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

Abstract

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 μg (m3 STP)−1 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‐μm 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 PO43− 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 NO3− and PO43−. This atmospheric input probably occurs principally in the form of dry deposition.