Dark NO₂ Reduction on a Graphene Surface with Implications for Soot Aging and HONO Formation

Soot, primarily composed of elemental carbon (EC) and organic carbon (OC), is ubiquitous in PM_2.5. In the atmosphere, the heterogeneous interaction between NO₂ and soot is not only an important pathway driving soot aging but also of central importance to nitrous acid (HONO) formation. It is commonly believed that the surface redox reaction between reductive OC and NO₂ dominates the night aging of soot and the conversion of NO₂ to HONO. However, completely differing from the currently popular explanation, we find here that the redox reaction between EC and NO₂ can also drive the conversion of NO₂ to HONO during soot aging. By combining in situ experiments with density functional theory (DFT) calculations, we proposed that the surface carbon vacancy defects on graphite/graphene-like EC should be a type of potential primary adsorption and reactive sites inducing the heterogeneous reduction of NO₂. We suggested a new mechanism that NO₂ is reduced to form HONO on surface vacancy defects through the splitting of H₂O molecules, and the carbon atoms adjacent to surface vacancy are simultaneously oxidized to form hydroxyl-functionalized EC. This novel finding provides insights into the chemical mechanism driving the NO₂-to-HONO conversion and rapid soot aging, which expands our knowledge of the heterogeneous chemistry of soot in the atmosphere.