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

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.