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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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.
210Pb; 210Po; Aerosol; Air; Atmospheric particulate; Residence time
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 . 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.
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 . 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 PM2.5 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 PM2.5 or PM10 and can collect the atmospheric particulate of aerodynamic diameters ≤ 2.5 μm (AP2.5) 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.
|Sample code||Aerodynamic diameter, μm||Sampling site||Sampling date||Sampling volume, m3||Mass concentration of particle, μg m-3||Latitude (N)||Longitude (E)|
|AP 7||≤ 2.5||Via Machiavelli, Taranto||11-11-2008||1621.36||19.16||40°29'18.76”||17°13' 33.47"|
|AP 8||≤ 2.5||Via Machiavelli, Taranto||12-11-2008||1632.73||13.91||40°29'18.76”||17°13' 33.47"|
|AP 9||≤ 2.5||Via Machiavelli, Taranto||13-11-2008||1338.25||4.73||40°29'18.76”||17°13' 33.47"|
|AP 10||≤ 2.5||Cisi, Taranto||14-11-2008||1565.65||5.87||40°29' 39.74"||17°13' 33.47"|
|AP 11||≤ 2.5||Cisi, Taranto||15-11-2008||216.03||3.84||40°29' 39.74"||17°13' 33.47"|
|AP 12||≤ 2.5||Cisi, Taranto||16-11-2008||1606.58||5.03||40°29' 39.74"||17°13' 33.47"|
|AP 1||≤ 10||Via Machiavelli, Taranto||11-11-2008||1609.99||30.68||40°29'18.76”||17°13' 33.47"|
|AP 2||≤ 10||Via Machiavelli, Taranto||12-11-2008||1608.85||22.01||40°29'18.76”||17°13' 33.47"|
|AP 3||≤ 10||Via Machiavelli, Taranto||13-11-2008||1331.43||9.82||40°29'18.76”||17°13' 33.47"|
|AP 4||≤ 10||Cisi, Taranto||14-11-2008||1586.11||14.86||40°29' 39.74"||17°13' 33.47"|
|AP 5||≤ 10||Cisi, Taranto||15-11-2008||204.66||21.47||40°29' 39.74"||17°13' 33.47"|
|AP 6||≤ 10||Cisi, Taranto||16-11-2008||1590.66||11.85||40°29' 39.74"||17°13' 33.47"|
|AP 13||≤ 10||Castel Romano (Roma)||19/29-11-2008||550.346||23.40||41°42'11.37"||12°26' 52.77"|
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].
|Sample code||Aerodynamic diameter, μm||Sampling site||Sampling date||Sampling volume, m3||Latitude (N)||Longitude (E)|
|AP14||≤ 2.5||Alto Adige, Taranto||4/5-5-2009||110.4||40°27’38.58”||17°15’49.04”|
|AP15||≤ 2.5||Alto Adige, Taranto||6/7-5-2009||110.4||40°27’38.58”||17°15’49.04”|
|AP16||≤ 2.5||Via Machiavelli, Taranto||4/5-5-2009||110.4||40°29’18.76”||17°13’33.15”|
|AP17||≤ 2.5||Via Machiavelli, Taranto||6/7-5-2009||110.4||40°29’18.76”||17°13’33.15”|
|AP18||≤ 10||Via delle Sorgenti, Statte||4/5-5-2009||110.4||40°33’44.95”||17°12’12.33”|
|AP19||≤ 10||Via delle Sorgenti, Statte||6/7-5-2009||110.4||40°33’44.95”||17°12’12.33”|
|AP20||≤ 10||Via delle Sorgenti, Statte||8/9-5-2009||110.4||40°33’44.95”||17°12’12.33”|
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.
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 .
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 (T1/2: 120 h) by a low background β-counter sometime after separation (about one month being suitable). The 210Pb activity concentration (CPb-210) in air sample (Bq m–3) was calculated according to the following equation:
where, ABi-210 is the net count rate of 210Bi (cps); λBi, 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).
|Sample No.||Sample weight, g||Pb yield, %||210Pb, Bq kg-1|
|IAEA–315-1||2.44||91.7||32.7 ± 1.4|
|IAEA–315-2||2.48||96.2||32.7 ± 1.4|
|IAEA–315-3||2.46||88.6||29.4 ± 1.3|
|IAEA–315-4||2.54||90.0||28.8 ± 1.3|
|IAEA–315-5||2.56||93.4||30.9 ± 1.3|
|IAEA–315-6||2.53||91.7||29.6 ± 1.3|
|Mean ± 1SD Range||2.44-2.48||91.9 ± 2.6
|30.7 ± 1.7
*: The recommended value (95% confidence interval) of 210Pb are 30.1 (26.0-33.7) Bq kg-1.
