Environ. Sci. Technol. 2005, 39, 1519-1522
Evidence for Anthropogenic 210Po in the Urban Atmosphere of Seoul, Korea G U E B U E M K I M , * ,† Y O U N G - L I M H O N G , † JAEHO JANG,† INSUNG LEE,† DONG-WOON HWANG,‡ AND HAN-SOEB YANG‡ School of Earth & Environmental Sciences/RIO, Seoul National University, Seoul 151-742, and Department of Oceanography, Pukyong National University, Busan 608-737, Korea
We have measured the concentrations of 210Po, 210Pb, SO42-, Na+, and 34S in precipitation samples from two metropolitan cities, Seoul and Busan, Korea. The δ34S values ranged from 0 to 10‰ in most Seoul and Busan precipitation samples, indicating major contributions from industrial sources to S levels. A high level of excess 210Po, which is not produced by 210Pb decay in the troposphere, was observed in both regions. The excess 210Po activities in some samples from Busan, a harbor city, were influenced strongly by sea salt (i.e., sea-surface microlayer) which could be traced using [Na+] and δ34S values. In Seoul precipitations, we observed a good correlation between nonsea-salt SO42- and excess 210Po, suggesting that both species are controlled mainly by the same factor. This correlation and the δ34S values indicate that the major source for both species in this region is likely to be anthropogenic, rather than from traditionally suggested sources such as soil resuspension, stratospheric air intrusion, sea sprays, volcanic emissions, and biogenic emissions.
Introduction In the atmosphere, gaseous 222Rn (t1/2 ) 3.8 days) produces a series of decay products, 210Pb (t1/2 ) 22.3 years), 210Bi (t1/2 ) 5 days), and 210Po (t1/2 ) 138 days). These Rn daughters are readily adsorbed onto aerosols and then grow in during the residence of the aerosols in the troposphere. Thus, their disequilibria (i.e., 210Pb/222Rn, 210Bi/210Pb, and 210Po/210Pb) in the troposphere have been utilized for determining the residence times of aerosols (1-4). However, in general, the residence times (10-300 days) of aerosols determined from 210Po/210Pb disequilibria were much longer than those (2-20 days) based on other methods including aerosol deposition models (1-3, 5, 6), indicating additional inputs of 210Po into the troposphere. The determination of additional inputs of 210Po is important not only for understanding the geochemistry of 210Po and its proxies such as S, Se, and Te, but also for evaluating the toxicity of aerosols by radioactive elements. Naturally occurring 210Po is an R emitter. Although R particles are not sufficiently energetic to pass through a person’s outer skin, they easily penetrate the unprotected lining and pass * Corresponding author phone: +82-2-880-7508; fax: +82-2-8766508; e-mail:
[email protected]. † Seoul National University. ‡ Pukyong National University. 10.1021/es049023u CCC: $30.25 Published on Web 02/09/2005
2005 American Chemical Society
through living cells when released within the lungs (7, 8). Thus, 210Po together with other radon daughters in the air may cause lung cancer, especially when there is exposure to high activities at point-source regions. The diverse sources of excess 210Po (the observed 210Po activity minus the 210Po activity produced from 210Pb in the troposphere) have been indicated in previous studies. First, the intrusion of stratospheric aerosols may be important since 210 Po is almost in equilibrium with 210Pb in the stratosphere due to the long residence time (2-24 months) in air (1). The fraction of tropospheric 210Po which mixes down from the stratosphere was found to be 0.2-7% (1, 9). Second, resuspension of humus or top soils may be important since 210Po is nearly in equilibrium with 210Pb in these materials (9, 10). Third, 210Po is unusually higher in the areas where volcanic activity has influence. Lambert et al. (11) found that 210 Po/210Pb activity ratios were about 5-40 in volcanic plumes from Mt. Etna. Fourth, in coastal areas of the ocean, sea spray from the sea-surface microlayer, which has a high activity of 210Po relative to 210Pb, may be important (2, 12, 13). In addition, Kim et al. (14) suggested the influence of biovolatile 210Po in the eutrophic coastal region, Chesapeake Bay, due to gas exchange when the wind speed is high, similar to dimethyl sulfide (DMS) and dimethyl selenide (DMSe) (15, 16). Momoshima et al. (17) reported the formation and emission of volatile Po compound occurred in relation to biological activity of microorganisms in culture medium as well as in natural seawater. Moore et al. (9) documented that anthropogenic sources of 210Po from phosphate fertilizer dispersion, a byproduct of gypsum, lead production, cement and other metal production, and fossil fuel burning, could constitute up to 7% of the total 210Po flux to the atmosphere in Boulder, CO (9). As such, Carvalho et al. (18) reported that 210Po/210Pb ratios could be increased in precipitation by industrial emission of 210Po in Lisbon, Portugal. Thus, in this study, we measured 210Po and 210Pb activity, together with the concentrations of other chemical species including non-sea-salt S and 34S, to evaluate the anthropogenic and natural contributions to excess 210Po in the urban precipitations from two different metropolitan cities in Korea.
