1. Introduction
Ambient air is composed of major constituents, nitrogen and oxygen, and small amounts of argon and carbon dioxide. Water vapour and trace gases are also present. It is, in fact, not a gas but an aerosol with suspended particulate matter. The particle sizes ranges from several nm for molecular clusters to about 100 μm for fog droplets and dust particles. Particles larger than 100 μm cannot stay suspended in air and may therefore not be considered as aerosols (Colbeck, 1998). Classification of aerosols with respect to their particle size differs depending on the purpose of use as well as the author (Wen, 1996; Colbeck, 1998; Turner & Colbeck, 2008; Busek & Adachi, 2008; Kumar et al., 2010b). Terms of ultrafine (<100 nm), fine (<1000 nm) and coarse (>1000 nm) particles are the preferred terms used by toxicologists and regulatory bodies. Aerosol scientists, on the other hand, refer mostly to different modes (Kumar et al., 2010b): nucleation (1–30 nm), Aitken (20–100 nm) and accumulation mode (90–1000 nm). Nevertheless, the borders are not strictly fixed and may differ from author to author. The term nano is not applied univocally either. Although it may refer to any particle of <1 μm size, it is reserved for particles of <300 nm (Kumar et al., 2010b), <100 nm (Busek & Adachi, 2008; Nowack & Bucheli, 2001), <50 nm (Morawska et al., 2008), or even smaller (Anastasio & Martin, 2001; Shi et al., 2001). In any case, monodisperse aerosols are very rare. They are mostly polydisperse with a particle size distribution which is mathematically described by either a differential or an integral distribution function
where
Similarly as for the number concentration, also size distributions of other property can be expressed, such as particle surface, volume, mass, activity (for radioactive aerosols) and others. For the same aerosol they appear to differ considerably from one another, in respect to both the number of peaks and peak positions in the particle size region (Colbeck, 1998; Turner & Colbeck, 2008; Busek & Adachi, 2008). For instance, the plot of the number distribution shows a maximum at smaller particle sizes than the plot of the volume (or mass) distribution, because the smaller particles, though abundant in number, have smaller volumes (smaller cube of diameter). For the same reason, the maximum in the surface distribution appears at larger particle sizes than that in the number distribution.
1.1. General aerosols
The term ‘general’ is used here to comprise all particles, carrying or not carrying radioactive elements (in our case radon short-lived decay products), although the contribution of the latter is minimal, as will be seen later. Atmospheric particles have natural and anthropogenic sources (Baltensperger & Nyeki, 1998; Nowack & Bucheli, 2001; Dobos et al., 2009; Kumar et al., 2010b). Forests and sea surfaces are permanent generators of aerosol particles (O’Dowd et al., 1997). Particles are also produced continuously in the atmosphere by condensation of semi-volatile organics (O’Dowd et al., 2002), and caused by photochemical reactions and gas → particle conversion (Vakeva et al., 1999; Holmes, 2007; Kumar et al., 2009). Volcanic eruptions (Amman & Burtcher, 1990), forest fires (Makkonen et al., 2010) and dust storms (Schwikowski et al., 1995) are, temporally, very strong sources, but mostly local and of short duration.
Particulates are emitted from a number of various human activities (Baltensperger & Nyeki, 1998; Kumar et al., 2010b). They are released at huge amounts through the chimneys of thermal power plants burning fossil fuel or biomass, incinerators, mineral mining and milling facilities and some others. In urban areas an important or even major particle source is traffic (Shi et al., 2001; Han et al., 2005; Agus et al., 2007; Young & Keeler, 2007; Busek & Adachi, 2008; Morawska et al., 2008; Kumar et al., 2010b). While particles emitted by vehicles using conventional and bio fuel are mostly smaller than 150 nm, debris generated by the tyre–road interaction fall into the 2.5–10 μm range. Nanoparticles are also produced intentionally (Kumar et al., 2010a) to be used as constituents in electronics, medicines, pharmaceuticals, cosmetics, paints and a variety of other consumer products. Nanotechnology is increasing fast and so is the possibility for the nanoparticles to appear in air of workplaces and to be released into the outdoor atmosphere and enter the living environment (Mackay & Henry, 2009).
