Sulfate content correlates with iron concentrations in ambient air pollution particles

Current levels of air pollution particles in American cities can increase human mortality. Both the mechanism of injury and the responsible components are not known. We have postulated that injury following air pollution particle exposure is produced through a generation of oxygen-based free radicals catalyzed by metals present in the particles. As a result of its abundance in the atmosphere, sulfate appears to potentially be the most successful ligand to complex metal cations. We tested the hypothesis that (1) some portion of iron in ambient air pollution particles is present as sulfate and (2) this relationship between iron and sulfate results from the capacity of the latter to function as a ligand to mobilize the metal from the oxide. Concentrations of sulfate and iron in acid extracts of 20 filters (total suspended particles) from Utah were measured using inductively coupled plasma emission spectroscopy. In vitro oxidant generation was also measured using thiobarbituric acid-reactive products of deoxyribose. There were significant correlations between sulfate content, iron concentrations, and oxidant generation. Agitation of calcium sulfate with iron(III) oxide produced concentrations of water-soluble, catalytically active iron. We conclude that some portion of iron in the atmosphere is present as a sulfate. This relationship between sulfate and iron concentrations is likely the product of SO42- functioning as a ligand for the meal after its mobilization from an oxide by photoreduction. There were also associations between sulfate content, iron concentrations, and oxidant generation. However, sulfates had no capacity to support electron transport unless they were present with iron.