Stable Lead Isotope Compositions In Selected Coals From Around

Jan 23, 2009 - The phasing out of leaded gasoline in many countries around the world at the end of the last millennium has resulted in a complex mixtu...
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Environ. Sci. Technol. 2009, 43, 1078–1085

Stable Lead Isotope Compositions In Selected Coals From Around The World And Implications For Present Day Aerosol Source Tracing M . D ´I A Z - S O M O A N O , † M . E . K Y L A N D E R , ‡ ´ PEZ-ANTO ´ N,† I. SUA ´ REZ-RUIZ,† M. A. LO M . R . M A R T ´I N E Z - T A R A Z O N A , † M . F E R R A T , § B . K O B E R , * ,| A N D D . J . W E I S S §,⊥ ´ (INCAR-CSIC), Oviedo, Instituto Nacional del Carbon 33011, Spain, Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden, Earth Science and Engineering, Imperial College, London, SW7 2BP, United Kingdom, Institute of Earth Sciences, University of Heidelberg, 69120 Heidelberg, Germany, and Mineralogy, The Natural History Museum, London, SW7 5PD, United Kingdom

Received June 30, 2008. Revised manuscript received November 29, 2008. Accepted December 12, 2008.

The phasing out of leaded gasoline in many countries around the world at the end of the last millennium has resulted in a complex mixture of lead sources in the atmosphere. Recent studies suggest that coal combustion has become an important source of Pb in aerosols in urban and remote areas. Here, we report lead concentration and isotopic composition for 59 coal samples representing major coal deposits worldwide in an attempt to characterize this potential source. The average concentration in these coals is 35 µg Pb g-1, with the highest values in coals from Spain and Peru and the lowest in coals from Australia and North America. The 206Pb/207Pb isotope ratios range between 1.15 and 1.24, with less radiogenic Pb in coals from Europe and Asia compared to South and North America. ComparingthePbisotopicsignaturesofcoalsfromthisandprevious studies with those published for Northern and Southern Hemisphere aerosols, we hypothesize that coal combustion might now be an important Pb source in China, the eastern U.S., and to some extent, in Europe but not as yet in other regions including South Africa, South America, and western U.S. This supports the notion that “old Pb pollution” from leaded gasoline reemitted into the atmosphere or long-range transport (i.e., from China to the western U.S.) is important. Comparing the isotope ratios of the coals, the age of the deposits, and Pb isotope evolution models for the major geochemical reservoirs suggests that the PbIC in coals is strongly influenced by the depositional coal forming environment.

1. Introduction Despite the phasing out of leaded gasoline in many countries worldwide, there is clear evidence that Pb remains a dominant atmospheric pollutant. For example, Pb emissions from * Corresponding author e-mail:[email protected]. † Instituto Nacional del Carbo´n (INCAR-CSIC). ‡ Stockholm University. § Imperial College. | University of Heidelberg. ⊥ The Natural History Museum. 1078

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 4, 2009

stationary fuel driven sources have increased from 10 577 t y-1 in 1988 to 11 690 t y-1 in 1995 (1), the north-south gradient in aerosol Pb concentrations over the Atlantic has disappeared (2) likely due to the increasing industrialization in the Southern Hemisphere, and ice cores in North America reveal increasing [Pb] in the North Pacific atmosphere since the 1970s (3). Given the toxic nature of Pb and its effect on human and environmental health (4), continued monitoring of emission strengths and the identification of new sources is key to maintaining good air quality. Stable lead isotope ratios, in combination with concentration data, have been extremely successful in identifying sources of Pb in atmospheric aerosols at local to global scales (5-7). Coal combustion is a potential major source of Pb in the environment due to its importance as a major energy source worldwide and the possible elevated trace element concentrations of coals (8, 9). In 2004 alone, 94.8 tons of lead compounds were emitted by combustion installations (>50 MW) in Europe, i.e., 12% of the total emissions of lead compounds into the air (10). Coal was suggested to be a significant pollutant source in several major cities in China (11, 12) and in the Okefenokee swamp in Georgia in the eastern U.S. (13). Lead concentration and isotope ratio data from the world’s major coal deposits is, however, seriously lacking. This limits source assessment of Pb in the environment, particularly in remote areas (2, 14-17). In an effort to fill this knowledge gap and to constrain the possible importance of coal as a Pb source, we recently discussed Pb concentrations ([Pb]) and isotope ratios (PbIC) in Spanish coals of different characters and origins (18). Here we extend this work geographically and present analyses from major coal deposits worldwide.

