Saharan aerosols over the tropical North Atlantic — Mineralogy

Abstract

Quantitative aerosol samples were collected on filters during three major Saharan dust outbreaks as they passed across the tropical North Atlantic Ocean during the summer of 1974. The sampling sites were: Sal Island, Cape Verde Islands; Barbados, West Indies; Miami, Florida. The dominant mineral group was mica—illite; the mean concentration at each station ranged from 54% at Sal Island to 64% at Barbados. Quartz was the second most abundant mineral with concentrations ranging from 20% at Sal Island to 14% at Barbados and Miami. Also present were kaolinite, chlorite, microcline, plagioclase and calcite — all at concentrations less than 10%. At each sampling site, the composition of the dust was relatively constant during all three dust episodes. However, there were significant differences between the mean composition at Sal Island and those at Barbados and Miami; the most notable difference was that the concentration of quartz decreased and that of the clays increased during the transit of the Atlantic. These changes are ascribed to the more rapid removal of quartz (relative to the clays) because of its relatively larger mass median diameter and, consequently, its greater settling velocity in the atmosphere. The dusts did not contain any primary halite or gypsum; abundant amounts of gypsum were measured in the samples but this gypsum was found to be a conversion product of calcite reacting with atmospheric sulfur species. The mineralogy of the Saharan dusts is similar to that of the clay fraction of sediments in the tropical North Atlantic with the exception of montmorillonite which was barely detectable in the dusts; this discrepancy may be ascribed, at least in part, to differences in analytical techniques. The mineralogy of Saharan dust is similar to that of an average shale. This similarity can be attributed to genetic factors and to the effect of winnowing.