Characterization of iron speciation in urban and rural single particles using XANES spectroscopy and micro X-ray fluorescence measurements: investigating the relationship between speciation and fractional iron solubility

Soluble iron in fine atmospheric particles has been identified as a public health concern by participating in reactions that generate reactive oxygen species (ROS). The mineralogy and oxidation state (speciation) of iron have been shown to influence fractional iron solubility (soluble iron/total iron). In this study, iron speciation was determined in single particles at urban and rural sites in Georgia USA using synchrotron-based techniques, such as X-ray Absorption Near-Edge Structure (XANES) spectroscopy and microscopic X-ray fluorescence measurements. Soluble and total iron content (soluble + insoluble iron) of these samples was measured using spectrophotometry and synchrotron-based techniques, respectively. These bulk measurements were combined with synchrotron-based measurements to investigate the relationship between iron speciation and fractional iron solubility in ambient aerosols. XANES measurements indicate that iron in the single particles was present as a mixture of Fe(II) and Fe(III), with Fe(II) content generally between 5 and 35 % (mean: ∼25 %). XANES and elemental analyses (e.g. elemental molar ratios of single particles based on microscopic X-ray fluorescence measurements) indicate that a majority (74 %) of iron-containing particles are best characterized as Al-substituted Fe-oxides, with a Fe/Al molar ratio of 4.9. The next most abundant group of particles (12 %) was Fe-aluminosilicates, with Si/Al molar ratio of 1.4. No correlation was found between fractional iron solubility (soluble iron/total iron) and the abundance of Alsubstituted Fe-oxides and Fe-aluminosilicates present in single particles at any of the sites during different seasons, suggesting solubility largely depended on factors other than differences in major iron phases.