The temporal stability in lead isotopic signatures at selected sites in the Southern and Northern Hemispheres

Geochimica et Cosmochimica Acta, Vol. 66, No. 8, pp. 1375–1386, 2002 Copyright © 2002 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/02 $22.00 ⫹ .00

Pergamon

PII S0016-7037(01)00862-6

The temporal stability in lead isotopic signatures at selected sites in the Southern and Northern Hemispheres A. BOLLHO¨ FER*,† and K. J. R. ROSMAN Department of Applied Physics, Curtin University of Technology, Perth, WA 6845, Australia (Received December 27, 2000; accepted in revised form November 27, 2001)

Abstract—A recent survey by Bollho¨fer and Rosman (2000, 2001) has defined the extent to which Pb isotopic ratios in aerosols vary on a global scale. However, it is also important for some applications to know how stable these signatures are. Here we report time series from 38 sites distributed worldwide in which aerosols have been sampled for periods of between 4 months and 4 yr. Apart from a few sites that have atypical conditions, European sites exhibit variations of ⬍0.6% in the 206Pb/207Pb ratio. There is, however, evidence of seasonal variations at sampling sites closer to Eastern Europe that probably reflect an enhanced westward transport of pollution in winter. The variability in Canada and the United States is now larger than before due to a decrease of airborne Pb levels coupled with an increase in the variety of industrial sources. The temporal changes observed in the United States do not exhibit a seasonal pattern. One site in Winnipeg, Canada, showed an extremely large variation, probably the result of seasonal changes influencing the direction of movement of local smelting emissions. Temporal variations in mainland Australia are comparatively small, with a typical range of 0.2% in the 206Pb/207Pb ratio and isotopic ratios that indicate leaded petrol was still a major source of atmospheric Pb over the sampling period. Copyright © 2002 Elsevier Science Ltd Due to Pb’s known toxicity, action was taken from the 1970s to reduce air Pb levels with the introduction of unleaded petrol and stricter emissions regulations (e.g., United States Environmental Protection Agency, 1996). In the United States, Canada, Japan, and most countries of the European Community, the use of leaded petrol has now (i.e., 2000) ceased, and most of the Pb polluting the atmosphere is emitted by industrial processes. This has led to a change in Pb isotopic composition of atmospheric aerosols (Rosman et al., 1994; Veron et al., 1999a,b). Also, as introduction dates vary, this led to emission imbalances and the dilution of atmospheric Pb from major source areas. As a consequence, meso- and long-scale transport of Pb and other pollutants to less-polluted environments can now be detected (Rosman et al., 1999; Aberg et al., 1999; Mukai et al., 1993, 1994). Recently, Jaffe et al. (1999) reported that Asian anthropogenic emissions even influence the concentration of certain atmospheric species in the western United States. Temporal variations, sometimes amounting to a significant shift in the Pb isotopic composition, have been found to occur in some regions (Bollho¨fer and Rosman, 2000, 2001; Simonetti et al., 2000). The changes can occur due to economic changes in a country (e.g., the introduction of unleaded petrol), urban activities that impact regional pollution (e.g., change in industrial activities), or (seasonal) transboundary transport of pollutant Pb from other countries. This paper presents detailed time series of the 206Pb/207Pb, 208Pb/207Pb, and 206Pb/204Pb ratios measured at 38 sites in the Southern and Northern Hemispheres to illustrate the stability of the isotopic signatures. It also uses the existing Pb isotope dataset, available meteorological data, and information on economic development to explain temporal changes and/or seasonal variations in Pb isotopic signatures.

1. INTRODUCTION

The pollution of the atmosphere with Pb is mainly caused by industrial activities such as mining, smelting, waste incineration, and the burning of petrol (Nriagu and Pacyna, 1988). It is a complex phenomenon depending on the market share of leaded petrol, the magnitude of industrial emissions, atmospheric circulation, and transport and deposition mechanisms. The potential of lead isotopes as a tracer of atmospheric emissions depends on the temporal variability of these factors and, hence, on isotopic records and the maintenance of aerosol collection programs to monitor the Pb isotopic composition of source regions. It is important that variations of the source isotopic signatures are well known as demonstrated in studies of Greenland snow by Rosman et al. (1993, 1994). Most of the data available today, however, are either short-term measurements or restricted to a limited region. Pb isotopic fingerprinting takes advantage of the fixed stable Pb isotopic composition of Pb ore bodies, which can differ by more than 30% in their natural 206Pb/207Pb or 208Pb/207Pb ratios, depending on their age and initial U, Th, and Pb content (Doe, 1970). Mixing of ores during alkyllead production or industrial processing reduces these natural variations, but they are still large compared with the measurement precision available. For example, Bollho¨fer and Rosman (2000, 2001) have recently reported significant geographic variations in the isotopic composition of Pb in the atmosphere of both hemispheres. In contrast to Pb concentrations or elemental ratios, Pb isotopes are more reliable as a source tracer as they are not fractionated during transport and deposition processes.

2. METHODS

* Author to whom correspondence should be addressed ([email protected]). † Present address: Environmental Research Institute of the Supervising Scientist (ERISS), Environmental Radioactivity, Locked Bag 2, Jabiru, NT 0886, Australia.

2.1. Sampling Sites The aerosol collection kits were distributed worldwide. Most sites are located within well-known cities in urban residential areas; how1375

1376

A. Bollho¨ fer and K. J. R. Rosman

ever, population size and density at the sampling sites may vary. None of the sites of this study is located in highly industrialized areas. Camaragibe, Brazil, is located in a rural area ⬃30 km west of Recife. Each filter usually collected air for a period of 2 months. Maps showing the locations of the sampling sites are presented in Bollho¨ fer and Rosman (2000, 2001). 2.2. Aerosol Collection Samples were collected using small diaphragm-type aquarium pumps modified to invert the airflow. The sampling kits consisted of the pump, 2 to 3 m of PVC tubing, and a plastic housing fitted with a 30 to 60-␮m pore-size Teflon backing and a 0.45-␮m pore-size Teflon front filter. The volume of the air passing through the filter was determined with an uncertainty ⬃10% for a pump operating at 240V/ 50Hz and ⬃30% at 110V/60Hz. After sampling, the monitors were sent back in batches of two to Curtin University in Perth. A filter segment was leached in warm 0.4M HBr for 1 h under HEPA-filtered air flow, and sequential leaching showed that 95% of the lead collected on the filters was leached off using this procedure. Ion-exchange chemistry was used to purify the lead before isotopic analysis. For more details, see Bollho¨ fer et al. (1999). 2.3. Mass Spectrometry The measurements were carried out using a VG354 multicollector thermal ionisation mass spectrometer. Samples were loaded on zonerefined single Rhenium filaments, together with a phosphoric acid/ silica gel ionisation enhancer. Measurements were normalised to NIST SRM981, which indicated a discrimination of 0.12 ⫾ 0.05% per mass unit for measurements on the Faraday Cups and 0.24 ⫾ 0.06% per mass unit for measurements on the Daly collector. The internal standard deviation of a single measurement was negligible compared with the uncertainty in the discrimination. 3. RESULTS AND DISCUSSION

Table 1 gives all 206Pb/207Pb, 208Pb/207Pb, and 206Pb/204Pb ratios and the concentrations measured at the different sites. As Pb levels might differ by several orders of magnitude on a local scale, they provide only limited evidence of pollution levels, although isotopic ratios tend to be representative. At some of the sampling sites, no long-term trends can be established due to a lack of previous data, whereas others do not exhibit seasonal variations, or both. Nevertheless, the results are presented, as they may prove useful for geochemists as a transient tracer. 3.1. South America and South Africa Long-term data measured in Punta Arenas at the southern tip of South America reveal that there was a slight increase from 206 Pb/207Pb ratios of ⬃1.07 from 1994 to 1995 to ⬃1.08 from 1997 to 1999 (Bollho¨ fer and Rosman, 2000). This increase is similar to what was observed in South Africa and indicates a common supplier for alkyllead, as leaded petrol was in use almost exclusively in both regions in 1993 (Associated Octel, 1995). However, the increase could also be because of a relative increase from industrial sources due to a slow phasing out of leaded petrol in both regions. In Brazil no leaded petrol is used and the Pb isotopic composition of aerosols shows a relatively large spread, which was about the same in 1995 and 1998 (Bollho¨ fer and Rosman, 2000). In Recife, in northeast Brazil, seasonal variations are noticeable, with low ratios during the southern autumn and higher ratios in spring. The sampling site in Camaragibe is located ⬃30-km west of Recife and shows the same variations, indicating that they are of a regional rather than a local nature.

