An important difference between anthropogenic and natural emissions is that anthropogenic emissions are often the result of high temperature processes, which result in the release of lead into the fine fraction of aerosols. These fine particles can be transported long distances before being deposited, mixing on hemispheric and global scales [Bollhofer and Rosman (2000); Bollhofer and Rosman (2001); Bollhofer and Rosman (2002); Boutron (1995); Simonetti et al. (2003)]. Atmospheric lead fluxes follow the primary anthropogenic sources and prevailing winds; with greatest lead deposition occurring in densely populated urban areas but reaching and affecting remote places as well. For example, Duce et al. 
Table 3 Worldwide emissions of lead from mobile sources in 1995 (in tonnes, taken from [Pacyna and Pacyna (2001)]).
North America South America Australia
19 507 6 852 32 996 10 414 4 866 2 000
19 507 11 992 44 293 15 780 7 270 2 000
found atmosphere-ocean fluxes over the North Atlantic Ocean being five-fold higher (0.08-1.03mgm-2 y-1 versus 0.02-0.2mgm-2 y-1) than those found over the Pacific Ocean highlighting the significance of North American emissions.
Most studies assessing sources of atmospheric aerosols have been conducted on limited regional scale in urban and remote areas. A classic study around an urban area was conducted in Canada. The 206Pb/207Pb ratio of integrated Canadian and US aerosols demonstrated that 24-43% of the anthropogenic lead aerosols close to Toronto were derived from the US [Sturges and Barrie (1987)]. A more recent study conducted in Sicily showed substantial 'natural' lead pollution (e.g. from volcanoes) can occur, accounting up to 30% at Mt. Etna and 80% at Vulcano Island [Monna et al. (1999)]. An early study in a remote area has been conducted at the tropical South Pacific island of American Samoa. At this site, Patterson and Settle constructed an atmospheric mass balance demonstrating that natural volcanic emissions and soil dusts accounted for about 1% of the total lead fluxes [Patterson and Settle (1987)].
Three recent studies by Bollhofer and Rosman [2000-2002] defined the extent to which lead isotopic ratios in aerosols vary on a global scale. They showed first that there are significantly different lead isotope ratios on a regional scale, for example, in the Northern Hemisphere, during the 1990s, the least radiogenic compositions were found in France and Spain (206Pb/207Pb between 1.097 and 1.142) and the most radiogenic in the United States (206Pb/207Pb between 1.173 and 1.231). Second, countries where leaded petrol is still marketed (e.g. Asia, Africa and Eastern Europe), automobile emissions dominate the lead isotope signature. In the US, however, the impacts of the phasing out of leaded petrol are already detectable. Whereas in the early 1990s the 206Pb/207Pb ratio was fairly uniform across the country, recent samples show that east (1.173-1.231) and west (1.1591.188) coast aerosols differed. The reason for this is unclear but one suggestion is that there is long distance transport of aerosols from Asia that are characterised by high lead concentrations and low 206Pb/207Pb ratios [Bollhofer and Rosman (2000); Bollhofer and Rosman (2001)]. Time series from 38 globally distributed sites revealed significant seasonal variations at sampling sites close to Eastern Europe that probably reflect an enhanced westward transport of pollution in winter. They also showed that the temporal variability in Canada and North America is now larger than before due to decreased airborne lead levels coupled with an increase in industrial sources. Temporal variations on mainland Australia are comparatively
small with a typical range of 0.2% in 206Pb/207Pb ratio and isotopic ratios that indicate that leaded petrol is still a major source. Figure 6 shows the long-term data measured at selected sites in South America, South Africa and Mexico. The ratios at Punta Arenas and South Africa show similar 206Pb/207Pb ratios and indicate a common supplier of alkyllead. There is also a notable increase of 206Pb/207Pb ratio at both sites compared to data from 1994 [Bollhofer and Rosman (2000)], which could be due to a relative increase from industrial sources because of the slow phasing out of leaded gasoline in both regions. In Recife in north eastern Brazil, seasonal variations are noticeable with low ratios during the southern autumn and higher ratios in spring. lead isotopes in aerosols collected in Mexico City are very stable and show variation of below 0.3%.
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