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. Iron is the most abundant transition element in airborne particulate matter (PM), primarilyexisting as Fe(II) or Fe(III). Generally, the fraction of water-soluble ironis greater in urban areas compared to areas dominated by crustal emissions.To better understand the origin of water-soluble iron in urban areas,tailpipe emission samples were collected from 32 vehicles with emissioncertifications of Tier 0 low emission vehicles (LEV I), Tier 2 lowemission vehicles (LEV II), ultralow emission vehicles (ULEVs),super-ultralow emission vehicles (SULEVs), and partial-zero emission vehicles(PZEVs). The components quantified included gases, inorganic ions, elementalcarbon (EC), organic carbon (OC), total metals, and water-soluble metals.Naphthalene and intermediate-volatility organic compounds (IVOCs) werequantified for a subset of vehicles. The IVOCs quantified contained 12 to 18carbons and were divided into three subgroups: aliphatic, single-ringaromatic (SRA), and polar (material not classified as either aliphatic orSRA). Iron solubility in the tested vehicles ranged from 0 % to 82 %(average  30  %). X-ray absorption near-edge structure (XANES)spectroscopy showed that Fe(III) was the primary oxidation state in 14 ofthe 16 tested vehicles, confirming that the presence of Fe(II) was not themain driver of water-soluble Fe. The correlation of water-soluble iron withsulfate was insignificant, as was correlation with every chemical componentexcept naphthalene and some C12–C18 IVOCs with  R2  values as high as0.56. A controlled benchtop study confirmed that naphthalene aloneincreases iron solubility from soils by a factor of 5.5 and that oxidizednaphthalene species are created in the extract solution. These resultssuggest that the large driver in water-soluble iron from primary vehicletailpipe emissions is related to the organic composition of the PM. Wehypothesize that, during the extraction process, specific components of theorganic fraction of the PM are oxidized and chelate the iron into water.

Water-soluble iron emitted from vehicle exhaust is linked to primary speciated organic compounds