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

A survey by Bollhoefer 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 percent in the Pb-206/Pb-207 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 U.S. 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 U.S. 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 percent in the Pb-206/Pb-207 ratio and isotopic ratios that indicate leaded petrol was still a major source of atmospheric Pb over the sampling period. (Author)