2. Experimental Section 2.1. Coal Samples. Fifty-nine coal samples were chosen from the INCAR-CSIC (Instituto Nacional del Carbon, Oviedo, Spain) sample bank to represent important coal deposits around the world, including different Spanish basins (16), Peru (14), Australia (2), Portugal (4), Romania (1), New Zealand (1), U.S. (10), Mexico (1), Canada (2), Indonesia (3), Hungary (1), Czech Republic (1), China (1), and South Africa (2). Most of the samples were bituminous. The ages of the deposits range from 360 to 1.8 million years. Information for the individual samples is given in Table 1. 2.2. Experimental Procedures. Coal samples were first washed, then dry ashed in a muffle furnace, and finally digested using a microwave oven (Milestone MLS 1200) and an HF/HNO3 acid mixture at the INCAR-CSIC. Lead concentrations and isotope analysis were measured using a quadrupole ICP-MS (ICP-Q-MS, Hewlett-Packard model HP4500) at the INCAR-CSIC and a multicollector-ICP-MS (ICPMC-MS, GVi Instruments model IsoProbe) at Imperial College London, respectively. Before analysis by MC-ICP-MS samples were passed through an ion exchange column to separate the Pb from the matrix. Details of the analytical procedures are given elsewhere (5, 18-20). The accuracy of the concentration measurements was (5%, assessed using the standard reference coal SARM 20. The accuracy of the isotope ratio measurements was (0.3% for all ratios including the isotopes 206Pb, 207Pb, and 208Pb, quantified measuring 10 coal samples on both mass spectrometers. The precision was (5% for concentration measurements determined by repeat analysis of selected coal samples and 0.2 and 0.02% for isotope ratios using Q-ICPMS and MC-ICP-MS, respectively, determined by repeat measurements of the NIST SRM 981 Pb standard. 10.1021/es801818r CCC: $40.75

 2009 American Chemical Society

Published on Web 01/23/2009

VOL. 43, NO. 4, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1079

Spain Portugal Portugal Portugal Portugal Czech Republic

Czech Republic Romania Hungary United Kingdom (England and Wales) United Kingdom (Scotland)

Ireland

Poland

Germay

Switzerland

17 18 19 20 21 22

23 24 25

28

29

30

31

33 34 35 36 37 38 39

Peru Peru Peru Peru Peru Peru Peru

32 Belgium median average std dev

27

26

Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain Spain

country

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

number

this this this this this this this

study study study study study study study

Farmer et al. (1999) 21 Farmer et al. (1999) 21 Farmer et al. (1999) 21 Farmer et al. (1999) 21 Farmer et al. (1999) 21 Chiaradia et al. (2000) 23 Walraven et al. (1997) 26

this study this study this study this study this study this study this study this study this study this study this study this study this study this study this study this study Diaz Somoano et al. (2007) 5 this study this study this study this study this study Novak et al. (2003) 23 this study this study

ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-Q-MS ICP-MC-MS ICP-Q-MS ICP-Q-MS

excluded Ballycastle

ICP-MC-MS ICP-MC-MS

ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS

ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-MC-MS ICP-MC-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-MC-MS ICP-MC-MS ICP-Q-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS

