Atmospheric Residence Times of the Fine-aerosol in the Region of South Italy Estimated from the Activity Concentration Ratios of 210Po/210Pb in Air Particulates

¹National Institute of Environmental Protection and Research, Via V. Brancati 48, 00144 Roma, Italy

²Roma Third University,Via Corrado Segre 4/6, 00146 Roma, Italy

*Corresponding Author:

Guogang Jia
National Institute of Environmental Protection and Research
Via V. Brancati 48, 00144 Roma, Italy
Email: guogang.jia@apat.it

Received date: September 11, 2014; Accepted date: October 30 2014; Published date: November 04, 2014

Citation: Jia G, Jia J (2014) Atmospheric Residence Times of the Fine-aerosol in the Region of South Italy Estimated from the Activity Concentration Ratios of 210Po/210Pb in Air Particulates. J Anal Bioanal Tech 5:216 doi: 10.4172/2155-9872.1000216

Copyright: © 2014 Jia G, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

The activity concentrations of ²¹⁰Po and ²¹⁰Pb in atmospheric particulate samples collected in South Italy (Taranto) in Nov 2008 and in May 2009 were determined. The corrected activity concentrations of ²¹⁰Pb and ²¹⁰Po in sampling time were in the range of 11.9-122 μBq m^-3 and of 300-1105 μBq m^-3, respectively. The ²¹⁰Po/²¹⁰Pb activity concentration ratios were in the range of 0.0273-0.174. Based on the ²¹⁰Po/²¹⁰Pb activity concentration ratios in air particulates, the atmospheric residence times of aerosol in South Italy were estimated, which are ranged from 8.89 to 49.7 days. The calculated residence times are very useful for simulating and modeling the atmosphere transport process of the inorganic and organic pollutants in air in the studied region.

Keywords

²¹⁰Pb; ²¹⁰Po; Aerosol; Air; Atmospheric particulate; Residence time

Introduction

Since long-range atmosphere transport is one of the major pathways of pollutants, studies on the fate and transport of inorganic and organic atmosphere pollutants are potentially significant. Aerosol-active radionuclides are ideal tracers for the study of atmosphere transport processes of troposphere and stratosphere aerosols. These radioactive tracers can be classified into three different groups according to their origin: (1) cosmogenic radionuclides (⁷Be, ²²Na, ³²P, ³³P, ³⁵S, etc.), which are produced in the upper atmosphere (stratosphere and troposphere) by spallation processes of light atomic nuclei (e.g. nitrogen and oxygen) when they absorb primary (mostly protons) and secondary (neutrons) cosmic radiation [1-5]; (2) artificial radionuclides (⁸⁹Sr, ⁹⁰Sr, etc.), produced by nuclear weapons tests, nuclear power plants, nuclear fuel reprocessing facilities, even some nuclear accidents [6,7]; and (3) natural radionuclides (²²²Rn, ²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po, etc.), produced during element evolution in the earth [8-11].

Due to the fact of continuous existence in the atmosphere, the natural radionuclides (²²²Rn, ²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po) have widely been used as powerful and preferable tracers to study the atmosphere transport processes of aerosols. It was reported that more than 99% of the ²²²Rn in the atmosphere are derived from emanation mainly from the continents. Once the ²²²Rn escapes from the rocks and minerals in the upper crust of the earth, it starts its journey in the atmosphere via diffusion and advection [12]. On its pathway, some of ²²²Rn undergo radioactive decay. The atmospheric ²²²Rn is not removed by either physical or chemical means due to its inert nature. The mean life of ²²²Rn (τ: 5.53 d) is comparable to the transit time of air masses across major continents and/or ocean, but much shorter compared to the mixing time scale of the atmosphere, and hence, it is widely dispersed in the atmosphere. The activities of the long-lived progenies, ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po in the atmosphere to a large extent are governed by their production rates, rates of radioactive decay and removal by atmospheric aerosol scavenging.

Over the past six decades, the distribution of ²²²Rn and its progenies (²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po) have provided very rich information as tracers to quantify several atmospheric processes in the aspects of (1) source tracking and transport time scales of air masses, (2) removal rate constants and residence times of aerosols, (3) stability and vertical movement of air masses, (4) physical and chemical behavior of analog species, and (5) washout ratios and deposition velocities of aerosols [4,8,9,11-17].

