Deposition rate of particulate and dissolved aluminum derived from saharan dust in precipitation at Miami, Florida

Abstract

Precipitation was collected for a 1‐year period in Miami, Florida. The deposition rate of Al in samples containing Saharan dust was 10.1 μg/cm2, this flux is equivalent to a mineral deposition rate of 126 μg/cm2 per year, a value comparable to the mineral accumulation rate in sediments of the tropical North Atlantic. Mineral deposition rates in rain were highly variable, with 22% of the total occurring in 1 day and 68% occurring in 4 days in two separate dust episodes. The volume‐weighted average concentration of dissolved Al in dust‐related rain events was 9.8 μg/L; if normalized to the total volume of rain that fell that year, the average would have a minimum value of 3.0 μg/L. The dissolved Al fraction (defined as all Al that passes through a 0.45‐μm filter) ranged from 0.5 to 48%, with a volume‐weighted mean of 5%; solubility tended to increase with decreasing rain pH and decreasing mineral concentration. If the 5% solubility applies to all Saharan dust deposited in the tropical Atlantic, then the annual deposition rate of soluble Al in this region would be 2–8×1011 g, a rate commensurate with that carried by the Amazon; on a global basis the annual input to the oceans of soluble Al in dust would be 20–40×1011 g, a rate comparable to that carried by rivers. These atmospheric input rates of dissolved Al are sufficient to account for many of the distribution features of Al in the oceans.