Introduction

It is generally accepted that the main source of 210Pb (T 1/2 = 22.3 y) and its decay products: 210Bi (T 1/2 = 5.03 d) and 210Po (T 1/2 = 138 d) in the lower layer of the atmosphere is a radioactive decay of gaseous 222Rn escaping from the soil [1]. The vast majority of all 222Rn daughters, short and long-lived, present in the air become attached to aerosol particles. Therefore, as long as half a century ago, Burton and Stewart proposed using the behavior of these and other airborne radionuclides to trace the fate of aerosols and investigate the physico-chemical processes occurring in the atmosphere [2]. In particular, the radioactive disequilibrium between 210Pb and 210Bi or 210Po is still widely utilized for calculating the residence times and evaluation of the aerosol sources [3–9].

Since atmospheric aerosols can be transported over long distances, 210Pb and its progeny concentrations in the particular site may not be connected strictly with the radium activity in adjacent soils, but they may also depend on meteorological conditions such as temperature, wet deposition (rainfall) and wind intensity. Therefore, their seasonal fluctuations are well documented [1, 10, 11]. Concentrations of 210Pb and/or 210Po in the atmosphere have been reported for many locations and the data are gathered in the 1988 UNSCEAR report [12]. The world average concentrations for 210Pb and 210Po in surface air were evaluated as equal to 500 and 50 μBq/m3, respectively. These two radionuclides can be also released from other natural sources such as volcanic plumes [13] or in a mine environment [14]. Other sources of the increase in the concentrations of 210Pb and/or 210Po in the local atmosphere are some industrial processes, particularly the production and use of agricultural fertilizers [15], biomass burning [16], and coal burning in coal-fired power plants or for domestic heating [17, 18].

Although the radiological impact from airborne routine discharges of a modern coal-fired power plant is generally negligible for the whole population [17], it must be taken into account that both 210Pb and 210Po easily volatilize during combustion. Their enrichment factor in escaping fly ashes in comparison to radioactivity in crude coal may reach a value up to 31 [19]. In the case of insufficient fly ash removal systems or intensive direct use of coal for domestic heating, such emissions may influence the urban air activity concentration of these two isotopes to some degree. Such an influence on the 226Ra levels in the air of Lodz was previously confirmed [20]. Within Lodz there are three big coal-fired power plants and several smaller local power plants with insufficient fly ash removal systems. The main power and heating plants in Lodz produce 26 mln GJ of energy per year using 1.35 mln Mg of coal [21] and emitting 18307.8 Mg of fly ash [22].

The main aim of this work was to measure the seasonal fluctuation of 210Pb and 210Po activities in urban air in the vicinity of the coal-fired power plant in Lodz during the autumn and winter period of 2008, as well as in spring and part of summer 2009, in order to evaluate their concentrations and corresponding effective dose from inhalation for the city inhabitants. Additionally, determination of the 210Pb and 210Po specific activity concentrations in the air particulates allows the calculation of their residence times. On the other hand, comparing these activities with those for escaping fly ash should give valuable information concerning the input of coal combustion to the total 210Pb and 210Po budget in urban air.

Experimental

Sample collection

The air particulate matter samples were collected in the centre of the city, 500 m from the EC2 power plant in two ways. The total suspended matter (TSM) samples were taken weekly by the aerosol sampling system with the ASS-500 station according to the slightly modified procedure described elsewhere [20, 23]. During a 1-week sampling period 2.5–6 g of the dust was captured on a PVC-Petranov type filter, each with an area of ~1600 cm2. Before γ-spectrometry counting of 210Pb, three filter discs (3.4 cm in diameter) with dust were cut out for 210Po determination.

The size-fractioned aerosol samples were collected using an Eight-Stage Cascade Impactor 20-80 Mark II type (Andersen Instruments). For the average air flow rate of 80 dm3/h, the following fractions with 50% cutoff aerodynamic diameters were separated: over 10.0 μm, 5.8 μm, 3.6 μm, 1.8 μm, 1.0 μm, 0.58 μm, 0.36 μm, 0.18 μm and 0.1 μm. A final filter collected all particles with diameters smaller than 0.1 μm.

Radioactivity measurement

The filters from the ASS-500 station were oven-dried for 12 h at 105 °C and after cutting of three small discs for 210Po determination they were pressed to the standard cylinder forms with a diameter of 5.2 cm and 0.4 cm in height. The nitrocellulose filters from the impactor stages were put down directly on the detector. The activity concentration of 210Pb was determined inside a 747E-type 10 cm lead shield using a Canberra GX3020 spectrometry system equipped with a High Purity Germanium (HPGe) calibrated detector with a resolution of 2.0 keV at an energy of 1.33 MeV and a relative energy efficiency of 30%. A special 2002 CSL preamplifier system for lowering the background in the 210Pb energy region of 46.3 keV was additionally installed. The calculated detection limit, according to Currie criterion [24] for 210Pb determination for samples from the ASS-500 station was 0.8 μBq/m3 (80,000 s counting time and 50,000 m3 of the filtered air) and 29 μBq/m3 for impactor samples (160,000 s counting time and 1000 m3 of filtered air).

