Characterization of Soluble Iron in Urban and Rural Aerosols Using Synchrotron Technology and Online Measurements

Iron is one of the most abundant transition metals in atmospheric aerosols, and it exists in a variety of chemical forms. Water-soluble iron is of particular interest, because it catalyzes reactions that produce reactive oxygen species, which in turn influence the atmospheric oxidant capacity and human health. Aerosol derived water-soluble iron is also a vital nutrient for phytoplankton growth in vast areas of the ocean. To date, acid-processing of dust particles, photochemical reactions and combustion processes are hypothesized to be the predominant sources of soluble iron; however, the contribution of each source to soluble iron variability is uncertain. In this study, Teflon filters (24 hour) PM2.5 were collected in several rural and urban sites in the Southeast and analyzed using synchrotron based fluorescence X-ray microscopy and iron XAFS spectroscopy to provide information on oxidation state as well as elemental composition of iron containing aerosol particles. Oxidized and reduced iron particles were observed at all the sites, with the majority of the iron occurring as various iron oxides. For some study sites, simultaneous online measurements of soluble iron were made using a Particle into Liquid Sampler (PILS) and Liquid Waveguide Capillary Cell (LWCC). Comparison of solubility measurements with synchrotron data suggests a relation between iron solubility and aerosol particle chemistry.