The tropospheric residence times are variable and are largely affected by the climate. In this paper, ²¹⁰Po and ²¹⁰Pb were utilized as atmospheric tracers and the residence times of aerosol in South Italy were calculated based on the activity concentration ratios of ²¹⁰Po/²¹⁰Pb in air. This is a preliminary attempt in the region of South Italy (Mediterranean Sea). The calculated residence times were compared with that reported in other regions by other researchers and the discordant residence times of aerosols were discussed. The obtained data are very useful for simulating and modeling the atmosphere transport process of the inorganic and organic pollutants in air in the studied region.

Materials and Methods

Apparatus and reagents

Bismuth-210 for ²¹⁰Pb determination was measured by a 10-channel low-level β-counter (Berthold LB770, Germany). The instrument and reagent background of the counter for ²¹⁰Pb measurement is of ≤ 0.0053 cps and the counting efficiency was 48.2% that was calibrated with a PbSO₄ precipitate source obtained from a standard ²¹⁰Pb solution. Po- 210 was determined by alpha spectrometry (Canberra, U.S.A.) with a counting efficiency of 31.2% and a background of ≤ 6 × 10^-6 s^-1 in the interested energy region.

A Perspex disk holder for polonium deposition was specially designed to fit 100-250 ml beakers [18]. Silver foil with a thickness of 0.15 mm was used for ²¹⁰Po spontaneous deposition and it was cut into disks of 23 mm in diameter. Large volume air sampler was the Model Thermo G10557 equipped with analyzer of PM_2.5 or PM₁₀.

Polonium-209 solution standard as a tracer for ²¹⁰Po determination by α-spectrometry and ²¹⁰Pb solution standard for β-instrument calibration, the reference material (IAEA-315) for quality control and the BIO-RAD-AG 1-X4 resin (100-200 mesh) for Pb separation were supplied by the Amersham (UK), the IAEA and the Bio-Rad Laboratories (Canada), respectively. Pb(NO₃)₂ to prepare the carrier solution for Pb separation and all other reagents were analytical grade.

Sites and sampling

Italy is a peninsula state in Mediterranean Sea surrounded by Tirreno Sea, Ionio Sea and Adriatico Sea, and has a very mild climate with rich precipitation in north and center, and less rich precipitation in south. The most sampling sites were located at near Taranto City and one site at the Castel Romano of Roma (CSM Roma), south of Italy. For determination of the residence time of the aerosol, atmospheric particulate samples were collected in 11-29 Nov, 2008 (the first sampling campaign) and in 4-9 May 2009 (the second sampling campaign). The atmospheric particulate samples were taken by a large volume (about 67 m³ h^-1) sampler. The sampler was equipped with an analyzer of PM_2.5 or PM₁₀ and can collect the atmospheric particulate of aerodynamic diameters ≤ 2.5 μm (AP_2.5) or ≤ 10 μm (AP₁₀) with a glass-microfiber filter of dimensions of 20 cm × 25 cm (Whatman GF/A cat. N. 1820- 866). During sampling, the velocity of the air for both samplers was about 1 m³ min^-1. The detailed information about the sampling sites was given in Tables 1 and 2. It was assumed that the collection efficiency for ²¹⁰Po and ²¹⁰Pb were constant and same. In such a case, problems in collection efficiency will not affect the residence time of aerosols, calculated from the activity concentration ratios of ²¹⁰Po/²¹⁰Pb.

Table 1: Characteristics of the sampling sites for atmospheric particulate (AP) collected in the sampling campaign of Nov 2008 [the range (mean) mass concentration of particulate: 3.84 – 30.68 (13.6 ± 8.4) μg m^-3].

Table 2: Characteristics of the sampling sites for atmospheric particulate (AP) collected in the sampling campaign of May 2009.

Anion-exchange resin column preparation

The anion-exchange resin, BIO-RAD-AG 1-X4 (100-200 mesh), was sequentially treated with 6 M NaOH, 6 M HCl and distilled water to remove any fine particles as well as other unexpected components. Twelve grams of the resin were then loaded in an ion-exchange column (13 mm internal diameter and 200 mm length). Before use, the column was conditioned with 20 ml of 1.5 M HCl for Pb separation.

