acknowledge the financial support from the Spanish Nuclear Safety Council (Consejo de Seguridad Nuclear), under contract number 2014.0227, as some of the measurements were carried out in the framework of the European Sparse Network for environmental radioactivity monitoring.
References Alegría, N., Herranz, M., Idoeta, R., Legarda, F., 2010. Study of 7Be activity concentration in the air of northern Spain. J. Radioanal. Nucl. Chem. 286, 347e351. http://dx.doi.org/10.1007/s10967-010-0710-6. Arnold, D., Vargas, A., Ortega, X., 2009. Analysis of outdoor radon progeny concentration measured at the Spanish radioactive aerosol automatic monitoring network. Appl. Radiat. Isot. 67 (5), 833e838. http://dx.doi.org/10.1016/ j.apradiso.2009.01.042. Baciu, A.C., 2005. Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania). J. Environ. Radioact. 83 (2), 171e189. http://dx.doi.org/10.1016/j.jenvrad.2005.02.015. Berthold Technologies GmbH & Co. KG, 2010. Moving Filter Particulate Monitor BAI 9100 D Operating Manual (Bad Wildbad, Germany). Chen, Jing, 2005. A review of radon doses. Radiat. Prot. Manag. 22 (4), 27e31. ticas Consejo de Seguridad Nuclear (CSN), 2013. INT-04.32 Red de estaciones automa gica ambiental (REA) del CSN. Operacio n y resultados. An ~ os de vigilancia radiolo 2010 y 2011. EFFTRAN, 2015 (written by Vidmar, T.). http://www.efftran.com. Florea, N., Duliu, O.G., 2012. Eighteen years of continuous observation or Radon and Thoron progenies atmospheric activity. J. Environ. Radioact. 104, 14e23. http:// dx.doi.org/10.1016/j.jenvrad.2011.10.002. Forkapic, S., Mrda, D., Veskovic, M., Todorovic, N., Bikit, K., Nikolov, J., Hansman, J., 2013. Radon equilibrium measurement in the air. Rom. J. Phys. 58 (Suppl. l), 140e147. Herranz, M., Idoeta, R., Abelairas, A., Legarda, F., 2006. Radon fixation for determination of 224Ra, 226Ra and 228Ra via gamma-ray spectrometry. Radiat. Meas.
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135 41 (4), 486e491. http://dx.doi.org/10.1016/j.radmeas.2005.09.007. Herranz, M., Idoeta, R., Legarda, F., 2008. Evaluation of uncertainty and detection limits in radioactivity measurements. Nucl. Instrum. Methods Phys. A 595 (2), 526e534. http://dx.doi.org/10.1016/j.nima.2008.07.105. Herranz, M., Rozas, S., Idoeta, R., Alegría, N., 2014. Effective dose in SMAW and FCAW welding processes using rutile consumables. J. Radiol. Prot. 34 (1), 133e148. http://dx.doi.org/10.1088/0952-4746/34/1/133. ISO 11929, 2010. Determination of the Characteristic Limits (Decision Threshold, Detection Limit and Limits of the Condifence Interval) for Measurements of Ionizing Radiation e Fundamentals and Application. International Organization for Standardization, Geneva. ISO/IEC Guide 98, 1995. Uncertainty of Measurement. International Organization for Standardization, Switzerland. Kobayashi, Tsuneo, 2002. Temporal variation of radon progeny ratio in outdoor air. Health Phys. 83 (2), 287e292. http://dx.doi.org/10.1097/00004032-20020800000015. Kojima, H., 1996. The equilibrium factor between radon and its progeny in the lower atmosphere. Environ. Int. 22 (Suppl. 1), 187e192. http://dx.doi.org/10.1016/ S0160-4120(96)00107-9. Kulwant, Singh, Manmohan, Singh, Surinder, Singh, Sahota, H.S., Papp, Z., 2005. Variation of radon (222Rn) progeny concentrations in outdoor air as a function of time, temperature and relative humidity. Radiat. Meas. 39 (2), 213e217. http://dx.doi.org/10.1016/j.radmeas.2004.06.015. Laboratoire National Henri Becquerel, 2015. Recommended Data. http://www. nucleide.org/DDEP_WG/DDEPdata.htm.
135
pez-Pe rez, M., Ramos-Lo pez, R., Perestelo, Nayra R., Duarte-Rodríguez, X., Lo rez, S., Cuevas, E., Herna ndez-Armas, J., 2013. Arrival of Bustos, J.J., Alonso-Pe radionuclides released by the Fukushima accident to Tenerife (Canary Islands). J. Environ. Radioact. 116, 180e186. http://dx.doi.org/10.1016/ j.jenvrad.2012.09.011. Person, B.R.R., 2014. 210Po and 210Pb in the terrestrial environment. Curr. Adv. Environ. Sci. CAES 2 (1), 22e37. €rfer, J., Zock, Ch., Wendt, J., Reineking, A., 1999. Radon parameters in Porstendo outdoor air. In: 2 International Symposium on Technologically Enhanced Natural Radiation Book of Abstracts, 124, Brazil. Stajic, J.M., Nikezic, D., 2015. The accuracy of radon and thoron progeny concentrations measured through air filtration. J. Environ. Radioact. 140, 50e58. http:// dx.doi.org/10.1016/j.jenvrad.2014.11.002. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2000. Sources and Effects of Ionising Radiation, vol. I. Annex B. United Nations, New York. Veleva, B., Valkov, N., Batchvarova, E., Kolarova, M., 2010. Variation of short-lived beta radionuclide (radon progeny) concentrations and the mixing processes in the atmospheric boundary layer. J. Environ. Radioact. 101 (7), 538e543. http://dx.doi.org/10.1016/j.jenvrad.2009.08.008. Winkler, R., Ruckerbauer, F., Trautmannsheimer, M., Tschiersch, J., Karg, E., 2001. Diurnal and seasonal variation of the equilibrium state between short-lived radon decay products and radon gas in ground-level air. Radiat. Environ. Biophys. 40 (2), 115e123. http://dx.doi.org/10.1007/s004110100096.