Pb isotopic signatures in aerosols collected in Mexico City, with alkyllead being the major source of atmospheric Pb, are very stable and show a variation of ⬍0.3% (95% confidence intervals; n ⫽ 6). 3.2. Australia Compared to earlier measurements, the isotopic composition of the aerosols does not exhibit a temporal trend apart from aerosols collected in Queensland and the Northern Territory. The reasons have been discussed in Bollho¨ fer and Rosman (2000). Pb isotopic ratios measured in Melbourne and Perth (Rosman, unpubl. data) are slightly more radiogenic during the Southern Hemisphere summer and lower during winter. The opposite trend, although very small in magnitude, is apparent in the Darwin samples with lower radiogenic Pb isotopic ratios during the wet season (November to March). The reason for this trend is possibly a difference in the amount of crustal Pb, with comparatively higher isotopic ratios than the industrial and petrol Pb collected on the filters. This contribution is likely to be enhanced during the dry, dustier months (May to October in Darwin, November to April in Perth) as compared to wetter months. Figure 1 shows the seasonal trends observed in Australia. 3.3. Asia and the Near East Previous data are not available for Abu Dhabi (UAE), Islamabad (Pakistan), or Bandung (Indonesia) for comparison. However, it is worth noting that Pb isotopic compositions at these sites are similar, and unleaded petrol usage was still negligible in 1993 (Associated Octel, 1995). Leaded petrol appears to be the major source of atmospheric Pb, and the similar composition is consistent with a single supplier of alkyllead. No leaded petrol is available in Japan. Data from Hiroshima, Japan, measured from 1998 to 1999 are, however, remarkably stable. Mukai et al. (1993) indicated that there were only minor variations in the isotopic ratios measured in Japanese cities from the early to the late 1980s and reported average 206Pb/ 207 Pb (208Pb/207Pb) ratios for this period of 1.159 ⫾ 0.005 (2.433 ⫾ 0.020). The comparison with our new data shows that the isotopic composition up to 1999 remains the same. 3.4. Europe Figure 2 shows the results of the Pb isotopic and concentration measurements from 1998 to 1999. Low isotopic ratios typical for alkyllead used in France and the U.K. (Monna et al., 1997) are accompanied by higher airborne Pb levels, indicating a significant contribution from the burning of leaded petrol with low radiogenic isotopic ratios. In northern Italy, the Pb isotopic composition of leaded petrol is higher with 206Pb/207Pb (208Pb/ 207 Pb) ratios of 1.156 (2.428) (Bollho¨ fer and Rosman, 2001), which is probably responsible for the higher ratios measured in Venice. No leaded petrol was available in Germany during the period sampled. Industrial Pb in Europe, such as Pb emitted by urban incinerators, has a relatively narrow range of isotopic ratios. Recent studies give 206Pb/207Pb (208Pb/207Pb) ratios of 1.143 to

Temporal stability in Pb isotopic signatures

1377

Table 1. Isotopic composition and concentration of Pb for the various time series investigated in the Northern and Southern Hemispheres. Uncertainties for the isotopic ratios are 95% confidence intervals. Concentrations encountered provide only limited evidence of the pollution levels as they can differ by several orders of magnitude on a local scale. Date on 1 Punta Arenas, Chile 23-Nov-96 23-Jan-97 23-Mar-97 23-May-97 23-Jul-97 23-Sep-97 23-Nov-97 23-Jan-98 22-Feb-98 22-Apr-98 22-Jun-98 2 Sao Paulo, Brazil 16-Oct-95 25-Apr-97 26-Nov-97 23-Dec-97 23-Feb-98 23-Apr-98 23-Jun-98 24-Aug-98 23-Oct-98 3 Recife, Brazil 20-Jan-98 20-Mar-98 25-May-98 25-Jul-98 25-Sep-98 25-Nov-98 4 Camaragibe, Brazil 7-Mar-98 7-May-98 7-Sep-98 8-Nov-98 9-Jan-99 5 Quito, Ecuador 13-Mar-98 1-May-98 1-Jul-98 2-Sep-98 4-Nov-98 4-Jan-99 6 Mexico City 20-Apr-98 20-Jun-98 20-Aug-98 20-Oct-98 20-Dec-98 20-Feb-99 7 Cape Town 28-Feb-98 28-Apr-98 28-Jun-98 8 Melbourne, Australia 4-Nov-94 30-Sep-94 31-Aug-94 6-Dec-94 3-Feb-95 9-Jan-95 5-Feb-98 5-Apr-98 9-Jun-98 8-Aug-98 28-Oct-98 30-Dec-98

Date off

conc. [ng/m3]

⫾

206 207

⫾

208 207

⫾

206 204

⫾

23-Jan-97 23-Mar-97 23-May-97 23-Jul-97 23-Sep-97 23-Nov-97 23-Jan-98 22-Feb-98 22-Apr-98 22-Jun-98 22-Aug-98

63.3 96.0 61.2 52.9 50.4 16.4 39.1 43.5 34.8 7.9 9.2

4.3 6.5 4.1 3.6 3.4 1.1 2.6 2.9 2.3 0.5 0.6

1.0845 1.0866 1.0755 1.0897 1.0852 1.0833 1.0918 1.0810 1.0904 1.0937 1.0867

0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0006 0.0044 0.0006 0.0009

2.3536 2.3589 2.3498 2.3599 2.3615 2.3555 2.3670 2.3552 2.3191 2.3732 2.3623

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0279 0.0012 0.0026

16.81 16.86 16.57 16.91 16.83 16.80 16.91 16.82 16.82 17.13 17.20

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.08 0.31 0.08 0.21

5-Nov-95 17-May-97 28-Nov-97 23-Feb-98 23-Apr-98 23-Jun-98 24-Aug-98 23-Oct-98 23-Dec-98

51.6 49.9 3.2 26.5 40.6 67.2 58.9 39.6 11.1

13.8 13.3 0.8 3.5 5.4 9.0 7.9 5.3 1.5

1.1768 1.1665 1.1548 1.1835 1.1811 1.1762 1.1709 1.1680 1.1698

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4284 2.4292 2.4250 2.4420 2.4374 2.4299 2.4290 2.4280 2.4364

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.39 18.23 18.03 18.57 18.56 18.42 18.31 18.24 18.26

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

20-Mar-98 20-May-98 25-Jul-98 25-Sep-98 25-Nov-98 25-Jan-99

30.4 31.3 30.7 40.8 7.3 7.3

4.1 4.2 4.1 5.4 1.0 1.0

1.1471 1.1412 1.1451 1.1487 1.1564 1.1511

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4206 2.4161 2.4234 2.4315 2.4380 2.4259

