Ambient Size Distributions and Lung Deposition of Aerosol Dithiothreitol-Measured Oxidative Potential: Contrast between Soluble and Insoluble Particles.

Ambient particulate matter may upset redox homeostasis, leading to oxidative stress and adverse health effects. Size distributions of water-insoluble and water-soluble OP (dithiothreitol assay, measure of oxidative potential per air volume) are reported for a roadside site and an urban site. The average water-insoluble fractions were 23% and 51%, and 37% and 39%, for fine and coarse modes at the roadside and urban sites, respectively, measured during different periods. Water-soluble OP was unimodal, peaked near 1-2.5 μm due to contributions from fine-mode organic components plus coarse-mode transition metal ions. In contrast, water-insoluble OP was bimodal, with both fine and coarse modes. The main chemical components that drive both fractions appear to be the same, except that for water-insoluble OP the compounds were absorbed on surfaces of soot and non-tailpipe traffic dust. They were largely externally mixed and deposited in different regions in the respiratory system, transition metal ions predominately in the upper regions and organic species, such as quinones, deeper in the lung. Although OP per mass (toxicity) was highest for ultrafine particles, estimated lung deposition was mainly from accumulation and coarse particles. Contrasts in the phases of these forms of OP deposited in the respiratory system may have differing health impacts.