16 33 61

1.181

1.174

-

2.106

2.084

2.0564 2.0623 2.0526 2.1025 2.0294 2.0463 2.0701

-

-

-

28

1.177

2.075

-

-

2.0714 2.0345 2.0439

2.0597 2.0585 2.0842 2.0951 2.0378

2.0729 2.1102 2.0812 2.1027 2.0912 2.1012 2.0947 2.0689 2.0326 2.0935 2.0996 2.0956 2.0376 2.0786 1.9822 2.0172

Pb/206Pb

208

South America 142-65 9 1.2058 142-65 1.2097 142-65 7 1.2096 142-65 89 1.1928 142-65 9 1.2197 142-65 5 1.2093 142-65 62 1.2046

-

-

Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous

-

-

9

1.1784

1.1810

1.1840

1.1900 1.2212 1.2097

1.1910 1.2002 1.2010 1.1847 1.1779 1.2068

1.1855 1.1551 1.1860 1.1830 1.1957 1.1701 1.1778 1.1847 1.1892 1.1840 1.1720 1.1744 1.2217 1.1842 1.2444 1.2217

Pb/207Pb

206

2.0786 2.0708 0.0313

-

-

32

24

11

19

17 15 17 8 58

Europe 8 7 17 5 4 3 54 25 30 132 301 6 7 5 3

[Pb] µg g-1

1.176 1.1845 1.1895 0.0184

-

-

-

-

65-1.8 23-5.3

Tertiary Miocene -

24-1.8 24-1.8 24-1.8 24-1.8 354-290

Tertiary-Neogene Tertiary-Neogene Tertiary-Neogene Tertiary-Neogene Carboniferous

-

354-290 354-290 354-290 354-290 354-290 354-290 354-290 354-290 354-290 354-290 354-290 354-290 142-65 142-65 -

age M.a.

Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Createceous Createceous -

time period/ epoch

92.0 87.0 84.8 87.0 55.5

80.5 76.0 85.9 79.1 81.3 86.8 70.8 76.5 82.3 91.00 72.5 65.3 80.5 80.5 -

5.07 4.86 4.78 5.59 5.71 5.11 5.66

-

-

-

-

-

-

86.7 94.4 93.3 94.0 88.6 95.3 97.8

-

-

-

-

-

-

-

3.1 2.7 1.9 1.3 6.4 1.6 0.6

-

-

-

-

-

-

-

9.2 3.9

3.0 6.0 8.1 3.0 5.5

8.0 0.0 4.7 11.5 8.8 9.0 9.2 18.8 0.00 0.00 10.6 10.1 1.8 1.8 -

10.2 2.9 4.8 4.7 5.0 3.1 1.6

-

-

-

-

-

-

-

0.4 3.3

5.0 7.0 7.1 10.0 39.0

11.5 24.0 9.4 9.4 9.9 4.2 20.0 4.7 17.7 9.00 16.9 24.6 17.7 17.7 -

3.58 2.04 2.05 7.25 6.78 8.21 13.88

-

-

-

-

-

-

-

37.86 50.7

40.24 46.81 38.92 33.81 25.05

35.59 8.06 33.27 19.86 31.12 37.4 33.78 40.82 8.62 9.71 35.46 28.6 38.66 38.65 -

14.48 3.47 4.65 3.98 7.60 11.37 4.76

-

-

-

-

-

-

-

8.19 15.9

33.81 19.58 37.63 44.13 11.3

11.35 4.09 6.35 49.07 6.08 5.05 4.78 43.40 23.43 27.89 11.49 28.16 13.9 13.9 46.46 49.31