0.0012 0.0012 0.0012 0.0013 0.0012 0.0012

17.87 17.77 17.89 18.10 18.11 18.14

0.02 0.02 0.02 0.04 0.02 0.04

7-May-98 7-Jul-98 8-Nov-98 9-Jan-99 9-Mar-99

12.7 13.0 5.5 6.4 3.7

1.7 1.7 0.7 0.8 0.5

1.1428 1.1468 1.1522 1.1544 1.1513

0.0006 0.0006 0.0006 0.0006 0.0006

2.4170 2.4215 2.4321 2.4329 2.4283

0.0012 0.0012 0.0012 0.0012 0.0012

17.80 17.89 18.04 18.03 17.95

0.02 0.02 0.02 0.02 0.02

1-May-98 1-Jul-98 2-Sep-98 4-Nov-98 4-Jan-99 5-Mar-99

22.0 35.0 15.0 7.1 6.1 17.5

5.9 9.3 4.0 1.9 1.6 4.7

1.1306 1.2044 1.1955 1.1785 1.1809 1.1915

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3941 2.4520 2.4473 2.4489 2.4479 2.4613

0.0012 0.0012 0.0012 0.0014 0.0012 0.0012

17.63 18.96 18.95 18.38 18.48 18.70

0.02 0.02 0.02 0.12 0.02 0.02

20-Jun-98 20-Aug-98 20-Oct-98 20-Dec-98 20-Feb-99 20-Apr-99

30.4 40.5 51.0 88.0 35.0 29.5

4.1 5.4 6.8 11.7 4.7 3.9

1.1899 1.1944 1.1952 1.1975 1.1948 1.1888

0.0006 0.0006 0.0006 0.0006 0.0006 0.0015

2.4597 2.4617 2.4621 2.4635 2.4635 2.4581

0.0012 0.0012 0.0012 0.0012 0.0012 0.0019

18.62 18.69 18.70 18.73 18.71 18.47

0.02 0.02 0.02 0.02 0.02 0.13

28-Apr-98 28-Jun-98 2-Sep-98

74.7 142.7 82.3

5.0 9.6 5.5

1.0854 1.0898 1.0858

0.0005 0.0005 0.0005

2.3534 2.3584 2.3563

0.0012 0.0012 0.0012

16.87 16.99 16.93

0.02 0.02 0.02

6-Dec-94 4-Nov-94 30-Sep-94 9-Jan-95 2-Mar-95 3-Feb-95 5-Apr-98 9-Jun-98 8-Aug-98 15-Oct-98 30-Dec-98 30-Mar-99

61.8 68.4 44.5 32.8 39.2 49.5 16.0 24.0 6.2 11.6 13.4 12.3

4.2 4.6 3.0 2.2 2.6 3.3 1.1 1.6 0.4 0.8 0.9 0.8

1.0676 1.0681 1.0671 1.0692 1.0714 1.0716 1.1022 1.0994 1.0997 1.0904 1.0963 1.1008

0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0006 0.0006 0.0006 0.0007 0.0024 0.0006

2.3416 2.3448 2.3524 2.3389 2.3445 2.3420 2.3688 2.3680 2.3683 2.3683 2.3730 2.3681

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0015 0.0061 0.0012

16.53 0.02 16.58 0.02 16.69 0.02 16.53 0.02 16.60 0.02 16.59 0.02 17.14 0.02 17.14 0.02 17.13 0.02 16.80 0.07 17.30 0.09 17.11 0.02 (Continued)

1378

A. Bollho¨ fer and K. J. R. Rosman Table 1. Continued. Date on

9 Darwin, Australia 1-Feb-98 1-Apr-98 1-Jun-98 5-Aug-98 9-Oct-98 30-Nov-98 10 Townsville, Australia 7-Apr-98 5-Jun-98 12-Aug-98 14-Oct-98 11 Smithton, Australia 10-Jun-98 25-Jun-98 30-Jun-98 14-Jul-98 3-Feb-99 16-Feb-99 2-Mar-99 16-Mar-99 30-Mar-99 13-Apr-99 27-Apr-99 30-Jun-99 12 Hobart, Australia 2-Jul-99 4-Feb-00 13 Hamilton, New Zealand 29-Jan-99 29-Nov-98 14 Abu Dhabi, UAE 26-Nov-97 9-Feb-98 9-Apr-98 9-Jun-98 15 Islamabad, Pakistan 5-Jan-98 5-Mar-98 10-Nov-98 8-Jul-98 8-May-98 10-Sep-98 16 Bandung, Indonesia 19-May-98 19-Jul-98 17-Sep-98 17-Nov-98 16-Jan-00 16-Mar-99 17 Hiroshima, Japan 16-Jan-98 18-Mar-98 18-May-98 26-Sep-98 25-Nov-98 25-Jan-99 18 Haifa, Israel 21-Sep-98 8-Apr-98 25-Jan-98 19 Constance Germany 28-Nov-97 28-Jan-98 30-Mar-98 28-May-98 28-Jul-98 30-Sep-98

Date off

conc. [ng/m3]

⫾

206 207

⫾

208 207

⫾

206 204

⫾

1-Apr-98 1-Jun-98 6-Aug-98 9-Oct-98 30-Nov-98 31-Jan-99

9.8 12.1 5.2 7.8 9.4 5.3

0.7 0.8 0.4 0.5 0.6 0.4

1.0740 1.0756 1.0772 1.0763 1.0758 1.0723

0.0005 0.0005 0.0005 0.0005 0.0005 0.0005

2.3424 2.3452 2.3462 2.3423 2.3465 2.3444

0.0012 0.0012 0.0012 0.3918 0.0012 0.0012

16.90 16.71 16.71 16.55 16.69 16.64

0.02 0.02 0.02 0.02 0.02 0.02

5-Jun-98 12-Aug-98 14-Oct-98 14-Dec-98

10.2 8.5 8 7.7

0.7 0.6 0.5 0.5

1.0820 1.0839 1.0839 1.0845

0.0005 0.0005 0.0005 0.0005

2.3531 2.3539 2.3555 2.3562

0.0012 0.0012 0.0012 0.0012

16.77 16.80 16.82 16.86

0.02 0.02 0.02 0.02

17-Jun-98 30-Jun-98 14-Jul-98 22-Jul-98 16-Feb-99 2-Mar-99 16-Mar-99 30-Mar-99 13-Apr-99 27-Apr-99 30-Jun-99 24-Aug-99

N/A 10.4 19.3 12.1 N/A N/A 8.5 16.3 12.3 16.1 5.0 4.0

0.6 1.1 0.8 1.1 0.3 0.3

1.0997 1.1107 1.0994 1.0887 1.1090 1.1110 1.1088 1.0999 1.1058 1.1103 1.1084 1.1022

0.0006 0.0006 0.0006 0.0005 0.0010 0.0010 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3616 2.3643 2.3628 2.3556 2.3750 2.3770 2.3735 2.3683 2.3726 2.3752 2.3715 2.3669

0.0013 0.0013 0.0012 0.0012 0.0020 0.0020 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.10 17.04 17.06 16.86 17.24 17.24 17.20 N/A 17.19 17.22 17.23 17.10

