A Novel Mechanism for NO2-to-HONO Conversion on Soot: Synergistic Effect of Elemental Carbon and Organic Carbon

Soot, mainly composed of elemental carbon (EC) and organic carbon (OC), plays an important role in the formation of atmospheric nitrous acid (HONO) through the heterogeneous reaction with nitrogen dioxide (NO2). In this study, we found that fresh soot exhibits a much higher HONO yield than its EC or OC components alone. This does not support the previously proposed hydrogen-abstraction mechanism for HONO formation at a single reductive site on the soot surface. Based on our observations of infrared (IR) spectroscopy and flow tube experiments, we propose a new mechanism that involves the synergistic participation of two sites: one located at OC and the other at EC. They provide a proton and an electron necessary for NO2 reduction to HONO, respectively, via a proton-coupled electron transfer pathway. This mechanism highlights the important roles of OC and EC in HONO release from soot and underscores the significant implications of soot compositions in atmospheric oxidative capacity.

Automated summary

Soot, composed of elemental carbon (EC) and organic carbon (OC), plays a crucial role in the formation of atmospheric nitrous acid (HONO). Fresh soot exhibits a higher HONO yield than its EC or OC components alone, suggesting a new mechanism involving the synergistic participation of two sites: one located at OC and the other at EC. This mechanism highlights the importance of OC and EC in HONO release from soot.