WRF-ARW/HERMES/CMAQ modelling system is applied with high resolution (1 km², 1 h) to assess changes in Madrid and Barcelona (Spain) air quality due to hybrid cars introduction. The substitution of the oldest cars by hybrids is effective to reduce NO<sub align="right"> 2 and PM<sub align="right"> 2.5 levels in the conurbations (up to 35%/12% in Madrid downtown, when 30% of cars are changed). In VOC-limited areas, as Barcelona, the NO<sub align="right"> x emissions abatement locally increases tropospheric ozone concentration. Downwind, however, the precursors' emissions reduction leads to lower O<sub align="right"> 3 levels. Air quality models, applied with high resolution, prove to be useful tools to manage urban air quality.