1.55 4.88 4.07 3.17 0.68

0.68 0.48 1.57 1.08 0.63 0.68 0.47 0.34 0.66 1.14 1.22 1.15 3.36 3.90 -

81.81 94.51 91.87 76.48 74.40 72.27 70.38

-

-

-

-

-

-

-

0.70 0.61 0.63 17.96 14.89 14.93 23.11

-

-

-

-

-

-

-

n.a. 1.6 56.77 2.1

44.27 51.18 36.59 33.73 78.26

72.74 88.13 79.86 40.95 81.5 77.18 81.71 77.86 85.80 89.67 73.08 59.39 68.79 69.00 -

vitrinite liptinite inertinite VM %, Ash %, C %, Stot vol % vol % vol % db db daf %, bs

0.70 90.4 0.25 92.8

0.16 0.30 0.20 0.19 1.07

0.89 2.54 0.99 0.89 1.04 0.78 0.82 0.90 3.20 2.47 0.66 0.60 0.38 0.40 -

R0 %

Pb/206Pb Isotopic Ratios of the 59 Coal Samples as well as Additional Data from the Literaturea

208

comment

Pb/207Pb and

206

source

TABLE 1. Lead Concentrations and

anthracite anthracite anthracite anthracite anthracite anthracite anthracite

-

-

-

-

-

-

-

A A A A A A A

bituminous lignite

lignite lignite lignite lignite bituminous

bituminous anthracite C bituminous bituminous bituminous bituminous bituminous bituminous anthracite B anthracite C bituminous bituminous subsub-

rank

1080

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 4, 2009

Peru Peru Peru Peru Peru Peru Peru

country

Mexico U.S. U.S. U.S. U.S. U.S. U.S. U.S. U.S. U.S. U.S. U.S.

Indonesia Indonesia Indonesia China China

China

China

China

China

China

China

63 64 65 66 67

68

69

70

71

72

73

61 Canada 62 Canada median average std dev

49 50 51 52 53 54 55 56 57 58 59 60

48 Columbia median average std dev

40 41 42 43 44 45 46

number

TABLE 1. Continued

this study this study this study this study Chen et al. (2005) 11 Chen et al. (2005) 11 Chen et al. (2005) 11 Chen et al. (2005) 11 Duzgoren-Atdin et al. (2007) 28 Duzgoren-Atdin et al. (2007) 28 Duzgoren-Atdin et al. (2007) 28

this study this study this study this study this study this study this study this study this study this study this study Chow and Earl (1972) this study this study

this study this study this study this study this study this study this study Farmer et al. (1999) 21

source

ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-Q-MS

ICP-MC-MS ICP-MC-MS

ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS ICP-MC-MS

ICP-MC-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS ICP-Q-MS

comment

65-1.8 65-1.8 65-1.8 -

Tertiary Tertiary Tertiary -

-

-

-

-

-

-

22 -

Asia

1.1800

1.1700

1.1410

1.1610

1.1670

1.1600

1.1822 1.1803 1.1881 1.1546 1.1630

1.2118 1.2142 1.2038 1.2059 0.0116

-

-

-

-

-

-

2.0897 2.1013 2.0878 2.1249 -

2.0419 2.0413 2.0494 2.0517 0.0171

-

5 5 7 9 6

2.0304 2.0640 2.0829 2.0625 2.0494 2.0694 2.0668 2.0447 2.0464 2.0201 2.0526 -

North America 142-65 11 1.2314 354-290 8 1.1987 354-290 9 1.1907 354-290 16 1.1945 354-290 14 1.2109 354-290 23 1.1938 354-290 2 1.1960 354-290 7 1.2047 354-290 5 1.2107 354-290 4 1.2215 354-290 5 1.2029 1.2010

2.0609 2.0723 2.0659 2.0605 2.0594 2.0606 2.0587

Cretaceous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous Carboniferous -