Methods

Leaching of ²¹⁰Pb and ²¹⁰Po from the air samples of glass-fiber filter: The air samples together with 25 mg Pb²⁺ carrier, 0.050 Bq of ²⁰⁹Po tracer, 60 ml of conc. HNO₃ and 10 ml of conc. HCl were added to a 250 ml beaker, which were then heated at 240°C to leach the analytes for 30 min. The sample solution was evaporated to incipient dryness and 40 ml of 72% HClO₄ were added. The solution was evaporated to fuming to destroy all organic matters until a colourless residue was obtained. Three 6 ml portions of conc. HCl were consecutively added to change the solution medium and evaporated to dryness. The residue was finally dissolved with 20 ml of conc. HCl and some water. The obtained solution was filtered through a Millipore filter paper (pore size: 0.1 μm: diameter: 47 mm) and collected in a 100 ml of tarred beaker. The quantity of the leaching solution was obtained by the gravimetric method.

Separation and determination of ²¹⁰Po: Twenty percent of the leaching solution obtained from Leaching of ²¹⁰Pb and ²¹⁰Po from the air samples of glass-fiber filter were put in a 100 ml beaker. Five ml of 20% hydroxylamine hydrochloride and 5 ml of 25% sodium citrate solution were added. The pH of the solution was adjusted to about 1.5 with 1:1 (v/v) ammonia. The solution was diluted to 50 ml, heated and stirred on a hot-plate magnetic stirrer (10 min). A Perspex holder with a silver disk was placed on the beaker and the silver disk was immersed into the solution. Any air bubbles trapped beneath the disk were removed by manipulation of the stirrer bar. The polonium deposition was continued for 4 h at 85-90°C then the disk was removed, washed with distilled water and acetone, dried and assayed by alpha spectrometry [19].

Separation and determination of ²¹⁰Pb: Eighty percent of the leaching solution obtained from Leaching of ²¹⁰Pb and ²¹⁰Po from the air samples of glass-fiber filter were neutralized to pH 1.0 - 1.5 with ammonia solution, and 10-20 g of NH₄Ac were added and dissolved by heating. Four ml of 0.5 M Na₂S were added, and in this case PbS and FeS was precipitated while most of Ca²⁺ and Mg²⁺ will remain in the solution. The precipitates together with solution were transferred to a centrifuging tube and centrifuged at 4000 rpm for 5 min. After centrifugation, the supernatant was discarded and the black precipitate was dissolved with 3 ml of conc. HCl and 21 ml water. Digestion was made by adding 2 ml of 30% H₂O₂, then the solution was filtered through a Millipore filter paper (pore size: 0.1 μm; diameter: 47 mm).

The obtained solution was passed through a pre-conditioned anionexchange resin column at room temperature and at a free flow rate. After washing with 40 ml of 1.5 M HCl, Pb was eluted with 60 ml of distilled water at free flow rate, and the separation time of the pair ²¹⁰Pb/²¹⁰Bi was recorded. Two ml of conc. H₂SO₄ were added to the collected eluant, which was then evaporated until fuming to destroy the organic matters by oxidation with 1 ml of 30% H₂O₂. Both the precipitate and the solution were centrifuged. The supernatant was discarded and the precipitate was filtered on a weighed filter paper with a diameter of 24 mm (Whatman 42). The filter together with the precipitate was dried at 110°C until constant weight (about 1 h) and weighed again to calculate the lead chemical yield.

Lead-210 was determined by measuring the in-growth activity of its progeny ²¹⁰Bi (T_1/2: 120 h) by a low background β-counter sometime after separation (about one month being suitable). The ²¹⁰Pb activity concentration (C_Pb-210) in air sample (Bq m_–3) was calculated according to the following equation:

   (1)

where, A_Bi-210 is the net count rate of ²¹⁰Bi (cps); λ_Bi, the ²¹⁰Bi decay constant (min^-1); t, the ²¹⁰Bi in-growth time after ²¹⁰Pb separation (min); η, the detection efficiency for ²¹⁰Bi; Y, the chemical yield; and V, the sampling volume (m³) for air.