Table 3: Experimental values of 210Pb activity concentrations (corrected to the date of 1st 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.
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 (PM2.5), 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) .
|Sample code||Sampling site||Aerodynamic diameter, μm||Po yield, %||210Po||Pb yield, %||210Pb||210Po/210Pb|
|AP7||Via Machiavelli||≤ 2.5||92.5||130 ± 6||84.5||975 ± 41||0.133|
|AP8||Via Machiavelli||≤ 2.5||96.9||214 ± 9||85.2||1054 ± 45||0.203|
|AP9||Via Machiavelli||≤ 2.5||90.2||83.5 ± 4.2||91.0||362 ± 15||0.231|
|AP10||Cisi||≤ 2.5||99.8||75.0 ± 3.7||90.0||464 ± 20||0.162|
|AP11||Cisi||≤ 2.5||96.6||65.9 ± 6.8||90.3||444 ± 20||0.148|
|AP12||Cisi||≤ 2.5||96.7||45.8 ± 2.7||82.8||298 ± 13||0.154|
|AP1||Via Machiavelli||≤ 10||91.2||131 ± 7||94.8||1083 ± 46||0.121|
|AP2||Via Machiavelli||≤ 10||96.2||226 ± 11||91.4||1099 ± 46||0.206|
|AP3||Via Machiavelli||≤ 10||100.0||77.9 ± 3.8||95.8||358 ± 15||0.217|
|AP4||Cisi||≤ 10||93.0||81.5 ± 3.9||91.4||484 ± 21||0.169|
|AP5||Cisi||≤ 10||91.6||77.2 ± 7.5||90.3||520 ± 23||0.149|
|AP6||Cisi||≤ 10||100.1||43.1 ± 2.0||87.2||331 ± 14||0.130|
|AP13||CSM Roma||≤ 10||93.1||48.5 ± 3.3||84.2||399 ± 17||0.122|
|Mean ± 1SD Range||- -||- -||95.2 ± 3.5 90.2-100.1||100 ± 60 43.1-226||89.1 ± 4.1 82.8-95.8||605 ± 318 298-1099||0.165 ± 0.038 0.121-0.231|
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.
|Sample code||Sampling site||Aerodynamic diameter, μm||Po yield, %||210Po||Pb yield, %||210Pb||210Po/210Pb|
|AP14||Alto Adige||≤ 2.5||100.3||246 ± 15||95.5||707 ± 31||0.348|
|AP15||Alto Adige||≤ 2.5||86.2||221 ± 13||100||664 ± 29||0.333|
|AP16||Via Machiavelli||≤ 2.5||100||218 ± 12||100||610 ± 27||0.357|
|AP17||Via Machiavelli||≤ 2.5||71.9||154 ± 11||100.3||641 ± 28||0.240|
|AP18||Via delle Sorgenti, Statte||≤ 10||88.5||193 ± 12||95.1||627 ± 27||0.308|
|AP19||Via delle Sorgenti, Statte||≤ 10||96.9||171 ± 13||91.0||716 ± 31||0.239|
|AP20||Via delle Sorgenti, Statte||≤ 10||90.8||265 ± 16||100.9||955 ± 41||0.277|
|Mean ± 1SD Range||- -||- -||90.7 ± 10.0 71.9-100.3||210 ± 40
|97.5 ± 3.7
|754 ± 118 610-955||0.300 ± 0.049 0.239-0.357|
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 (APb210) were corrected by equation 2:
where, AtPb210 was obtained at the 210Pb determination time (Tables 4 and 5); λPb210, the 210Pb decay constant; t, the time from air sampling to analyzing.