2 41 61 72

1.2014 1.1943 1.2000 1.2022 1.2002 1.2048 1.2040

95.0 72.1 60.3 60.0 67.7 52.5 59.5 72.8 51.2 79.6 80.3 -

-

90.3 93.4 89.5 88.8 86.3 97.8 80.6

-

-

-

-

-

-

0.35 0.38 3.94 1.32 -

-

-

-

-

-

-

94.7 93.9 99.8 44.9 -

-

-

-

-

-

-

3.3 3.2 0.0 0.9 -

1.8 0.4

0.7 0.0 13.0 11.1 3.4 2.4 9.2 6.4 8.4 1.9 0.2 -

-

3.8 2.2 6.7 3.3 8.9 0.0 0.0

-

-

-

-

-

-

2.0 2.9 0.2 49.1 -

56.8 52.3

4.3 27.9 26.7 28.9 29.0 45.1 31.3 20.8 40.4 18.5 19.6 -

-

5.9 4.4 3.8 7.9 4.8 2.2 19.4

-

-

-

-

-

-

47.83 46.96 5.56 20.34 -

21.5 22.14

17.09 34.15 14.61 15.37 33.79 32.28 26.1 17.4 21.48 -

-

2.10 28.22 29.76 32.8 30.78 26.42 8.99

-

-

-

-

-

-

2.77 1.60 1.25 9.56 -

8.3 8.13

47.38 5.22 4.41 8.15 54.23 39.95 6.2 8.46 14.88 5.12 5.51 -

-

6.11 17.44 18.19 6.39 38.02 9.96 46.36

0.78 0.72 0.43 1.05 0.83 0.69 n.a. 0.83 0.64 -

-

2.15 5.01 8.91 2.79 2.45 4.38 0.46

-

-

-

-

-

-

73.26 76.33 93.37 n.a. -

-

-

-

-

-

-

0.53 0.29 0.46 0.7 -

82.03 0.25 81.99 0.26

43.45 76.83 39.27 52.62 82.57 80.63 76.14 87.64 87.74 -

-

90.21 68.02 65.25 77.38 51.25 74.41 49.27

vitrinite liptinite inertinite VM %, Ash %, C %, Stot vol % vol % vol % db db daf %, bs

1.20 41.4 1.20 47.3

1.07 1.56 0.88 0.56 1.00 1.20 0.80 0.90 1.00 1.30 1.60 -

-

4.91 0.72 0.67 0.83 0.67 1.03 2.50

Pb/206Pb R0 %

208

2.077 2.0606 2.0623 0.0158

-

-

16 226 206 58 65 82 24

Pb/207Pb

206

1.2190 1.2046 1.2052 0.0076

142-65 142-65 142-65 142-65 142-65 142-65 142-65

Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous Cretaceous

[Pb] time period/epoch age M.a. µg g-1

-

-

-

-

-

-

subsubanthracite B bituminous -

bituminous bituminous

bituminous bituminous bituminous bituminous bituminous bituminous bituminous bituminous bituminous bituminous bituminous -

-

anthracite A bituminous bituminous bituminous bituminous bituminous anthracite C

rank

VOL. 43, NO. 4, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1081

country

Australia Australia

South Africa

South Africa

81

82

this study this study Farmer et al. (1999) 21 Monna et al. (2006) 30 Monna et al. (2006) 30

this study this study Chiaradia et al. (1997) 22 this study

Duzgoren-Atdin et al. (2007) 28

source

ICP-MC-MS ICP-MC-MS

ICP-Q-MS

ICP-Q-MS ICP-Q-MS

comment

-

17 11 11 6

11

1.2159 1.2118 1.2128 0.0082

1.2050

2.0440 2.0440 2.0385 0.0293

2.0490

2.0750

-

1.2060

1.9954 2.0289

South Africa 299-251 1.2252 299-251 1.2118

Permian Permian 6

2.0839 2.0631 2.0698 0.0122

1.2110 1.1726 1.2055 1.1987 0.0176

65-1.8

Tertiary

2 5 5 3

2.0623 2.0631

Oceania 299-251 8 1.2057 299-251 5 1.2053

Permian Permian

22 22

-

-

-

-

-

-

-

0.71 26.7 0.71 53.4

1.11 97.0

-

-

-

6.9 8.2

2.0

0.7 0.0

-

-

-

-

58.5 29.4

1.0

46.7 31.4

-

-

-

-

30.87 34.51

-

23.1 19.1

-

-

-

-

7.97 12.1

0.57

9.80 9.70

-

-

-

-

-

-

-

-

-

77.76 0.71 73.08 0.53

-

80.25 0.54 80.83 0.59

-

vitrinite liptinite inertinite VM %, Ash %, C %, Stot vol % vol % vol % db db daf %, bs

