A numerical kinetic aerosol model is utilized to study conditions under which secondary sulfuric acid aerosol can be formed in an urban atmosphere. This model incorporates the mechanisms of photochemical reactions, heteromolecular nucleation, heteromolecular condensation and thermal coagulation. Various atmospheric conditions are used to study the effects of production rate of sulfuric acid gaseous molecules, relative humidity, temperature, initial particle size distribution and kinetic coefficient for nucleation and condensation growth on the formation of new secondary sulfuric acid aerosol. Our model calculations show that the rate-limiting factor for the formation of these secondary aerosols is the amount of the sulfuric acid gaseous molecules available in the atmosphere.