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

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 Cu 2+ 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 Cu 2+ showed either antagonism or synergism with BrC against DTT consumption, Cu 2+ displayed antagonism with most BrC against • OH generation. Fluorescence quenching experiments provided evidence of complexation between Cu 2+ and water-soluble organic compounds (WSOCs, from ambient PM 2.5 ), which was influenced by Cu 2+ 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 Cu 2+ . Our results suggest the interactions of BrC and copper play a crucial role in PM OP and highlight complexation in evaluation of PM OP.