Quality control: Following approaches can be used to review the quality of a radio analytical method: (1) to analyze the certified reference materials or similar matrices and to compare the obtained results with the recommended values, and (2) to participate in the intercomparison activities between different international laboratories.

For the purpose of quality control, the reference material IAEA- 315 Marine Sediment supplied by the IAEA was used. About 2 g of the reference material were analyzed following the recommended procedure of this paper. The precision was evaluated by the relative standard deviation obtained from a set of six analyses. The accuracy was assessed by the term of relative bias, which reflects the difference between the experimental mean and recommended value of ²¹⁰Pb activity concentration. Due to the presence of unsupported ²¹⁰Pb in the IAEA-315, the fraction of unsupported ²¹⁰Pb had to be corrected to the base date.

The obtained ²¹⁰Pb activity concentrations in the IAEA-315 were shown in Table 3. The mean ²¹⁰Pb concentration in the IAEA-315 was found to be 30.7 ± 1.7 Bq kg^-1 (decay correction to the date of 1st Jan. 1993). It was observed that the relative standard deviation is ± 5.5% for ²¹⁰Pb. Since all being less than ± 10% the precision for the analyses is well acceptable as far as such a low activity is concerned. The relative bias obtained from the analyses was +2.0% for ²¹⁰Pb, showing that the mean activity concentration of ²¹⁰Pb is in good agreement with the recommended value of 30.1 Bq kg^-1 (the 95% confidence interval: 26.0- 33.7 Bq kg^-1).

Table 3: Experimental values of ²¹⁰Pb activity concentrations (corrected to the date of 1^st Jan. 1993) in the IAEA–315 Marine Sediment*.

Due to its short half-life, the reference materials for ²¹⁰Po are not available. The quality control for ²¹⁰Po analyses in this laboratory was carried out through participating in the intercomparison activities organized by the IAEA in 29 March 2007. The samples for intercomparison were a set of five water samples. The obtained activity concentrations of ²¹⁰Po were all in good agreement with the values given by the IAEA.

Detection limits: Taking into account the 3σ of the blank count rates, the counting efficiencies of the instrument, the radiochemical yields, the in-growth or decay factor (²¹⁰Pb: 100%) and the sampling volume, the detection limit, or more precisely, the minimum detectable activity (MDA) of the method for air samples was 1.7 μBq m^-3 for both ²¹⁰Po and ²¹⁰Pb.

Results and Discussion

²¹⁰Po and ²¹⁰Pb concentrations in atmospheric particles:

The atmospheric particulate masses of ≤ 2.5 μm in an aerodynamic diameter are considered as the most harmful particles from health physics point of view, as they can be easily inhaled and dissolved in lung. Two kinds of atmospheric particulate samples were taken, one with a atmospheric particulate mass concentration in the fraction of an aerodynamic diameter ≤ 2.5 μm (PM_2.5), and another in the fraction of ≤ 10 μm (PM₁₀). The uncorrected ²¹⁰Po and ²¹⁰Pb concentrations in μBq m^-3 in atmospheric particulate collected in 11-29 Nov. 2008 at the site Taranto (AP 1-12) and CSM Roma (AP13) and analyzed in 9-19 Dec. 2008 were reported in Table 4 (the first sampling campaign), and that collected at site Taranto (AP 14-20) in 4-9 May 2009 and analyzed in 24-26 June 2009 were given in Table 5 (the second sampling campaign) [20].

Table 4: The ²¹⁰Po and ²¹⁰Pb activity concentrations in μBq m^-3 in atmospheric particulate samples AP 1-12 collected at Taranto in 11-17 Nov. 2008 and AP 13 at CSM Roma in 19-29 Nov. 2008 and calculated at the determination time of 9-19 Dec. 2008.

As the sampling sites are far from the laboratory and the sample analyses were done with a delay of some days, consequently, the activity concentrations of ²¹⁰Pb in air in Tables 4 and 5 were underestimated due to the ²¹⁰Pb decay, and the activity concentrations of ²¹⁰Po were overestimated due to its in-situ production from ²¹⁰Pb-²¹⁰Bi decay. Therefore, the activity concentrations of ²¹⁰Pb and ²¹⁰Po given in Tables 4 and 5 were corrected to the sampling time.

