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Atmospheric Environment 54 (2012) 80e85

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Atmospheric Environment

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Significant emissions of 210Po by coal burning into the urban atmosphereof Seoul, Korea

Ge Yan, Hyung-Mi Cho, Insung Lee, Guebuem Kim*

School of Earth & Environmental Sciences/RIO, Seoul National University, Seoul 151-747, South Korea

a r t i c l e i n f o

Article history:Received 14 November 2011Received in revised form28 December 2011Accepted 26 February 2012

Keywords:Po-210SourceSeoulWet depositionHealth riskCoal burning

* Corresponding author. Tel.: þ82 2 880 7508; fax:E-mail address: gkim@snu.ac.kr (G. Kim).

1352-2310/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.atmosenv.2012.02.090

a b s t r a c t

We conducted a year-round survey of precipitation samples to investigate the sources of excess 210Po inthe urban atmosphere of Seoul, Korea. The dominant fraction of 210Po in our samples, independent of thein-situ decay of tropospheric 210Pb, was linked with anthropogenic processes. Using vanadium andpotassium as tracers, the excess 210Po was mainly attributed to combustion of coal, with minor contri-butions from biomass burning. The annual integrated amount of 210Po deposited over the Seoul area viaprecipitation was estimated to be 1.75 � 1010 Bq yr�1, which might represent a potential public healthrisk in the vicinity of major point sources, due to its highly adverse biological effects. Since the world coalconsumption is growing, the magnitude of coal burning derived 210Po is expected to increase in thefollowing decades, which should be carefully monitored.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Generally the atmospheric 210Po (t1/2 ¼ 138 days) is consideredto be produced naturally by the in-situ decay of 210Pb (t1/2 ¼ 22.3years) via 210Bi (t1/2 ¼ 5 days) in the atmosphere. 210Pb, in turn, isderived from 222Rn (t1/2 ¼ 3.8 days), a noble gas that is predomi-nantly emanated from the Earth’s continental crust. These particle-reactive 222Rn descendants rapidly get attached to ambient aerosolsand hereafter their fates are closely linked with those of carryingaerosols, which are subject to removal by wet and dry deposition.Thus, the scavenging rates of atmospheric aerosols have beendetermined by their radioactive disequilibria (Hussain et al., 1998;McNeary and Baskaran, 2007; Poet et al., 1972). However, thesignificant discrepancy between aerosol residence times yieldedfrom 210Po/210Pb ratios and other radioactive tracer pairs(210Pb/222Rn and 210Bi/210Pb) reveals that a large fraction of atmo-spheric 210Po is derived independently from disintegration oftropospheric 210Pb. Over the past several decades, this excess 210Pohas been attributed to a number of natural and man-madeprocesses, including intrusion of stratospheric aerosols (Mooreet al., 1976; Poet et al., 1972; Tokieda et al., 1996), resuspension ofsurface soil (Turekian and Cochran, 1981), eruption of volcanic

þ82 2 876 6508.

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plumes (Lambert et al., 1982), plant exudation (Moore et al., 1976),generation of sea sprays from the sea-surface micro-layer (Baconand Elzerman, 1980), and emission of biovolatile 210Po speciesfrom the eutrophic ocean (Hussain et al., 1998; Kim et al., 2000),along with additional anthropogenic origins that comprise fossilfuel combustion, waste incineration, biomass burning, phosphatefertilizer dispersion, and industrial production (Carvalho, 1995;Kim et al., 2005; Moore et al., 1976).

Identifying the sources of excess 210Po in the atmosphere is ofgreat importance, owing to its pivotal role in multiple respects.Firstly, it can be used as a surrogate for elements within the samegroup in the periodic table to study atmospheric geochemistry.Secondly, it is an effective indicator for tracing air-borne pollutantslike Hg and for investigating several atmospheric processes such asaerosol removal rate and washout ratios (McNeary and Baskaran,2007). More importantly, 210Po is a potential health hazard owingto the highly energetic cell disrupting alpha particles (5.3 MeV)emitted during its decay process. Because inhalation accounts forthe majority of radiation received by human body (73% of theaverage total dose from background radiation) (HPS, 2010), air-borne 210Po poses a considerable health risk by damaging humaninternal structures, thus increasing the risk of cancer (Evans et al.,1981; Harely and Pasternack, 1981). Besides, once 210Po in the airis assimilated into the flora and fauna, it may be bio-concentratedin the food chain and, thus, pose further threat to human health(Daish et al., 2005).

