Delivery of anthropogenic bioavailable iron from mineral dust and combustion aerosols to the ocean

Abstract

Atmospheric deposition of anthropogenic soluble iron (Fe) to the ocean has been suggested to modulate primary ocean productivity and thus indirectly affect the climate. A key process contributing to anthropogenic sources of soluble Fe is associated with air pollution, which acidifies Fe-containing mineral aerosols during their transport and leads to Fe transformation from insoluble to soluble forms. However, there is large uncertainty in our estimate of this anthropogenic soluble Fe. Here, we, for the first time, interactively combined laboratory kinetic experiments with global aerosol modeling to more accurately quantify anthropogenic soluble Fe due to air pollution. We firstly examined Fe dissolution kinetics of African dust samples at acidic pH values with and without ionic species commonly found in aerosol water (i.e., sulfate and oxalate). We then constructed a new empirical scheme for Fe release from mineral dust due to inorganic and organic anions in aerosol water, by using acidity as a master variable. We implemented this new scheme and applied an updated mineralogical emission database in a global atmospheric chemistry transport model to estimate the atmospheric concentration and deposition flux of soluble Fe under preindustrial and modern conditions. Our improved model successfully captured the inverse relationship of Fe solubility and total Fe loading measured over the North Atlantic Ocean (i.e., 1-2 orders of magnitude lower Fe solubility in North African- than combustion-influenced aerosols). The model results show a positive relationship between Fe solubility and water soluble organic carbon (WSOC)/Fe molar ratio, which is consistent with previous field measurements. We estimated that deposition of soluble Fe to the ocean increased from 0.05-0.07 Tg Fe yr-1 in preindustrial era to 0.11-0.12 Tg Fe yr-1 in present days, due to air pollution. Over the High Nitrate Low Chlorophyll (HNLC) regions of the ocean, the modeled Fe solubility remains low for mineral dust (< 1 %) in a base simulation but is substantially enhanced in a sensitivity simulation, which permits the Fe dissolution for mineral aerosols in the presence of excess oxalate under low acidity during daytime. Our model results suggest that human activities contribute to about half of the soluble Fe supply to a significant portion of the oceans in the Northern Hemisphere, while their contribution to oceans in high latitudes remains uncertain due to limited understanding of dust Fe dissolution under pristine conditions.