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 measured concentrations, correlation matrices, enrichment factors, and the distribution of lead between dissolved and particulate fractions. In Niger, mean 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 Central African Republic, high individual sample (600) and mean (10.5) crustal enrichment factors indicate a noncrustal source for lead in rain. Very weak correlations between lead and other elements (e.g., Rb) for both total and dissolved fractions argue against a biogenic source and suggest an anthropogenic source. In Cameroon, individual sample and mean crustal enrichment factors are greater than 10, indicating a noncrustal source for lead in rainwater. We suggest the possibility of a biogenic lead source that would partly explain its correlation to Rb in dissolved and total rain fractions. For all stations, lead crustal enrichment factors decrease with increases in aluminium concentrations (i.e., mineral dust) and with increases in the fraction of soluble lead in rainwater. Thus lead in rain appears to be made up primarily of two types of particles; mineral dust particles with low solubility and highly soluble anthropogenic and/or biogenic particles.