Apart from size, aerosol particles also differ markedly in their shape, structure and chemical composition, which is beyond the scope of this paper.
When breathing air, aerosol particulates are deposited partly on the walls of the respiratory tract. Mathematical simulations have shown that their deposition depends strongly on the particle size (Hofmann & Koblinger, 1990; Alföldy et al., 2009). Thus, for instance (Oberdörster et al., 2005), about 90 % of the inhaled 1 nm particles are deposited in the nasopharyngeal region and the rest, in the tracheobronchial region with no deposition in the alveolar region. 5 nm particles are almost equally deposited in all three regions. On the other hand, half of the 20 nm particles are deposited in the alveolar region and the remaining half in the other two regions equally. Physical translocation and clearance in the respiratory tract are also size-dependent. Aerosol particles enter the body also by ingestion and absorption by skin. This uptake is more effective for smaller particles than for larger ones, nonetheless minor compared with inhalation. Because the ratio of the numbers of surface to bulk atoms increases exponentially with reducing size, smaller particles are expected to be chemically and biochemically more reactive, and thus potentially more toxic, than larger ones (Oberdörster et al., 2005). It has been recognised that nanoparticles cause oxidation stress, pulmonary inflammation and cardiovascular events, although the mechanisms of these detrimental effects are not entirely understood (Nowack & Bucheli, 2001; Oberdörster et al., 2005; Nel et al., 2006; Kumar et al., 2010b).
1.2. Radioactive aerosols – radon short-lived decay products
Atoms of several radionuclides attach to the atmospheric particles which thus appear as radioactive aerosols (Schery, 2001; Papastefanou, 2008). They are of different origin and may be classified as follows (Papastefanou, 2008): (i) cosmogenic: 7Be, 22Na, 32P and 35S; (ii) radon and thoron short-lived decay products; (iii) fission products: 89Sr, 90Sr, 103Ru, 131I, 132Te, 137Cs and 140Ba; (iv) originated from high-energy accelerators: 7Be, 22Na, 24Na and 52Mn; (v) plutonium isotopes from nuclear weapons testing and reactor accidents; and (vi) mine particulate matter containing radionuclides of the uranium, thorium and actinium radioactive decay chains. We shall focus here on radon short-lived decay products.
There are three primordial radioactive decay chains in the earth crust (Nero, 1988): the thorium chain starting with 232Th, the uranium chain starting with 238U, and the actinium chain starting with 235U. In each chain, radioactive transformation of radium results in formation of radon, the only gas in the entire decay chains. Thus, 220Rn (thoron, half-life
Radon α-transformation is followed by a radioactive chain of its short-lived decay products (RnDP): 218Po (α, 3.05 min) → 214Pb (β, 26.8 min) → 214Bi (β, 19.7 min) → 214Po (α, 164 µs) (Nero, 1988). Initially, they appear mostly as positive ions (Chu & Hopke, 1988; Porstendörfer & Reineking, 1992; Hopke, 1996) which react with molecules of trace gases and vapours (mostly water) in air, are partially oxidized and form small charged clusters. They also get neutralized (Goldstein & Hopke, 1985; Porstendörfer & Reineking, 1992; Hopke, 1996) by (i) recombination with ion pairs produced by α, β and γ emissions and recoil atoms during radioactive transformations of airborne radionuclides, as well as by background γ and cosmic rays, (ii) electron scavenging by OH radicals formed by radiolysis of water molecules, and (iii) charge transfer from molecules of lower ionisation potential. The state of 218Po and 218PoO2 at the moment of its α-transformation is decisive because it determines the initial characteristics and behaviour of the subsequent members in the chain. For 218Po clusters in 50 % humid air at an ionisation rate of 3.2 pC kg–1 s–1 (45 μR h–1), the rates of the above three processes of neutralisation are 0.07×10–2 s–1, 1.07×10–2 s–1 and0.4×10–2 s–1, respectively, and more than half of the clusters are neutral (Dankelmann et al., 2001). These processes are accompanied and followed by attachment of clusters (Hopke, 1996; Dankelmann et al., 2001; Schery, 2001; Pagelkopf & Porstendörfer, 2003; Papastefanou, 2008), both charged and already neutralised, to the background atmospheric aerosol particles. Because of the recoil energy gained during α-transformation of the attached parent atoms, a considerable proportion of the RnDP atoms desorbs from the aerosol particles (Mercer, 1976; Porstendörfer, 1984), the phenomenon being more pronounced for smaller aerosol particles (Tu et al., 1994).