Table 5: The ²¹⁰Po and ²¹⁰Pb activity concentrations in μBq m^-3 in atmospheric particulate samples collected at Taranto in 4-9 May 2009 and calculated at the determination time of 24-26 June 2009.

The activity concentrations of ²¹⁰Pb at sampling time (A_Pb210) were corrected by equation 2:

   (2)

where, A^t_Pb210 was obtained at the ²¹⁰Pb determination time (Tables 4 and 5); λ_Pb210, the ²¹⁰Pb decay constant; t, the time from air sampling to analyzing.

The ²¹⁰Po activity concentration (A_Po210ⁱ ) produced from ²¹⁰Pb (A_Pb210) decay through ²¹⁰Bi at the sample determination time (t) can be calculated from equation 3, which was derived from Bateman’s equation [21]:

   (3)

where, A_Pb210 can be obtained from equation 2. In the bracket of equation 3, the first and second terms describe the ²¹⁰Po fraction derived from ²¹⁰Pb and ²¹⁰Bi, and the last term the fraction of ²¹⁰Po decayed. Therefore, the ²¹⁰Po activity concentration (A_Po210, Bq m^-3) present at the sampling time can be obtained from:

   (4)

where, A_Po210^t was the activity concentrations of ²¹⁰Po at the determination time given in Tables 4 and 5.

The corrected activity concentrations of ²¹⁰Pb and ²¹⁰Po in sampling time were reported in Tables 6 and 7. It was indicated that in the first sampling campaign (Table 6) the obtained ²¹⁰Po activity concentrations in samples of PM_2.5 and PM₁₀ were in the range of 11.9–112 (mean: 37.5 ± 38.0) μBq m^-3 and 13.3–99.8 (37.0 ± 30.4) μBq m^-3, that of ²¹⁰Pb in the range of 300–1060 (603 ± 330) μBq m^-3 and 333–1105 (614 ± 337) μBq m^-3, and the ²¹⁰Po/²¹⁰Pb activity concentration ratios in the range of 0.0273–0.113 (0.0596 ± 0.0389) and 0.0274–0.1247 (0.0620 ± 0.0376), respectively.

Table 6: The ²¹⁰Po and ²¹⁰Pb activity concentrations in μBq m^-3 in atmospheric particulate samples AP 1-12 collected at Taranto in 11-17 Nov. 2008 and AP 13 at CSM Roma in 19-29 Nov. 2008 and corrected to the sampling time and the calculated residence times.

Table 7: The ²¹⁰Po and ²¹⁰Pb activity concentrations in μBq m^-3 in atmospheric particulate samples collected at Taranto in 4-9 May 2009 and corrected to the sampling time and the calculated residence times.

At first glance of the data in Table 6 and the weather record, the activity concentrations of ²¹⁰Po and ²¹⁰Pb were highly variable, in particular, depending on the variability of weather conditions encountered during the sampling period. During rain events less particulate matter was collected, and thus, also lower activity concentrations of ²¹⁰Po and ²¹⁰Pb were detected. The second characteristic of the data in Table 6 was the great difference between the ²¹⁰Pb and ²¹⁰Po activity concentrations. Removal of ²¹⁰Pb and ²¹⁰Po from the atmosphere occurs mainly by wet and dry deposition of the carrier aerosol and their radioactive decay. Since the residence times of the atmospheric aerosols are much shorter than the half-life of the ²¹⁰Po progeny, this species cannot reach secular equilibrium with its predecessors. Therefore, ²¹⁰Po atmospheric concentration is much lower than that of ²¹⁰Pb and the average concentrations of ²¹⁰Po in surface air were observed to be 6.3 ± 3.7% of that of ²¹⁰Pb in the region of South Italy.

In the UNSCEAR reports [22] the reference concentrations in air were about 50 μBq m^-3 (ranged from 12 to 80 μBq m^-3) for ²¹⁰Po and 500 μBq m^-3 (ranged from 28 to 2250 μBq m^-3) for ²¹⁰Pb, respectively, and they were site specific. It was reported that the yearly average concentrations of ²¹⁰Pb in surface air over Europe were about 200-700 μBq m^-3. Therefore, the obtained concentrations of ²¹⁰Po and ²¹⁰Pb in this study should be well in agreement with the reported values.