1.11 52.6 1.44 68.6

-

Pb/206Pb R0 %

208

2.0955 2.1009 0.0171

-

Pb/207Pb

206

1.1720 1.1685 1.1683 0.0134

-

[Pb] time period/epoch age M.a. µg g-1

-

-

-

bituminous bituminous

bituminous

bituminous bituminous

-

rank

a Given are basic structural, geographical, and geological informations where available. R0 is the vitrinite reflectance, VM is the volatile matter, db is the dry basis, daf is the dry ash free basis and Stot and C are total sulphur and carbon concentrations. (s) stands for information not available. The classification of the coals follows ISO 11760:2005 Diaz et al.

83 South Africa median average std dev

South Africa South Africa

79 80

77 Australia 78 New Zealand median average std dev

75 76

74 China median average std dev

number

TABLE 1. Continued

FIGURE 1. Isotope signatures for coals from different regions represented using a box diagram showing the median and the percentiles. The median and average are very close (see Table 1). The calculated averages of the 206Pb/207Pb ratios (with errors given at the 95% confidence interval) are 1.19 ( 0.04 for Europe (n ) 32, E), 1.21 ( 0.2 for North and South America (n ) 14, N-Am, and n ) 15, S-Am, respectively), 1.17 ( 0.03 for Asia (n ) 12), 1.20 ( 0.03 for Oceania (n ) 4, Oc) and 1.21 ( 0.02 for South Africa (n ) 5, S-Afr).

3. Results and Discussion 3.1. Lead Isotopic Composition and Concentration in the Coal Samples. The [Pb] and PbIC for the coal samples are given in Table 1. Concentrations range between 1.6 and 301 µg g-1. Three groups are distinguished: (i) coals with [Pb] 100 µg g-1 from Spain (Puertollano Basin), China (Xiqu deposit), and Peru. The PbIC ranges between 1.15 and 1.24 for 206Pb/207Pb and between 1.98 and 2.12 for 208Pb/206Pb. The [Pb] and PbIC agree well with published data of coal or coal fly ash of the same or adjacent countries (11, 18, 21-30). We note that a considerable spread within coals from the same country can occur, e.g., 206Pb/207Pb ranges between 1.126 and 1.252 in U.S. coals (24) or between 1.130 and 1.268 in Spanish coals (18). Considerable spread in values is also seen on a continental level (see Supporting Information Figure 1). We tested statistically for possible correlation between PbIC and organic (vitrinite, lipinite, inertinite) and inorganic (ash) components, and S and C content (Table 1) (Supporting Information Figure 2 and Supporting Information Table 1). We found no correlations at the 95% confidence level, except to a small degree with liptinite. This is in line with previous work on Spanish coals (18) and reflects the fact that Pb is associated with various chemical forms in coals (8). Figure 1 plots the isotope signatures for the different regions using a box diagram. The calculated averages (with errors given at the 95% confidence interval) are 1.19 ( 0.04 for Europe (n ) 32), 1.21 ( 0.01 and 1.21 ( 0.02 for North and South America (n ) 14 and n ) 15, respectively), 1.17 ( 0.03 for Asia (n ) 12), 1.20 ( 0.03 for Oceania (n ) 4), and 1.21 ( 0.02 for South Africa (n ) 5). To test whether the means of these regions are statistically different, i.e., whether we can confidently state that a particular region has more or less radiogenic Pb, a series of one-tailed t tests were 1082