For 210Po determination, the cut out filters from the ASS-500 station and filters from each cascade impactor stage (after 210Pb determination) were mineralized with the mixtures of HNO3, HF and H2O2 in the teflon vessels of the microwave digestion device Muliwave 3000 (Anton Peer). Afterwards, the digestion solutions were filtered and slowly evaporated at a temperature of 100º C. Dry residue was dissolved in 50 mL of 1 M HCl and from this solution 210Po, prior to α-spectrometry measurement, was separated by the most widely used technique: spontaneous deposition on silver discs [25]. Activity of 210Po was determined using an α-spectrometry system with PIPS detector (Canberra Packard). Detection limits (for 80,000 s counting time) were equal to 1.19 and 0.30 μBq/m3 for samples from the ASS-500 station and cascade impactor, respectively.

Quality assurance

The accuracy of the analytical procedure was evaluated by checking the activities of 210Pb and 210Po radionuclides in two standard reference materials: IAEA Soil 327, IAEA Sediment 300 and IAEA material distributed for intercomparison measurements-Soil CU-2006-03. The obtained values for 210Po: 58.2 ± 5.9 for Soil 327 and 337.1 ± 24 Bq/kg for Sediment 300 were close to the certified values of 58.8 and 340.5 Bq/kg, respectively. Similarly, a good compliance was obtained for 210Pb analysis in Soil 327—57.6 ± 3.8 Bq/kg and Soil CU-2006—251.6 ± 4.4, while the certified values were: 58.8 and 250.5 Bq/kg, respectively.

Results

210Pb and 210Po in total suspended matter (TSM) collected by ASS-500 station

The fluctuations of the 210Pb and 210Po activity in TSM of urban air of Lodz during the sampling period from October 2008 to July 2009 are shown in Fig. 1a and b. The highest activities were measured during the 2008/2009 autumn–winter season (calendar weeks 42–10) up to 1.232 and 0.431 mBq/m3 for 210Pb and 210Po, respectively. The average values of 210Pb and 210Po in this time were 0.597 ± 0.069 and 0.077 ± 0.018 mBq/m3. Generally, slightly lower activity for both isotopes was observed during end of spring-summer season (weeks: 11–30). The average values found in summer were 0.516 ± 0.115 and 0.057 ± 0.011 mBq/m3 for 210Pb and 210Po, respectively.

Fig. 1
figure 1

Activities of 210Pb (a) and 210Po (b) during sampling periods

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These small seasonal differences in the values of 210Po and 210Pb concentrations are statistically significant (t-test, 95% confidence level). Such seasonal trend for 210Pb was also observed for Japan [26], other western European countries (e.g. in Germany) as well as in Egypt [27, 28]. It can be explained by fact that 222Rn and its daughter isotopes enter the stratosphere creating a significant stratospheric 210Pb reservoir [29]. Therefore, 210Pb is also efficiently transported to the surface by the vertical atmospheric motion that is strong in the beginning of spring and autumn, resulting in the enhancements of surface concentration of 210Pb [26].

However, it seems that the additional reason for seasonal differences in the concentrations of 210Po in Lodz urban air could be the longer residence times for aerosol particles during the winter period.

The average resident time of the aerosols was estimated on the basis of the activity of 210Po and 210Pb isotopes by methods described elsewhere [3, 4, 30] from Eq. 1.

$$ {\frac{{{\text{A}}_{{{}^{210}{\text{Po}}}} }}{{{\text{A}}_{{{}^{210}{\text{Pb}}}} }}} = {\frac{{T_{\text{R}}^{2} }}{{(T_{\text{R}}^{{}} + 1/\lambda_{\text{Bi}} )(T_{\text{R}}^{{}} + 1/\lambda_{\text{Po}} )}}} $$
(1)

where T R is the average resident time of the aerosols, λBi,Po is radioactive decay constants for 210Bi and 210Po, respectively

The results of T R determinations for the 10-month sampling period in 2008 and 2009 are presented on Fig. 2.

Fig. 2
figure 2

Residence time of aerosols in winter and spring 2008–2009 in Lodz

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The calculated resident times ranged from 7 to even 120 days (the average value equals 36.1 days). A few factors influence the residence time of particles in the air. Generally, particles with aerodynamic sizes ranging from 0.1 to 10 μm last comparatively longer and move freely with air masses. However, washing out of the solid matter by rain plays an important role in their existence in the air. Additionally, wet fallout also increases the probability of condensation and growth of the aerosol particles by coagulation, which increases the deposition velocity of the aerosol particles, and consequently reduces the residence time values. Therefore, the values of the weekly rainfall [mm] in the sampling periods are additionally included in Fig. 2.