The total concentration of RnDP in air is reported as the equivalent equilibrium activity concentration (
where
On the world average, RnDP contribute about half to the effective dose (radon contribution is minor) a member of the general public receives from all natural radioactivity (United Nations Scientific Committee on the Effects of Atomic Radiation [UNSCEAR], 2000, thereafter UNSCEAR, 2000), and are a major cause of lung cancer, second only to cigarette smoking (Darby et al., 2005). For this reason, their levels in living and working environments are of great social concern and scientific challenge.
In contrast to general aerosols, the health detrimental effects of radioactive aerosols have been known for a long time and their dosimetry well elaborated. For general purposes of radiation protection, the International Commission of Radiological Protection in its Publication 65 recommends for radon dosimetry a dose conversion factor (
On the other hand, Birchall and James (Birchall & James, 1994; Marsh et al., 2002) elaborated the dosimetric approach of calculating the dose conversion factor (
where
In addition, Porstendörfer (1996) proposed relationships between
Because
1.3. Size distribution of radon short-lived decay products
For RnDP, as for other radioactive aerosols (Schery, 2001; Papastefanou, 2008), the activity size distribution
The two distributions differ because the attachment coefficient
Here,
According to a review by Porstendörfer & Reineking, (1992), the activity median diameter (AMD) of the RnDP clusters falls in the range of 0.9 nm to 30 nm, while the activity median aerodynamic diameter (AMAD) of the aerosol particles carrying RnDP attached, falls in the range of 50 nm to 500 nm. In a radon chamber containing carrier aerosol, AMD values of 0.82, 0.79, 1.7 and 0.82 nm were obtained for unattached 218Po, 214Pb, 214Bi and 214Po, respectively (Fukutsu et al., 2004). The border between unattached and attached is not fixed. Thus, for indoor air, RnDP associated to particles smaller than 20 nm, grouped around 5 nm (Tu et al., 1991), and particles in the 0.5–1.5 nm size range may be considered as unattached RnDP (Hopke et al., 1992). Measurements in indoor air also showed that within the unattached region of <10 nm, two (with AMD of 0.8 and 4.2 nm) or even three activity distribution peaks (0.6, 0.85 and 1.25 nm) may appear (Porstendörfer, 1996; Porstendörfer, 2001). In addition, RnDP appeared in the nucleation mode (attached to particles of 14–40 nm), the accumulation mode (210–310 nm), and the coarse mode (3000–5000 nm) (Porstendörfer, 2001). In an intercomparison experiment carried out in a test chamber, the AMD values of the unattached RnDP were found in the range from 0.53 to 1.76 nm, followed by a gap until about 50 nm when the attached RnDP appeared (Cheng et al., 2000).
As reviewed by Porstendörfer & Reineking, (1992),
Thus,
Elevated
2. Experimental
2.1. Measuring techniques
2.1.1. Radon decay products
Individual activity concentrations (
In order to facilitate comparison of number concentrations of general aerosols and RnDP aerosols, the activity concentrations (
2.1.2. General aerosols
Number concentration and size distribution of general aerosol particles have been measured with a Grimm Aerosol SMPS+C instrument, Series 5.400. A long Vienna DMA unit was used for 10–1100 nm and a medium DMA unit, for 5–350 nm size range. The DMA unit separates charged particles into 44 channels, based on their electrical mobility which depends on the particle size and electrical charge: the smaller the particle and the higher its electrical charge the higher is its mobility. Particles enter the CPC unit containing a heater saturator in which alcohol vapour molecules condense onto the entering particles, thus causing them to grow into droplets. The droplets are then detected with a laser beam (DLS detection) and counted. The frequency of measurement is one in seven minutes for the long, and one in four minutes for the medium DMA unit. The instrument gives the total number concentration
2.2. Site description
Two experimental sites were selected: the Postojna Cave and a dwelling in the suburban area.