Experimental Section Two sampling sites were chosen for this study, the roof of the Daeyeon campus building (about 200 m from the shore) at Pukyong National University (PKNU) in Busan (35.1°N,127°E) and the roof of the Gwanak campus building at Seoul National University (SNU) in Seoul (37.5°N,129°E), Korea, from June 2002 to June 2003 (Figure 1). Seoul is located in the middle of the Korean peninsula, with a population of 10 million and an area of 759 km2. Busan, a harbor city, is located on the southeastern coast of Korea, with a population of 4 million and an area of 605 km2. Four plastic buckets (diameter 33 cm) were used to collect large-volume precipitation samples for analysis of 210Po, 210Pb, 34S, and other chemical species during the same sampling period. In addition, bulk samples for the total 210Pb fluxes were collected by continuously exposing a bucket to the atmosphere for 2 weeks. The radionuclide samples were acidified (pH < 1), and then the 209Po spike and Pb carrier were added. After complete desorption of Po and Pb from the surface of the buckets was allowed (about 2 weeks for bulk samples, and 1 day for wet event samples), the samples were quantitatively transferred to glass beakers and evaporated at ∼80 °C until the volume was reduced to 10 mL. The sample solution was adjusted to VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. A map showing the two study sites in Seoul (SNU) and Busan (PKNU) metropolitan cities. Seoul is located in the middle of the Korean peninsula, with a population of 10 million and an area of 759 km2. Busan is located on the southeastern coast of Korea, with a population of 4 million and an area of 605 km2. 0.5 M HCl, and then the Po was spontaneously plated onto a silver disk at 90 °C after addition of ∼0.5 g of ascorbic acid. The 210Po source was counted using a silicon barrier detector coupled to a multichannel analyzer. The remaining solution was purified further for Pb, and then the 210Po produced from 210Pb was analyzed for 210Pb determination. The details of the analysis, counting, and calculations for 210Po and 210Pb are described by Kim et al. (19). The water samples were measured for Na+ by using ICP-AES (inductively coupled plasma atomic emission spectrometry) and for SO42- using ion chromatography. For the analysis of δ34S, large-volume precipitation samples (about 20 L) were collected. Soon after collection, the sample was filtered through a 0.45 µm Millipore filter, and then 1-2 mL of concentrated HNO3 was added. After 10% BaCl2 was added to precipitate BaSO4, the sample was heated for 1 day at a temperature lower than the boiling point. The BaSO4 precipitates were collected by centrifugation, dried, and weighed to calculate chemical recovery. After further purification (20), δ34S was determined by using mass spectrometry, VG Isotech Prism. The δ34S (SO42-) values are given in permil (‰) units, on the basis of the CDT international standard.
FIGURE 2. (a) Biweekly record of total depositional fluxes of 210Pb and precipitation amounts and (b) a plot of 210Pb flux versus precipitation amounts at the Busan station (PKNU) from June 2002 to June 2003.
Results and Discussion The highest fluxes of 210Pb were observed at the Busan station during the summer monsoon season and during the spring in association with dust storms, the so-called “Asian Dust” events (Figure 2a). The total (wet + dry) depositional flux of 210Pb was calculated to be 0.019 Bq cm-2 yr-1. This flux is lower than that reported in the eastern Asia regions (0.0270.033 Bq cm-2 yr-1) (21, 22), but higher than that found in America (0.013-0.022 Bq cm-2 yr-1) (23-25) and European regions (26). The higher fluxes in the eastern Asia regions appear to be due to the prevailing incursions of westerlies from the large Asian landmass. In general, the variation of the 210Pb flux is correlated to the precipitation amount (r2 ) 0.60, p < 0.05) (Figure 2b), indicating that the precipitation amount exerts a major control on 210Pb deposition in the atmosphere, as documented by previous studies (23, 27, 28). To determine the dilution effect of precipitation on the 210Pb and 210Po fluxes, we plotted the specific concentrations 1520
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FIGURE 3. Precipitation amounts versus the activities of (a) 210Pb and (b) 210Po from the Seoul (SNU) and Busan (PKNU) stations from June 2002 to June 2003. of those nuclides versus the precipitation amount (Figure 3). For this plot, we combined individual samples into one when they were collected within 2 days. The specific concentrations of 210Pb and 210Po in precipitation decreased sharply as the
FIGURE 4. 210Po versus 210Pb activities in precipitation from the Seoul (SNU) and Busan (PKNU) stations from June 2002 to June 2003. amount of precipitation increased at both sampling stations. This efficient removal of aerosols by initial precipitation is typical for most reactive chemical species in the atmosphere (23, 25, 29). The activity ratio of 210Po to its parent 210Pb ranges from 0.01 to 0.70 (averaging 0.15) in the precipitation at both stations. This value is much higher than that expected from the aerosol residence time of about 10 days in the atmosphere (1-4) (Figure 4). This suggests the occurrence of excess 210Po in precipitation, which could be from various sources, such as the resuspension of humus or top soils (10), the mixing of stratospheric aerosols (1), volcanic emissions (11), sea spray from the oceanic surface layer (2, 12), and the sea-air exchange of biovolatile Po (14). First, the contribution of sea spray to excess 210Po was evaluated by calculating