| Effects of ammonia on the homogeneous gas phase hydrolysis reactions of nitrogen dioxide (NO2) and sulfur dioxide (SO2), respectively, are investigated by quantum mechanical calculations using density functional theory and ab initio molecular orbital theory. The equilibrium geometric and thermodynamic parameters of all monomers and clusters in the two reaction systems are analyzed in the presence and absence of ammonia, respectively. It is found that ammonia plays an essential role in the formation of nitrous acid (HONO) from the gas-phase hydrolysis of NO 2. The nighttime accumulation of HONO is attributed to the removal of HNO3 from the gas phase by NH3 to form particulate ammonium nitrate, while the daytime accumulation of HNO3 is attributed to the photolysis of HONO. For the hydrolysis of SO2 to form sulfurous acid, ammonia dramatically reduces the activation energy for reaction, from 20.0 kcal mol-1 to 10.0 kcal mol-1, and leads to particulate ammonium bisulfite (NH4HSO3) which is ultimately oxidized and converted into sulfate aerosols in the atmosphere. |