2.2.1. Postojna Cave
The Postojna Cave is the largest show cave in Slovenia. Like the majority of karst caves, it is only naturally ventilated. Its air temperature is practically constant around 9°C all the year round and relative air humidity is close to 100 %. It is a practically horizontal cave. The air flow differs considerably in summer and winter. In winter, when the cave temperature is higher than outside, cave air is released from the cave into the outdoor atmosphere due to the air draught caused by the ‘chimney effect’ (Hakl et al., 1997; Kertész et al., 1999), thus allowing fresh and cold outdoor air to enter the cave through low lying openings. This effect is not operative in summer, when the outside temperature is higher than in the cave, and air draught is minimal or reversed. The air speed is low, never exceeding 1.5 m s–1 along the main entrance corridor and being much less or zero in other parts. As a result of different natural ventilation regimes, the cave interior, at least along the main air paths, is less wet in winter than in summer.
The cave is open for visitors every day from 10h to 16h in winter and from 9h to 18h in other seasons. The daily number of visitors is around 3500 in summer and from fifty to several hundred in winter, totalling about half a million a year. Visitors ride an electric train for the first 2 km from the entrance to the train stop in the cave, walk 1.8 km on an 8-shaped route and return back to the train in about an hour and a half. The lowest point, our study site, is in the middle of the walking route. The walking path is paved with a special concrete containing silica sand to prevent slippery steps when wet.
The major source of particulate matter in the cave air is the inflow of fresh outside air in winter time. Deposition of dust is observed on the surfaces in the main corridor all the way from the entrance to the train stop, but not further. Another source is the railway. Particles originate from rusting of iron parts, grinding of sand, and rotting and damages of wooden sleepers. The particles are lifted and resuspended by the air draught caused by the train running at a speed of 1.6 m s–1 through the narrow corridors and galleries. Also human activity, both visits and maintenance work at the cave infrastructure, is a potential source.
In the cave, radon and RnDP have been monitored systematically for years in order to estimate radiation doses for the personnel and to keep them below an acceptably low level (Vaupotič, et al., 2001). In addition to elevated radon levels, as found in many caves worldwide (Hakl et al., 1997; Cigna, 2005; Field, 2007; Vaupotič, 2010; Kávási et al., 2010),
Previous radon measurements (Vaupotič, et al., 2001; Vaupotič, 2008b; Gregorič et al., 2011) pointed out that the difference in air temperature outside and in the cave played a dominant role in governing both diurnal and seasonal variations of the environmental conditions in the cave. Radon levels were lowest in winter when the cave temperature is higher than outdoors, because radon-rich air is released from the cave into the outdoor atmosphere and fresh outdoor air with low radon concentration is driven into the cave. This effect is not operative in summer, when the outdoor air temperature is higher than in the cave, and the resulting radon levels in the cave air are higher. As an example, at the lowest point the following average values in summer and in winter, respectively, were obtained in 1999 (Vaupotič, 2008b):
Several 5–10 day measurements were carried out with the EQF devices in summer 2009 and winter 2010 at the lowest point along the guided walking route in the cave. Within these periods, the SMPS+C instrument with the long DMA unit was also used, but for several hours only during morning visits, because the instrument is not designed for practically 100 % air humidity and therefore its operation was minimized.
Bearing in mind that RnDP in the Postojna Cave are attached to aerosols larger than 100 nm (Butterweck et al., 1992), we selected 50 nm as the border between the unattached and attached RnDP. We were thus interested in concentrations of particles smaller than 50 nm (
2.2.2. Dwelling
A farm in Zalog, a suburb of Ljubljana, was selected for our experiment. It comprised a residential house and several accompanying farm buildings. They were built in 1987 of concrete and brick. The family lives in the ground floor of the residential house, while the first floor is left unoccupied. One person lives temporarily in a small 20 m2 flat in the basement, its floor lying 1.2 m below the courtyard level. It consists of a kitchen, living room, bathroom and corridor. A door and a window of the kitchen look at the courtyard in front of the house, while the other door connects it to other rooms. Central heating using hot water radiators is based on burning wood. There is no air conditioning. The farm is situated at the end of a small farmers’ village, at a distance of about 20 m from the Ljubljanica River and with the nearest neighbour about 50 m away. The village is surrounded by fields. Across the river at a distance of about 500 m, the main railway Ljubljana – Zagreb runs along a hill covered by forest. The Ljubljana waste water purification plant is about 400 m out of the village and several small industrial plants are several kilometres away.
