Lead precipitation fluxes at tropical oceanic sites determined from ²¹⁰Pb measurements

Concentrations of lead, ²¹⁰Pb, and ²¹⁰Po were measured in rain selected for least influence by local sources of contamination at several tropical and subtropical islands (Enewetak; Pigeon Key, Florida; and American Samoa) and shipboard stations (near Bermuda and Tahiti). Ratios expressed as ng Pb/dpm ²¹⁰Pb in rain were 250-900 for Pigeon Key (assuming 12% adsorption for ²¹⁰Pb and no adsorption for lead), depending on whether the air masses containing the analyzed rain came from the Caribbean or from the continent, respectively; about 390 for the northern Sargasso Sea downwind from emissions of industrial lead in North America; 65 for Enewetak, remote from continental emissions of industrial lead in the northern hemisphere; and 14 near Tahiti, a remote location in the southern hemisphere where industrial lead emissions to the atmosphere are much less than in the northern hemisphere. (The American Samoa sample yielded a higher ratio than Tahiti; the reason for this is not clear but may be due to local Pb sources.) The corresponding fluxes of lead to the oceans, based on measured or modeled ²¹⁰Pb precipitation fluxes, are about 4 ng Pb/cm²y for Tahiti, 10 for Enewetak, and 270 for the Sargasso Sea site, and between 110 to 390 at Pigeon Key.