Chemical signals of epiphytic lichens in southwestern North America; natural versus man-made sources for airborne particulates

Ambient airborne particulate matter (PM) in southwestern North America cons ists of naturally derived desert dust, plus anthropogenic inputs from sever al sources. Epiphytic lichens (Usnea sp.) in this region are a useful biomo nitor for the airborne PM because they derive nutrients and moisture largel y from incorporated atmospheric aerosols, and not by absorption from the ho st tree limb from which they are suspended. Using a broad-based sampling st rategy from southern Chihuahua, Mexico, to northern New Mexico, USA, we sho w that select elemental abundance ratios and lead isotopes from epiphytic l ichens are useful for distinguishing between sources of airborne PM, and fo r gauging anthropogenic inputs into desert ecosystems. Abundance patterns o f the trace elements La, Nd, and Sm in the lichens suggest origination from continental crust, but rare earth elements display a pronounced enrichment relative to the major element Fe by a factor of about 5. This enrichment a ppears related to geologic weathering, aeolian transport, and grain-size bi ases toward trace-element-rich mineral grains in the arid setting. Using th e metal Pb as an indicator of human inputs, epiphytic lichens typically sho w Pb enrichments by a factor of about 25-60 over typical upper crustal valu es. Regional-scale differences in Pb isotope ratios of these lichens relate to different pollutant sources in southwestern North America. (C) 1999 Pub lished by Elsevier Science Ltd. All rights reserved.