A method for measuring total aerosol oxidative potential (OP) with the dithiothreitol (DTT) assay and comparisons between an urban and roadside site of water-soluble and total OP

Abstract

An automated analytical system was developed for measuring the oxidative potential (OP) with the dithiothreitol (DTT) assay of filter extracts that include both water-soluble and water-insoluble (solid) aerosol species. Three approaches for measuring total oxidative potential were compared. These include using methanol as the solvent with (1) and without (2) filtering the extract, followed by removing the solvent and reconstituting with water, and (3) extraction in pure water and performing the OP analysis in the extraction vial with the filter. The water extraction method (the third approach, with filter remaining in the vial) generally yielded the highest DTT responses with better precision (coefficient of variation of 1–5 %) and was correlated with a greater number of other aerosol components. Because no organic solvents were used, which must be mostly eliminated prior to DTT analysis, it was easiest to automate by modifying an automated analytical system for measuring water-soluble OP developed by Fang et al. (2015). Therefore, the third method was applied to the field study for the determination of total OP. Daily 23 h filter samples were collected simultaneously at a roadside (RS) and a representative urban (Georgia Tech, GT) site for two 1-month study periods, and both water-soluble (OPWS-DTT) and total (OPTotal-DTT) OP were measured. Using PM2. 5 (aerodynamic diameter < 2.5 µm) high-volume samplers with quartz filters, the OPWS-DTT-to-OPTotal-DTT ratio at the urban site was 65 % with a correlation coefficient (r) of 0.71 (N = 35; p value < 0.01), compared to a ratio of 62 % and r = 0. 56 (N = 31; p value < 0.01) at the roadside site. The same DTT analyses were performed, and similar results were found using particle composition monitors (flow rate of 16.7 L min−1) with Teflon filters. Comparison of measurements between sites showed only slightly higher levels of both OPWS-DTT and OPTotal-DTT at the RS site, indicating both OPWS-DTT and OPTotal-DTT were largely spatially homogeneous. These results are consistent with roadway emissions as sources of DTT-quantified PM2. 5 OP and indicate that both soluble and insoluble aerosol components contributing to OP are largely secondary.