Impacts on iron solubility in the mineral dust by processes in the source region and the atmosphere: A review

Mineral dust is a complex entity containing a range of iron minerals including poorly crystalline to crystalline iron oxides to clay minerals. Important progress has been made to characterize iron mineralogical compositions in the dust recently. These include the quantification of the content of crystalline hematite and goethite, which appear to show a regional variation in North Africa as a result of the differences in the degree of chemical weathering. Fractional Fe solubility (dissolved to total iron, FFS) in the atmospheric aerosols has been reported to range from 0.1% to 80%. However, FFS is usually less than 0.5% in the non-atmospherically-processed dust, suggesting that FFS can be enhanced by atmospheric processes. One of the atmospheric processes, gravitational settling of dust, which has been previously hypothesized to cause the abovementioned enhancement of FFS during dust transport has been shown to be insignificant. Cycling of dust particles in the clouds, in which pH is usually higher than 4, and in the aerosol phase, in which pH is usually substantially lower, can significantly affect iron speciation and FFS. Laboratory experiments showed that a significant amount of iron (>0.5%) can only be solubilized in the dust when pH is lower than 4. These laboratory data suggest that acid processing rather than cloud processing might be a prime mechanism to cause an increase in FFS in the dust during transport. Further laboratory studies, field measurements, and modelling are needed to increase the ability of models to quantify the atmospheric processing of iron in the dust.