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Review
Radiological characterisation and radon equilibrium factor in the outdoor air of a post-industrial urban area S. Rozas*, R. Idoeta, N. Alegría, M. Herranz Department of Nuclear Engineering and Fluid Mechanics, University of the Basque Country (UPV/EHU), Alameda Urquijo s/n, Bilbao 48013, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 24 September 2015 Accepted 27 September 2015 Available online xxx
The radiological characterisation of outdoor air is always a complicated task due to the several radioactive emissions coming from the different radionuclides and also because of the very short half-lives of radionuclides in the natural radioactive series. In some places, this characterisation could result in unusual values because the natural presence of radionuclides with terrestrial origin can be modified by manmade activities. Nonetheless, this characterisation is useful not only for air quality control purposes but also because radon and its progeny in the outdoor air are the main contributors to human exposure from natural sources. In this study, we have carried out air particle sampling, followed by gamma-ray spectrometry, alpha spectrometry and beta counting determinations for this purpose. Subsequently, the outdoor air has been radiologically characterised through the obtained data and using a pre-existing analytical method to take into account the radioactive decays of short half-life radionuclides during sampling, sample preparation and measuring times. Bilbao was chosen to carry out this work. It is a medium-sized town located in northern Spain, close to the Atlantic Ocean and at sea level. This city has a recent industrial past as there were numerous steel mills and other heavy industries, including some quarries, and some open pit mines close to it, which concluded in a remediation program. So, it is a place where the air is potentially modified by manmade activities. The obtained results show that activity concentration values for long-lived radionuclides that precede radon and thoron are in the order of 106 Bq m3 and long-lived ones after radon are around 104 Bq m3. Thoron progeny are around 2 102 Bq m3 and radon progeny are around 1.8 Bq m3. The mean radon equilibrium factor was 0.18. All of these values are close to the minimum UNSCEAR values, but show some variability, which highlights the importance of determining activity concentrations for each naturally occurring radionuclide and the equilibrium factor in the outdoor air. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Air quality control Radiological characterisation Natural radioactivity Radon equilibrium factor
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.1. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.2. Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 2.3. Activity determination in filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 2.4. Activity concentration determination in outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 2.5. Radon equilibrium factor determination in outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 3.1. Activity concentrations in the outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 3.2. Radon equilibrium factor in the outdoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
* Corresponding author. E-mail address: [email protected] (S. Rozas). http://dx.doi.org/10.1016/j.jenvrad.2015.09.023 0265-931X/© 2015 Elsevier Ltd. All rights reserved.
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4.
127
3.3. Influence of environmental parameters on radon equilibrium factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
1. Introduction Radionuclides existing in the outdoor air have cosmogenic or terrestrial origins. The cosmogenic isotopes are mainly 7Be, 14C and 3 H (UNSCEAR, 2000), and the terrestrials are 137Cs, 40K and other radionuclides from the natural radioactive series. All of them are usually present in sampled aerosols, albeit to a different extent. The concentration of terrestrial radionuclides in the lower layers of the atmosphere is heavily dependent on their removal from soils and, in the case of Rn and its progeny, also on the emanation of Rn from soil surfaces. Dilution factors, such as radioactive decay, alter these concentrations, which can also be affected by atmospheric mixing phenomena, season, time, height and location, as many studies have revealed (Arnold et al., 2009; Baciu, 2005; Florea and Duliu, 2012; Kulwant et al., 2005; Veleva et al., 2010). Additionally, these concentrations could also be modified by manmade activities, such as mining, quarries and other types of industrial facilities, including NORM/TENORM industries. In this case, as the existence of radioactive equilibrium between radionuclides from the uranium and the thorium-series cannot be assumed, the possible presence of an activity-supported and an unsupported component for the progeny should be taken into account. The concentration of cosmogenic radionuclides also depends on atmospheric mixing phenomena, season, time, height and location, being lower at sea level. But as they do not belong to radioactive series and their half-lives are relatively long, it is easy to measure them and only in some cases, like in the case of nuclear industries, can their concentration in the outdoor air be affected by manmade activities. Bilbao is a town located in northern Spain, close to the Atlantic Ocean (2 560 W, 43 150 N) and at sea level (Fig. 1). The city is situated at the bottom of a valley that is open to the sea, and which has a recent industrial past. Close to the city there were numerous steel mills and other heavy industries, including some limestone quarries, and some, now closed, open pit mines. Therefore, this city, which suffered high levels of pollution until the 1990s, could have singular air radiological characteristics. In the late 90's and the early 00's most of these industrial activities were stopped, industries were abandoned, industrial wastes remained on land and mines and quarries stayed in the open air. In this context, the Spanish Government and the regional Basque Government started a big remediation program to recover soil to use it in other secondary activities (residential and small industries), which is almost finished nowadays. So, the radionuclides released during the development of these industrial and remediation activities might have undergone the processes of deposition, resuspension and dispersion and hence, they might be modifying the activity concentration of radionuclides with terrestrial origin in the air. These reasons have motivated us to obtain the radiological characterisation of Bilbao's air. As no nuclear facilities e which would have been able to modify the inventory of cosmogenic radionuclides e exist around this city, only terrestrial radionuclides have been considered in this work. Nevertheless, measurements carried out on cosmogenic radionuclides show that their activity concentrations are around 103 Bq m3 for 7Be, 103 Bq m3 for 3H