Lead-210 and ²¹⁰Po in atmosphere comes from several sources: (1) from volcanic dust [23,24], (2) from resuspended soil particles [25], (3) from ²²²Rn gas which is emitted from the ground into the atmosphere [10,17], and (4) from fossil fuel combustion, biomass burning and industrial processes including mining and smelting of uranium, phosphate, lead and iron ore [17,26]. The first three categories are natural sources, while the fourth is anthropogenic.

The contribution of ²¹⁰Pb and ²¹⁰Po in air from the first category source is negligible in this study, as there were no any volcanic eruptions in the surrounding region during the investigation period. The contribution from the second category source is also negligible, as it is usual that the ²¹⁰Po and ²¹⁰Pb activities in the resuspended soil particles in air should be in equilibrium (i.e. ratio is about 1.0), and in these measurements the ²¹⁰Po/²¹⁰Pb ratios were well below that value. The contribution from the fourth category source cannot be excluded, but it seems less important if compared with that in the control site (AP13) in most days of the sampling. In fact, the ²²²Rn gas exhalation from the ground into the atmosphere (the third category) is the most important source contribution of ²¹⁰Po and ²¹⁰Pb in the obtained samples. As shown in Table 6, at the same site the activity concentrations of ²¹⁰Po and ²¹⁰Pb found in the sample category of PM₁₀ did not differ from the corresponding values found in the sample category of PM_2.5, thus it was showed that almost all of ²¹⁰Po and ²¹⁰Pb were found only in the atmospheric particulate fraction with aerodynamic diameters ≤ 2.5 μm (PM_2.5), and this conclusion is consistent with that reported by Marley et al. [11]. Studies showed that, when inhaled, smaller particles can be more toxic than a comparable mass of larger particles of the same material, as the health effects are directly linked to their bigger particle surface area and higher solubility [27].

The second sampling campaign for atmospheric particle samples was carried out at 4 sites in Taranto in 4-9 May 2009. As shown in Table 7, the corrected ²¹⁰Po activity concentrations in the samples of PM_2.5 and PM₁₀ were in the range of 34.2-122 (mean: 92.8 ± 39.7) μBq m^-3 and 37.1-102 (70.6 ± 32.5) μBq m^-3, that of ²¹⁰Pb in the range of 627-727 (674 ± 42) μBq m^-3 and 644-982 (787 ± 175) μBq m^-3, and the ²¹⁰Po/²¹⁰Pb activity concentration ratios in the range of 0.0519-0.174 (0.137 ± 0.057) and 0.0504-0.113 (0.0891 ± 0.0338), respectively. Compared with the results in Table 6 (the first sampling campaign), the concentrations of ²¹⁰Pb (Table 7) in the samples of the second sampling campaign seem unchanged, while that of ²¹⁰Po were doubled. The ²¹⁰Po/²¹⁰Pb ratio variation could be a characteristic of the seasonal variation of ²¹⁰Po and ²¹⁰Pb in the atmosphere of the region.

The atmospheric residence time of aerosol in the studied region

In this work ²¹⁰Po and ²¹⁰Pb were used as the atmospheric tracers, and the residence times of aerosol in the region of South Italy were calculated based on the activity concentration ratios of ²¹⁰Po/²¹⁰Pb in the atmospheric particles.

Theoretical basis for calculation of the atmospheric residence time of aerosol: Lead-210 is the progeny of ²²²Rn. The main decay chain of ²¹⁰Pb is shown in Figure 1.

Figure 1: The decay chain of ²¹⁰Pb.

Lead-210, ²¹⁰Bi and ²¹⁰Po, all are particle-reactive and can be rapidly adsorbed to atmosphere aerosol after they are produced with average attachment time of 40 s to 3 min [28]. If all ²¹⁰Pb in the sampling site is produced from the decay of ²²²Rn in the air and all ²¹⁰Po is derived from ²¹⁰Pb (i.e. no additional source of ²¹⁰Po and ²¹⁰Pb), then using a simple steady-state model, the disequilibrium between ²¹⁰Po and ²¹⁰Pb can be utilized to determine the residence time of the aerosols. In a well-mixed, isolated atmospheric sample where the rate of supply of radon gas is constant, the rate of change of decay product concentration with the elapsed time (t) can be expressed as a differential equation [4,6,29,30]:

   (5)

where N_p and N_d are the atom concentrations of parent and daughter products; λ_p and λ_d, their respective decay constant (reciprocal lifetime); λ_r, the removal rate constant of aerosol. Under equilibrium conditions,

   (6)

In case where the activity concentration ratios of products are separated by an intermediate decay product, such as the ratio of ²¹⁰Po to ²¹⁰Pb via ²¹⁰Bi, the ratio was given by Robbins [31]:

   (7)

or

   (8)

where, A_Po210 and A_Pb210 is the respective activity concentration (Bq m^-3) of ²¹⁰Po and ²¹⁰Pb in air; λ_Pb210 (0.00008537 d^-1), λ_Bi210 (0.1383 d^-1), and λ_Po210 (0.002009 d^-1), the decay constant of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po, respectively.

The residence time of atmospheric aerosols in days, T_r, was defined as the reciprocal of the removal rate constant of aerosol, i.e., T_r=1/λ_r, therefore,

   (9)

Since the ratio of A_Po210/A_Pb210 increases exponentially with time, a value of the mean tropospheric or atmospheric residence time of the associated aerosols can be calculated from equation 9.

The atmospheric residence times of aerosol in the region of South Italy: The calculated residence times of aerosol in the region of South Italy based on the activity concentration ratios of ²¹⁰Po/²¹⁰Pb in the atmospheric particles and on the equation 9 were also given in Tables 6 and 7. It was summarized that the calculated residence times of aerosol collected in the first sampling campaign were ranged from 8.89 to 35.3 days with a mean value of 19.2 ± 9.6 days, and that in the second sampling campaign from 15.8 to 49.7 days with a mean value of 33.6 ± 14.2 days. The residence times were not classified as the particulate size, as the residence times as a function of the particulate size were not observable. The variations of the residence time observed in the two sampling campaigns could be a reflection of the seasonal variations.

Discussion about the residence times of aerosol obtained from different techniques and regions: Martell and Moore [10,14] discussed the available evidence concerning tropospheric aerosol removal rates and concluded that residence times in the troposphere are 1 week or less. After literature investigation, the residence times obtained from different techniques and regions were summarized in Table 8. It was shown that the residence times are extremely variable, as they are dependent on the sources, meteorology, composition, transport and mixing, as well as wet and dry removal rates of the associated aerosols to which the radionuclides are attached. In fact, the removal of particulate matter is primarily by precipitation scavenging, and this suggests that seasonal variation of the precipitation should contribute to seasonal fluctuations of tracer concentrations in surface air. This effect should be in addition to those of seasonal transport within the stratosphere and seasonal changes of stratosphere mass. It was Hvinden et al. [32] that first recognized the important effect of precipitation rate on seasonal and latitudinal variations of radioactive fallout.

Table 8: The atmospheric residence times (ART) of aerosol obtained from different techniques and regions.

It was considered that the concept of a single residence time for the whole troposphere is not valid, as it consists of different layers like the planetary boundary layer, the middle and upper troposphere, the polar atmosphere etc., with different scavenging and mixing properties [33]. The value of residence time obtained with the various radioactive tracers will therefore refer to their mean production levels, e.g., ⁷Be, ³²P, for upper troposphere, and ²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po for the planetary boundary layer, etc. [34].

As shown in first 4 lines in Table 8, the residence times of atmospheric particles, which were associated with ⁷Be-aerosol, vary from 2.6 – 35.4 days. Among the data, Shapiro and Forbes-Resha [35] estimated a mean atmospheric aerosol residence time of 35.4 days in two year measurements at Fullerton, California USA, while Winkler et al. [36] estimated the residence times of 5-6 days in 46 measurements sampled during >1 year period in ground level air at Neuherberg, Germany, being 6-times difference with the data of Shapiro and Forbes- Resha [35].

As shown in Table 8, the ⁷Be/³²P and ⁷Be/³⁵S ratios in air has been used to estimate the tropospheric residence times, and the obtained mean tropospheric residence time was 44.1 and 42.8 days, respectively. Due to the numerous sampling numbers, it seems less variations of the mean residence time were observed. However, the comparison of ⁷Be with ¹³⁷Cs of stratospheric origin in surface air indicated the presence of a stratospheric component of ⁷Be [37,38], the magnitude of which is varying and time dependent. Hence the tropospheric residence times calculated from the ⁷Be/³²P and ⁷Be/³⁵S ratios are less reliable. The same disputes or even worse reputation also happened to ⁷Be/²²Na ratios.

