Single-particle oxidation state and morphology of atmospheric iron aerosolsle

Sixty-three iron-containing particles from five field campaigns (PELTI, ACE-Asia, MILAGRO (urban location and aloft on NCAR C-130), INTEX-B) were characterized with scanning transmission X-ray microscopy and near-edge X-Ray absorption fine structure L-edge spectroscopy (NEXAFS-STXM). Particle sizes ranged from 0.2 to 4.5 mu m and were found in many types of morphologies. Iron was found to exist in many different oxidation states with average Fe(II) to Fe( II) + Fe(III) ratios ranging from 0.0 to 0.73. Twenty-two inclusions and six agglomerations were found. For 29 particles, concurrent (spatially resolved) carbon K-edge absorption spectra were collected; X-ray images suggest that in some instances, there are clear phase barriers between iron and carbonaceous regions in agglomerations and irregular particles. Occurrences of Fe(II) fractions and organic functional group abundances among particles appeared primarily in two clusters, one group high in both values and the other low in both, though consistent correlations between the two variables within each particle were not observed. The reduced form of iron on particle surfaces was observed in 16 particles, possibly suggesting atmospheric processing. For this limited set of particles, neither inferred particle source nor surface processing was by itself a strong predictor of overall Fe(II) fraction, indicating that both are important for variables contributing to the observed Fe(II) fraction in the atmosphere. In addition, seven spherical particles from the ACE-Asia campaign showed an iron shell with an absence of iron toward the core. These particles have carbon spectra characteristics consistent with tarballs described by Posfai et al. (2004), Hand et al. (2005), and Tivanski et al. (2007), which were previously identified as homogeneous carbon spherules. In this work, NEXAFS-STXM has detected heterogeneities in iron distribution and redox state over individual particles, showing the existence of a variety of types of iron particles in the atmosphere. Such information can be useful in improving models of iron particles, including deposition and fate in seawater.