0.02 0.02 0.02 0.02 0.02 0.02 0.08 N/A 0.07 0.02 0.02 0.02

2-Sep-99 6-Apr-00

12.3 11.4

0.8 0.8

1.1118 1.0984

0.0006 0.0005

2.3750 2.3652

0.0012 0.0012

17.30 17.08

0.02 0.02

13-Mar-99 29-Jan-99

11.3 6.3

0.8 0.4

1.1039 1.1118

0.0006 0.0006

2.3839 2.3981

0.0012 0.0012

17.15 17.36

0.00 0.02

26-Jan-98 9-Apr-98 9-Jun-98 9-Aug-98

194.6 117.8 82.2 75.1

13.1 7.9 5.5 5.1

1.0973 1.1001 1.0997 1.0959

0.0005 0.0006 0.0005 0.0005

2.3667 2.3660 2.3655 2.3610

0.0012 0.0012 0.0012 0.0012

17.08 17.10 17.10 17.02

0.02 0.02 0.02 0.02

5-Mar-98 5-May-98 10-Jan-99 10-Sep-98 8-Jul-98 10-Nov-98

193.3 135.2 278.0 151.1 161.9 565.3

13.0 9.1 18.7 10.2 10.9 38.1

1.0913 1.1005 1.1091 1.1081 1.1095 1.1139

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3686 2.3807 2.3793 2.3834 2.3788 2.3842

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

16.96 17.20 17.29 17.27 17.27 17.35

0.02 0.02 0.02 0.02 0.02 0.02

19-Jul-98 19-Sep-98 17-Nov-98 17-Jan-99 16-Mar-99 16-May-99

150.7 147.8 104.4 113.6 266.8 234.1

40.2 39.4 27.8 30.3 71.2 62.4

1.1030 1.0912 1.1055 1.0939 1.0896 1.0912

0.0006 0.0005 0.0006 0.0005 0.0007 0.0005

2.3711 2.3608 2.3739 2.3632 2.3744 2.3596

0.0012 0.0012 0.0012 0.0012 0.0018 0.0012

17.15 16.96 17.20 17.04 17.51 16.95

0.02 0.02 0.02 0.02 0.16 0.02

18-Mar-98 18-May-98 26-Sep-98 25-Nov-98 25-Jan-99 25-Mar-99

33.3 30.2 26.0 34.9 30.5 36.2

4.5 4.0 3.5 4.7 4.1 4.8

1.1573 1.1616 1.1602 1.1560 1.1574 1.1562

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4434 2.4406 2.4412 2.4353 2.4393 2.4438

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.10 18.16 18.17 18.02 18.06 18.10

0.02 0.02 0.02 0.02 0.02 0.02

22-Nov-98 8-Jun-98 25-Mar-98

36.6 24.7 50.4

2.5 1.7 3.4

1.1310 1.1282 1.1359

0.0006 0.0006 0.0006

2.3993 2.3926 2.4013

0.0012 0.0012 0.0012

17.65 17.59 17.70

0.02 0.02 0.02

28-Jan-98 30-Mar-98 28-May-98 28-Jul-98 30-Sep-98 2-Dec-98

6.0 4.8 4.4 2.8 5.1 8.8

0.4 0.3 0.3 0.2 0.3 0.6

1.1474 1.1479 1.1466 1.1378 1.1443 1.1448

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4279 2.4273 2.4217 2.4112 2.4160 2.4194

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.94 0.02 17.95 0.02 17.89 0.02 17.74 0.02 17.84 0.02 17.84 0.02 (Continued)

0.7 1.3 0.8

Temporal stability in Pb isotopic signatures

1379

Table 1. Continued. Date on 20 Plo¨ n, Germany 22-Nov-97 22-Jan-98 24-Mar-98 24-May-98 24-Jul-98 23-Sep-98 21 Grenoble, France 15-Nov-95 16-Feb-96 16-May-96 27-Sep-96 9-Dec-97 5-Feb-98 7-Apr-98 6-Jun-98 11-Aug-98 13-Oct-98 22 Avignon, France 7-Dec-97 30-Jan-98 6-Apr-98 14-Jun-98 16-Aug-98 11-Oct-98 23 Valencia, Spain 14-Jan-98 16-Mar-98 13-May-98 14-Jul-98 14-Sep-98 13-Nov-98 24 Venice, Italy 12-Jan-98 12-Mar-98 12-May-98 13-Jul-98 18-Sep-98 18-Nov-98 25 Rome, Italy 26-Jan-98 16-Feb-98 15-Apr-98 15-Jun-98 18-Aug-98 16-Dec-98 26 Oxford, U.K. 27-Dec-97 27-Feb-98 20-Apr-98 20-Jun-98 20-Aug-98 20-Oct-98 27 Moscow, Russia 8-Dec-98 8-Feb-99 8-Apr-99 8-Jun-99 4-Aug-99 4-Oct-99 28 Davis, USA 28-Jan-98 15-Mar-98 5-Apr-98 26-Apr-98 17-May-98 7-Jun-98

Date off

conc. [ng/m3]

⫾

206 207

⫾

208 207

⫾

206 204

⫾

22-Jan-98 24-Mar-98 24-May-98 24-Jul-98 23-Sep-98 23-Nov-98

4.5 1.5 6.4 3.4 3.6 2.9

0.3 0.1 0.4 0.2 0.2 0.2

1.1647 1.1545 1.1567 1.1471 1.1513 1.1618

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4399 2.4287 2.4307 2.4176 2.4209 2.4367

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.20 18.02 18.06 17.89 17.98 18.15

0.02 0.02 0.02 0.02 0.02 0.02

15-Feb-96 16-May-96 27-Sep-96 17-Nov-96 5-Feb-98 7-Apr-98 6-Jun-98 11-Aug-98 30-Oct-98 24-Dec-98

142.3 69.3 52.3 100.4 35.1 23.5 26.7 29.1 11.5 12.6

9.6 4.7 3.5 6.8 2.4 1.6 1.8 2.0 0.8 0.9

1.1343 1.1278 1.1248 1.1236 1.1046 1.1098 1.1155 1.1136 1.1134 1.1122

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4048 2.4052 2.3987 2.3980 2.3863 2.3819 2.3884 2.3858 2.3854 2.3808

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.68 17.76 17.52 17.48 17.28 17.25 17.38 17.35 17.35 17.29

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

30-Jan-98 6-Apr-98 14-Jun-98 16-Aug-98 11-Oct-98 26-Dec-98

12.3 10.8 25.5 21.1 28.5 11.9

0.8 0.7 1.7 1.4 1.9 0.8

1.1340 1.1265 1.1291 1.1334 1.1241 1.1315

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3942 2.3904 2.3975 2.4024 2.3956 2.3930

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.69 17.56 17.60 17.69 17.55 17.62

0.02 0.02 0.02 0.02 0.02 0.02

16-Mar-98 13-May-98 14-Jul-98 14-Sep-98 13-Nov-98 14-Jan-99

19.7 31.8 35.7 28.2 25.7 12.6

1.3 2.1 2.4 1.9 1.7 0.9

1.1209 1.1282 1.1232 1.1152 1.1137 1.1205

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3957 2.4054 2.3971 2.3896 2.3853 2.4003

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.46 17.59 17.48 17.36 17.31 17.54

0.02 0.02 0.02 0.02 0.02 0.02

12-Mar-98 12-May-98 12-Jul-98 18-Sep-98 18-Nov-98 19-Jan-99

21.9 17.3 20.6 22.0 19.3 12.6

1.5 1.2 1.4 1.5 1.3 0.9

1.1597 1.1534 1.1516 1.1485 1.1488 1.1564

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4299 2.4232 2.4206 2.4187 2.4300 2.4276

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.13 18.03 17.98 17.95 18.09 18.05

0.02 0.02 0.02 0.02 0.02 0.02

16-Feb-98 15-Apr-98 15-Jun-98 15-Aug-98 16-Oct-98 14-Feb-99

157.1 76.2 81.3 95.6 45.5 129.1

10.6 5.1 5.5 6.4 3.1 8.7

1.1147 1.1183 1.1199 1.1205 1.1164 1.1080

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3800 2.3832 2.3857 2.3871 2.3828 2.3758

