Heterogeneous formation of EPFRs from aromatic adsorbates on the carbonaceous particulate matter

Abstract

Pyrogenic carbonaceous materials of atmospheric particulate matter (PM) have recently aroused intensive interest, due to their critical role in the formation of environmentally persistent free radicals (EPFRs). This study focuses on the heterogeneous formation of EPFRs on the graphene oxide (GO), the dominant component of carbonaceous PM. First-principles calculations have been performed to investigate the interaction between 2-chlorophenol (2-CP), an important precursor of EPFRs, and GO to find the stable adsorption configurations. The climbing image nudged elastic band (CI-NEB) method and Dimer method were employed to explore the decomposition route of 2-CP to generate different types of EPFRs on the GO surface. The results indicate that aromatic precursors tend to undergo homolytic fission rather than heterolytic splitting to form EPFRs induced by surface oxygen-containing groups and carbon defects on the carbonaceous PM. Surface OH groups and water, both of which are related to ambient humidity, play essential roles in producing EPFRs. The formation mechanism of EPFRs over GO unveiled in this study, which represents dramatic differences from the conventional mechanism on the metal oxide surfaces, may account for a previously overlooked source of EPFRs encountered in atmospheric PM.