Enhancement of N2O4 on porous glass at room temperature:: A key intermediate in the heterogeneous hydrolysis of NO2?

The heterogeneous hydrolysis of NO2 at surfaces in the atmosphere is believed to be a significant source of MONO, a key OH precursor in urban areas. However, the mechanism of this reaction is not known. The uptake of 2.9 Torr of NO2 in N-2 at a total pressure of 508 Torr on a porous glass surface with varying amounts of surface-adsorbed water was studied using FTIR at 294 K. The ratio of N2O4 to NO2 was enhanced on the glass surface relative to the gas phase. On a relatively dry surface, the formation of surface-adsorbed HNO3 was observed over a period of similar to 20 h, likely due to the reaction with small amounts of water on the surface. Gas-phase NO and N2O were also generated, When larger amounts of water were initially present on the surface, surface-adsorbed HNO3 was formed immediately, as well as gas-phase NO, N2O, and MONO. Although the NO2 concentrations used in the present studies are much larger than those found in the atmosphere, this work suggests that N2O4, should be considered as a key intermediate in the heterogeneous NO2 to form HONO.