G. Yan et al. / Atmospheric Environment 54 (2012) 80e85 81

Recently, 210Po was found to be mainly anthropogenic in theurban atmosphere of Seoul (Kim et al., 2005). However, due to thediversity and complexity of human activities in the urban area, theprimary origins of atmospheric 210Po still remain ambiguous,rendering our knowledge of its health impacts insufficient. There-fore, in the present study, we aimed at identifying the principalsources of 210Po in Seoul and obtaining an approximate estimationof its health effect.

2. Materials and methods

Precipitation samples (rain and snow) were collected on anevent basis from October 2009 to September 2010, on the rooftopof a four-storey building at a university campus in Seoul (37.5�N,127�E), which is a representative metropolis characterized bya high population density (over 10 million residents in an area of605.36 km2) and numerous environmental issues. Three high-density polyethylene buckets were used to collect 210Po and210Pb samples. Trace elements and major ions were sampledsynchronously using polypropylene vessels mounted on a sup-porting rack placed 1.2 m above the rooftop. Prior to use, theabove-mentioned collectors were cleaned with diluted hydro-chloric acid and thoroughly rinsed with ultra-pure water. Theapparatus was manually deployed prior to onset of precipitationand retrieved after cessation. Immediately after collection, thesamples were transported to the laboratory located within thesame building, where they were processed and preservedaccording to the elements to be analyzed. The snow samples wereallowed to thaw at room temperature in a laminar flow clean roombefore further treatment.

The activities of 210Po were measured using the same method asthat used by Kim et al. (2005), in which the Po isotopes wereautoplated onto silver discs at 90 �C and subsequently countedusing an alpha spectrometer. For the determination of 210Pb, theresidue solution after 210Po plating was concentrated to 3 mL andquantitatively transferred to counting vials and the final volumewas brought to 8 mL. A Ge co-axial detector (Canberra) was used,and its counting efficiency (cpm/dpm ratios, at 46.5 keV) wasdetermined using the same volume of NIST traceable 210Pbstandard.

To minimize the risk of contamination, ultra-clean protocolswere followed for the collection and handling of trace elementsamples (Church et al., 1990). Using a class-100 clean bench, thesamples were transferred to pre-cleaned HDPE bottles and werethen acidified with 6 M ultra-pure HNO3, and sealed in cleandouble bags. The trace elements (aluminum and vanadium) weremeasured using an inductively coupled plasma mass spectrom-eter (ICP-MS, Model: X-II, Thermo Inc., UK) equipped witha MicroMist nebulizer (Glass expansion, USA). Rh and Tl wereincluded as internal standards to correct for any instrumentalinstability.

The samples for major ions were filtered through Whatman0.7 mM GF/F glass fiber filters, and stored frozen at �20 �C in pre-cleaned polypropylene conical tubes until analysis. The anions(SO2�

4 ) were determined by suppressed ion chromatography(Model ICS 2000); the cations (Naþ and Kþ) were measured usingthe ICP-MS.

3. Results and discussion

3.1. 210Po and 210Pb in precipitation samples

The 210Po activities in the precipitation showed small varia-tions throughout the sampling period, except a few high values insummer, whereas relatively higher activities were observed in

winter time for 210Pb (Fig. 1a). These activities cannot be linkeddirectly to their source strengths, since precipitation amountcould exert a significant influence on the abundance of wetdepositional species. The corresponding ratios of 210Po/210Pb werefound to be low in winter and high in summer (Fig. 1b), whichmight be attributed to their independent source strengthvariations.

The monthly integrated wet depositional fluxes ranged from1.0 to 5.2 mBq cm�2 month�1 for 210Pb and from 0.10 to0.77 mBq cm�2 month�1 for 210Po (Fig. 2). Relatively higher fluxesfor 210Pb were observed in October 2009, and March, June, July,August, September 2010, as compared to those during the rest ofthe year. A similar trend is displayed for 210Po, with the exception ofMarch and October when the fluxes were relatively low. Theprecipitation amount was low and relatively constant from October2009 to April 2010, and it gradually increased from May toSeptember in 2010, in accordance with the climate regime for thisregion (Korean Meteorological Association, www.kma.go.kr).