and 102 Bq m3 for 14C. From a dosimetric point of view, the 14C and 3H contents are negligible. The remaining radionuclide, 7Be, has been previously analysed by this group (Alegría et al., 2010). To carry out this characterisation, aerosols and particles in the air were sampled by means of a filter and measured by gamma spectrometry. Therefore, only the attached fraction of radionuclides has been collected. In any case, as some authors suggest, most of the activity in the outdoor air is in the attached fraction and this frac€rfer et al., 1999). tion ranges from 88% to 96% (Porstendo In these filters, gamma emitter radionuclides were determined. However, to obtain activity concentrations in the sampled air, the obtained activity concentration of the short-lived radionuclides must be time-corrected, taking into account radioactive decays and ingrowths during sampling, sample preparation and measuring times. To make these time corrections, as the presence in the air of short-lived radon progeny depends also on the presence of radon in the air, Rn in air has been obtained by direct measurements. With these Rn and gamma spectrometry data and the use of a mathematical treatment developed by us and used for dose assessment in NORM industries (Herranz et al., 2014), the activity concentration of gamma emitters in sampled aerosols and particles was finally obtained. This characterisation, repeated weekly for a year, also allowed us to analyse the variations in radionuclide concentration. Additionally, for the most contaminated filters this process was completed carrying out radiochemical isolations in the filters, followed by alpha spectrometry and beta counting measurements, in order to obtain the activity concentration of alpha and beta emitters. The data obtained provided us with the isotopic ratios and relationships between radionuclides. Finally, it should be taken into consideration that, although their concentrations in the outdoor air are very low, radon and its progeny are the most important contributors to human exposure from natural sources. As the equivalent dose is mainly due to the short-lived progeny of radon, which is not always in radioactive equilibrium to its parent, effective dose assessment is usually carried out by measuring radon concentration and applying an equilibrium factor. Therefore, the most accurate estimation of this equilibrium factor is the guarantee of an accurate dose assessment (Forkapic et al., 2013; Chen, 2005, Kojima, 1996; UNSCEAR, 2000). Using the data obtained in the radiological characterisation of the outdoor air, equilibrium factors for radon and its progeny can be assessed and the values obtained can be compared with those typically used. In summary, the main objective of this work is the radiological characterisation of the outdoor air in a post-industrial urban area like Bilbao and, also, the determination of the equilibrium factor, considering in detail all its contributors. The laboratory where this work was carried out is accredited by ENAC (Spanish National Accreditation Body) since 2003, under the ISO/IEC standard 17,025, for the determinations carried out in this work. Thus, the procedures are validated and used routinely in national and international intercomparisons and proficiency test exercises. The materials and methods used to achieve the stated objectives and the obtained results are presented below.
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Fig. 1. Bilbao's location, indicated by an A on a Google map, 2015.
2. Materials and methods 2.1. Sampling The aerosol sampling station ASS 500 was used to collect aerosols from the outdoor air. For a week, a nominal air flow rate of 0.1389 m3 s1 was forced to pass through a filter by an air pump. All the air filtered throughout a week was monitored by means of a calibrated air flow totalizer. The filter used was a PTI type G3 polypropylene, with an active surface of 0.41 0.41 m2. Some experiments have been carried out to obtain the retention efficiency of the filter in the particular conditions of the sampling; a retention efficiency of 95.5% has been obtained for particles size larger than pez-Pe rez et al., 0.1 mm. This value matches those from reference (Lo 2013) in which the collection efficiency is analysed using the same filter, same equipment and same time collection as that of this work. This collection efficiency has been applied to assess the radionuclides activity concentration. This radiological control has been done since the late 90's in the framework of the Dense and Sparse Networks of the European Community. In Spain these networks depend on the Spanish Nuclear Safety Council (CSN) and our laboratory was selected by the CSN to run both networks in Bilbao, where water, soil and also air are controlled in a routine basis. The location of the sampling points and hence the location of aerosol sampling station were adopted in accordance with the CSN as representative places of the quality of the different environmental constituents. At the same place as the aerosol sampling station, radon activity
concentration values were obtained by the moving filter particulate monitor BAI 9100D, manufactured by Berthold, which continuously measures alpha and beta particulate activity in the presence of radon and applies the Alpha Beta Pseudo coincidence Difference (ABPD) method (Berthold, 2010) to provide radon activity concentration values every 10 min. This monitor belongs to the Automatic Stations Network to provide the radon reference values for the environmental radioactivity monitoring to the Spanish Nuclear Safety Council (CSN, 2013). These values complete, in a continuous way, the data obtained in the Sparse and Dense Networks. 2.2. Measurement Once a week, the filter was removed from the aerosol sampling station ASS 500, always at the same time, dried in an oven at 90 C for 10 min, folded, wrapped in a plastic bag and put in a polypropylene box. This preparation prevents losses of ingrowing radon, as measurements carried out weeks after sampling proved. Thereupon, the sample was analysed at three different times by gamma-ray spectrometry. As the objective is to determine activity concentrations in outdoor air, the first measurement was carried out immediately after sampling (early measurement) and establishing a short counting time (1800 s), in order to obtain a picture of the gamma emitters present in the air and to prevent the complete decay of the unsupported part of short-lived radionuclides in the filter. The two following measurements were carried out a week later and some weeks after the early measurement (delayed measurements) so as to obtain a picture of other radionuclides, whose
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concentration determinations are presented in Table 1. These intensities and other nuclear data have been taken from the database of Recommended Data from Laboratoire National Henri Becquerel (2015). In order to assess the activity values of 228Ra, 228Th, 224Ra and 226 Ra in filters from their progeny, the values were measured one week and once again some weeks after the early gamma-ray spectrometry, when the unsupported part of short-lived radionuclides had disappeared (Herranz et al., 2006). It was done as follows: From the measurements done a week after sampling, the activity values of 228Ra and 224Ra in filters were obtained. The values of 228Ra were determined through its daughter 228Ac, both radionuclides having reached secular equilibrium. Those of 224Ra were determined through its daughters 212Pb and 212Bi, to avoid the interference with 241.997 keV from 214Pb. For this measurement, it was considered that unsupported 212Pb and 212Bi had disappeared and 224Ra, 212Pb and 212Bi had reached a transient equilibrium with an activity ratio of 1.14. Results obtained for 224Ra were also corrected for its one-week ingrowth from 228Th. That 228Th was obtained from the second delayed measurement, when it was in equilibrium with 212Pb and 212Bi. Finally, second delayed measurements also provided the activity of 226Ra from its daughters 214Pb and 214Bi. These delayed measurements were also used to check the ability of the box to prevent radon losses by assessing the levels of 226Ra obtained by radioactive equilibrium with its progeny and 226Ra obtained by radiochemical methods. These results are shown in Table 4. In the cases of 234Th and 228Ac, the comparison of their activity values obtained through the three measurements allowed us to get conclusions on the existence of equilibrium between 234Th and 238 U and between 228Ac and 228Ra in the sampled air and, hence, to obtain the activities for 238U and 228Ra. Uranium (238U) and thorium (232Th) decay schemes are presented in Figs. 2 and 3, respectively, together with the method used for the determination of each radionuclide in the series. These delayed measurements also allowed us to determine long-lived radionuclides obtained directly from their respective gamma photopeak, like 137Cs, 40K and 210Pb, with better statistics and lower detection limits than in early measurements.
