Dispersal of atmospheric lead in northeastern North America as recorded by epiphytic lichens

Abstract

Epiphytic lichens collected from trees within northeastern North America (ca. 500,000 km 2) have been used as natural filters of airborne lead in order to document the spatial distribution of this metal in the atmosphere and identify its principal sources. Lichens record an integrated signal over a few years of atmospheric fall-out and thus minimise any signals due to variable (seasonal) atmospheric circulation patterns, and biases related to discrete events focusing atmospheric pollution from a single point source. The Pb/Al ratios measured in lichens are 20–600 times higher than that of upper crustal rocks, indicating that most of their Pb is of anthropogenic origin. Hence, their Pb isotopic composition is directly related to that of the pollution sources. The 206Pb/ 207Pb values for the lichens vary from 1.146 to 1.206 and these are interpreted as results of mixing between Canadian and USA industrial lead. Pb isotope ratios of lichens define a coherent geographic distribution since samples from the US have high 206Pb/ 207Pb, typical of US industrial Pb, whereas lichens sampled in northern Québec have the lowest 206Pb/ 207Pb, typical of Canadian industrial Pb. Lichens sampled along the St. Lawrence Valley have intermediate isotopic composition, which is interpreted as a result of mixing between USA and Canadian sources. USA industrial Pb contributes 75–35% of the total atmospheric Pb in this area and the contribution decreases from Montréal towards the Gulf of St. Lawrence. Overall, geographic Pb isotope gradients may be sharp and indicate a strong control by both sources and prevailing meteorological conditions. From these results, it is possible to draw the first Pb isotopic distribution map of atmospheric Pb pollution over northeastern North America.