Lead in rainwaters from intertropical Africa: Natural or anthropogenic origin?

As part of an investigation of lead in precipitation, we sampled rainwater over a 2 year period at four locations in intertropical Africa. We clearly identified crustal dust and anthropogenic sources for lead in rain using me asured concentrations, correlation matrices, enrichment factors, and the di stribution of lead between dissolved and particulate fractions. In Niger, m ean and most individual sample crustal enrichment factors are less than 10, suggesting the dominance of the crustal dust source for lead in rainwater at that site. Nevertheless, the few enrichment factors greater than 10 and an estimate of the concentration of noncrustal lead in rainwaters from this site suggest anthropogenic sources may be important as well. In the Centra l African Republic, high individual sample (600) and mean (10.5) crustal en richment factors indicate a noncrustal source for lead in rain. Very weak c orrelations between lead and other elements (e.g., Rb) for both total and d issolved fractions argue against a biogenic source and suggest an anthropog enic source. In Cameroon, individual sample and mean crustal enrichment fac tors are greater than 10, indicating a noncrustal source for lead in rainwa ter. We suggest the possibility of a biogenic lead source that would partly explain its correlation to Rb in dissolved and total rain fractions. For a ll stations, lead crustal enrichment factors decrease with increases in alu minium concentrations (i.e., mineral dust) and with increases in the fracti on of soluble lead in rainwater. Thus lead in rain appears to be made up pr imarily of two types of particles; mineral dust particles with low solubili ty and highly soluble anthropogenic and/or biogenic particles.