activities can be determined through the equilibrium with their progeny, establishing a longer counting time (250,000 s) to get better statistics and lower detection limits. The detector used was an extended range coaxial germanium detector with a carbon composite window (Canberra, XtRa model) that permits an energy measuring range from 3 keV to 3 MeV with a 90% relative efficiency and a resolution of 2.2 keV (FWHM) at 1.33 MeV peak. Obtained spectra were processed by EG&G ORTEC Gamma Vision 6.01 commercial software and calibrations in energy and efficiency were made using a calibration source with the same geometry and composition as the sample, spiked with a certified multi gamma-ray emitter standard, from 47 to 1836 keV. Summing corrections were applied by means of the software EFFTRAN (Vidmar, 2015). Finally, after the gamma-ray spectrometry measurements in the filters with the highest activity, 210Pb, 210Po, 226Ra and isotopic U and Th were determined by radiochemical methods. To achieve these determinations, aliquots of the most contaminated filters were treated by cold lixiviation with aqua regia (for Po determination) and by acid digestion with a microwave system (for U, Th, Ra and Pb determinations). U and Th were isolated from the resulting solution by means of Dowex AG1 X8 anion exchange resins, Ra and Pb by precipitation and Po by direct deposition. This way, 238U, 235U, 234U, 232Th, 230Th, 228Th and 210Po could be measured by alpha spectrometry and 226Ra and 210Pb by alpha/beta counting. Detectors used for alpha spectrometry were of the passivated implanted silicon (PIPS) type, manufactured by Canberra, with a nominal background of 0.1389 s1m2. For alpha/beta counting we used a gas flow proportional counter equipped with 7 cm-diameter detectors with a nominal background of 0.00167 s1 for alpha counting and 0.0167 s1 for beta counting. For each measurement, uncertainty and detection limits were obtained. Combined uncertainties were obtained by means of the GUM (ISO/IEC Guide 98, 1995), considering all sources of uncertainty: calibration (standard certificate, preparation, counting and calibration fitting for gamma spectrometry), sample and background counting, sample preparation and a 3% for sampling. Decision thresholds and detection limits were obtained following ISO 11929 standard (ISO 11929, 2010); specific formulae used were presented in a paper (Herranz et al., 2008). 2.3. Activity determination in filters
2.4. Activity concentration determination in outdoor air The activity of radon progeny in the filters was obtained from the early gamma-ray spectrometry measurements. To do this, the possible ingrowth of 214Pb and 214Bi from 226Ra and 222Rn, and of 212 Pb and 212Bi from 228Th, 224Ra and 220Rn in the filters was considered negligible due to the short time between the end of sampling and the measurement (less than 1 h). This short time implies that the supported activity of 214Pb/214Bi and 212Pb/212Bi in the filters would never be more than 0.7% of 226Ra and 3% of 224Ra activity at the start of the measurement. Other short-lived radionuclides, like 228Ac and 234Th were also analysed in this early measurement. Photopeak efficiency and photon emission intensities of the energy peaks of 214Pb, 214Bi, 212Pb and 212Bi used for activity
Table 1 Photopeak efficiency and photon emission intensity of energy peak of
214
Pb,
214
Bi,
The activity concentration of radionuclides in the outdoor air could be assessed by the knowledge of the total air volume filtered and the activity values obtained in the filter. The activity content of short-lived Rn progeny radionuclides in filters was obtained at the measuring time from the early gammaray measurements. From these values, the activity concentration of these radionuclides in the outdoor air could be correctly obtained. To do this, we used a time correction based on Bateman equations, taking into account decay and ingrowth corrections during sampling time, measurement and the time lapse between them. This analytical method was developed for NORM industries dose assessment (Herranz et al., 2014).
212
Pb and
212
Bi used for activity concentration determination.
Radionuclide
Energy peak [keV]
εpi , photopeak efficiency [%]
Igi , photon emission intensity [%]
214
351.932 609.312 238.632 727.330
7.5 5.1 9.9 4.5
37.6 46.1 43.3 6.6
Pb 214 Bi 212 Pb 212 Bi
130
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Fig. 2. Simplified uranium decay scheme, with determined radionuclides and measurement techniques used.