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performed at the 95 and 99% confidence intervals (see Supporting Information Table 2). We find that the isotopic composition of North America cannot be distinguished from that of Europe and the composition of South Africa from that of Oceania at the 95% confidence interval. It is, however, statistically proved at the 99 and 95% confidence intervals, respectively, that North American and European coals are more radiogenic than Asian coal. Thus the identification and quantification of Pb from these centers to the North Pacific (coming from Asia and North America) and to Eurasia (coming from Europe and Asia) is possible, should coal become a main source of Pb in the atmosphere. This would be a similar situation to the 1980s when the contributions of Pb in North Atlantic surface waters and atmosphere coming from the U.S. and Europe could be calculated using the PbIC of gasoline of these source regions (31). 3.2. The Lead Isotopic Composition in Coals and the Age of the Deposits. Figure 2 plots the 208Pb/206Pb vs 207Pb/ 206 Pb ratios of the coals grouping them according to the geological ages of the deposits (Tertiary, 1.8-65 Ma; Cretaceous, 65-142 Ma; Permian, 251-299 Ma; and Carboniferous, 290-354 Ma). While the 208Pb/206Pb ratios are more influenced by the “effective” Th/U ratios in the coals (U/ Th-geochemistry dependent), the 207Pb/206Pb ratios are more time-related (dependent on geochronological features of the samples). The plot shows that the roughly age-grouped coal data vary significantly along both diagram axes, rather unrelated to the ages derived from stratigraphical arguments. The accumulation of radiogenic Pb isotopes in the investigated coals is therefore controlled by the geochemistry (Th/ U) and the geochronology of the coals alike. Figure 2 compares the coal data of the present study with reference lines according to a Pb isotope evolution model. From the various available models of Pb isotope evolution in crustal environments we selected the model of Kramers and Tolstikhin (37) which distinguishes between the evolution of average young and average old upper crust and average crustal erosion areas. The stippled lines demonstrate model Pb isotope evolution in 100 Ma steps over the last 500 Ma. The reference procedure indicates that the generation of heterogeneous Pb isotope distributions in the coals, which make them suitable for isotope finger printing concepts, are not monocausal. Heterogeneous Th/U-distributions, different geochronologies, mixing of Pb components with different evolutional histories and open-system U-Th-Pb-behavior could have influenced the observed Pb isotope patterns. We propose the following potentially relevant phenomena: (i) specific in-growth of radiogenic Pb through the decay of U and Th in the coals since the formation of the deposits, a commonly observed process in ore deposits (32); (ii) the different dominant chemical forms of Pb in coal (i.e., amount of Pb fixed in PbS), influencing chemical behavior and mobility of Pb in the coals; (iii) possible selective leaching of Pb isotopes out of the deposit as observed in soils and coals (33-35); (iv) allochtonous sources like volcanic emissions or atmospheric dust deposition; and (v) transfer of Pb components during the peat and coal forming events and in the post depositional environments, e.g., from other adjacent crustal areas (36-38). These complex processes are difficult to identify or even quantify. This does, however, not affect the present day source discrimination aspect of the PbIC. 3.3. Lead Isotopic Composition in Coals and Aerosols. Most trace metals are associated with aerosols in the atmosphere (39). Volatile elements such as Pb are mainly found in particles smaller than 1 µm. Figure 3 plots the 206Pb/ 207 Pb ratios of coals from this study and previous studies (Table 1) and of aerosols collected during the 1990s and later in geographically relevant regions (2, 11-16, 28, 30, 40-42). The aerosol signatures are noticeably different from the coal

FIGURE 2. 208Pb/206Pb vs. 207Pb/206Pb plot of the coal ratios grouped according to the geological age of the deposits, including the Tertiary-Neogene (1.6-23 million years, M.a.), Cretaceous (65-145 M.a.), Permian (250-290 M.a.), and Carboniferous (290-362 M.a.). Also shown are isotope evolution curves of a Pb isotope evolution model proposed by Kramers and Tolstikhin (47) as a reference for the discussion of the spread in the Pb isotope compositions of the analyzed coals (see text for details).

