Urban PM2.5 oxidative potential: Importance of chemical species and comparison of two spectrophotometric cell-free assays

Oxidative potential (OP) of particulate matter (PM) - defined as the capacity of PM to oxidize target molecules generating reactive oxygen species (ROS) - has been proposed as a more health relevant metric than PM mass. In this study two cell-free methods were used to assess the OP of PM filters collected at an urban site and to evaluate correlation with PM mass and PM composition. Among the different assays existing, two inexpensive and user-friendly methods were used both based on spectrophotometric measurements of depletion rate of target reagents oxidized by redox-active species present in PM. One assay measures the consumption of dithiothreitol (OPDTT) and the other the ascorbate (OPAA). Although both assays respond to the same redox-active species, i.e., quinones and transition metals, no correlations were found between OPDTT and OPAA responses to compounds standard solutions as well as to ambient samples. When expressed in relation to air volume, OPDTT m-3 strongly correlates with PM2.5 mass whereas no correlation was found for OPAA m-3 with PM2.5. When expressed on mass basis, both OPDTT ug-1 and OPAA ug-1 show a strong dependence on the sample composition, with higher OP for summer samples. OPDTT m-3 were highly correlated with the determined metals (Cu, Zn, Cr, Fe, Ni, Mn) whereas OPAA m-3 showed only moderate correlation with Cu and Mn. Thus, the two assays could potentially provide complementary information on oxidative potential characteristic of PM. Consequently, the combination of the two approaches can strengthen each other in giving insight into the contribution of chemical composition to oxidative properties of PM, which can subsequently be used to study health effects.