The activity concentration of long-lived radionuclides in the outdoor air can be directly assessed from their activity in filters, the air volume crossing the filter during the sampling time e mean value of 96,070 m3 e and the collection efficiency of the filter.
2.5. Radon equilibrium factor determination in outdoor air Following UNSCEAR formulae (2000), the equilibrium equivalent radon concentration for 222Rn (EEC222Rn) is determined by Eq. (2):
EEC222Rn ¼ 0:105C1 þ 0:515C2 þ 0:380C3
(2)
222
Fig. 3. Simplified thorium decay scheme, with determined radionuclides and measurement techniques used.
Then, by means of EEC222Rn and the Rn concentration mean value, the radon equilibrium factor (F) is assessed in the following manner:
We assumed that the activity of short-lived Rn progeny, during early gamma-ray measurements in filters, was unsupported by Ra and Rn. This is based on the low Ra content in filters obtained by delayed measurements (Table 2), the short time needed for the whole process and the fact that any potential Rn in filters was removed in the sample heating process. Nevertheless, we assumed that some of these short-lived Rn progeny were partially supported by the other short-lived Rn progeny. The analytical method used provides theoretical counts, obtained during the measurement time. These theoretical counts equal the counts obtained from the early gamma spectrometry for a measurement time tm, letting us determine the activity concentration in the outdoor air for radionuclide i, by Eq. (1):
F ¼ EEC222Rn =Cm
Z Cpi ¼ εpi Igi
tm 0
li Ni ðQi ; tÞdt
(1)
where Cpi is the experimental peak net area of radionuclide i (counts); εpi is the full energy peak efficiency of radionuclide i, summing corrections included; Igi is the photon emission intensity of radionuclide i; tm is the measurement time (s); Ni(Qi,t) is the amount of nuclei of radionuclide i at any time during measurement in the filter, which depends on Qi, the concentration of radionuclide i in the sampled air.
(3)
where F is the equilibrium factor; Cm is the radon concentration mean value (Bq m3); C1, C2 and C3 are the activity concentrations in the outdoor air for 218Po, 214Pb and 214Bi, respectively (Bq m3). Once the activity concentrations of 214Pb and 214Bi were determined, to apply these formulae, we needed the values for the activity concentration of 218Po in the outdoor air. We assumed that the activity concentration of 218Po was two times higher than that of 214Pb (Kulwant et al., 2005; Stajic and Nikezic, 2015; Tsuneo Kobayashi, 2002) with the same relative uncertainty. To assess the mean activity concentration of 222Rn in the air, the definition of the lapse time of interest is key. 214Pb and 214Bi activity values used to obtain EEC222Rn have negligible supported components regarding the unsupported ones (Table 2). Therefore, since the week-long sampling time is longer than the half-lives of 214Pb and 214Bi, the concentration values for both radionuclides come from the air sampled at the end of the sampling time and we assumed that these values remained constant over this time. As the radon content in the atmosphere is very variable, it is more precise to take 222Rn mean values corresponding also to the end of the sampling time. The last time value was chosen to be equal to 2 h, because the unsupported 214Pb and 214Bi sampled before that moment would have decayed by the measuring time to values
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135
the situation in each week. Compared to the ranges of activity concentration published by UNSCEAR (2000), which are based on activity concentrations from different regions and countries from all over the world, the obtained mean activity concentrations are close to the reported minimum values. Therefore, it can be concluded that the possible radionuclides released during the development of industrial and remediation activities, described in the Introduction section, have not produced a significant impact on the activity concentration of radionuclides in the air. Furthermore, it can be seen that in the uranium-series, 226Ra has an activity concentration of around 106 Bq m3, 210Pb is two orders of magnitude higher than 226Ra, and 222Rn and its progeny are, at least, six orders of magnitude higher than 226Ra. This behaviour is also found in the thorium-series, where the activity concentrations of 212Pb and 212Bi are around four orders of magnitude higher than the activity concentration of 224Ra determined analytically. These values agree with the ones from UNSCEAR (2000). 40K is around 105 Bq m3 and 137Cs is around 107 Bq m3, found only in 6 samples. Finally, 212Pb and 212Bi are found to be at complete equilibrium in 57% of samples; close to the equilibrium in 16% of them, taking into account associated relative uncertainties, which are around 5%. In 27% of samples, 212Bi was observed below detection limits. 214Pb and 214Bi were found close to the equilibrium in 80% of samples and in disequilibrium in 10% of them, considering also their uncertainties. 214Bi was also found below detection limits in 10% of cases.
below the detection limit. After determining 212Pb and 212Bi activity concentrations, the equilibrium equivalent radon concentration for 220Rn (EEC220Rn) could also be calculated following Eq. (4):
EEC220Rn ¼ 0:913C1 þ 0:087C2
(4)
where C1 and C2 are the activity concentrations in the outdoor air for 212Pb and 212Bi, respectively (Bq m3). 3. Results and discussion 3.1. Activity concentrations in the outdoor air From 28 June 2013 to 27 June 2014, the outdoor air of Bilbao was characterised every week, following the previously described methodology. Obtained results are shown in Fig. 4, which illustrates activity concentration variation along time, in Bq m3, for 226Ra, 222Rn, 214 Pb, 214Bi, 210Pb, 212Pb, 212Bi, 40K and 137Cs. 234Th, 137Cs, 228Ac and 224 Ra were found below detection limits almost every time. For all radionuclides, their corresponding mean value along with standard deviation (SD) and values reported by UNSCEAR (2000) are presented in Table 2, where it can be seen that the obtained results fall within the range of these reference values. As it can be seen in Fig. 4, the activity concentration of each radionuclide varies smoothly within an order of magnitude over time, from sample to sample. Therefore, the mean value represents
Fig. 4. Activity concentrations (Bq m3) in the outdoor air for
226
Ra,
222
Rn,
214
Pb,
131
214
Bi,
210
Pb,
212
Pb,
212
Bi,
40
K and
137
Cs from sampling between 28/06/2013 and 27/06/2014.