FIGURE 3. Plot showing the range of 206Pb/207Pb ratios measured in aerosols (b) from various locations around the world after 1990 and in coals (O). Data taken from several sources see text for details. Regions where aerosols and coals were sampled are underlined. signatures with the main exceptions of China, the eastern U.S., and to some degree, western Europe. We suggest that over the past two decades: (i) coal was not the most important source of atmospheric Pb in these regions despite the phasing out of leaded gasoline in most of these regions; (ii) leaded gasoline contributions from fuel combustion and recycling of Pb deposited in the environment or other sources are still dominant in Europe, South Africa, Oceania, and Indonesia; and (iii) coal could have been a major source of Pb in China and the eastern U.S. during this time period. We note that the phasing out of leaded petrol happened at different times and rates, which affects our interpretation. We note also that a full assessment of local

and regional Pb sources is warranted before concrete conclusions about the importance of coal contributions can be made. Export and import of coal across regions further complicates our interpretation (e.g., Australian ore imported to Europe). Our conclusions are however, largely in line with previous studies. In South Africa, Monna and co-workers (30) measured low radiogenic Pb in lichens in Johannesburg (sampled in 2001 and 2003) and suggested that domestic coal burning, initially suspected to account for the Pb content in the air, only acts as a minor source, even in townships. Domestic coal burning might have been responsible for the more radiogenic Pb in aerosols collected in the Indian Ocean off the coast of South Africa in 2002 (2) due to specific wind conditions during the sampling campaign. Leaded gasoline was still used in South Africa (4) at the time of these sampling campaigns (2, 14, 30) which likely overprinted coal signatures. Nonradiogenic Pb was measured in aerosols in Australia (14, 16, 43) during the 1990s. Though unleaded petrol was introduced in Australia in 1986, the market share of leaded petrol in Australia and New Zealand was still more than 50% in 1993. In Indonesia, the non radiogenic Pb in the aerosols (collected in the 1990) (15) are likely due to the presence of leaded gasoline as the phase-out of leaded gasoline began in Indonesia only in 2001 (44). The suggestion that coal is an important Pb source in China and the eastern U.S. agrees with studies in large Chinese cities such as Shanghai (11, 42), Tianjin (12), and Guangzhou (28); a study of atmospheric Pb deposition in the Okefenokee Swamp in Georgia, U.S. (13); and data from aerosols collected at Tampa, New York, and Woods Hole (16). Finally, we note that the PbIC of the Chinese coals overlap well with western U.S. aerosols, supporting the idea that long-range transport of aerosols from China to the U.S. via the westerlies occurs (3, 16, 45). The Pb emissions of Asia exceeded those of North America 3- to 4-fold in 1989 (16). A complex situation is present in Europe where a significant overlap between coal and aerosols is found in Western Europe but not in Eastern Europe. This might represent the effect of air mass transport from Russia (15, 46), the recycling of gasoline Pb deposited in the environment (40, 41) or the complex political and economical pattern of Europe with different countries adopting different strategies for energy generation. Our data suggest that with the phasing out of leaded gasoline on a global scale, the influence of coal becomes VOL. 43, NO. 4, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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more pronounced. This database should provide a tool when tracing pollution sources and atmospheric transport processes on a global scale.

Acknowledgments We thank the Royal Society and the Spanish Research Council (INCAR-CSIC) for making this collaboration possible. We acknowledge the very thorough reviews of three anonymous referees. This paper is dedicated to Sofia, Laura, and Pablo.

Supporting Information Available The statistical treatment of the data is described as well as a comment on the notation/use of isotope ratios is provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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