Strontium-90 has also been used for atmospheric studies; however, there are few data available for estimation of the residence times in our collected literatures

Radon-222 is a short-life, inert gaseous and natural occurring radionuclide. In an ideal enclosed air mass (closed system with respect to these nuclides), the residence times of aerosols obtained from the activity ratios of ²¹⁰Pb/²²²Rn, ²¹⁰Bi/²¹⁰Pb, and ²¹⁰Po/²¹⁰Pb are expected to agree with each other. But a large number of studies have also indicated discordance between the residence times obtained from these three pairs. The discrepancies are due to deviations from these ideal conditions. Each method has its own advantages and disadvantages. For instance, the advantages of ²¹⁰Bi/²¹⁰Pb method are that the mean life of ²¹⁰Bi (7.2 d) is comparable to the mean residence time of aerosols and water vapor in the lower atmosphere and is less sensitive to extraneous sources than is ²¹⁰Po/²¹⁰Pb method, and the disadvantage is the time sensitive nature of this pair [39]. Therefore, it has been welldocumented that the ²¹⁰Bi/²¹⁰Pb-based residence time is always lower than the ²¹⁰Po/²¹⁰Pb-based residence times. The advantages of ²¹⁰Po/²¹⁰Pb method are less time sensitive and the measurement techniques are more reliable, and the disadvantage is that volatile nature of ²¹⁰Po could result in additional sources of ²¹⁰Po to the atmosphere which could alter the residence time based on ²¹⁰Po/²¹⁰Pb pair (such as large amounts of ²¹⁰Po are released to the atmosphere from major volcanic events) [24,40]. Moreover, both methods could be affected by finite amount of resuspended dust or soil which will have higher ²¹⁰Bi/²¹⁰Pb and/or ²¹⁰Po/²¹⁰Pb activity ratio in surface air sampling. Recent results from the distribution of these nuclides in size-fractionated aerosols appear to yield consistent residence time in smaller-size aerosols, possibly suggesting that the residence time discrepancies in larger size aerosols are derived from resuspended dust. Therefore, if applied properly, ²²²Rn and its progeny products could be considered as the ideal tracers for atmospheric studies, as vertical distribution profiles showed a more even distribution throughout the troposphere and stratosphere for their concentrations than for other atmospheric radionuclides [16]. As shown in Table 8, the mean values of the atmospheric residence times, estimated from the ²¹⁰Bi/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb activity concentration ratios in the collected literatures, were 43.9 and 35.2 days, respectively.

Due to the fact in Table 6 that (1) the ²¹⁰Po or ²¹⁰Pb activity concentrations in samples of PM_2.5 and PM₁₀ at the same site are nearly in the same level, and (2) the ²¹⁰Po/²¹⁰Pb activity ratios are much less than 1, it is judged that the particulate size in the collected samples is ≤ 2.5 μm and the contribution from resuspended soil in the samples is negligible. Therefore, the calculated atmospheric residence times of aerosol in South Italy, being 8.89-35.3 (mean: 19.2 ± 9.6) days in the first sampling campaign and 15.8-49.7 (mean: 33.6 ± 14.2) days in the second sampling campaign, were real reflection of the atmosphere transport characteristics or behaviours of the inorganic and organic atmosphere pollutants in the region of the Mediterranean Sea.

Conclusion

Based on the ²¹⁰Po/²¹⁰Pb activity concentration ratios, the estimated atmospheric residence times of aerosol in the region of South Italy were ranged from 8.89 to 35.3 days with a mean value of 19.2 ± 9.6 days in the first sampling campaign in Nov 2008 and from 15.8 to 49.7 days with a mean value of 33.6 ± 14.2 days in the second sampling campaign in May 2009. After comparison with the data in literatures, it is concluded that the technique used in the study is reliable. The calculated atmospheric residence times of aerosol in South Italy, were real reflection of the atmosphere transport characteristics or behaviors of the inorganic and organic atmosphere pollutants in the region of the South Mediterranean Sea.

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