Measurements were performed in the kitchen of the basement flat (Smerajec et al., 2010) in which a preliminary radon survey had shown elevated radon activity concentration, ranging from 600 Bq m–3 to 1000 Bq m–3, as compared with the national winter average of 121 Bq m–3 for a thousand dwellings (Humar et al., 1992). Both instruments were operated simultaneously from October 2010 to January 2011, covering temporal human activities, e. g., cooking, washing, opening window and door, etc. Here, the medium DMA unit was used. Because it is designed for >5 nm sizes, the size distribution of the unattached RnDP could not be evaluated. Therefore the assumption (Tu et. al., 1991; Shimo & Saito, 2000) was adopted that the attached RnDP in indoor air are associated with particles larger than 20 nm. Thus, the fraction of the general aerosol particles related to the unattached RnDP was expressed by
3. Results and discussion
Results are presented separately for the Postojna Cave in summer and in winter, and for the dwelling.
3.1. Postojna Cave
3.1.1. Summer results
For one of the measurement carried out in summer, Fig. 1 shows the size distribution of general aerosols (a) before visits started and (b) during regular visits. Their geometric mean values (GMpd) of 26 and 31 nm did not differ significantly.
The decrease in total number concentration (
Figure 4a shows the radon activity concentration (
Number concentrations of RnDP atoms are several orders of magnitude lower than the total number concentration of general aerosols (Fig. 2). Therefore, we do not believe that the reason for high
smaller ones is four to ten times higher than that of the larger ones. Thus, the probability of RnDP atoms being associated with smaller particle-clusters is favoured, even without the fact that the ratio of the number of surface atoms, as adsorption sites, to the total number of atoms is substantially greater on the smaller particles than on the larger ones (Oberdörster et al., 2005).
3.1.2. Winter results
The number size distribution of general aerosols, (a) before visits started and (b) during regular visits, was measured in winter (Fig. 7). The related GMpd values, i. e., 110 nm and 113 nm, respectively, are substantially higher than in summer (Fig. 1). They are similar to those in the Bozokov Dolomite Cave in the Czech Republic (Thinová et al., 2005; Rovenská et al., 2008) but smaller than in a limestone cave in Australia (Solomon et al., 1992). The total concentration of general aerosols and concentrations of aerosol particles of various diameters is displayed in Fig. 8. In contrast to the situation in summer (Fig. 2), the total concentration gradually increased during visits, from 2000 cm–3 to 2800 cm–3 and is similar to the initial value in summer of (2700 cm–3), but was much higher than during visits in summer (700 cm–3). Higher concentration in winter cannot be understood as a result of higher air ionisation due to the radioactivity present (Pashenko & Dublyansky, 1997), because both
Figure 10a shows
Under winter conditions (Fig. 12), the number concentration of the <50 nm particles is about twenty times lower than that of larger ones, and their total surface area is about four hundred times smaller than that of larger ones. The probability for RnDP atoms to be associated with smaller particles is thus substantially reduced as compared with the summer conditions, and consequently, the majority of RnDP atoms appear in the attached form.
These results show why
3.2. Dwelling
Only results for selected period are presented here. Fig. 13a shows the total number concentration (2-h average values) of general aerosols (
For the above period, Fig. 15 shows diurnal variation in
Figure 16 shows the time series of a)
As expected, opening the window, both
On closing the window, only a slight increase in GMpd was observed (Fig. 20a), as shown also in Fig. 21. GMpd steadily increases and thus
An abrupt and very large increase in
Figure 24a shows diurnal variations of
fold lower than that of the larger ones (
4. Conclusion
Radon decay products (RnDP) and general aerosols were monitored in parallel in the air of the Postojna Cave (size range 10–1100 nm) in summer and in winter and in a dwelling (size range 5–350 nm) in a suburban area in an autumn – winter period, focused on the unattached fraction of RnDP (
In the outdoor air at the dwelling,
In the dwelling,
Acknowledgments
The study was financed by the Slovenian Research Agency within the project contract no. J1-0745. The author thanks Ms. Mateja Smerajec for her help in operating the Grimm instrument and for data evaluations.
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