The heterogeneous hydrolysis of NO 2 at surfaces in the atmosphere is believed to be a significant source of HONO, a key OH precursor in urban areas. However, the mechanism of this reaction is not known. The uptake of 2.9 Torr of NO 2 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 N 2 O 4 to NO 2 was enhanced on the glass surface relative to the gas phase. On a relatively dry surface, the formation of surface-adsorbed HNO 3 was observed over a period of ∼20 h, likely due to the reaction with small amounts of water on the surface. Gas-phase NO and N 2 O were also generated. When larger amounts of water were initially present on the surface, surface-adsorbed HNO 3 was formed immediately, as well as gas-phase NO, N 2 O, and HONO. Although the NO 2 concentrations used in the present studies are much larger than those found in the atmosphere, this work suggests that N 2 O 4 should be considered as a key intermediate in the heterogeneous hydrolysis of NO 2 to form HONO.
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Barney et al. (1999) studied this question.