Surface Uptake of Exogenous Volatile Organic Compounds Enhances the NO₂-to-HONO Conversion on Soot

Heterogeneous reduction of nitrogen dioxides (NO2) on soot has been suggested to be a source of nitrous oxide (HONO) in the atmosphere. However, conventional wisdom says that the contribution of NO2 reduction on soot to HONO formation is limited due to the rapid deactivation of surface reductive sites in the atmosphere. On the contrary, we show that adsorbed anthropogenic and biogenic volatile organic compounds (VOCs), collectively called exogenous VOCs (EVOCs), can persistently promote the heterogeneous conversion of NO2 to HONO on soot. The surface uptake of NO2 driven by H2O, coupled with the hydrogen reduction of NO2 supported by EVOCs on elemental carbon (EC), is proposed to be the key pathway causing the persistent production of HONO. Furthermore, field observation conducted during biomass burning seasons in the North China Plain also showed that the presence of EVOCs promotes HONO production significantly. Our results imply that the continuous adsorption and conversion of EVOCs on soot surfaces may have a remarkable enhancing impact on the persistent conversion of NO2 to HONO in the atmosphere.