The parallel Naples airshed model (PNAM) is a parallel software package for the numerical simulation of photosmog episodes in urban scale domains. It solves the atmospheric diffusion equations, which model the air pollution dynamics in a Eulerian approach, using a symmetric time‐splitting, where the advection is separated from the (coupled) diffusion and chemistry. Presents some results of a numerical simulation of a severe photochemical smog episode, which occurred in the Naples area. A preliminary comparison with measured data is reported, as a first step toward the validation of PNAM. Some parallel performance results, obtained on an IBM SP machine, are also shown.