Table 2 Mean values obtained, with their standard deviation (SD), and range of values (UNSCEAR, 2000) for activity concentrations of 212Pb, 212Bi, 226Ra, 222Rn, 218Po, 214Pb, 214Bi, 210Pb, 40 K and 137Cs in outdoor air from 28/06/2013 to 27/06/2014. Radionuclide
Mean value and SD of activity concentration [Bq m3]
Activity concentration range from UNSCEAR [Bq m3]
212
0.025 ± 0.015 0.019 ± 0.013 1.0 106 ± 8.2 13.2 ± 10.8 3.9 ± 3.2 2.0 ± 1.6 1.6 ± 1.4 2.6 104 ± 1.5 1.5 105 ± 8.5 2.7 107 ± 1.2
0.02e1 0.01e0.7 1 106 1e100 1e5 1e5 1e5 2 104e1 103 e 5 107e5 106
Pb Bi Ra 222 Rn 218 Po 214 Pb 214 Bi 210 Pb 40 K 137 Cs 212 226
10
7
104 106 107
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Following the method described in Section 2.2, filters with high natural radioactive content were analysed by alpha spectrometry and alpha/beta counting to determine activity concentrations for other alpha and beta emitters. This way, a complete radiological characterisation was carried out for some filters. A complete set of values for the dissolved filter with the highest content is shown in Table 3. These results can be compared with those reported by UNSCEAR (2000). Results from Table 3 confirm that all the radionuclides before radon are in the same order of activity concentration, 106 Bq m3, and so are long-lived radionuclides after radon, at 104 Bq m3. However, activity concentration mean values for radon and its short-lived progeny are around 13 and 1.8 Bq m3, respectively. Regarding the thorium-series, and as expected, radionuclides before thoron are in the same order of activity concentration, 106 Bq m3, but activity concentration mean values for the shortlived progeny of thoron are around 0.02 Bq m3. If attention is paid to alpha spectrometry results, there is an activity concentration ratio of 1.00 ± 0.15 between 232Th and 228Th, 1.11 ± 0.12 between 238U and 234U and 1.34 ± 0.42 between 228Ra and 224Ra; hence, each pair of radionuclides is in equilibrium and close to the corresponding values reported by UNSCEAR (2000). Between 234Th and 230Th the activity concentration ratio is 0.87 ± 0.08, and therefore, they are close to the equilibrium. The activity concentration found for 210Po is two times lower than that for 210Pb, probably due to the fact that some 210Po could have been removed when the filter was being dried in an oven at 90 C for 10 min (about 50% of it is vaporised at 50 C within 45 h (Person, 2014)), and to the different chemical behaviour of Pb and Po in the atmosphere. This filter heating does not affect lead, because although this radionuclide also presents some volatility, it is at a higher temperature (1750 C). On the other hand, as shown in Table 4, the activity concentration values for 226Ra obtained by alpha/beta counting and by gamma secular equilibrium with its progeny match fairly well, which demonstrates that the sample preparation for gamma-ray spectrometry prevents radon losses.
3.2. Radon equilibrium factor in the outdoor air Following the methodology described in Section 2.5, equilibrium equivalent radon concentrations (EEC222Rn and EEC220Rn) were obtained and the radioactive equilibrium factor for F was assessed. Fig. 5 shows the equilibrium factor and EEC220Rn/EEC222Rn ratio obtained for each day of filter removal. Corresponding mean values, with their standard deviation (SD), and values reported by UNSCEAR (2000) are presented in Table 5. As it can be seen in Fig. 5, the value of the equilibrium factor, F, varies rapidly, with 0.51 and 0.06 being its maximum and minimum values, respectively. There are two abnormal values, 0.01 and 0.05, which were not taken into account. This proves the importance of determining the equilibrium factor in each specific place, as it varies considerably from day to day and sometimes, it is quite far from the typical outdoor equilibrium factor of 0.6 (UNSCEAR, 2000). EEC220Rn/EEC222Rn values are around 0.018, with maximum and minimum values being 0.077 and 0.002, respectively. Since this ratio is strongly affected by 212Pb and 214Pb activity concentrations, it can be roughly understood as the ratio of 212Pb to 214Pb. These mean values can be compared to the range proposed by UNSCEAR (2000) as shown in Table 5. In this Table it can be seen that both F and EEC220Rn/EEC222Rn obtained mean values are close to the minimum values reported by UNSCEAR (2000). 3.3. Influence of environmental parameters on radon equilibrium factor In order to explain the equilibrium factor variability from sample to sample, and taking into account that natural radionuclides are strongly affected by environmental parameters, a set of such parameters e temperature, relative humidity, precipitation, pressure, irradiation and wind speed and direction e were controlled by a weather station located close to the aerosol sampling station. See Table 6.
Table 3 Air activity concentrations of the filter with the highest content, with their uncertainties (coverage factor ¼ 1), and UNSCEAR air activity concentrations. Radionuclide 232
Th Ra 228 Th 224 Ra 212 Pb 212 Bi 235 U 238 U 234 Th 234 U 230 Th 226 Ra 214 Pb 214 Bi 210 Pb 210 Po 228
Determination method
a spectrometry g spectrometry a spectrometry g spectrometry g spectrometry g spectrometry a spectrometry a spectrometry g spectrometry a spectrometry a spectrometry g spectrometry g spectrometry g spectrometry g spectrometry a spectrometry
Activity concentration in air [Bq m3] 6
1.19 10 ± 1.26 1.85 106 ± 1.05 1.19 106 ± 1.27 2.48 106 ± 7.60 5.13 102 ± 2.99 3.62 102 ± 9.25
UNSCEAR air activity concentration [Bq m3]
7
5 107 1 106 1 106 e 2 102e1 1 102e7 101 5 108 1 106 e e 5 107 1 106 1e5 1e5 2 104e1 103 5 105
10 107 107 108 104 104 108 108 108 107 107
106 106
*DL ¼ Detection Limit.
