The Effects of Particle Size, Relative Humidity, and Sulfur Dioxide on Iron Solubility in Atmospheric Particulate Matter

The current study focuses on studying how iron (Fe) solubility is affected by particle size, relative humidity, and exposure to sulfur dioxide (SO2). Fe, the most abundant transition metal in atmospheric particulate matter, plays a critical role in the atmospheric sulfur cycle and is a micronutrient for phytoplankton in remote regions of the ocean. To mimic oceanic particles, iron-containing minerals (hematite, magnetite, goethite, and illite) were resuspended with sodium chloride and size-segregated on Teflon filters into five different size fractions: 10-2.5 μm, 2.5-1.0 μm, 1.0-0.5 μm, 0.5-0.25 μm, and <0.25 μm. Mineral phases were then exposed to 5 ppm SO2 in air at marine environment humidity (>80%) and arid environment humidity (24%). Trials with no SO2 ­were also performed as comparisons. Total Fe was determined by using microwave-assisted acid digestion and soluble Fe was determined by extracting the samples in a simulated cloud water buffer (pH 4.25, 0.5 mM acetate, 0.5 mM formate, and 0.2 mM ammonium nitrate). Both total and soluble Fe concentrations were determined via inductively-coupled plasma mass spectrometry (ICP-MS). We found that, as particle size decreased, Fe percent solubility increased for hematite, magnetite, and goethite. The percent solubility of Fe in these mineral phases steadily increased from 0.5-10% as particle size decreased. In contrast, the Fe percent solubility in illite was relatively constant for the largest four size fractions but increased dramatically in the smallest size fraction. The percent solubility of Fe in illite ranged from 5-20% as the particle size decreased. Additionally, increased Fe solubility was linked to increased relative humidity with higher percent solubility generally observed in all mineral phases for the samples exposed at the higher humidity. No correlation was observed for the effects of the SO2 on Fe percent solubility. The likely lack of Fe-SO2 interactions were also supported by synchrotron-based x-ray spectroscopy. These results help further the knowledge of how the solubilization of particulate Fe is affected by atmospheric transport.