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.37 17.42 17.46 17.47 17.39 17.24

0.02 0.02 0.02 0.02 0.02 0.02

27-Feb-98 20-Apr-98 20-Jun-98 20-Aug-98 20-Oct-98 20-Dec-98

2.5 12.4 11.8 24.2 5.4 9.4

0.2 0.8 0.8 1.6 0.4 0.6

1.1237 1.1235 1.1245 1.1183 1.1220 1.1227

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.3922 2.3940 2.3933 2.3871 2.3956 2.3902

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.50 17.50 17.50 17.42 17.50 17.49

0.02 0.02 0.02 0.02 0.02 0.02

8-Feb-99 8-Apr-99 8-Jun-99 4-Aug-99 4-Oct-99 8-Dec-99

11.9 27.6 31.1 20.1 25.1 18.6

3.2 7.4 8.3 5.4 6.7 5.0

1.1631 1.1596 1.1609 1.1596 1.1599 1.1591

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4426 2.4431 2.4406 2.4428 2.4393 2.4399

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.19 18.19 18.12 18.13 18.06 18.09

0.02 0.02 0.02 0.02 0.02 0.02

15-Mar-98 5-Apr-98 26-Apr-98 17-May-98 7-Jun-98 28-Jun-98

2.2 4.1 4.8 4.4 2.9 16.2

0.6 1.1 1.3 1.2 0.8 4.3

1.1863 1.1750 1.1811 1.1692 1.1746 1.1846

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4416 2.4361 2.4425 2.4339 2.4334 2.4546

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.54 0.02 18.35 0.02 18.47 0.02 18.24 0.02 18.33 0.02 18.50 0.02 (Continued)

1380

A. Bollho¨ fer and K. J. R. Rosman Table 1. Continued. Date on

28 Davis, USA 1-Aug-98 30-Sep-98 2-Dec-98 2-Feb-99 29 Berkeley, USA 1-Jan-98 28-Feb-98 29-Apr-98 28-Jun-98 29-Aug-98 7-Nov-98 30 Pasadena, USA 9-Nov-97 16-Jan-98 14-Mar-98 31 Tampa, USA 11-Dec-97 11-Feb-98 9-Apr-98 11-Jun-98 11-Aug-98 32 Woodshole, USA 21-Jan-98 23-Mar-98 25-May-98 6-Aug-98 12-Oct-98 33 New York, USA 21-Jan-98 23-Mar-98 25-May-98 34 Toronto, Canada 20-Aug-98 20-Jun-98 10-Apr-98 10-Feb-98 15/11/98 15-Jan-99 35 Victoria, Canada 6-Mar-98 1-May-98 30-Jun-98 1-Sep-98 2-Nov-98 4-Jan-99 36 Calgary, Canada 25-Jun-98 25-Aug-98 27-Oct-98 28-Dec-98 2-Mar-99 6-May-99 37 Winnipeg, Canada 16-Jun-98 15-Aug-98 9-Oct-98 22-Dec-98 22-Feb-99 4-May-99 38 Newfoundland, Canada 15-Jun-98 14-Aug-98

Date off

conc. [ng/m3]

⫾

206 207

⫾

208 207

⫾

206 204

⫾

30-Sep-98 2-Dec-98 2-Feb-99 1-Apr-99

3.7 4.7 4.5 3.3

1.0 1.3 1.2 0.9

1.1599 1.1652 1.1751 1.1623

0.0012 0.0021 0.0006 0.0006

2.4334 2.4306 2.4373 2.4306

0.0026 0.0045 0.0012 0.0012

18.21 18.21 18.44 18.12

0.04 0.04 0.02 0.02

28-Feb-98 29-Apr-98 28-Jun-98 29-Aug-98 7-Nov-98 1-Feb-99

3.6 3.9 2.8 4.1 4.3 3.5

1.0 1.0 0.7 1.1 1.2 0.9

1.1588 1.1660 1.1604 1.1736 1.1716 1.1658

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4261 2.4392 2.4362 2.4390 2.4338 2.4297

0.0012 0.0013 0.0014 0.0012 0.0012 0.0012

18.10 18.32 18.20 18.33 18.35 18.17

0.02 0.02 0.05 0.02 0.02 0.02

16-Jan-98 14-Mar-98 24-May-98

6.8 4.5 7.00

1.8 1.2 1.9

1.1809 1.1741 1.1742

0.0006 0.0006 0.0006

2.4392 2.4351 2.4390

0.0012 0.0012 0.0012

18.45 18.37 18.39

0.02 0.02 0.02

11-Feb-98 9-Apr-98 11-Jun-98 11-Aug-98 11-Oct-98

4.2 2.6 9.6 3.4 2.4

1.1 0.7 2.6 0.9 0.7

1.2105 1.2080 1.2314 1.2159 1.2131

0.0006 0.0006 0.0006 0.0006 0.0006

2.4603 2.4595 2.4703 2.4624 2.4621

0.0012 0.0012 0.0012 0.0012 0.0012

19.02 19.01 19.39 19.06 19.05

0.02 0.02 0.02 0.02 0.02

23-Mar-98 25-May-98 6-Aug-98 12-Oct-98 27-Nov-98

3.7 3.1 2.5 5.6 10.4

1.0 0.8 0.7 1.5 2.8

1.1955 1.1733 1.1801 1.1762 1.1797

0.0006 0.0006 0.0006 0.0006 0.0006

2.4511 2.4413 2.4386 2.4375 2.4404

0.0012 0.0012 0.0012 0.0012 0.0012

18.69 18.33 18.43 18.36 18.43

0.02 0.02 0.02 0.02 0.02

23-Mar-98 25-May-98 6-Aug-98

106.3 48.8 67.6

28.3 13.0 18.0

1.2232 1.1970 1.2141

0.0006 0.0006 0.0006

2.4636 2.4589 2.4621

0.0012 0.0012 0.0012

19.19 18.89 19.04

0.02 0.02 0.02

20-Oct-98 20-Aug-98 11-Jun-98 10-Apr-98 15-Jan-99 15-Mar-99

12.3 15.7 11.9 12.9 N/A 10.3

3.3 4.2 3.2 3.4

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4334 2.4263 2.4185 2.4306 2.4325 2.4328

