Trace elements in the atmosphere of American Samoa: Concentrations and deposition to the tropical South Pacific

As part of the SEAREX (Sea‐Air Exchange) Program, samples of aerosol particles, rain, and dry deposition were collected from an 18‐m‐high tower on American Samoa, in the tropical South Pacific. The mineral aerosol concentration at Samoa (∼10 ng m −3 ) was among the lowest ever recorded for the near‐surface troposphere; it was far lower and also less variable than the mean concentrations observed during the high‐ and low‐dust seasons at Enewetak (∼1000 and ∼100 ng m −3 , respectively), the site of a previous SEAREX experiment in the tropical North Pacific. Atmospheric concentrations of Cu, I, Pb, Sb, Se, and Zn were higher than those expected from mineral aerosol or atmospheric sea salt. These anomalously enriched elements (AEEs) generally exhibited lower concentrations at Samoa, but for Se and I, which may have important biological sources in the oceans, the differences in concentrations at the two sites were small. Higher enrichments of the AEEs relative to oceanic and crustal sources occurred at Samoa, suggesting that the atmospheric residence times of these elements are longer than those of mineral particles or that their sources are more widespread than those of mineral dust. Wet deposition appears to be quantitatively more important than dry deposition for the air‐to‐sea transfer of mineral dust, and recycled sea spray evidently accounts for a significant fraction of the airsea exchange of the enriched trace elements.