Air-to-sea flux of soluble iron: is it driven more by HNO₃ or SO₂? – an examination in the light of dust aging

Abstract. Aeolian dust provides the major micronutrient of soluble Fe to organisms in certain regions of the global ocean. In this study, we conduct numerical experiments using the MOZART-2 atmospheric chemistry transport model to simulate the global distribution of soluble Fe flux and Fe solubility. One of the mechanisms behind the hypothesis of acid mobilization of Fe in the atmosphere is that the coating of acidic gases changes dust from hydrophobic to hydrophilic, a prerequisite of Fe mobilization. We therefore include HNO₃, SO₂ and sulfate (SO₄²⁻) as dust transformation agents in the model. General agreement in Fe solubility within a factor of 2 is achieved between model and observations. The total flux of soluble Fe to the world ocean is estimated to be 731–924×10⁹ g yr⁻¹, and the average Fe solubility is 6.4–8.0%. Wet deposition contributes over 80% to total soluble Fe flux to most of the world oceans. Special attention is paid to the relative role of HNO₃ versus SO₂ and sulfate. We demonstrate that coating by HNO₃ produces over 36% of soluble Fe fluxes compared to that by SO₂ and sulfate combined in every major oceanic basin. Given present trends in the emissions of NO_x and SO₂, the relative contribution of HNO₃ to Fe mobilization may get even larger in the future.