During the summer 1998, a strong pollution event was documented over Paris as part of the Etude et Simulation de la Qualit¿ de l'air en Ile-de-France (ESQUIF) project (second intensive observation period (IOP2)). From 7 to 9 August 1998 the pollution event changes from a well-marked ozone plume issued from Paris to a more general pollution over the whole Ile-de-France region. Using a three-dimensional chemistry-transport model and its adjoint part, the sensitivity of ozone, Ox, and NOx peaks to model parameters is investigated. For two locations, Paris and a suburban site, the influence of both meteorological and chemical model parameters on the simulated field concentrations is hourly quantified for each day. Processes leading to a urban polluted event are compared. It is shown that the pollutant concentrations are mainly driven by traffic and solvent surface emissions and meteorological parameters such as temperature. Since the adjoint approach is limited to infinitesimal model perturbation, some scenario simulations are carried out to evaluate the linearity of the impact of the most sensitive parameters within the uncertainty range. It is shown that the sensitivities determined from the adjoint approach can be extrapolated until their uncertainty ranges except for the wind speed.