Table 4 226 Ra activity concentrations (Bq m3), with their uncertainties (coverage factor ¼ 1), obtained by two methods. Radionuclide 226
Ra
Determination method
a/b counting g spectrometry
Activity concentration in air [Bq m3] 6
7
3.53 10 ± 2.84 10 3.49 106 ± 4.01 107
% Relative difference 2.40
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135
Fig. 5. Equilibrium factor between and 27/06/2014.
222
Rn and its progeny (F) and
220
Rn/222Rn equilibrium equivalent concentration ratio (EEC220Rn/EEC222Rn), from sampling between 28/06/2013
Table 5 Mean values, with their standard deviation (SD), for F and EEC220Rn/EEC222Rn and range proposed by UNSCEAR from 28/06/2013 to 27/06/2014.
F EEC220Rn/EEC222Rn
133
Obtained mean and SD values
UNSCEAR
0.18 ± 0.07 0.018 ± 0.014
0.20e1 0.01e0.08
In an attempt to fit an exponential and linear function to the set of equilibrium factor values and environmental parameters by the method of least squares, no clear correlations were obtained. We have sorted the data from different environmental parameters into bins and the average value of the factor F within each bin can be seen in Fig. 6. From these results it can be seen a weak but significant correlation between both parameters, as other authors also suggest (Winkler et al., 2001). In the case of the relationship with wind speed, no clear correlation is found for factor F, although, EEC220Rn and EEC222Rn both show a negative exponential dependence on wind speed. As it can be seen in Fig. 6, F increased at this location if the wind was blowing from north-west direction, this fact being highly dependent on specially favoured atmospheric conditions of each location (Winkler et al., 2001). It is also observed in the same figure that the equilibrium factor presents a weak negative correlation with irradiation. No clear correlations are obtained when attempting to find any relationship with pressure and humidity.
4. Conclusions Combining Bateman equations customised for joining collection, ingrowth and decay of radionuclides during sampling with
early and delayed gamma-ray measurements after sampling, a yearlong full radiological picture of the air of a post-industrial urban area (Bilbao) is obtained. The radiological characterisation throughout a year in the outdoor air of Bilbao shows a low committed impact of past manmade activities on the radiological quality of the outdoor air. Therefore, as activities obtained are close to the minimum values collected by UNSCEAR, it can be concluded that the possible radionuclides released in the past in Bilbao during the development of the industrial and remediation activities do not have a significant impact in the activity concentration of radionuclides with terrestrial origin in the air. Despite the variability of the results from sample to sample, we observed that radionuclides before radon and thoron are in the same order of activity concentration, 106 Bq m3, as well as longlived radionuclides after radon, which are about 104 Bq m3. However, the activity concentration values for 222Rn short-lived progeny are around 1.8 Bq m3, and those for 212Pb and 212Bi are around 0.02 Bq m3. There is a remarkable difference in activity concentration between radionuclides before and after radon and thoron that can be related to radon and thoron emanation from soil. In any case, activity concentration values are close to the values presented in the literature (Arnold et al., 2009; Baciu, 2005; Florea and Duliu, 2012; Kulwant et al., 2005; UNSCEAR, 2000; Veleva et al., 2010). To obtain the specific value of the equilibrium factor between radon and its progeny, the activity concentration values for 222Rn were around 13 Bq m3. The calculated equilibrium factor values were around 0.18, close to the reference minimum values and below the suggested value of 0.6. These obtained factors showed some variability, which highlights the importance of determining the equilibrium factor in outdoor air in each specific place. Finally,
Table 6 Mean, maximum and minimum recorded values of environmental parameters during the last 2 h of weekly samplings from 28/06/2013 to 27/06/2014. Environmental parameter
Mean value
Maximum value
Minimum value
Temperature [ C] Relative humidity [%] Pressure [hPa] Irradiation [W m2] Wind speed [m s1] Wind direction [ ] Precipitation [mm], registered only 9 days
18.75 68.14 1012.24 185.75 0.43 208.60 0.32
31.23 90.65 1028.00 792.25 2.20 e 6.90
9.97 42.65 993.42 0.02 0.15 e 0.10
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Fig. 6. Correlations between environmental parameters (temperature, relative humidity, pressure, irradiation and wind speed and direction) and radon equilibrium factor (F). R2 corresponds to the linear least squares fitting.
the obtained EEC220Rn/EEC222Rn mean value was also close to the minimum value from the range provided by UNSCEAR (2000). We have tried to analyse the influence of a set of environmental parameters on radon equilibrium factor. In this sense, we have to remark that we have collected data for this factor F weekly throughout a year and that some environmental parameters are found to be within a narrow range of values, as the wind speed. Consequently, these facts have not allowed us to obtain any clear correlations for some parameters like wind speed, humidity and pressure. However, a negative correlation between the equilibrium factor and temperature as well as irradiation was observed and the north-west wind direction seems to increase this factor F.
Acknowledgement Authors acknowledge the financial support from the Spanish Nuclear Safety Council (Consejo de Seguridad Nuclear), under contract number 2014.0227, as some of the measurements were carried out in the framework of the European Sparse Network for environmental radioactivity monitoring.
