Inhalation of particulate matter (PM) is related to adverse health effects and PM-induced oxidative stress is one important reason. The oxidative potential (OP) of PM has been attributed mainly to transition metals (TMs) while quinones and atmospheric Humic-like substances (HULIS) also make some contributions. Previous studies often treat each PM component as an independent part in estimating the contribution to OP. However, certain PM fractions interact with each other, for example, the synergistic effects on hydroxyl radical (·OH) in Fe/quinones mixtures have gained some attention. Considering the abundant functional groups in HULIS, HULIS is hypothesized to affect OP reactivity by TMs through its ability of chelating TMs. However, these possible effects on the measured OP are...[ Read more ]

Inhalation of particulate matter (PM) is related to adverse health effects and PM-induced oxidative stress is one important reason. The oxidative potential (OP) of PM has been attributed mainly to transition metals (TMs) while quinones and atmospheric Humic-like substances (HULIS) also make some contributions. Previous studies often treat each PM component as an independent part in estimating the contribution to OP. However, certain PM fractions interact with each other, for example, the synergistic effects on hydroxyl radical (·OH) in Fe/quinones mixtures have gained some attention. Considering the abundant functional groups in HULIS, HULIS is hypothesized to affect OP reactivity by TMs through its ability of chelating TMs. However, these possible effects on the measured OP are rarely investigated. Work in this thesis is motivated by this research gap and summarized below:

● A moderate binding affinity of Cu with HULIS was measured using a fluorescence quenching method confirming the existence of chelation interaction of HULIS with TMs.

● The mixture effects of HULIS and certain TMs on the measured OP were probed using the cell-free dithiothreitol (DTT) assay (OP_DTT). Suppression effect was observed in mixtures of HULIS and a low level of Cu, while enhancement was found in mixtures of HULIS and a high level of Mn. This work demonstrates the importance and complex nature of mixture effects between atmospheric organics such as HULIS and Cu/Mn on the measured OP_DTT.

● As DTT assay does not directly measure the generation of reactive oxygen species (ROS), an OP assay that quantifies both ·OH formation (OP_·OH) and ascorbic acid (AA) depletion (OP_AA) was adopted to evaluate the mixture effects of atmospherically relevant organics and certain TMs. The experiments were conducted in the phosphate buffer containing AA in three types of organic-metal mixture systems. (1) For simple lab prepared metal (Fe and Cu) and organic solutions (i.e. carboxylic acids and imidazole derivatives), it was found that carboxylic acids enhanced the activities of TMs while imidazole derivatives suppressed them. (2) For mixtures of above-mentioned metals and HULIS extracted from ambient or biomass burning source samples, negligible interactions were observed in ambient HULIS-metal mixtures, while the biomass burning HULIS unambiguously affected the OP_AA and OP_·OH of Fe/Cu. The effect was metal-specific and was related to the different chemical compositions of HULIS. (3) For the mixtures of metal solution from ambient PM_2.5 (i.e. the hydrophilic fraction of water-extracted PM_2.5 from solid phase extraction) and ambient HULIS, a suppression effect was observed. In addition to the mixture effect, a good correlation between OP_AA and OP_·OH were observed while the proportion of OP_·OH/ OP_AA was found to differ among the redox reaction systems (i.e. Cu complex versus Fe complex, metal solution versus HULIS solution), indicating varying sensitivity of the OP_AA and OP_·OH.

In summary, we observed mixture effects in mixtures of atmospherically relevant organics and TMs in terms of the measured OP quantities (i.e. OP_DTT, OP_AA, OP_·OH). There are both consistency and difference between different OP endpoints. Our work demonstrates the importance and complexity of interactions between PM compositions and the advantages of co-measurement of ROS formation and antioxidant depletion for a better assessment of OP of PM.