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Hydrolysis of NO 2 dimer is of great interest in atmospheric chemistry because it can form HONO, a major source of OH in polluted urban atmospheres. Herein, the reaction mechanism and kinetics for the hydrolysis of trans -ONONO 2 (the favorable route in the hydrolysis of NO 2 dimer) catalyzed by catalyst X (X = H 2 SO 4 , H 2 SO 4 ⋯ H 2 O, and (H 2 SO 4) 2) have been studied theoretically. The calculated results show that the addition of X not only increases the stabilization energy of the reactant complex, but also decreases the energy barrier by over 7.0 kcal⋅mol−1. Kinetic simulations show that H 2 SO 4 ⋯ H 2 O exerts the strongest catalytic role as compared with H 2 SO 4 , and (H 2 SO 4) 2 with its pseudo-first-order rate constant k t ′ (SW) respectively larger by 6–5 and 10–11 orders of magnitude. Additionally, compared with the rate constant of t -ONONO 2 + H 2 O reaction, the k t ′ (SW) of t -ONONO 2 + H 2 SO 4 ··· (H 2 O) 2 reaction increase by 13 times at 298 K. Therefore, the present work may suggest that formation of nitrous acid from the hydrolysis reaction of t -N 2 O 4 is promoted significantly by pre-existing aerosols of small clusters of sulfuric acid. Image 1 • A catalytic effect of H 2 SO 4 , H 2 SO 4 ⋯ H 2 O and (H 2 SO 4) 2 on the t -ONONO 2 + H 2 O reaction has been investigated. • All of H 2 SO 4 , H 2 SO 4 ⋯ H 2 O and (H 2 SO 4) 2 catalysts can substantially reduce the energy barriers of the reaction. • The H 2 SO 4 ⋯ H 2 O catalyst has the most obvious catalytic effect on the t -ONONO 2 + H 2 O reaction. [ABSTRACT FROM AUTHOR]