The 210Po activity concentration (APo210i ) produced from 210Pb (APb210) decay through 210Bi at the sample determination time (t) can be calculated from equation 3, which was derived from Bateman’s equation :
where, APb210 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 (APo210, Bq m-3) present at the sampling time can be obtained from:
where, APo210t 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 PM2.5 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.
|Sample code||Sampling site||Aerodynamic diameter, μm||210Po||210Pb||210Po/210Pb||Residence time, d|
|AP7||Via Machiavelli||≤ 2.5||26.7 ± 1.6||981 ± 42||0.0273||9.89|
|AP8||Via Machiavelli||≤ 2.5||112 ± 7||1060 ± 45||0.1055||30.1|
|AP9||Via Machiavelli||≤ 2.5||41.0 ± 2.7||364 ± 15||0.1128||32.0|
|AP10||Cisi||≤ 2.5||20.3 ± 1.3||467 ± 20||0.0434||14.0|
|AP11||Cisi||≤ 2.5||13.0 ± 1.5||447 ± 20||0.0291||10.4|
|AP12||Cisi||≤ 2.5||11.9 ± 0.9||300 ± 13||0.0398||13.1|
|AP1||Via Machiavelli||≤ 10||29.8 ± 2.0||1089 ± 46||0.0274||9.93|
|AP2||Via Machiavelli||≤ 10||99.8 ± 6.5||1105 ± 47||0.0903||26.0|
|AP3||Via Machiavelli||≤ 10||44.9 ± 2.9||360 ± 15||0.1247||35.3|
|AP4||Cisi||≤ 10||40.0 ± 2.5||486 ± 21||0.0823||24.0|
|AP5||Cisi||≤ 10||15.4 ± 1.6||523 ± 23||0.0295||10.5|
|AP6||Cisi||≤ 10||15.5 ± 1.0||333 ± 14||0.0466||14.9|
|AP13||CSM Roma||≤ 10||13.3 ± 1.1||401 ± 17||0.0331||11.4|
|Mean ± 1SD
|39.2 ± 33.2
|626 ± 327
|0.0632 ± 0.0372
|19.2 ± 9.6
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.
|Sample code||Sampling site||Aerodynamic diameter, μm||210Po||210Pb||210Po/210Pb||Residence time, d|
|AP14||Alto Adige||≤ 2.5||122 ± 9||727 ± 31||0.168||47.9|
|AP15||Alto Adige||≤ 2.5||106 ± 8||683 ± 30||0.155||44.1|
|AP16||Via Machiavelli||≤ 2.5||109 ± 8||627 ± 28||0.174||49.7|
|AP17||Via Machiavelli||≤ 2.5||34.2 ± 2.8||659 ± 29||0.0519||16.2|
|AP18||Via delle Sorgenti, Statte||≤ 10||72.8 ± 5.6||644 ± 28||0.113||32.1|
|AP19||Via delle Sorgenti, Statte||≤ 10||37.1 ± 3.3||736 ± 32||0.0504||15.8|
|AP20||Via delle Sorgenti, Statte||≤ 10||102 ± 8||982 ± 42||0.104||29.7|
|Mean ± 1SD
|83.4 ± 35.8
|722 ± 121
|0.117 ± 0.052
|33.6 ± 14.2
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  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 , (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 PM2.5, 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 (PM2.5), and this conclusion is consistent with that reported by Marley et al. . 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 .
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 PM2.5 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.
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 . 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]:
where Np and Nd are the atom concentrations of parent and daughter products; λp and λd, their respective decay constant (reciprocal lifetime); λr, the removal rate constant of aerosol. Under equilibrium conditions,
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 :
where, APo210 and APb210 is the respective activity concentration (Bq m-3) of 210Po and 210Pb in air; λPb210 (0.00008537 d-1), λBi210 (0.1383 d-1), and λPo210 (0.002009 d-1), the decay constant of 210Pb, 210Bi and 210Po, respectively.
The residence time of atmospheric aerosols in days, Tr, was defined as the reciprocal of the removal rate constant of aerosol, i.e., Tr=1/λr, therefore,
Since the ratio of APo210/APb210 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.  that first recognized the important effect of precipitation rate on seasonal and latitudinal variations of radioactive fallout.
