The photochemical production of HONO during the heterogeneous hydrolysis of NO2

The heterogeneous hydrolysis of NO2 in thin water films, a major source of HONO and hence OH radicals in polluted urban atmospheres, has been previously reported to be photoenhanced (H. Akimoto, H. Takagi and F. Sakamaki, Int. J. Chem. Kinet., 1987, 19, 539, ref. 1) which has important implications for OH production both in environmental chambers and in the lower atmosphere. We report here studies of the impact of 320-400 nm radiation on HONO formation during the heterogeneous NO, hydrolysis at 296 K. The experiments were carried out in a borosilicate glass cell using long path Fourier transform infrared (FTIR) spectroscopy with three initial NO2 concentrations (20, 46, and 54 ppm) at relative humidities of 33, 39, and 57%, respectively. Nitrous acid was first allowed to accumulate from NO2 hydrolysis in the dark, and then the mixture of reactants and products was irradiated. The measured concentration-time profiles of the gases were compared to the predictions of a kinetics model developed for this system. The initial loss of HONO upon irradiation was consistent with its photolysis and known secondary gas phase chemistry without any photoenhancement. While the fundamental NO2 heterogeneous hydrolysis is not itself photoenhanced, there is clear evidence in these experiments for the generation of gas phase HONO by photolysis of adsorbed HNO3 formed during the heterogeneous hydrolysis. The mechanisms and atmospheric implications of HONO as well as NO2 formation by the photolysis of surface-adsorbed HNO3 are discussed.