It is evident from Fig. 2 that the residence times are generally not closely correlated with rainfall value and we could not observe the seasonal changes in the residence times, except for some maxima during longer period with clear weather.

210Pb and 210Po in the size fractioned air particulate matter collected by a cascade impactor

The mass distribution of aerosols particles versus their diameters is presented on Fig. 3.

Fig. 3
figure 3

Mass distribution of aerosol fractions

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The size profile of the collected aerosols indicates a domination of fine aerosol particles in the urban air. The predominant part of aerosol dust consists of particles with a diameter below 1 μm (~75%) with 60% input of particles with a diameter of 0.36 μm. Similar size distribution profiles were obtained for other urban areas, for example Mexico City [3]. Such small particles are usually emitted from energetic coal combustion or from motor vehicles [30–32]. On the other hand, particles with a diameter >2.5 μm are usually associated with soil resuspension [33], and their input to the total mass of the Lodz urban air is usually below 11%.

A similar picture has been observed for the activity size distribution (Bq/m3) of 210Pb and 210Po (Fig. 4). It suggests that these radionuclides are predominantly absorbed on the surfaces of suspended particulates in the air.

Fig. 4
figure 4

Activity 210Pb (a) and 210Po (b) in different aerosol fractions

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Therefore, it seems to be interesting to compare not only their activity concentrations in the air (Bq/m3) but also the specific activities of these two radionuclides (Bq/g of dust matter) for different fractions of aerosols. As is evident form Fig. 5, the specific activities of these radionuclides are strongly related each to other, with the highest specific activities for fractions in the range 0.36–0.58 μm.

Fig. 5
figure 5

Specific activities of 210Pb and 210Po in different aerosol fractions (Bq/g) in February (a) and July (b)

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The data from Fig. 5 suggest that there is one source of both isotopes and their maximum activity occurs for particles with an aerodynamic size of about 0.4 μm.

The calculated aerosol residence times on the basis of 210Po/210Pb ratios for the 2-week sampling periods in winter (February) and summer (July) are presented in Table 1.

Table. 1 The 210Po and 210Pb ratios and aerosol residence times-T R for different particle fractions
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As was expected in both periods, the 210Po/210Pb ratio values were higher in the case of fine particles with diameters <0.58 μm. However, the average 210Po/210Pb ratio value for all fractions was close to 0.11 (in February) and 0.144 (in July) and similar to the average value for TSM samples collected by the ASS-500 station. The residence times calculated for fine particle fractions with diameters <0.58 μm were longer and equal to 38 days for February and even 55 days for July. Therefore, the slightly higher concentrations of 210Po during the winter season cannot be explained by a difference in the residence times, because those were lower during February.

In order to check the possible coal combustion influence on the higher 210Pb and 210Po concentrations, the specific activities of both radionuclides were compared for some local environmental samples (Table 2)

Table 2 Specific activities of 210Po and 210Pb (Bq/g) in urban aerosols, surface soil and fly ash samples
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The observed specific activities of both radionuclides in fly ash samples were much lower than those in the air particulate matter samples. Therefore, intense coal combustion in this region does not influence the 210Po and 210Pb levels in the urban air. Enhanced emission of fly ashes with relatively low specific activity of both radionuclides causes a 65% decrease in their specific activities during the winter season for fine fraction aerosols. These data are consistent with other measurements of these two radionuclides in the environmental samples near a coal-fired power station [34].

Conclusions

The average activity concentrations during the autumn/winter season was equal to 0.077 and 0.597 mBq/m3 for 210Po and 210Pb, respectively, whereas during spring/summer season these activities decreased to values of 0.057 for 210Po and 0.516 mBq/m3 for 210Pb. However, in spite of the seasonal reduction of these activities, the average 210Po/210Pb ratio was constant and equal to 0.12. The calculated resident times varied from 7 to 120 days with the average value of 36 days, which was comparable to values reported in the other urban areas in USA, Greece or Egypt (4, 9, 27). The use of a 9-stage cascade impactor allows confirmation that nearly 60% of the total activities of both radionuclides and 60% of the total mass were collected in the first two fractions of the fine dust. No influence of coal combustion on the activity concentrations in the urban air was observed.

Since these values only slightly exceed their reference levels reported by UNSCEAR [35], corresponding age-weight annual committed effective dose from inhalation of 210Po and 210Pb for Lodz inhabitants is a negligible ~6 μSv, only.