Atmospheric pollutant dispersion around an urban area using trace metal concentrations and Pb isotopic compositions in epiphytic lichens

Epiphytic lichen samples were collected in and around a city of 300 000 inhabitants in NE France to study the dispersion and fallout of Cd, Cu, Zn and Pb. Lichens hanging in small tree branches within a 15 km radius from the city centre, together with bus air filter aerosols and unleaded gasoline samples, were analysed to evaluate the relative contribution of anthropogenic metals. On an average, Pb and Cd concentrations and calculated enrichment factors in lichens showed a radial distribution from downtown, decreasing rapidly within 4 or 5 km and then less so towards more rural areas, whereas it is more diffuse for Cu and Zn despite the more elevated values close to the city centre. Metal concentrations can be interpreted in the light of major wind directions and/or secondary pollution sources to the atmosphere. Lichens sampled in the city and close to high traffic roads had 206Pb/207Pb ratios of about 1.13. This ratio was consistently higher (up to 1.157) for lichens sampled at only five kilometres from the city centre. On the other hand, urban aerosols (on bus air filters) have homogeneous 206Pb/207Pb ratios of 1.153±0.0031.153±0.003. The variation in Pb isotopic composition in lichens can be interpreted as the result of mixing between different industrial sources and old Pb pollution from leaded gasoline combustion re-emitted into the atmosphere. Combining elemental concentrations and isotopic ratios allowed a better understanding of the atmospheric deposition of metals and related sources in urban areas.