In general, the variation patterns of 210Pb and 210Po fluxes werein agreement with those of the precipitation amount (Fig. 2),implying that wet deposition is an important mechanism control-ling the removal of these particle-reactive radionuclides from theatmosphere. However, discrepancies were observed between thevariations of precipitation and 210Pb in October 2009 and March2010, when the 210Pb fluxes were remarkably high, in contrast withlow precipitation amounts. The unusually high values of 210Pbfluxes are likely to be associated with air masses containing high222Rn activities due to strong influences from the landmass orelevated scavenging efficiencies during these time periods. Never-theless, the correlation between 210Po monthly flux and precipi-tation amount was much more significant compared with that for210Pb (r2 ¼ 0.83 for 210Po vs. r2 ¼ 0.41 for 210Pb). Therefore, it ispostulated that the atmospheric 210Po flux was subject to control ofsources independent of 210Pb, and the strengths of these sourceswere probably less variable than those for 210Pb during thesampling year.

3.2. Sources of excess 210Po in atmosphere of Seoul

If all 210Po in our precipitation samples was derived fromatmospheric 210Pb, assuming similar removal rates for bothspecies, the “aerosol residence time” based on the 210Po/210Pbratios were calculated to be 8e699 days (average of 74 days,excluding the 10 samples with exceptionally high 210Pb activitieswith respect to 210Po) (Wallner et al., 2002), which is muchlonger than those estimated from other radiotracer pairs in the222Rn decay chain (Kim et al., 2000; Papastefanou and Bondietti,1991). Since previous studies suggest the tropospheric aerosolresidence times vary within a narrow range on a global scale(Balkanski et al., 1993; Papastefanou, 2006), we may expecta similar value for our wet depositional samples. If the averageaerosol residence time is assumed to be 10 days (Balkanski et al.,1993), the vast majority of 210Po in our samples cannot be sup-ported by the in-situ decay of 210Pb in the troposphere (Fig. 3),implying the existence of additional sources of 210Po in theatmosphere. In order to clarify the complementary sources of210Po, we conducted correlation analyses between excess 210Poand several chemical species present in the precipitationsamples, which were selected as proxies for the potential sourcesdocumented in previous studies. Excess 210Po was calculated bysubtracting the fraction derived from tropospheric 210Pb from themeasured 210Po activities, which equates to ca. 2.8% of themeasured 210Pb activities and corresponds to a 10-day residencetime of aerosols (our calculation suggest the distributionbehavior of excess 210Po do not vary dramatically for the

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Fig. 1. Temporal trends of 210Pb and 210Po activities (a) and 210Po/210Pb activity ratios (b) in individual precipitation samples collected in Seoul during 2009e2010.

G. Yan et al. / Atmospheric Environment 54 (2012) 80e8582

residence time ranging from 5 to 15 days). To avoid any inter-ference by the different scavenging efficiencies of these chemicalspecies by precipitation, event integrated depositional amountinstead of concentration/activity was employed for the correla-tion analysis. As the chemical constituents of precipitationusually originate from more than one source, the dominant

Fig. 2. Monthly precipitation amount and integrated wet depositiona

sources were extracted by excluding contributions from minorsources, using the following equations:

½X�crustal ¼ ½Al�sample � ð½X�=½Al�Þcrust (1)

½X�marine ¼ �Na

�sample � ð½X�=�Na�Þseawater (2)

l fluxes of 210Pb and 210Po observed in Seoul during 2009e2010.

Fig. 3. Plot of 210Po versus 210Pb activities in precipitation samples collected in Seoulduring 2009e2010. The solid line corresponds to the expected distribution curve ofdata points, assuming a 10-day residence time.

Fig. 4. Plots showing correlations between event-integrated wet depositional amountof excess 210Po and various source tracers: sea-salt sodium (a), aluminum (b), non-sea-salt sulfate (c), and non-crustal vanadium (d) for individual events.

G. Yan et al. / Atmospheric Environment 54 (2012) 80e85 83

where [X]crustal and [X]marine denote the components from theEarth’s crust and seawater, respectively, for a given chemicalspecies X. It is assumed that, in our samples, the sodium ions arederived exclusively from seawater, and aluminum ions are ofcrustal origin only. The ratios of species X with respect to sodiumand aluminum based on the compositions of seawater and theupper continental crust can be found in the literature (Keene et al.,1986; Taylor and McLennan, 1995).

