Dissolved input of manganese to the ocean: Aerosol source

Authors

  • Cécile Guieu,

  • Robert Duce,

  • Richard Arimoto


Abstract

Aerosol particle samples representative of polluted air, dust-laden air, and clean marine air were collected in marine regions and used in a series of dissolution studies. These samples were exposed to seawater for varying lengths of time and to deionized (Milli-Q®) water at various values of pH. The percentage of aerosol Mn dissolved in Milli-Q® water increased from 55 to 80% between pH 8 and pH 2 for pollution aerosols. Less dissolution occurred with the mineral aerosol particles, for which the dissolved Mn increased from 25 to 50% between pH 8 and pH 2. As similar behavior is found for particles collected in clean marine air, we conclude that the dissolution process for aerosol particles from a remote marine area, where crustal Mn dominates pollution Mn, is controlled by the background mineral content. The kinetics of Mn dissolution in seawater are rapid for all the samples: a concentration plateau is reached after 10 min or less of exposure. Release of dissolved Mn from polluted aerosols in seawater was approximately twice the value obtained with mineral particles (55 and 30%, respectively). Apparently, no additional Mn dissolution beyond that which has clearly taken place in rain occurs when rainwater enters the ocean. A positive relationship evidently exists between the total Mn concentration of the aerosol and the dissolved concentration after exposure in seawater. Extrapolating the relationship from the available data suggests that the dissolved saturation value is approximately 60 nmol L−1. Considering the different behavior found for the different types of particles, at least two cases must be considered when assessing mass balances or calculating atmospheric fluxes of Mn to the ocean. Based on the results of our dissolution studies, the resulting dissolved fluxes of Mn of atmospheric origin during a pulse of Saharan dust range from 0.16 to 0.33 μmol m−2d−1 over the duration of the pulse. The total dissolved Mn flux from anthropogenic sources in western North Europe is estimated to be 0.3 μmol m−2d−1. Fluxes of dissolved Mn of mineral and anthropogenic origin resulting from this calculation in the two examples selected are of the same order of magnitude. The spatial and temporal patterns of these different sources and the dynamics of the upper water column have to be taken into account to estimate quantitatively the impact of atmospheric input on Mn concentrations in the water column.

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