We have quantified the impact that an introduction of electric vehicles into the car fleet has on air quality (regarding NO and NO₂) using a numerical dispersion model. An experimental campaign is conducted close to a highway in Milan, Italy. Meteorological parameters and chemical concentrations are measured along with the traffic emissions. We use a Lagrangian Stochastic Dispersion Model to create numerical simulations of the chemical reactions and dispersion involving pollutants from the highway. To evaluate the air pollution reductions, emission scenarios with different rates of electric vehicles introduction are simulated. We have found that only a significant replacement (50%) of non-electric vehicles with electric ones yields a remarkable reduction of the pollutant concentrations. However, even with lower electric vehicles introduction rates, the air quality improvements may be relevant during intense pollution episodes. The results provide useful information to decision makers and public administrators for planning measures to modify the car fleet composition aiming to improve the urban air quality.