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

Abstract

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.

SO2 Photoaging Enhances the Surface Conversion of NO2-to-HONO on Elemental Carbon

“…The fast oxidation of OC with reducing capacity is the key factor leading to the rapid deactivation of soot. In soot particles, however, element carbon (EC) composed of inorganic carbonaceous material with graphene-like structure also is a type of important composition. , Especially, a most recent work from Chen et al elucidated that the NO 2 -to-HONO conversion reduction on soot is mainly driven by the synergistic role of both elemental carbon (EC) and OC rather than OC alone . This indicates that the surface chemistry involving plentiful EC can also contribute HONO production.…”

“…16,17 Especially, a most recent work from Chen et al elucidated that the NO 2 -to-HONO conversion reduction on soot is mainly driven by the synergistic role of both elemental carbon (EC) and OC rather than OC alone. 18 This indicates that the surface chemistry involving plentiful EC can also contribute HONO production. Moreover, differing from the irreversible photooxidation of OC, some recent work verified that either light irradiation or the surface oxidation of SO 2 can remarkably elevate the reducing capacity of oxidated carbonaceous materials.…”