Sea spray from the surface micro-layer was the first potentialsource to be taken into consideration, since Seoul is located onlyapproximately 30 km to the east of the Yellow Sea and the pre-vailing wind systems for Seoul are westerlies (southwesterly in thesummer and northwesterly in the winter). The randomly distrib-uted data points in the plot of sea-salt sodium (the fraction derivedfrom soil was excluded using Eq. (1)) against excess 210Po (Fig. 4a)suggest that the marine influence was insignificant. In addition toseawater, contribution of the re-suspension of top soil was evalu-ated using aluminum as a tracer. The lack of correlation indicatesthe insignificance of the Earth’s crust as a source of excess 210Po(Fig. 4b). The total contribution from these two sources was esti-mated to be less than 2% for each individual event (except two), byassuming the constant 210Po/Al ratio of 3.4 Bq g�1 in top soil (Taylorand McLennan, 1995; Tokieda et al., 1996) and 210Po/Na ratio of0.45 Bq g�1 in surface micro-layer of the ocean (Bacon andElzerman, 1980; Pattenden et al., 1980). Volcanic plumes andstratospheric aerosols have been suggested to be other naturalsources of excess 210Po in the atmosphere. However, the contri-butions from them in Seoul were excluded, considering theirtransient nature (Kim et al., 2000; Poet et al., 1972).

Sulfate is generally considered to come mainly from anthropo-genic sources in urban areas and is thus normally used as a pollu-tion indicator in atmospheric studies (Matsumoto and Uematsu,2004). The significant positive correlation of excess 210Po withnon-sea-salt sulfate (from which the marine fraction was sub-tracted using Eq. (2)) (Fig. 4c) indicates anthropogenic origins forthese chemical species. The dominant fractions of sulfate in theatmosphere are known to be derived by gas-particle conversionthrough homogenous nucleation, condensation, and coagulation oftheir precursors SO2 (Han et al., 2008). In Seoul and neighboringareas, these gaseous pollutants are emitted by industries, powerplants, domestic heating, and vehicle traffic (Chae et al., 2004). Inparticular, the first two sources together account for ca. 86% of thetotal SO2 emission budget in the adjacent province (Chae et al.,2004), which surrounds Seoul city and is thus expected to bea crucial source area. Because all the aforementioned processes

relate to fossil fuel combustion, we infer that the dominant excessatmospheric 210Po in Seoul is of fossil fuel origin, with power plantsand industries as the main potential contributors.

Vanadium has been widely used as a reliable fingerprint foremissions from fossil fuel combustion (Nriagu and Pacyna, 1988;

G. Yan et al. / Atmospheric Environment 54 (2012) 80e8584

Tsukuda et al., 2005). Non-crustal vanadium (contribution fromsurface soil was eliminated using Eq. (1)) was found to correlatestrongly with the excess 210Po (Fig. 4d), indicative of fossil fuel asthe common source for both species. Fossil fuel consumption isascribed to four primary areas: transportation, household heatingin the winter season, coal-/oil-burning power generation plants,and industries. Since vanadium in fossil fuels is generally removedduring the refining process (Hope, 1997; Lowenthal et al., 2000),motor vehicles and house heating, which consume only refinedproducts (gasoline and diesel for vehicles and natural gas forheating in Korea), cannot contribute significantly to vanadiumcontent in our samples. On the other hand, vanadium is enriched incrude oil and coal (major fuels for power generation and industrialproduction) and their fly ashes (Tsukuda et al., 2005). Further,vanadium is also found in plant tissues, and is likely to be releasedinto the air during the burning process.

