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

Photosensitized renoxification of HNO3 isfound to produceHONO in an unexpectedly high yield, which has been considered an importantsource for atmospheric HONO. Conventionally, the production of HONOis ascribed to the secondary photolysis of the primarily formed NO2. In this study, by using humic acid (HA) as a model environmentalphotosensitizer, we provide evidence of the direct formation of NO2 in its electronic excited state (NO2*) as a keyintermediate during the photosensitizing renoxification of HNO3. Moreover, the high HONO yield originates from the heterogeneousreaction of the primarily formed NO2* with the co-adsorbedwater molecules on HA. Such a mechanism is supported by the increaseof the product selectivity of HONO with relative humidity. Furtherluminescence measurements demonstrate clearly the occurrence of anelectronic excited state (NO2*) from photolysis of adsorbedHNO(3) on HA. This work deepens our understanding of theformation of atmospheric HONO and gives insight into the transformationof RNS. This work proposes a new mechanismthat enhances the understandingof the formation of atmospheric HONO from the renoxification of nitrate.