Tracing sources of atmospheric pollution in Western Canada using the Pb isotopic composition and heavy metal abundances of epiphytic lichens

Abstract

The Pb isotopic composition and trace metal concentrations of epiphytic lichens collected from tree branches within northwestern North America are reported here, with a latitudinal coverage extending from the Beaufort Sea (Arctic circle) to the Canada–USA border. Overall, the trace metal concentrations and Pb isotope compositions correlate with latitudinal position, since lichens retrieved north of latitude 60°N are characterized by low enrichment factors (EF) (mainly between 10 and 30) for heavy metals (i.e. Pb, Zn) and radiogenic 206Pb/207Pb isotope values (∼1.170–1.180). Samples collected further south are characterized by higher EF for heavy metals and much lower Pb isotopic compositions (i.e. 206Pb/207Pb⩽1.150). Lichens retrieved in the immediate vicinity of major urban centers (i.e. Calgary, Alberta and Victoria, British Columbia) record distinct Pb isotopic values compared to the regional signal measured in adjacent (remote) samples. The total variation defined by the Pb isotopic compositions of the lichens may be attributed to the mixing of atmospheric particulates and aerosols derived from at least four end-member components, three anthropogenic and one natural. The latter is the predominant signal recorded in lichens retrieved north of 60°N, and is similar in Pb isotopic composition to a natural component identified in aerosols collected during the autumn season of 1994 at Alert (Canadian High Arctic). In contrast, samples collected further south reflect in-part mixing between Canadian and USA anthropogenic sources of atmospheric Pb. The third (unradiogenic) anthropogenic end-member most probably represents atmospheric emissions originating from one of the world's major Zn/Pb smelters located at Trail, British Columbia.