Role of dust alkalinity in acid mobilization of iron

摘要

Atmospheric processing of mineral aerosols by acid gases (e.g., SO₂,HNO₃, N₂O₅, and HCl) may play a key role in thetransformation of insoluble iron (Fe in the oxidized or ferric (III) form)to soluble forms (e.g., Fe(II), inorganic soluble species of Fe(III), andorganic complexes of iron). On the other hand, mineral dust particles have apotential of neutralizing the acidic species due to the alkaline bufferability of carbonate minerals (e.g., CaCO₃ and MgCO₃). Here wedemonstrate the impact of dust alkalinity on the acid mobilization of ironin a three-dimensional aerosol chemistry transport model that includes amineral dissolution scheme. In our model simulations, most of the alkalinedust minerals cannot be entirely consumed by inorganic acids during thetransport across the North Pacific Ocean. As a result, the inclusion ofalkaline compounds in aqueous chemistry substantially limits the irondissolution during the long-range transport to the North Pacific Ocean: onlya small fraction of iron (<0.2%) dissolves from hematite in thecoarse-mode dust aerosols with 0.45% soluble iron initially. On the otherhand, a significant fraction of iron (1–2%) dissolves in the fine-modedust aerosols due to the acid mobilization of the iron-containing mineralsexternally mixed with carbonate minerals. Consequently, the modelquantitatively reproduces higher iron solubility in smaller particles assuggested by measurements over the Pacific Ocean. It implies that thebuffering effect of alkaline content in dust aerosols might help to explainthe inverse relationship between aerosol iron solubility and particle size.We also demonstrate that the iron solubility is sensitive to the chemicalspecification of iron-containing minerals in dust. Compared with the dustsources, soluble iron from combustion sources contributes to a relativelymarginal effect for deposition of soluble iron over the North Pacific Oceanduring springtime. Our results suggest that more comprehensive data forchemical specificity of iron-rich dust is needed to improve the predictivecapability for size-segregated soluble iron particles.