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

18.26 18.12 17.91 18.22

2.8

1.1674 1.1622 1.1503 1.1661 1.1707 1.1629

18.18

0.02 0.02 0.02 0.02 0.02 0.02

1-May-98 30-Jun-98 1-Sep-98 2-Nov-98 4-Jan-99 26-Feb-99

2.3 1.4 1.5 3.4 4.7 2.1

0.6 0.4 0.4 0.9 1.3 0.6

1.1454 1.1468 1.1456 1.1488 1.1411 1.1488

0.0006 0.0006 0.0006 0.0006 0.0006 0.0006

2.4153 2.4161 2.4137 2.4188 2.4107 2.4183

0.0012 0.0012 0.0012 0.0012 0.0012 0.0012

17.84 17.85 17.90 17.94 17.79 17.95

0.02 0.02 0.02 0.02 0.02 0.02

25-Aug-98 27-Oct-98 28-Dec-98 2-Mar-99 6-May-99 6-Jul-99

2.2 2.6 2.5 3.1 2.6 1.3

0.6 0.7 0.7 0.8 0.7 0.3

1.1583 1.1617 1.1715 1.1607 1.1598 1.1606

0.0006 0.0006 0.0006 0.0006 0.0007 0.0008

2.4188 2.4324 2.4319 2.4278 2.4311 2.4293

0.0012 0.0012 0.0012 0.0012 0.0014 0.0016

N/A 18.20 18.30 18.15 18.25 17.97

0.09 0.02 0.02 0.02 0.07 0.03

15-Aug-98 19-Oct-98 22-Dec-98 22-Feb-99 4-May-99 13-Jul-99

2.2 2.3 2.3 4.9 3.3 1.8

0.6 0.6 0.6 1.3 0.9 0.5

1.1563 1.1451 1.1312 1.0943 1.1698 1.1551

0.0008 0.0011 0.0008 0.0027 0.0006 0.0006

2.4168 2.4181 2.4109 2.3652 2.4380 2.4282

0.0012 0.0031 0.0016 0.0053 0.0012 0.0012

18.06 17.68 17.57 16.98 18.28 18.00

0.02 0.21 0.05 0.10 0.02 0.05

14-Aug-98 15-Oct-98

1.3 1.1

0.3 0.3

1.1691 1.1691

0.0006 0.0137

2.4384 2.4309

0.0012 0.0015

18.27 18.21

0.02 0.03

Temporal stability in Pb isotopic signatures

1381

Fig. 1. 206Pb/207Pb ratio and Pb concentration time series measured in Australia in 1998 and 1999. Notice the logarithmic concentration axis.

1.155 (2.4266 –2.4362) for France (Monna et al., 1997), 1.154 to 1.156 (2.423–2.424) for Switzerland (Chiaradia and Cupelin, 2000), and 1.142 to 1.160 for Germany (Hamester et al., 1994). Steel metallurgy displays radiogenic ratios of 1.179 to 1.223 (2.421–2.450) (Alleman, 1997; Veron et al., 1999a). 3.4.1. Long-term trend A comparison is made with earlier data taken between 1981 and 1994 (Maring et al., 1987; Hopper et al., 1991; Grousset et al., 1994; Mukai et al., 1994; Bacon et al., 1996; Monna et al., 1997; Aberg et al., 1999; Kober et al., 1999; Rosman et al., 1999; Veron et al., 1999a) in the following discussion. The isotopic composition of aerosols in the U.K. has remained constant in the past ⬃20 yr, and no significant change could be discerned. The 206Pb/207Pb (208Pb/207Pb) ratios measured from 1995 to 1998, including data from Bollho¨ fer et al. (1999) and the ratios of 1.136 (2.412) measured in Braemar, Scotland (this study), were 1.108 to 1.136 (2.380 –2.412), which are similar to 1.096 to 1.129 (2.373–2.407) reported in earlier studies. The isotopic ratio of Oxford aerosols most probably represents a mixture of petrol and industrial Pb.

Assuming an average petrol 206Pb/207Pb ratio in the UK of 1.067 and ⬃1.15 for industrial Pb, we conclude that despite the introduction of unleaded petrol, the relative contribution of alkyllead to airborne Pb still accounts for approximately onethird of atmospheric Pb pollution at our sampling site. With the complete phasing out of leaded petrol, a shift in isotopic composition is expected, as the relative contribution of industrial Pb will increase. The 206Pb/207Pb (208Pb/207Pb) ratios measured in aerosols from Grenoble, Avignon, and other sampling sites in France from 1995 to 1999 were 1.105 to 1.137 (2.381–2.410) (Bollho¨ fer and Rosman, 2001). Ratios are higher than those reported for the early 1980s and those measured in leaded petrol. Assuming that the Pb isotopic signature of leaded petrol in France has not changed significantly over the past ⬃20 yr, the increase in isotopic ratios indicates an increasing relative contribution from industrial sources, which now amounts to ⬃70% in Avignon and ⬃40% in Grenoble. The highest ratios were measured in Germany, where airborne Pb concentrations were lowest. Although alkyllead is produced in Germany (Roskill Report, 1996; Landesgewer-

1382

A. Bollho¨ fer and K. J. R. Rosman

Fig. 2.

206

Pb/207Pb ratio and Pb concentration time series measured in Europe in 1998 and 1999.

beanstalt Bayern, pers. comm.), most probably from (Eastern) European and Russian Pb ores with relatively radiogenic signatures (Hopper et al., 1991), there is no leaded petrol available, and the Pb isotopic ratios reflect the composition of other sources. Comparing the concentrations at our German sampling sites with other European cities, we noticed that they are 5 to 10 times lower, an effect of the early introduction of unleaded petrol and stricter emissions regulations. Compared to airborne Pb isotopic ratios between 1980 and 1994 (Kober et al., 1999), the ratios are significantly more radiogenic in 1998. A relative increase in the amount of Pb from domestic industrial emissions or foreign emissions from the burning of alkyllead or industrial activity most likely influence the ratios measured in German aerosols and are responsible for the higher ratios. This influence is seen in the seasonal trends exhibited in time series from Plo¨ n and Constance. 3.4.2. Seasonal trend Time series of Pb isotopes in Moscow aerosols were taken in 1999 (Table 1). The 206Pb/207Pb (208Pb/207Pb) ratio of 1.160 ⫾ 0.002 (2.441 ⫾ 0.002) (95% confidence intervals, n ⫽ 6) is the same as ratios measured in other samples collected in Eastern

Europe in 1998 (Bollho¨ fer and Rosman, 2001) and are referred to as the Eastern European signature. The lack of seasonal variations suggests a very stable source in Moscow, which is most probably leaded gasoline or oil and coal combustion as suggested for the Czech Republic by Vile et al. (2000). Although there is some variability in Pb isotopic ratios measured at individual European sampling sites in 1998 (Fig. 2), the variations (95% confidence intervals, n ⫽ 6) are generally ⬍0.6%. No common seasonal pattern is evident at the French sampling sites, and also, the variations in Oxford are small. In Italy samples from Venice exhibit the lowest ratios in autumn, whereas Rome ratios are lowest in winter when concentrations are highest. The most obvious features can be observed at the German sampling sites. Isotopic ratios are highest in autumn and winter at Constance and Plo¨ n. In Constance, the higher ratios are correlated with slightly higher Pb concentrations. Figure 3 shows the general wind direction measured at the sampling site in Constance, with 0, 90, 180, and 270° indicating northerlies, easterlies, southerlies, and westerlies, respectively. The prevailing winds are westerlies, with some northeasterly influence in summer, autumn, and winter, although there is a negligible

Temporal stability in Pb isotopic signatures

1383

Fig. 3. Comparison of wind data measured in Constance (Martin Wessels, pers. comm.) with isotopic ratios measured in Constance (dotted line) and Plo¨ n (solid line), Germany; 0, 90, 180, and 270° indicate northerlies, easterlies, southerlies, and westerlies, respectively.

northeasterly influence in spring. The wind data indicate that Pb isotopic ratios, which represent a time-integrated measurement over 2 months, are higher with a northeasterly contribution during collection. 206Pb/207Pb data for the Plo¨ n sampling site are shown in the plot as well and follow a similar pattern. Although we cannot exclude local sources as being responsible for the pattern observed, we suggest that low ratios are due to transport of airborne Pb with Western European air masses. Higher ratios indicate a significant contribution from Eastern Europe, with isotopic ratios higher than average ratios measured in Western Europe. Westward mesoscale transport of pollution has been observed in southeastern Germany (Krautstrunk et al., 2000). A seasonal trend was evident for airborne Pb, Zn, Se, and other trace elements, with higher concentrations observed in winter, which was attributed to enhanced burning of coal and the transport of pollution from the Bohemian basin (Matschullat et al., 2000). Also, sampling sites in the European Arctic show higher pollutant concentrations in winter compared to summer. By the means of Pb isotopic fingerprinting, Rosman et al. (1999) and Aberg et al. (1999) have already shown that significant amounts of Pb are transported to southern Norway from Eastern Europe. Our data from Plo¨ n show that this transport is enhanced during winter, which is in general agreement with a recent study by Tuomenvirta et al. (2000). It was shown that although westerlies are dominant throughout the year in FennoScandia, southerly and easterly flow may occur in winter and spring, respectively, and therefore, may also influence the airborne Pb isotopic ratios at our sampling site in northeast Germany. 3.5. United States and Canada 3.5.1. Long-term trend Time series of isotopic ratios measured in the United States and Canada are shown in Figure 4. In the United

