Water-soluble iron correlation to primary speciated organics in low-emitting vehicle exhaust

Abstract. Iron is the most abundant transition element in airborne PM, primarily existing as Fe(II) or Fe(III). Generally, the fraction of water-soluble iron is greater in urban areas compared to areas dominated by crustal emissions. To better understand the origin of water-soluble iron in urban areas, tail-pipe emission samples were collected from 32 vehicles with emission certifications of Tier 0, low emission vehicles (LEV I), tier two low emission vehicles (LEV II), ultralow emission vehicles (ULEV), superultra-low emission vehicles (SULEV), and partial-zero emission vehicles (PZEV). Components quantified included gases, inorganic ions, EC/OC, total metals and water-soluble metals. In addition, naphthalene and various classes of C12–C18 intermediate volatility organic compounds (IVOC) were quantified for a subset of vehicles: aliphatic, single ring aromatic (SRA), and polar (material not classified as either aliphatic or SRA). Iron solubility in the tested vehicles ranged from 0–82 % (average = 30 %). X-ray absorption near edge structure (XANES) spectroscopy showed that Fe(III) was the primary oxidation state in 14 of the 16 tested vehicles, confirming that the presence of Fe(II) was not the main driver of water-soluble Fe. Correlation of water-soluble iron to sulfate was insignificant, as was correlation to every chemical component, except to naphthalene and some C12–C18 IVOCs with R2 values as high as 0.56. A controlled benchtop study confirmed that naphthalene, alone, increases iron solubility from soils by a factor of 5.5 and that oxidized naphthalene species are created in the extract solution. These results suggest that the large driver in water-soluble iron from primary vehicle tail-pipe emissions is related to the organic composition of the PM, indicating the organic fraction of the PM influences the behavior and solubility of iron.