Direct Formation of Electronic Excited NO₂ Contributes to the High Yield of HONO during Photosensitized Renoxification

Photosensitized renoxification of HNO3 is found to produce HONO in an unexpectedly high yield, which has been considered an important source for atmospheric HONO. Conventionally, the production of HONO is ascribed to the secondary photolysis of the primarily formed NO2. In this study, by using humic acid (HA) as a model environmental photosensitizer, we provide evidence of the direct formation of NO2 in its electronic excited state (NO2*) as a key intermediate during the photosensitizing renoxification of HNO3. Moreover, the high HONO yield originates from the heterogeneous reaction of the primarily formed NO2* with the co-adsorbed water molecules on HA. Such a mechanism is supported by the increase of the product selectivity of HONO with relative humidity. Further luminescence measurements demonstrate clearly the occurrence of an electronic excited state (NO2*) from photolysis of adsorbed HNO3 on HA. This work deepens our understanding of the formation of atmospheric HONO and gives insight into the transformation of RNS

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