States, lowest 206Pb/207Pb (208Pb/207Pb) ratios of 1.159 (⬃2.426) were measured in Berkeley, and ratios this low had not been reported previously (e.g., Sturges and Barrie, 1989; Flegal et al., 1993; Rosman et al., 1994). Also, from 1998 to 1999 there was a significant difference between isotopic ratios measured in aerosols from the East and the West Coasts of the United States. West Coast aerosols were less radiogenic than those from the East Coast and tended towards higher 208Pb/207Pb ratios. Generally, 206Pb/207Pb ratios measured in Canada are lower than U.S. ratios as reported earlier (e.g., Sturges and Barrie, 1987, 1989; Carignan and Gariepy, 1995). A decrease from 1.21 to 1.20 in 206Pb/207Pb isotopic ratios between 1983 and 1992 in trade westerlies in the Sargasso Sea has been reported by Veron et al. (1999b), whereas the composition of the trade easterlies remained comparatively constant at ⬃1.16. The decrease in isotopic composition of the westerlies coincides with a drastic decrease of vehicle Pb emissions in the United States from 62189 tonnes per year in 1980 to 1690 tonnes per year in 1990 (United States Environmental Protection Agency, 1996). By 1995 ⬃40% of the Pb emissions were from metal processing. This increasing influence of industrial Pb compared to alkyllead, which had very radiogenic ratios in the United States (Chow et al., 1975), is probably reflected in the further decrease in isotopic ratios measured in aerosols in our data. The 206Pb/207Pb ratios in the United States (1.19 ⫾ 0.02) are on average lower than those reported earlier. It seems plausible that due to the drastic decrease of airborne Pb levels in North American air, multiple industrial sources and long-range transport of pollution that went unnoticed in earlier studies can now be detected. For example, low ratios observed in Woods Hole might be due to the transport of pollution to the East Coast with the easterlies. On the other hand, emissions and the transport of pollution from China with the westerlies might account for the

1384

A. Bollho¨ fer and K. J. R. Rosman

Fig. 4. 206Pb/207Pb ratio and Pb concentration time series measured in the United States and Canada in 1998 and 1999.

lower 206Pb/207Pb and relatively enriched 208Pb/207Pb ratios observed on the West Coast (Bollho¨ fer and Rosman, 2001). 3.5.2. Seasonal trend Although the 206Pb/207Pb isotope ratio variability at the 95% confidence interval is relatively high at the individual sampling sites in the United States, i.e., from 0.5% at Berkeley and up to 2.7% in New York, the pattern is not seasonal except in Tampa, which exhibits highest radiogenic ratios during midsummer. In Canada, variations are ⬍0.6% (95% confidence interval) except for Winnipeg. In contrast to the densely populated European continent, where countries with comparatively strict emissions regulations adjoin countries where leaded petrol is still a major source of airborne Pb and emission regulations are poor, the Pb isotopic ratios in aerosols in the United States and Canada are more likely influenced by local point sources. Mesoscale transport of pollution on the order of 100 to 1000 km (from one country to the other) as observed in Germany, is less likely to have an effect, as overall Pb levels in North American air are low (United States Environmental Protection Agency, 1996).

For example, Tampa is home to one of the 10 biggest lead emitters in the United States: the Tampa Electric Co. (United States Environmental Protection Agency, 1996). The highly radiogenic Pb isotopic ratios in midsummer could be explained by an increasing demand for electrical power by the public due to the frequent use of air conditioners during this period. This would lead to elevated emissions from the power plant, which, provided the fossil fuels show more radiogenic Pb isotopic ratios, might lead to an increase in isotopic ratios in atmospheric aerosols in Tampa. At Winnipeg (Canada) large seasonal variations have been observed. These variations are similar to those in Quebec reported by Simonetti et al. (2000). They relate the low isotopic ratios measured in summer 1998 in precipitation sampled close to Montreal to smelter emissions located upwind from Sudbury. The variability implies a less-radiogenic additional source in winter, which could be local smelters refining relatively unradiogenic Canadian Pb. Sturges and Barrie (1989) report significantly lower isotope ratios for back trajectories passing the Northern Ontario Cu smelter regions. Similar reasons are most likely responsible for the less-radiogenic ratios observed at our sampling site.

Temporal stability in Pb isotopic signatures 4. CONCLUSIONS

Our study shows that the Pb isotopic composition of major source regions of Pb pollution, such as Eastern Europe, South Africa, Mexico, or Japan (Pacyna et al., 1995), are reasonably stable with time. This stability complements recent studies by Bollho¨ fer and Rosman (2000, 2001) that characterizes different regions on the globe according to their Pb isotopic ratios. However, North America, regarded as a major polluter of the Northern Hemisphere in the 1970s (Rosman et al., 1993), now shows greater variability in isotopic ratios than reported earlier. This reflects the increasing relative influence of other sources, as alkyllead has been eliminated from petrol. This makes emissions from the United States more difficult to identify on the one hand, but on the other hand, the overall low Pb levels allow other sources of pollution reaching the United States to be detected. Without a detailed knowledge of local meteorology and the local economy, it remains difficult to explain some of the temporal and seasonal patterns observed at our sampling sites. However, variations occurring at some of the sampling sites, especially in Europe, can be explained with the aid of meteorologic and literature data. Our study demonstrates the need for continuous, integrated sampling of Pb isotopes to establish the further usefulness of Pb isotopes as a source tracer. Acknowledgments—We thank colleagues and associates who assisted in collecting the aerosol samples: M. Wessels, J. Fietzke, K. van der Welde, P. and B. Flachaire, A. Feran, S. Caroli, C. Barbante, G. Cozzi, M. Frank, G. Ramendik, R. Cheyene, A. M. Bakhit, S. B. Saad, H. Hidaka, R. Tadday, T. Stokes, E. Callahan, F. Wuerthwein, B. PeuckerEhrenbrink, N. Frank, A. Rostami, M. Whiticar, K. and E. Deusch, L. K. Kadonaga, R. Krouse, A. L. Norman, R. Hesslein, M. A. Wadleigh, M. Cortez, M. Basei, A. N. Sial, S. Hong, M. Ruppenthal, M. Diantoro, A. Dick, J. P. Candelone, C. Devonport, R. Maas, R. Cirocco, B. Parsonson, and J. Wengrove. All members of the TIMS laboratory of the Centre of Excellence in Mass Spectrometry are thanked for their valuable input throughout. This work was funded by a large Australian Research Council (ARC) research grant with additional support from the Australian Bureau of Meteorology and the CSIRO Division of Atmospheric Research through the Cape Grim Baseline Air Pollution Station program. Gratefully acknowledged are three anonymous reviewers who greatly improved the manuscript. Associate editor: K. Kyser REFERENCES Aberg G., Pacyna J. M., Stray H., and Skjelkvale B. L. (1999) The origin of atmospheric lead in Oslo, Norway, studied with the use of isotopic ratios. Atmos. Environ. 33, 3335–3344. Alleman L. (1997) Apport des isotopes stables du plomb au suivi des traces metalliques en Mediterranee et en Atlantique Nord. The`se de doctorat, Universite´ de Droit, d’Economie et des Sciences d’AixMarseille. Associated Octel (1995) Worldwide Gasoline Survey 1992–1993. OPP No. 4. Bacon J. R., Jones K. C., McGrath S. P., and Johnston A. E. (1996) Isotopic character of lead deposited from the atmosphere at a grassland site in the United Kingdom since 1860. Environ. Sci. Technol. 30, 2511–2518. Bollho¨ fer A. and Rosman K. J. R. (2000) Isotopic source signatures for atmospheric lead in the Southern Hemisphere. Geochim. Cosmochim Acta 64, 3251–3262. Bollho¨ fer A. and Rosman K. J. R. (2001) Isotopic source signatures for atmospheric lead: The Northern Hemisphere. Geochim. Cosmochim. Acta 65, 1727–1740.

