Reaction of ammonia with SO3 as a potential source of sulfamic acid in the troposphere has been investigated by means of electronic structure and chemical kinetic calculations. Besides, the hydrolysis reaction, which is known to be a major atmospheric decay channel of SO3, has also been investigated. The catalytic effects of ammonia and water on both the reactions have been studied. Rate coefficients for all the studied reaction channels were calculated using the transition state theory employing pre-equilibrium approximation. Calculated rate coefficients for a number of catalyzed hydrolysis and ammonolysis processes were found to be much higher (by similar to 10(5) to similar to 10(9) times) than the gas kinetic limit at ambient temperature. With decrease in temperature because of negative temperature dependence of rate coefficients, that difference became even larger (up to similar to 10(16) times). Therefore, in order to remove the discrepancies, rate coefficients for all the studied reaction channels were calculated by means of the master equation. The results showed marked improvements, with only one channel showing a slightly higher rate coefficient above the gas kinetic limit. The rate coefficients for catalyzed channels obtained from the master equation also showed negative temperature dependence, albeit to a much smaller extent. The uncatalyzed ammonolysis reaction, similar to the corresponding hydrolysis, was found to be too slow to have any practical atmospheric implication. For both reactions, ammonia-catalyzed pathways have higher rate coefficients than water-catalyzed ones. Between hydrolysis and ammonolysis, the latter showed a higher rate coefficient. In spite of that, ammonolysis is expected to have negligible contribution in the tropospheric loss process of SO3 because of large difference in concentration values between water and ammonia in the troposphere.