Photoenhancement of the nitrous acid formation in the surface reaction of nitrogen dioxide and water vapor: Extra radical source in smog chamber experiments

The surface reaction of NO₂ and H₂O vapor to emit HONO into the gas phase was studied in the evacuable and bakeable photochemical chamber under the irradiation of UV-visible light (? 290 nm). Kinetic analysis of the NO, NO₂, and HONO with the aid of computer modeling strongly suggested that the formation of HONO by the surface reaction is photoenhanced. When a linear regression was assumed, the photoenhancement factor defined by {(k′₂₁/k₂₁) ? 1} was expressed as (6.8 ± 2.5)k₁ under our experimental conditions, where k₁ is the primary photolysis rate of NO₂, and k₂₁, k′₂₁ are the second-order-equivalent rate constants of the HONO formation reaction in dark and under irradiation, respectively. The discussion was made that this photocatalitic enhancement of HONO formation would explain the nature of the extra OH radical flux in the smog chamber experiments, which has been discussed as “unknown radical source” and has still been unexplained by the surface dark reaction of NO₂ and H₂O to emit HONO.