Fine particle mass concentration is the metric used to characterize exposure of populations to ambient aerosols and to relate this exposure to the consequent health impacts. As an alternative to mass, recent research has focused on the application of potentially more relevant health-related aerosol properties, such as aerosol oxidative potential (OP). Fine particle OP determined with the dithiothreitol assay (OP DTT) has been linked to emergency department visits for cardiorespiratory diseases in recent studies. While bulk measurements of OP are becoming more common, size resolved data provide unique insights on sources of aerosol OP and atmospheric processes that affect OP. Here we present contrasts in the size distributions of water-soluble OP DTT and water-insoluble OP DTT, along with size distributions of other key aerosol species, and show the data are consistent with two main chemical species contributing to both soluble and insoluble OP DTT forms; transition metal ions and oxidized aromatic species, such as quinones. Size distributions of these two forms of OP DTT differ. Insoluble OP DTT follows the typical bimodal fine-coarse mode distribution, whereas the water-soluble distribution is unimodel with a peak near the fine-coarse mode minimum at 1 to 2.5 μm. In this study region, for both the soluble and insoluble forms, aerosol OP DTT is largely generated by atmospheric processing. This includes oxidation of organic components and metals mobilization by acidic particles, implying that aerosol toxicity, as measured by this assay, is largely secondary and affected by varied multiphase processes.