Aging Oxidation Reactions on Atmospheric Black Carbon by OH Radicals. A Theoretical Modeling Study

Aging processes of black carbon (BC) particles require knowledge of their chemical reactivities, which have impact on cloud condensation nuclei (CCN) activities, radiant properties and health problems related to air pollutions. In the present work, interactions between several OH radicals with BC (modeled with a coronene molecule) were calculated by using DFT and PM6 codes as described by Mysak et al. Water interaction with BC was also included. Results show that OH radical adsorption is preferred on border sites, independent of the theoretical method employed. Potential energy curves using DFT(TPSS-D3) approach for OH chemisorption showed small-energy barriers, as reported in previous work with PM6. A dipole moment has been created, and the hydrophobic coronene surface is transformed to hydrophilic after the first OH chemisorption. Several stages were found in the BC aging by OH radicals, thus (a) Hydroxylation of coronene by several OH radical would lead to H abstractions directly from the substrate. (b) Abstraction of H from adsorbed OH (at the border sites) drives a C-C bond breaking and the formation of carboxyl groups. (c) Hydrogen abstraction from carboxyl group produces decarboxylation (CO2 plus water) as experimentally obtained. Potential energy curves of one of the reactive path were calculated with the PM6 method. The formation of products was confirmed using DFT. Coronene interaction with O-2 was also considered to have a realistic atmospheric environment.