the seawater fraction in precipitations using Na+ (unfortunately, major ion samples were collected only for 20 and 24 large events from the Seoul and Busan stations, respectively). If we assume that all Na+ ions originate from seawater, for high excess 210Po samples, about 0.21% and 0.03% of B14 and B17 samples from the Busan station and 0.01% and 0.01% of S16 and S17 samples from the Seoul station originate from seawater (Figure 4). This implies that the precipitation samples (particularly for B14) from the Busan station were influenced somewhat by sea spray. The seawater fraction in all Busan samples (n ) 24) is 0.02% on average relative to 0.004% for Seoul samples (n ) 20). Heyraud and Cherry (30) showed that 210Po/210Pb ratios range from 1 to 3 and from 2 to 80 for the surface microlayer and neuston, respectively. Thus, a small fraction of the sea spray can result in a large excess of 210Po in the coastal atmosphere. To subtract the contribution of sea spray to SO42-, we calculated the non-sea-salt (nss) SO42- using eq 1. This equation also assumes that all Na+ ions in the measured samples originate from seawater (31-34). Since Na+ itself
[nss-ion] ) [ion]sample -
( ) [ion]
[Na+]
[Na+]sample (1)
seawater
may be influenced by anthropogenic sources (35), this estimate is subject to large errors. However, this estimate at least clearly indicates that the sea-salt contribution to the Seoul samples is negligible. The excess 210Po and nss-SO42showed a large scatter at the Busan station, while there was a good correlation (r2 ) 0.69) between the two species at the Seoul station (Figure 5a). Although not depicted here, there was no correlation between nss-SO42- and its parent 210Pb, which is also of continental origin at the Seoul station. This suggests that there is a single major factor controlling both
FIGURE 5. Excess 210Po activities versus (a) non-sea-salt SO42concentration and (b) δ34S in precipitation from Seoul (SNU) and Busan (PKNU) stations. A 95% confidence interval is shown for the regression (a). In general, the δ34S value is approximately 21‰ for sea spray, 5‰ on average for volcanic emissions, and lower than -4‰ for most of the biogenic emissions (37, 38). Anthropogenic sulfates exhibit a wide range of values from 0 to ∼10‰. excess 210Po and nss-SO42- in the Seoul atmosphere. However, the sources of 210Po versus nss-SO42- appear to be more variable in the Busan samples, as indicated from the more variable S sources on the basis of the 34S values later (Po/S ratios may be variable in different S sources depending on the availability of 210Pb in the S sources). Since the source of excess 210Po in the atmosphere is so variable, being dependent on geographical conditions, the main factor controlling both species may be more easily determined for S. For this reason, we measured δ34S values in the precipitation samples. In general, the δ34S value is approximately 21‰ for sea sprays, 5‰ on average for volcanic emissions, and lower than -4‰ for most of the biogenic emissions (36-38). Anthropogenic SO42- exhibits a wide range of values from 0 to ∼10‰, depending on the relative contribution of combustion, fossil fuel refining, gypsum processing, and ore smelting (39). Although we have not been able to determine the specific source of the anthropogenic S in Seoul precipitations, the main source of SO42- in the Seoul atmosphere is confirmed to be anthropogenic on the basis of δ34S (Figure 5b). The anthropogenic sources for S are likely to be the burning of fossil fuels such as coals and petroleum oils, biomass burning, and/or high-temperature incineration. Moore et al. (9) estimated that anthropogenic sources of 210Po from phosphate fertilizer dispersion, as a byproduct of gypsum, lead production, cement and other metal production, and fossil fuel burning, could constitute up to 7% of the total 210Po fluxes to the atmosphere. Carvalho et al. (18) also reported that 210Po/210Pb ratios in some precipitation samples were clearly influenced by industrial emission although primary sources were not measured directly. Therefore, it is likely VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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that excess 210Po in the metropolitan city, in areas of Seoul, originates mostly from anthropogenic sources in association with burning and incineration processes. As shown in the case of the Busan samples, sea spray may be another minor source of excess 210Po in the Seoul atmosphere. Other previously suggested important sources of excess 210Po, such as soil resuspension (10), volcanic emission (11), stratospheric air intrusion (1), and biovolatilization from the coastal ocean (14), are not likely to be the main factor controlling both excess 210Po and SO42- in the Seoul atmosphere, as confirmed from the δ34S values. Therefore, we suggest that the most important source of excess 210Po in the urban atmosphere is anthropogenic. Since 210 Po (half-life 138 days) emits high-energy R particles and is strongly adsorbed onto lung cells if inhaled, the anthropogenic source of 210Po may represent another important heath-hazard component in the atmosphere, especially in point-source regions.
Acknowledgments We thank S. H. Hwang (PKNU), who collected precipitation samples for this study.
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Tables giving the activities of and excess concentrations of Na+, SO42-, and nss-SO42-, and δ34S values at the Busan and Seoul stations (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review June 28, 2004. Revised manuscript received December 15, 2004. Accepted December 15, 2004. ES049023U