1385

Bollho¨ fer A., Chisholm W., and Rosman K. J. R. (1999) Sampling aerosols for lead isotopes on a global scale. Anal. Chim. Acta 390, 227–235. Carignan J. and Gariepy C. (1995) Isotopic composition of epiphytic lichens as a tracer of the source of atmospheric lead emissions in Southern Quebec, Canada. Geochim. Cosmochim. Acta 59, 4427– 4433. Chiaradia M. and Cupelin F. (2000) Behaviour of airborne lead and temporal variations of its source effects in Geneva (Switzerland): Comparison of anthropogenic versus natural processes. Atmos. Environ. 34, 959 –971. Chow T. J., Snyder C. B., and Earl J. L. (1975) United Nations FAO and International Atomic Energy Association Symposium, Vienna, Austria (IAEA-SM-191/4), proceedings, pp. 95–108. Doe B. R. (1970) Lead Isotopes. pp. 137, Springer Verlag, Berlin, Heidelberg, New York. Flegal A. R., Maring H., and Niemeyer S. (1993) Anthropogenic lead in Antarctic seawater. Nature 365, 242–244. Grousset F. E., Quetel C. R., Thomas B., Buat-Menard P., Donard O. F. X., and Bucher A. (1994) Transient Pb isotopic signatures in the Western European Atmosphere. Environ. Sci. Technol. 28, 1605– 2608. Hamester M., Stechmann H., Steiger M., and Dannecker W. (1994) The origin of lead in urban aerosols—A lead isotope study. Sci. Total Environ. 146/147, 321–323. Hopper J. F., Ross H. B., Sturges W. T., and Barrie L. A. (1991) Regional source discrimination of atmospheric aerosols in Europe using the isotopic composition of lead. Tellus 43B, 45– 60. Jaffe D., Anderson T., Covert D., Kotchenruther R., Trost B., Danielson J., Simpson W., Berntsen T., Karlsdottir S., Blake D., Harris J., Carmichael G, and Itsushi U. (1999) Transport of Asian air pollution to North America. Geophys. Res. Lett. 26, 711–714. Kober B., Wessels M., Bollho¨ fer A., and Mangini A. (1999) Pb isotopes in sediments of Lake Constance, Central Europe constrain the heavy metal pathways and the pollution history of the catchment, the lake and the regional atmosphere. Geochim. Cosmochim. Acta 9, 1293–1303. Krautstrunk M., Neumann-Hauf G., Schlager H., Klemm O., Beyrich F., Corsmeier U., Kalthoff N., and Kotzian M. (2000) An experimental study on the planetary boundary layer transport of air pollutants over East Germany. Atmos. Environ. 34, 1247–1266. Maring M., Settle D., Buat-Menard P., Dulac F., and Patterson C. C. (1987) Stable lead isotope tracers of air mass trajectories in the Mediterranean region. Nature 330, 154 –156. Matschullat J., Maenhaut W., Zimmermann F., and Fiebig J. (2000) Aerosol and bulk deposition trends in the 1990’s, Eastern Erzgebirge, Central Europe. Atmos. Environ. 34, 3213–3221. Monna F., Lancelot J., Coiudace I. W., Cundy A. B., and Lewis J. T. (1997) Pb isotopic of airborne particulate material from France and the Southern United Kingdom: Implications for Pb pollution sources in urban areas. Environ. Sci. Technol. 31, 2277–2286. Mukai H., Furuta N., Fujii T., Amb Y., Sakamoto K., and Hashimoto Y. (1993) Characterization of sources of lead in the urban air of Asia using ratios of stable lead isotopes. Environ. Sci. Technol. 27, 1347–1356. Mukai H., Tanaka A., and Fujii T. (1994) Lead isotope ratio of airborne particulate matter as tracers of long-range transport of air pollutants around Japan. J. Geophys. Res. 99, 3717–3726. Nriagu J. O. and Pacyna J. M. (1988) Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 333, 134 –139. Pacyna J. M., Scholtz M. T., and Li Y-F. (1995) Global budget of trace metal sources. Environ. Rev. 3, 145–159. Roskill Report (1996) The Economics of Lead, 5th Edition. Roskill Information Services Ltd., London, England. Rosman K. J. R., Chisholm W., Boutron C. F., Candelone J. P., and Gorlach U. (1993) Isotopic evidence for the source of lead in Greenland snows since the late 1960s. Nature 362, 333–334. Rosman K. J. R., Chisholm W., Boutron C. F., Candelone J. P., and Hong S. (1994) Isotopic evidence to account for changes in the

1386

A. Bollho¨ fer and K. J. R. Rosman

concentration of lead in Greenland snow between 1960 and 1988. Geochim. Cosmochim. Acta 58, 3265–3269. Rosman K. J. R., Ly C., and Steinnes E. (1999) Spatial and temporal variation in isotopic composition of atmospheric lead in Norwegian moss. Environ. Sci. Technol. 32, 2542–2548. Simonetti A., Garie´ py C., Carignan J., and Poissant L. (2000) Isotopic evidence of trace metal sources and transport in eastern Canada as recorded from wet deposition. J. Geophys. Res. 105, 12263–12278. Sturges W. T. and Barrie L. A. (1987) Lead 206/207 isotope ratios in the atmosphere of North America: Tracers of American and Canadian emissions. Nature 329, 144 –146. Sturges W. T. and Barrie L. A. (1989) The use of stable lead 206/207 ratios and elemental composition to discriminate the origins of lead in aerosols at a rural site in Eastern Canada. Atmos. Environ. 23, 1645–1657. Tuomenvirta H., Alexandersson H., Drebs A., Frich P., and Nordli

P. O. (2000) Trends in Nordic and. Arctic temperature extremes and ranges. J. Climate 13, 977–990. United States Environmental Protection Agency (1996) National Air Pollutant Emission Trends, 1990 –1995. Office of Air Quality Planning and Standards, Research Triangle Park, NC., EPA-454/R-96 – 007. Veron A., Flament P., Bertho M. L., Alleman L., Flegal R., and Hamelin B. (1999a) Isotopic evidence of pollutant lead sources in Northwestern France. Atmos. Environ. 33, 3377–3388. Veron A. J., Church T. M., Rivera-Duarte I., and Flegal A. R. (1999b) Stable lead isotopic ratios trace thermohaline circulation in the subarctic North Atlantic. Deep-Sea Res. II, 919 –935. Vile M. A., Kelman Wieder R., and Novak M. (2000) 200 years of Pb deposition throughout the Czech Republic: Patterns and sources. Environ. Sci. Technol. 34, 12–21.