Deposition Rate Uf Particulate and Dissolved Aluminilll Derived from Saharan Dust in Precipitation at Miami, Florida

Precipitation was collected for a I-year period in Miami, Florida. The deposition rate of Al in samples containing Saharan dust was 10.1 ~g/cm2; this flux is equivalent to a mineral deposition rate of 126 ~g/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 ~g/L; if normalized to the total volume of rain that fell that year, the average would have a minimum value of 3.0 ~g/L. The dissolved Al fraction (defined as all Al that passes through a 0.45-vm 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 % solubili ty 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 x lOll 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 x lOll 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.