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

Abstract. An automated analytical system was developed for measuring the oxidative potential (OP) with the dithiothreitol (DTT) assay of filter extracts that include both watersoluble 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 . 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 (OP WS-DTT ) and total (OP Total-DTT ) OP were measured. Using PM 2.5 (aerodynamic diameter < 2.5 µm) high-volume samplers with quartz filters, the OP WS-DTT -to-OP Total-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 OP WS-DTT and OP Total-DTT at the RS site, indicating both OP WS-DTT and OP Total-DTT were largely spatially homogeneous. These results are consistent with roadway emissions as sources of DTT-quantified PM 2.5 OP and indicate that both soluble and insoluble aerosol components contributing to OP are largely secondary.

Use of Dithiothreitol Assay to Evaluate the Oxidative Potential of Atmospheric Aerosols

“…As a strong reducing agent [19], DTT is oxidized to form disulfides in the DTT assay when electrons are transferred from DTT to molecular oxygen through the redox reactions that can be accelerated in the presence of catalytic components (e.g., quinones and transition metals) in PM [18,24]. Over time, the remaining reduced DTT can be quantitatively determined with colorimetric analyses by adding 5,5 dithiobis (2-nitrobenzoic acid) (DTNB, Ellman's reagent) to immediately form the chromophoric 2-nitro-5-thiobenzoic acid (TNB), which has a large absorption coefficient (14,150 M −1 cm −1 ) at 412 nm [27].…”

“…Most often, extracted PM samples are incubated with buffer solutions (pH 7-7.4) and the DTT reagent for a specified amount of time before the reaction is terminated by adding DNTB. Some studies allow incubation for up to 30 or 60 min before absorption measurement (i.e., a fixed period of time) [28], while other studies measure the DTT consumption at different time intervals and estimate the fitted slope for DTT consumption over a period of reaction time [24,54,133,134]. The methods of calculation could lead to inconsistent DTT results.…”

“…Generally, the term of OP DTT implies the chemical reactivity and potential toxicity of PM constituents in regard to their oxidative properties when considering PM exposure and the associated health effects [18,24]. While PM is currently regulated by mass concentrations to protect environmental and public health, particle size, number concentrations, and chemical composition are also imperative matrices that are directly linked to the adverse health outcomes [11,23,25,26].…”

Development and field testing of an online instrument for measuring the real-time oxidative potential of ambient particulate matter based on dithiothreitol assay

“…MCs were originally designed for collecting the water-soluble gases with Henry's constant (K H ) > 10 3 M atm −1 (Cofer et al, 1985;Spaulding et al, 2002;Hennigan et al, 2009); however, few studies (Anderson et al, 2008;King and Weber, 2013) have also used these devices for collecting the water-soluble fraction of the ambient particles. Hennigan et al (2009) estimated the collection efficiency of the MC as ∼ 95 % using nitric acid aerosols at a flow rate of 21 L min −1 . King and Weber (2013) also calculated the collection efficiency of the MC by operating it simultaneously with a PILS and measuring the sulfate collected by both systems.…”

Measurements of Oxidative Potential of Particulate Matter at Belgrade Tunnel; Comparison of BPEAnit, DTT and DCFH Assays

“…Moreover, the DTT activity in the water-soluble fraction mostly depends on the metal content and organic species from combustion [46]. According to the literature, there are references that deal with the issue of soluble/insoluble part of PM, different extraction procedures as well as the correlation between soluble and insoluble fraction [47][48][49][50].…”