Bio-Availability of Iron Inputs to the Ocean Driven by Aerosol Chemistry in Dust

Tuesday, May 13, 2014 - 07:00
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Chemical transformation of iron (Fe) in mineral dust aerosols is a dominate source of the bio-available (soluble) Fe to the open ocean. Observational studies suggest a large variability in the aerosol dissolved Fe fraction within dust particles (0.01% to 80%): generally low near dust source regions and increases during transport. Despite the high temporal and spatial variability in observed Fe solubility, this fraction has been conventionally assumed constant (1-2%) in Earth System models (ESMs). Simulations of the varying Fe solubility and its effects on ocean ecosystem require the understanding of how anthropogenic aerosols mobilize insoluble Fe in mineral dust. Using a chemical transport model, we examine the Fe solubility in dust with a detail aerosol chemistry scheme that treats sulfate, nitrate, and ammonium reacting with alkaline dust minerals. The calculated fraction of dissolved Fe strongly depends on chemical speciation of dust aerosols and aerosol chemistry. Assuming a calcite-rich Asian dust composition (11% CaCO3, 5.5% MgCO3, and 5% Fe2O3), the model predicts a much lower Fe solubility (0.4%~1%) than that of aluminosilicate-rich particles (illite without CaCO3 and MgCO3), 1% to 10%. The estimated Fe solubility for calcite-rich dust composition is in good agreement with the cruise data for coarse particles (diameter>1.25åµm) over both the Asian-dust-dominate North Pacific (0.5å±0.02% vs. 0.6å±0.2%) and the African-dust-dominate Tropical Atlantic (0.5% vs. 0.3å±0.7%). However, results with aluminosilicate-rich particles have better estimates of Fe solubility in fine dust aerosols over Pacific (2.0å±0.2% vs. 1.7å±0.8%) and Atlantic (1.2% vs. 2.2å±7.1%). Sensitivity of Fe solubility to the mineralogy of iron is examined. A higher iron content in dust and a faster dissolution rate at the second dissolution stage for hematite lead to substantially higher soluble iron deposition than that for illite. In addition, bio-availability of iron to the ocean is also found to be sensitive to dust source functions. We are working on adding the acid-based iron dissolution scheme to the CAM5 aerosol module. A review on soluble Fe predictions from several ESMs including the CESM1.0 and a couple of chemical transport models implemented with detailed Fe dissolution schemes will be presented.

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