Estimates of atmospheric-processed soluble iron from observations and a global mineral aerosol model: Biogeochemical implications

Desert dust deposition to the ocean may be a significant source of biogeochemically important elements, specifically iron. The bioavailability of iron in the oceans requires it to be in a soluble form, and because atmospheric iron in desert dust is typically insoluble, understanding the atmospheric processes that convert insoluble iron to more soluble forms is essential. Understanding these relationships is especially important in remote ocean regions where iron may be the limiting nutrient. Observations of soluble iron from 2001 cruise-based aerosol measurements over the Atlantic and Pacific Oceans ranged from 0 to 45% (mean of 4 ± 9%) in the fine mode (<2.5 μm in diameter) and 0 to 87% (mean of 2 ± 10%) in the coarse mode. We test two simple hypotheses of soluble iron enhancement in the atmosphere using a global model of mineral aerosols. The first method assumes that iron solubility increases as iron is exposed to solar radiation, approximating photoreduction reactions that are important pathways for enhancement of soluble iron in the presence of acidic solutions. The second process imitates cloud processing of iron by increasing the amount of soluble iron when the mineral aerosol comes into contact with a cloud. Both methods resulted in similar average magnitudes of percent soluble iron compared to observations but did not capture specific events or have sufficient variability, perhaps because the model does not include aerosol interactions between species other than mineral dust or other processes that may be important.