|Method No.||Regions studied||Sampling time||Latitude-
|Aerodynamic diameter, μm||Tracer used||Sample numbers||Range of the ART, days||Mean of the ART, days||Reference|
|1||Thessaloniki Greece||Nov. 2006-June 2008||40.63°N-22.97°E||0.76-1.18||7Be||12||7.4 - 8.9||8.0|||
|2||Neuherberg Gemany||1996-1997||48.22°N-11.60°E||-||7Be||46||5 - 6||-|||
|4||Hongkong China||Nov.-March 2001||22.30°N-114.17°E||-||7Be||-||2.6 -11.8||-|||
|5||Leningrad USSR||1963-1966||59.92°N-30.33°E||-||7Be/32P||117||10 - 100||42 ±15|||
|6||Berne Austria||1965||51.78°N-11.73°E||-||7Be/32P||19||20 - 60||41 ±9|||
|7||Toulouse France||1967||43.62°N-1.45°E||-||7Be/32P||120||15 - 75||30 ±10|||
|8||Vilnius USSR||1967-1969||54.63°N-25.32°E||-||7Be/32P||24||8 - 47||26 ±9.3|||
|9||Bavarian Alps Austria||1970-1980||48.83°N-12°E||-||7Be/32P||3000||20 - 60||34 ±17|||
|10||Antarctic Cont.||1977-1981||-||-||7Be/32P||26||46 - 132||73 ±29|||
|11||Troposphere||1959-1965||10 - 70°N||-||7Be/32P||17||22 - 71||43 ±16|||
|12||Stratosphere||1959-1965||10 - 70°N||-||7Be/32P||21||47 - 93||64 ±14|||
|13||Leningrad USSR||1964-1966||59.92°N-30.33°E||-||7Be/35S||86||5 - 70||23 ±7.3|||
|14||Aspendale Australia||1967-1968||37.83°S-145°E||-||7Be/35S||8||30 - 150||78 ±38|||
|15||Bavarian Alps Austria||1973-1975||48.83°N-12°E||-||7Be/35S||600||10 - 60||31 ±10|||
|16||Upper atmosphere||1960-1961||-||-||7Be/35S||17||17 - 68||39 ±10.5|||
|17||Vilnius USSR||1965-1969||54.63°N-25.32°E||-||7Be/22Na||48||530 - 19000||4400 ±4400|||
|19||Troposphere||1960-1961||-||-||7Be/22Na||12||2630 - 14280||7200 ±3300|||
|20||Stratosphere||1960-1961||-||-||7Be/22Na||15||1550 - 11110||4500 ±2400|||
|21||BalboaGemany/USA||Feb. 1963||-||-||90Sr||-||<3d – months||-||[51,52]|
|22||ANL USA||June-July 1998||41.7°N-88.0°W||0.1-10||210Bi/210Pb||7||6 - 67||37.7 ±19.1|||
|23||PHX USA||May-June 1998||34.1°N-106.8°W||0.1-10||210Bi/210Pb||9||5 -78||43.7 ±20.3|||
|24||NM USA||May 1998||33.5°N-111.8°W||0.1-10||210Bi/210Pb||4||41 - 63||50.3 ±9.4|||
|25||Poker Flat USA||Jan.-Fab. 1996||65.1°N-147.5°W||-||210Po/210Pb||2||11.9 - 32||22 ±14|||
|26||Eagle USA||March 1996||65.9°N-141.2°W||-||210Po/210Pb||6||9.5 – 38.7||23 ±11|||
|27||ANL USA||June-August 1996||41.7°N-88.0°W||0.1-10||210Po/210Pb||19||27 - 66||44.4 ±10.6|||
|28||ANL USA||June-July 1998||41.7°N-88.0°W||0.1-10||210Po/210Pb||7||24 - 71||41.4 ±15.0|||
|29||PHX USA||May-June 1998||34.1°N-106.8°W||0.1-10||210Po/210Pb||10||16 - 73||47.1 ±17.8|||
|30||NM USA||May 1998||33.5°N-111.8°W||0.1-10||210Po/210Pb||4||44 - 61||51.0 ±8.4|||
|31||MC Mexico||Feb.-march 1997||19.5°N-99.0°W||0.1-10||210Po/210Pb||20||8 - 77||43.6 ±16.0|||
|32||Taranto Italy||11-17 Nov. 2008||40.49°N-17.20°E||<2.5-10||210Po/210Pb||13||9.89 - 35.3||19.2 ±9.6||This work|
|33||Taranto Italy||4-9 May 2009||40.49°N-17.20°E||<2.5-10||210Po/210Pb||7||15.8 - 49.7||33.6 ±14.2||This work|
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 . 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. .
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  estimated a mean atmospheric aerosol residence time of 35.4 days in two year measurements at Fullerton, California USA, while Winkler et al.  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 .
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 . 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 . 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 PM2.5 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.
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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