Distribution and geochemistry of aerosols in the tropical north Atlantic troposphere: Relationship to Saharan dust

The spatial distribution and water-soluble chemical composition of aerosols in the tropical North Atlantic troposphere were examined during the NASA Global Tropospheric Experiment/Atlantic Boundary Layer Experiment (GTE/ABLE 1) conducted in the vicinity of Barbados, West Indies. During the June 1984 study period the atmospheric aerosol at 1.5-3.5 km altitudes, the region composing the so-called Sahara air layer, consisted primarily of Saharan dust. The aerosol mass concentration reached 400 #g (m 3 STP)-x in the mainstream of a massive Saharan dust outbreak which passed through the longitude of Barbados between June 13 and 18, 1984. Measurements of the particle size distribution of the Saharan dust indicated that about 85% of the mass was contained in the 1- to 8-#m diameter range; a distribution that was consistent among the three dust outbreaks observed during ABLE. Enrichment of water-soluble aerosol chemical species in Sahara air compared to clean tropospheric air at >4 km altitude, ranged from about tenfold for NH4 + to more than a factor of 400 for NO3-. Calculations suggest that Saharan dust represented a significant source of airborne NO3-, non-sea-salt SO42-, and PO, 3- in boundary layer air over the tropical North Atlantic. Estimates of the annual input of Saharan dust-related chemical species to the equatorial Atlantic suggested that deposition of mineral aerosol to oligotrophic surface waters may be an important source of the nutrient species NO 3 - and PO, 3-. This atmospheric input probably occurs principally in the form of dry deposition. Aerosols over the world's oceans originate from both natural and anthropogenic sources. Various estimates of global aerosol production, although based on limited data (especially for natural sources), strongly suggest that mobilization of natural eolian material greatly exceeds anthropogenic-related emissions (see, e.g., Prospero et el. [1983]). Additional evidence demonstrating the importance of natural eolian sources has come from geochemical studies of deep-sea sediments. These studies suggest that soil material transported from arid regions by wind is primarily responsible for the distribution of certain clay minerals in oceanic sediments [Griffin and Goldberg, 1968; Windom, 1975; Chester et al., 1979; Prospero, 1981a]. It is important therefore to determine the spatial and temporal variability of major eolian sources and the subsequent long-range geochemical transport in the global atmosphere. The first direct measurements of soil dust in the atmosphere over the tropical North Atlantic were obtained by Delany et al. [1967]. It was hypothesized from studies of soil dust (mineral aerosol) at Barbados, West Indies, that Saharan dust originating from North Africa was a major source of natural eolian material to the tropical North Atlantic troposphere. Over the past two decades extensive documentation of spatial and temporal variability of Saharan mineral aerosol over the tropical North Atlantic has been obtained [Prospero and Carlson, 1970, 1972; Prospero et al., 1970, 1981; Chester et al.,