References Alegría, N., Herranz, M., Idoeta, R., Legarda, F., 2010. Study of 7Be activity concentration in the air of northern Spain. J. Radioanal. Nucl. Chem. 286, 347e351. http://dx.doi.org/10.1007/s10967-010-0710-6. Arnold, D., Vargas, A., Ortega, X., 2009. Analysis of outdoor radon progeny concentration measured at the Spanish radioactive aerosol automatic monitoring network. Appl. Radiat. Isot. 67 (5), 833e838. http://dx.doi.org/10.1016/ j.apradiso.2009.01.042. Baciu, A.C., 2005. Radon and thoron progeny concentration variability in relation to meteorological conditions at Bucharest (Romania). J. Environ. Radioact. 83 (2), 171e189. http://dx.doi.org/10.1016/j.jenvrad.2005.02.015. Berthold Technologies GmbH & Co. KG, 2010. Moving Filter Particulate Monitor BAI 9100 D Operating Manual (Bad Wildbad, Germany). Chen, Jing, 2005. A review of radon doses. Radiat. Prot. Manag. 22 (4), 27e31. ticas Consejo de Seguridad Nuclear (CSN), 2013. INT-04.32 Red de estaciones automa gica ambiental (REA) del CSN. Operacio n y resultados. An ~ os de vigilancia radiolo 2010 y 2011. EFFTRAN, 2015 (written by Vidmar, T.). http://www.efftran.com. Florea, N., Duliu, O.G., 2012. Eighteen years of continuous observation or Radon and Thoron progenies atmospheric activity. J. Environ. Radioact. 104, 14e23. http:// dx.doi.org/10.1016/j.jenvrad.2011.10.002. Forkapic, S., Mrda, D., Veskovic, M., Todorovic, N., Bikit, K., Nikolov, J., Hansman, J., 2013. Radon equilibrium measurement in the air. Rom. J. Phys. 58 (Suppl. l), 140e147. Herranz, M., Idoeta, R., Abelairas, A., Legarda, F., 2006. Radon fixation for determination of 224Ra, 226Ra and 228Ra via gamma-ray spectrometry. Radiat. Meas.
S. Rozas et al. / Journal of Environmental Radioactivity 151 (2016) 126e135 41 (4), 486e491. http://dx.doi.org/10.1016/j.radmeas.2005.09.007. Herranz, M., Idoeta, R., Legarda, F., 2008. Evaluation of uncertainty and detection limits in radioactivity measurements. Nucl. Instrum. Methods Phys. A 595 (2), 526e534. http://dx.doi.org/10.1016/j.nima.2008.07.105. Herranz, M., Rozas, S., Idoeta, R., Alegría, N., 2014. Effective dose in SMAW and FCAW welding processes using rutile consumables. J. Radiol. Prot. 34 (1), 133e148. http://dx.doi.org/10.1088/0952-4746/34/1/133. ISO 11929, 2010. Determination of the Characteristic Limits (Decision Threshold, Detection Limit and Limits of the Condifence Interval) for Measurements of Ionizing Radiation e Fundamentals and Application. International Organization for Standardization, Geneva. ISO/IEC Guide 98, 1995. Uncertainty of Measurement. International Organization for Standardization, Switzerland. Kobayashi, Tsuneo, 2002. Temporal variation of radon progeny ratio in outdoor air. Health Phys. 83 (2), 287e292. http://dx.doi.org/10.1097/00004032-20020800000015. Kojima, H., 1996. The equilibrium factor between radon and its progeny in the lower atmosphere. Environ. Int. 22 (Suppl. 1), 187e192. http://dx.doi.org/10.1016/ S0160-4120(96)00107-9. Kulwant, Singh, Manmohan, Singh, Surinder, Singh, Sahota, H.S., Papp, Z., 2005. Variation of radon (222Rn) progeny concentrations in outdoor air as a function of time, temperature and relative humidity. Radiat. Meas. 39 (2), 213e217. http://dx.doi.org/10.1016/j.radmeas.2004.06.015. Laboratoire National Henri Becquerel, 2015. Recommended Data. http://www. nucleide.org/DDEP_WG/DDEPdata.htm.
135
pez-Pe rez, M., Ramos-Lo pez, R., Perestelo, Nayra R., Duarte-Rodríguez, X., Lo rez, S., Cuevas, E., Herna ndez-Armas, J., 2013. Arrival of Bustos, J.J., Alonso-Pe radionuclides released by the Fukushima accident to Tenerife (Canary Islands). J. Environ. Radioact. 116, 180e186. http://dx.doi.org/10.1016/ j.jenvrad.2012.09.011. Person, B.R.R., 2014. 210Po and 210Pb in the terrestrial environment. Curr. Adv. Environ. Sci. CAES 2 (1), 22e37. €rfer, J., Zock, Ch., Wendt, J., Reineking, A., 1999. Radon parameters in Porstendo outdoor air. In: 2 International Symposium on Technologically Enhanced Natural Radiation Book of Abstracts, 124, Brazil. Stajic, J.M., Nikezic, D., 2015. The accuracy of radon and thoron progeny concentrations measured through air filtration. J. Environ. Radioact. 140, 50e58. http:// dx.doi.org/10.1016/j.jenvrad.2014.11.002. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2000. Sources and Effects of Ionising Radiation, vol. I. Annex B. United Nations, New York. Veleva, B., Valkov, N., Batchvarova, E., Kolarova, M., 2010. Variation of short-lived beta radionuclide (radon progeny) concentrations and the mixing processes in the atmospheric boundary layer. J. Environ. Radioact. 101 (7), 538e543. http://dx.doi.org/10.1016/j.jenvrad.2009.08.008. Winkler, R., Ruckerbauer, F., Trautmannsheimer, M., Tschiersch, J., Karg, E., 2001. Diurnal and seasonal variation of the equilibrium state between short-lived radon decay products and radon gas in ground-level air. Radiat. Environ. Biophys. 40 (2), 115e123. http://dx.doi.org/10.1007/s004110100096.