The interplay of Brown carbon (BrC) surrogates and copper: Implications for the oxidative potential of ambient...

Atmospheric particulate matter (PM) poses adverse effects on human via producing reactive oxygen species (ROS). Oxidative potential (OP, ability to generate ROS) can be induced by various chemicals in PM, while their interplay remains poorly characterized. Here, we systematically assessed influences of Cu²⁺ on OP of Brown carbon (BrC) (e.g., imidazoles) using dithiothreitol (DTT) assay. Results showed DTT consumption rate exhibited an initial rise and later decline (0.25 -0.56 µM/min) along with increase of BrC concentration (0.1 - 2 µM), while no general trend was observed for ^•OH formation. Although Cu²⁺ showed either antagonism or synergism with BrC against DTT consumption, Cu²⁺ displayed antagonism with most BrC against ^•OH generation. Fluorescence quenching experiments provided evidence of complexation between Cu²⁺ and water-soluble organic compounds (WSOCs, from ambient PM_2.5), which was influenced by Cu²⁺ concentration. Further parallel factor analysis of spectra showed that polycarboxylate-type humic acid-like substances (complexation site number (n): 0.46, complexation equilibrium constant (k): 0.51) and fulvic acid-like compounds (n: 0.42, k: 0.62) were the main components in WSOCs that complexed with Cu²⁺. Our results suggest the interactions of BrC and copper play a crucial role in PM OP and highlight complexation in evaluation of PM OP.