Understanding the mechanism of NO2 interaction on semiconductor surfaces such as TiO2 is a key step in designing the catalytic processes for conversion of NO2 to useful products. In the present work, through density functional theory calculations and NEB simulations, we have performed a comprehensive electronic structure study and established the reaction steps for efficient conversion of NO2 to HONO on TiO2 surface in the presence of water vapor. We predict the dimerization of NO2 to form a metastable N2O4. The latter's dissociation to NO+and NO3- complexes occurs in two pathways: (i) direct disproportionation reaction and (ii) through formation of NO2+and NO2- intermediates followed by O transfer. The introduction of H2O on a NO2 chemisorbed surface leads to the formation of nitrous acid through the interaction of NO+ with the water. The reaction pathways leading to formation of nitrous and nitric acids are formulated. © 2020 American Chemical Society.