Therefore, we employed a typical biomass burning indicator,anthropogenic potassium (for which the contributions from surfacesoil and sea-salt were subtracted using Eqs. (1) and (2) respec-tively), to evaluate this potential contribution. The data points werefound to be clustered in two regions, with a positive linear rela-tionship observed in each (Fig. 5). The slope drawn from the lowerregionwas found to be around 84, which is in good agreement withthe elemental ratio ranges between potassium and vanadium invarious types of coal (Bertine and Goldberg, 1971; Nadkarni, 1980).However, in contrast with coal, crude oil and their fly ashes containmuchmore vanadium than potassium (K/V<< 1) (Bacci et al., 1983;Hsieh and Tsai, 2003). The data points in the upper region witha remarkably large slope seemed to be associated with emissionsfrom biomass burning. Although we were unable to locate solidevidence to support this speculation, analysis on trace speciesemitted from cigarette smoke demonstrated that the content ofpotassium is about five thousand times higher than that of vana-dium (Kleeman et al., 1999), which provides us with somewhat ofa clue to the content of these species in biomass. In fact, Korea hasbeen suggested to be subject to influences from occasional massivebiomass burning events such as forest fires in Siberia and openburning of agricultural waste in rural and semi urban areas in Korea(Lee et al., 2005; Park and Cho, 2011). In contrast, the anthropogenicprocesses related to fossil fuel combustion, the major source ofvanadium, are essentially constant. This explains the distinctpatterns observed in Fig. 5 that in some samples, potassium ishighly enriched with respect to vanadium. Accordingly, it isconcluded that excess 210Po in the Seoul atmosphere is mainlyproduced by coal combustion, with secondary contributions frombiomass burning.

Fig. 5. Relationship between concentrations of anthropogenic potassium and non-crustal vanadium.

3.3. Wet depositional flux of excess 210Po in Seoul and itsimplication

The annual wet depositional flux of excess 210Po is calculated tobe 28.9 Bq m�2 yr�1, which amounts to 1.75 �1010 Bq yr�1 over theentire Seoul area if assuming uniform deposition rate. It offersa glimpse of the magnitude of 210Po being emitted into the air ofSeoul each year. This anthropogenic atmospheric 210Po might bringharm to the local population through inhalation or ingestion. Uponemission, 210Po is dispersed in the atmosphere and is readilyscavenged by wet and dry deposition. Accordingly, it is expectedthat air-borne 210Po could pose health risks to the public in thevicinity of major point sources rather than under normalcircumstances.

Since the utilization of thermal/coking coal is ubiquitous in theglobal extent, it is very likely that coal burning contributes signif-icantly to atmospheric 210Po, especially in urban regions around theworld. In some developing countries like China and India, whichrelies on coal as major fuel for electricity generation and industrialproduction, humans are likely to be more vulnerable to 210Poproduced by coal burning, particularly when the emissions are notproperly controlled. Indeed, the concerns over radioactive hazardfrom coal-fired power plants date back to the 1970s, the radiationdoses of radioactive waste (including 210Po) released from whichwere found to be greater than those from nuclear plants generatingthe same amount of energy (McBride et al., 1978). A latest work on210Po content in seafood affected by a coal burning power plantsuggested people living around might be exposed to radiationcrossing the safety limit through food uptake (Alam and Mohamed,2011). As world coal consumption is rapidly growing (an expectedincrement of over 50% by 2035) (EIA, 2011), we anticipate anascending level of 210Po being emitted into the atmosphere in thenear future, which should be routinely monitored especially inregions with significant point sources.

In conclusion, amongst the various origins which have beensuggested to be contributing to the excess atmospheric 210Po, coalburning was identified as the primary source in the urban atmo-sphere for the first time. The health threat posed by this coalburning derived 210Po is presumably a global issue. Our conclusioncan be further validated by conducting aerosol studies and directmeasurements of emissions from potential primary sources, as wellas by extending investigations to other areas in the world. Moreextensive studies, such as scavenging mechanisms and transport ofanthropogenic 210Po, might be required to advance our insight on210Po in the atmosphere.

Role of the funding source

This work was funded by the Korea Meteorological Adminis-tration Research and Development Program under Grant CATER2012-7170. G.Y. and H.-M.C. were partially supported by the BK21scholarship through School of Earth and Environmental Sciences,Seoul National University, Korea. The sponsor had no involvementin study design, data analysis or preparation of the manuscript.

Acknowledgements

We thank J.-W. Jeong, I.-T. Kim and Y.-H. Oh for their assistancewith sampling and analysis.

Appendix. Supplementary data

Supplementary data related to this article can be found online atdoi:10.1016/j.atmosenv.2012.02.090.

G. Yan et al. / Atmospheric Environment 54 (2012) 80e85 85

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