Influence of anthropogenic combustion emissions on the deposition of soluble aerosol iron to the ocean: Empirical estimates for island sites in the North Atlantic

The results of several recent studies challenge the reigning paradigm that continental soil dust provides the only significant atmospheric source of dissolved iron to the surface ocean. This evidence includes correlations between the operational solubility of aerosol iron and atmospheric loadings of black carbon and aluminum-normalized vanadium and nickel, each of which are associated with emissions from the combustion of fossil fuel oil. These observations suggest that the relative solubility of aerosol iron, hence the eolian flux of soluble iron to the surface ocean, may be significantly impacted by anthropogenic oil combustion products. Using recent field data from the Bermuda region, we have developed an empirical method to estimate the solubility of aerosol iron using bulk aerosol concentrations of Fe, V and Al. We apply this method to a large body of published data from the AEROCE program for North Atlantic island sites on Tenerife, Barbados, Bermuda and Ireland, where the relative proportions of anthropogenic aerosols range from minor to major, respectively. Our aerosol iron solubility estimates suggest that anthropogenic emissions contribute approximately 70% and 85% of the annual dry deposition of soluble iron to the surface ocean near Bermuda and Ireland, respectively, implying that human activities have profoundly affected the iron budget of the North Atlantic region. The annual mean dry deposition of soluble iron at Barbados and Izana is dominated by soil dust. The anthropogenic contribution at these two sites ranges from 12% to 30% and is highly dependent on the soil dust solubility of Fe employed in the model. The low end (∼12%) estimate appears to be more representative of these high-dust sites.